Fish behavior is often varied and complex within and between species. Sensory stimuli, cyclic influences, population density and structure, habitat quality, the availability and use of space, the potential for competition and coexistence, the need to avoid predators, foraging and diet, reproduction, and other factors all contribute towards the evolution of patterns of behavior and their use. Despite the great diversity of fish species, their wide patterns of geographical and spatial distribution, and their highly variable ecological requirements, there are a number of patterns of behavior common to all fishes, as well as unique adaptations that occur only in a few.
Sensory systems and behavior
The behavior of marine fishes is shaped by sensory information provided by any one of their senses, both singly and in combination. They use vision to detect prey, avoid predators, identify species, choose mates, communicate, engage in social and territorial interactions, select and use habitat, and navigate. Fishes use their inner ear to detect sounds made by conspecifics during communication, by approaching predators, or by other fishes as they feed. If the fish has a swim bladder, these sounds can be amplified. Low frequency sounds made by movement, including struggling, are detected by the lateral line. Fishes may communicate by rasping mouthparts, gill arches, or other organs, and amplifying the sounds with their swim bladders. Touch is important in prey detection, predator avoidance, social interactions, and courtship and spawning behavior. Olfaction (smell and taste) is important in many predators for the detection of prey. The barbels beneath the mouth of a goatfish are highly sensitive and allow this fish to taste, as well as feel, potential prey. The sense of taste also allows a fish to determine quickly where a prey item is palatable or toxic. Chemical cues are also utilized for navigation. For example, salmon utilize chemical cues to detect the natal stream, where they will return to reproduce and die. Some fishes, especially sharks, skates, and rays, and the electric freshwater fishes of the families Gymnotidae and Mormyridae, are capable of detecting minute electrical currents discharged by their prey. The patterns they detect allow them to pinpoint the location of the prey, even if lies buried under benthic sediments or is obscured by turbid or muddy water. Electroreception also allows some migrating fishes to determine their geographical position relative to Earth's magnetic field. Electricity is also used for communication in gymnotids, mormyrids, and some catfishes (Synodontidae and Ictaluridae).
Fish behavior is influenced by various cycles that govern such activities as habitat use, feeding, migration, and reproduction. Circadian rhythms derived from internal or endogenous 24-hour clocks control hormone releases and subsequent behaviors. Changes in light levels on a daily or seasonal basis are a principle factor influencing rhythms. Lunar periods control tidal cycles that influence patterns of local migration, feeding, and reproduction. Seasonal shifts in water temperature or other climatic variables trigger migratory and reproductive behaviors.
Fishes and other organisms possess an internal, or endogenous, clock that is set to a period of approximately 24 hours for a given day. This clock can be adjusted daily by some sort of trigger or stimulus. Two common stimuli are the onset of daylight and the constant progression of low and high tides. The clock governs a number of basic behaviors, such as the onset of movement, feeding, or courtship, along with the hormonal activity that influences or triggers these behaviors.
Most freshwater and marine fishes are diurnal, or active in daylight, during a 24-hour period. As dusk approaches, diurnal species seek shelter in which to rest or sleep and are replaced by nocturnal species that are active during the night. At dawn, these fishes retire to shelter or simply rest until dusk approaches again. Some fishes ignore the changeover between day and night and are more or less active for 24 hours. The dawn and dusk changeover periods, also known as crepuscular periods, also trigger pronounced reproductive or predatory activities in a number of species. Vertical migrations, in which deep-dwelling species rise hundreds or even thousands of feet in the water column at night, only to descend when daylight approaches, are also triggered during these times.
Tidal shifts, either the onset of low tide or high tide, and the corresponding movement of water off or onto a flat, tide pool, or other type of habitat, govern the movements of fishes within or between affected habitats. For example, as the tide falls in a tide pool, residents must move out of the pool and seek shelter elsewhere to avoid desiccation or thermal shock. As the tide returns, so do the fishes. Similarly, predators cue on the outgoing tide and move to locations where prey will gather or pass through as they move out of an affected habitat. Tidal shifts also trigger courtship and spawning behaviors that favor the movement of pelagic eggs and larvae off the reef or flat to avoid benthic predators or, alternately, to allow pelagic larvae to move back onto the reef or flat for settlement.
Temperate marine and freshwater fishes often spawn on a seasonal basis, usually during spring or summer, although others spawn in the autumn prior to the onset of winter. Spawning in spring or summer provides an opportunity for larvae to feed and grow before the falling temperatures of autumn and winter slow growth and activity rates. Fishes spawning in the autumn have eggs that may overwinter and hatch with the onset of spring. Spawning in river species is often timed to coincide with annual or seasonal flood cycles that trigger migrations, but that also provide feeding opportunities for juveniles and adults and the increased dispersal of young.
Tropical, and some temperate, species court and spawn in relation to phases of the moon. Some species are semilunar, in that they spawn every other week in relation to the new and full moon. Others are lunar, in that they spawn just once a month, either on the new or full moon. The actual day of spawning relative to moon phase may be variable, as a number of species spawn on the days on either side of the new or full moon, but reach a peak at a the height of the phase. Semilunar and lunar spawning may also be seasonal. For example, a number of groupers (Serranidae) form spawning aggregations once or twice a year, with the time of formation centered around a specific phase of the moon.
Many reef fishes, particularly those tropical species resident at low latitudes, court and spawn daily. Their reproductive cycle is regulated by daily shifts in light. Some species spawn at dawn, others at dusk or into the night, and still others during daylight, but the time of spawning shifts daily in relation to tidal phase.
Migratory behavior of marine and freshwater fishes may be controlled by annual, seasonal, lunar, or daily cycles that trigger movement from one location or depth to another. Pelagic fishes, such as marlins (Istiophoridae) or dolphinfishes (Coryphaenidae), migrate great distances annually. These species, and numerous others, track changes in water temperature and move from winter to summer grounds, or vice versa, for feeding and reproduction. Many river species, especially in larger rivers prone to flooding, migrate annually or seasonally to take advantage of the new spawning habitats and food sources made available when bottomlands are flooded. Fishes may migrate from one body of water to another for reproduction, and their progeny often migrate back from where their parents came. Diadromous, catadromous, and amphidromous migrations and subsequent recruitment of young may be triggered by annual or seasonal stimuli. Vertical migrations allow fishes to track the movements of potential prey as they migrate up and down in the water column.
Communication is an important component of fish behavior. The transmission and reception of information by a number of means facilitates social interaction, the partitioning of space, cooperative feeding, predation avoidance, and reproduction. Visual communication is important in all but the darkest or most turbid environments. Many freshwater and marine species possess color patterns that are helpful for species recognition, sex recognition, age determination, and for assessments of agonistic and reproductive states. Both black and white coloration and bright colors are utilized. Coral reef fishes, for example, are famous for their bright color patterns, or poster coloration. (Poster coloration, a term coined by Nobel laureate Konrad Lorenz, refers to the conspicuousness and potential advertising or function of bright color patterns in coral reef fishes. Such coloration is useful in intra- and interspecific communication during territorial interactions, aggregation formation and maintenance, or mating, and facilitates species recognition.) Color patterns may be permanent or temporary. The latter is under hormonal control in relation to the expression of certain behaviors. For example, some groupers assume temporary color patterns during social interactions. The detection of bioluminescent signals at night or in low-light habitats is another component of vision-based communication. Numerous deep-sea and deep-slope fishes utilize light flashes to communicate with conspecifics. Fishes also employ body and fin displays to communicate intentions during territorial encounters, courtship, and predation avoidance.
Several species of fishes communicate with sound. Sound production is used to warn predators, warn of predators, attract mates, attract conspecifics in school formation and maintenance, and to communicate intentions during agonistic, reproductive, and parental care interactions. Sound production also places fishes at risk from predation, as some predators have learned to locate sound producers and prey upon them.
Fishes produce chemical secretions known as pheromones, which may be detected by taste or smell. Chemoreception is significant for the recognition of conspecifics in catfishes (Ictaluridae), minnows (Cyprinidae), and other species. This recognition is important in establishing and maintaining social relationships, such as dominance hierarchies or territorial interactions. Parents and young in species that practice parental care of fry and juveniles, such as the cichlids (Cichlidae), employ chemical reception to identify each other.
Fishes make use of touch when communicating intentions during aggressive behavior, courtship behavior, and parental care. Electric communication in gymnotids, mormyrids, and some catfishes (Mochokidae and Ictaluridae) is also used for aggressive and courtship behaviors, and is especially helpful in waters where visual detection is greatly reduced or nonexistent. Electrical discharges made by these fishes are species specific. Variations in production properties, such as pulse length, interpulse length, frequency, and amplitude, allow these fishes to communicate or assess information about species identity, individual identity, sex, size, reproductive readiness, and level of agonistic behavior. The fishes also obtain information on the location of and distance between communicators in this way.
Behavior and habitat use
How fishes select and make use of habitat is determined by their behavior. In marine systems, especially coral and rocky reefs, pelagic larvae actively swim shoreward as they prepare to settle into a habitat. Prior to and during settlement they assess the suitability of that habitat. For example, damselfish (Pomacentridae) larvae settling onto a portion of a reef have been observed to reject this habitat, swim back up into the water column, and search for more suitable one. Post-larvae, juveniles, and adults all utilize a variety of patterns that allow them to compete or coexist with others already using a habitat. Agonistic behavioral displays are common. Sometimes, the behaviors involved are cryptic, in that fishes "sneak" into the habitat and become established without drawing attention to themselves. Size-structured schooling species likely join the school as larvae and exploit a repertoire of innate patterns that allow them to function cohesively with conspecifics. In shoals or mixed-species schools, members use a similar repertoire to join, maintain, or leave the aggregation. Solitary pelagic species employ a repertoire of behavioral patterns that allows them to swim, feed, and avoid predation in a habitat that provides little or no cover. If cover is present in the form of drifting pelagic algae, logs, or other flotsam or jetsam, small pelagic species seek shelter there. Some species, such as the sargassumfishes (Antennariidae), are adapted to life in floating sargassum, where they shelter from predators, ambush prey, and mate. Other fishes, such as juvenile butterfishes (Stromateidae), shelter within the tentacles of pelagic jellyfishes. Many small pelagic fishes recruit to floating structures, and larger predators are attracted in turn.
Benthic freshwater and marine species often adapt to specific conditions and make use of seemingly novel structures that provide shelter, feeding sites, or places for reproduction. In moderate or fast-moving streams, trouts and charrs learn to make use of rocks, logs, holes, undercut banks, and other forms of structure as shelter. Their swimming behaviors allow them to move up into the current, feed or chase off intruders, and return to their shelter sites. Sand-dwelling darters (Ammocrypta; Percidae) in slower moving streams rest on the sand, but bury into it to avoid predators. On coral reefs, various species use specific behaviors to burrow into sand, rubble, cobble, or mud in order to avoid predation, ambush prey, or rest. Fishes that employ burrows use those dug by other organisms or dig and maintain their own structures, often building multiple entrances or exits. They swim, hover, or rest near these burrows as they feed or engage in social interactions, but will dart quickly into an entrance if threatened. If resting inside a burrow, they may quickly escape via an alternate route if a predator is blocking or has entered one of the entrance points. These burrows are sometimes shared by more than one fish species or by invertebrates, such as snapping shrimps, in a symbiotic relationship. Other benthic fish use abandoned tubes made by polychaete worms or other invertebrates. These fishes swim out of these tubes to feed or mate, but return and move backward into them if threatened. Some tube-dwelling species use these structures as cryptic ambush sites from which they attack passing fishes. Shrublike corals, sea fans, and black corals all provide structures for a number of small reef species. These structures are shared because intra- and interspecific behavioral interactions define the use of space and reinforce order and structure within the coral head.
Agonistic behavior is employed by fishes to establish social dominance, defend territories, and ward off potential predators, and involves the use of displays given at increasing levels of intensity. The displays are fixed or modal action patterns, and the sequence of their use is often highly ritualized. The information communicated by a pattern or series of patterns in sequence is therefore recognized in the context of its use, and aggressive behavior leading to the injury of one or more parties in the interaction is often averted.
Dominance of one or more individuals by the agonistic behavior towards another of the same species occurs among groups of individuals, in shoals, and in schools. Dominance is expressed in either of two ways. First, a single individual may dominate all others who hold equal rank under the dominant fish. More commonly, a dominance hierarchy forms linearly, with a single alpha individual dominating others. The alpha is followed by a beta individual that dominates the remaining individuals, and so on. Dominance hierarchies such a this are often ordered on the basis of body size, with larger fishes dominating smaller fishes. They may also be ordered by different levels of aggressive behavior between individuals, with the more aggressive fishes dominating less aggressive fishes. Mating groups of sex-changing fishes also have hierarchies. For example, the mating group of the hawkfish Cirrhitichthys falco (Cirrhitidae) consists of a single dominant male, a large dominant female, and two or more subdominant females of variable size, which dominate one another on the basis of greater body size.
Agonistic behavior is used to defend living space, food resources, or mating groups and sites. This is known as territorial behavior. Territorial displays include the use of body displays, erect fins, color changes, sound production, chasing, or a combination of patterns. Defense may be against both intra- and interspecific intruders. The latter include competitors for food and space, but may also include potential predators of a defender's nest of eggs or free-swimming offspring. Territories usually consist of an area of relatively fixed size. The size of a territory varies within species and between species, usually as a function of size or sex, but will also vary in relation to the give and take of interactions with neighbors. A territory will often be nested in a much larger home range that is utilized by the fish or a group of fishes. Only that space that is actively defended by an individual is considered a territory. Territories may be permanent or temporary. For example, territories needed for courtship and spawning of a number of fishes are formed only during the breeding season or at certain times during the breeding season. Agonistic behavior increases during these times, but will often be absent during nonmating periods. Fishes may leave their home ranges or territories to form temporary multimale territories at courtship sites, known as leks, at which to attract females for mating. Some of these leks are "floating," in that their position may change relative to the location of females in the area. Territories required for feeding may be quite large, especially for larger predatory fishes, and may often greatly exceed the area defended for shelter space. Permanent territories often involve the defense of a shelter site. Males in single-male, multifemale, mating groups will defend their territories and those of the females contained within. Females, in turn, defend their smaller territories from intrusions by neighboring females within the same group. Mating sites of these fishes are defended in the same way. At the extreme end of territorial behavior is the defense of personal space, as seen in shoaling or schooling fishes that display to, or ward off, neighboring fishes who swim too close.
Territorial defense is also practiced by monogamous pairs of fishes. The best-known examples are the butterflyfishes (Chaetodontidae) on coral reefs. Pairs of butterflyfishes patrol
a territory and may encounter potential competitors for food or space. These competitors include conspecifics (also usually in pairs), as well as other species that may utilize the same resources. Defense involves an exchange of ritualistic displays culminating in the departure of the combatants. Highly territorial butterflyfishes, not usually in pairs, chase intruding competitors away; however, noncompetitive species are usually ignored. Clustering behavior, in which one or more individuals within a group, or mosaic, of territories rises up into the water column to assess others in the mosaic without engaging in agonistic interactions, is an interesting offshoot of territorial behavior, and has been observed in damselfishes.
Fishes form shoals or schools for protection from potential predators, foraging, overcoming the territorial defenses of individuals migration, and reproduction. Shoals are unorganized aggregations and may often be temporary in nature. They may consist of different species with a changing membership (heterospecific shoal), are relatively unstable, and may be dissolved and reformed quickly. Schools are organized or polarized aggregations that form permanently or temporarily. Generally, they are monospecific (contain only one species),
and membership is often age specific. Schools may dissolve at night for diurnal species, during daylight for nocturnally active species, or remain constant over a 24-hour period. Some schools dissolve under heavy attacks from predators, but reform afterward if sufficient numbers of fishes survive. The movements of schools are governed by a unique set of behaviors that determines position, synchronized movements, swimming speed, evasion, and flight.
Fishes have been found to be capable of transmitting traditional information socially. For example, certain benthic reef fishes follow predictable long-term routes between feeding and sheltering grounds as either darkness or dawn approaches. Experimental manipulations of the composition of a shoal of these fishes have shown that resident fishes can transfer information about the location of resting shelters to fishes new to the shoal during a dusk or dawn migration.
Fishes form symbiotic relationships with other fishes, or with invertebrates that share some common form of micro-habitat, food, or need. An excellent example of shared microhabitat is seen in the anenomefishes (Pomacentridae) and the burrowing shrimp gobies (Gobiidae). Anenomefishes live in close association with anenomes, flowerlike benthic invertebrates related to corals that have poisonous nematocycts in their tentacles and are capable of inflicting an injurious or deadly sting. The function of these nematocysts is to deter predators and immobilize prey. Anenomefishes have developed defenses against the effects of the sting and make use of the tentacles for shelter and nest sites. Part of the anenomefish's defense is behavioral, in that its undulating movements within the tentacles communicate to the anemone that it is neither a threat nor some form of prey to be stung and ingested. In exchange, the anemonefish defends the anenome against potential predators, such as butterflyfishes, that feed upon the anemone's tentacles. Anenomefishes may also "groom" the anenome by removing foreign matter and feces, as well as consuming potential parasites. Shrimp gobies share burrows with one or more species of snapping shrimps of the family Alpheidae. The shrimp, which is usually blind, constructs a burrow that may be shared with one or two individuals of a given species. The goby, or pair of gobies, guards the burrow at its entrance while the shrimp maintains it. If danger approaches, the shrimp is alerted as the goby or gobies signal it with flicks of the caudal fin. If the threat continues, the goby or gobies will dive headfirst into the burrow and the shrimp will retreat. Often there is species specificity between shrimps and gobies, and there is some evidence that both the goby and the shrimp settle out of the plankton together as post-larvae, prior to the construction of their shared burrow. The behavior exhibited by the anenomefishes and anenomes, and the shrimp gobies and the burrowing shrimps, is known as a mutualistic symbiosis, in that parties benefit from the relationship. Other examples of mutualistic behavior relating to cleaning and the control of parasites, and commensalistic behavior, such as the relationship between remoras and large pelagic fishes, are discussed in the section on feeding behavior.
Behavior is an essential component in the reproduction of fishes, and is essential for the identification of conspecifics, the attraction and selection of mates, the process of courtship and spawning, and, in a number of fishes, parental care. To perform these functions, fishes have developed a repertoire of behavioral patterns. These are often used in conjunction with some physical trait, such as a larger body size, elongated fin rays, larger hook-shaped lower jaws (kypes), well-developed humps on the forehead, and unique color patterns (temporary or permanent). These physical traits are coupled with one or more behavior patterns, whose use and intensity of use accentuate the traits. The products of sexual selection, these patterns and traits confer reproductive advantages to individuals in a given species population. Other patterns are essential for the physical act of spawning or breeding to occur, or for parental care to be successful.
Fishes identify conspecifics as potential mates by the recognition of species-specific morphological shape and form, color patterns, scents, or various behaviors. This recognition is not as easy as it seems because of variation in each as a function of sexual dimorphism or individual variation. Sometimes "mistakes" in species identification are made, and interspecific mating occurs. Most of these mating attempts likely fail, but occasionally functional offspring, or hybrids, result. Once species identity is recognized, the next step is to determine if the potential mate is of the opposite sex. Again, differences in morphology and color pattern or other factors accentuated by sexual dimorphism are recognized. If no such differences exist, then the use of behavioral patterns becomes an essential tool for the recognition of sex. Once sex is recognized, potential mates have to determine if they are attractive or suitable for one another and if they are ready to mate. Sexually selected traits and patterns are utilized to determine this. In many mating systems, females select mates. Females are attracted to males that appear stronger, fitter, and capable of providing the greatest investment for her offspring. Males utilize a series of behavioral patterns to convey this impression. They may also emphasize the quality of their resources, such as feeding or sheltering territories, nest sites and qualities, or spawning site locations. Females assess these attributes and select the most attractive male or males, accordingly. Males, for their part, may wish to mate with the largest female or the most females possible. Larger females tend to be more fecund, thus increasing a male's opportunity to fertilize more eggs, pass down his genes, and achieve greater fitness. Mating with more than one female also increases fitness. Males compete with other males for access to females. The quality of their behavioral and physical traits and their use in intermale interactions, convey an advantage to one male over another.
Courtship patterns are used to attract mates, assess spawning readiness, and facilitate spawning or mating. A male employs a series of patterns to attract passing females or initiate courtship with females that form part of his mating group, and courtship bouts ensue with variable success. For example, in single male, multifemale mating groups (called "harems" by some authors), a male courts and spawns with each of his females in succession. Unfortunately for the male, this is not always easy. Individual variation in readiness to spawn among females means that a male may have to make repeated visits to females within his group before spawning takes place, and there is no guarantee that he will be successful in spawning with all females during a given spawning period. Sometimes the male is so busy attempting to spawn with one female that too much time passes and the "spawning opportunity window" closes before he can mate with others in his group. Alternatively, especially at dusk when light levels are falling, the male may become the target of a predator as he moves between females to court. Predator avoidance is costly because spawning opportunities may be lost. Worse, from the male's perspective, is that he might become lost, and control of the mating group will pass to a sex-changing female within his group or to another male from outside the group. Females are also at risk while waiting to court and spawn, and this threat may affect spawning readiness.
If courtship is successful and the male is able induce the female to spawn or mate, another series of patterns comes into play, which help the pair (or group, if spawning is in an aggregation) synchronize their activities so that the spawning or mating event is successful. Some of these patterns are as gentle as a male nuzzling the female's abdomen during a paired pelagic ascent, as in some of the marine angelfishes (Pomacanthidae). Other patterns, such as a sharp-toothed male shark grabbing a female's flank so that he can insert his clasper inside her and attempt internal fertilization of her eggs, are more forceful.
Modes of reproduction classify how fishes reproduce physically. The mode itself is not specific to a single taxonomic group, but may be shared by many taxa regardless of their phylogenetic affiliation. Criteria are defined based upon the degree of parental care, if any, invested. Thus, fishes may fall into three general guild categories: those that do not guard offspring, those that guard offspring, or those that bear offspring. Nonguarding species spawn openly upon substrates, either pelagically or benthically, or they hide their broods. Open pelagic spawning occurs in the water column. Fishes swimming in the water column engage in courtship and release eggs. Benthic fishes swim upward into the water column and release eggs and sperm at the apex of the spawning rise or rush. Spawning may be paired or in groups. Open benthic spawning includes the release of eggs onto the substrate (rocks, cobble, etc.), with the resultant larvae being either pelagic or benthic. Benthic spawning may also occur on plants as an obligatory or nonobligatory function, or on sand. Brood hiders deposit their eggs on the bottom, in caves, on or in invertebrates (such as corals, bivalve mollusks, and crinoids), or on beaches during a tidal cycle. They may also deposit eggs on a substrate that is prone to annual desiccation, in which case the eggs are adapted to resist this and hatch out when wet conditions return.
Spawning aggregations are a specialized behavior, and are formed by migrations of fishes to specific sites for courtship and spawning. There are two general types of spawning aggregations: resident and transient. Resident aggregations occur locally, in that members of the aggregation are drawn from the general area in which the aggregation forms. These aggregations usually form on a daily, semilunar, or lunar frequency. Transient aggregations consist of fishes that are drawn from a much larger area and population. Some species of groupers, for instance, migrate hundreds of miles along coastal waters in the Gulf of Mexico and Caribbean, and the number of individuals forming the aggregations can be in the hundreds or even thousands. These aggregations usually form annually or seasonally in relation to the lunar period. Both kinds of aggregations form at sites that appear to facilitate mating and the dispersal of eggs and larvae.
Guarders include those fishes that choose the substrate they spawn upon with subsequent guarding of the offspring. The substrates chosen include rocks, plants, terrestrial structures (such as overhanging leaves or flooded grasses), or the water column. Nest spawners construct simple or complex nests to attract females, deposit eggs, and provide parental care. Nests are made from a variety of materials, including gravel and rock, sand, holes, plant materials (with or without a "glue" secreted by the male), anenomes, or bubbles. In the latter case, males of some species make the nest by blowing bubbles of air and mucous onto an object or even the underside of the surface of the water. Miscellaneous materials are also utilized in nest construction. Generally speaking, nest construction is more common in freshwater than in marine species.
Fishes that bear young are either external or internal bearers. External bearers include mouth brooders, pouch brooders, gill chamber brooders, forehead brooders, skin brooders, or brooders that transfer offspring between one individual and another. Fertilization may be external or internal depending upon the taxon. Internal bearers have internal fertilization. Oviparous fishes deposit egg cases on the substrate; these eggs hatch externally. Ovoviviparous fishes retain fertilized eggs until they hatch and then release offspring "live." Viviparous fishes retain fertilized eggs that, as embryos, develop internally and are also released live. There are two forms of this strategy. The first is yolk-sac viviparity, in which the egg's yolk sac is attached to the digestive system of the developing embryo. The second is placental viviparity, in which a placental connection between the mother and the developing embryo occurs. Not all internally fertilizing fishes have internal bearing, however. For example, glandulocaudine characins (family Characidae) and many catfishes of the family Auchenipteridae have internal fertilization but are egg scatters. Similarly, members of the genus Campellolebias (family Rivulidae) have internal fertilization but are egg hiders.
One specialized form of live bearing is parthenogenesis, in which young are produced by a female without the fertilization of eggs by males. There are two forms, gynogenesis and hybridogenesis. In gynogenesis, an egg is activated following mating with a male of another species, but no fertilization of that egg occurs. The egg develops within its mother and is born as a female identical to the mother. In hybridogenesis, mating with a male of another species also occurs, but the egg is fertilized. The male's genetic component is discarded and the egg develops into a female identical to the mother.
Fishes select sites for reproduction in a variety of ways. For instance, salmons, trouts, and charrs make short or long-distance migrations to spawning habitat. Spawning occurs in gravel beds or other suitable substrates and includes the preparation of a redd, a depression made into the substrate where eggs are deposited, fertilized, and buried. In other fishes, nest sites may be placed within the territory of a male (or less commonly, a female) and are defended after spawning takes place between the nest owner and one or more mates. Nest sites may also be located within a home range of a temporary or permanent pair of fishes. There the eggs are deposited and fertilized, after which the site is abandoned. In some species, there is no nest at all, and eggs are merely broadcast over a suitable substrate. Spawning sites of pelagic species follow similar rules. Sites may be within a territory or cluster of territories, and are defended against intruders (usually same-sex rivals). Sites may also be used on a regular basis, due to some physical feature that may favor pelagic spawning, but are not defended.
Parental care of eggs and offspring is most often practiced by males than females, and is more commonly seen in freshwater rather than marine species. In some species of African cichlids, such as members of the genus Lamprologus, care is provided by helpers that are usually related to the parents and offspring. These helpers forego the opportunity to breed at this time, but manage to realize some sort of evolutionary fitness by learning parenting behaviors and by protecting offspring that carry a fraction of their own genes.
Care behaviors provide defense and maintenance of offspring. Parents protect the eggs and larvae in their mouths, brood pouches, or other structures, but also attack intruders that attempt to prey upon them. Defense also includes herding or shielding larvae or post-larvae from attacks. Maintenance behaviors include blowing on eggs to provide them with oxygen and to remove detritus or other undesirable objects (including dead eggs). Parents also provide alternative food sources for growing larvae. For example, some cichlids secrete a skin mucous that provides nutrition for their young, who ingest this by "glancing off" the flanks of their parents where the mucous is deposited. Intertidal species may wrap their bodies around an egg mass to shield it from desiccation during low tide. A freshwater tetra, Copella sp. (Lebiasinidae) from South America, lays its eggs on the leaves of overhanging terrestrial plants to avoid predation, and splashes water on the egg mass to provide it with oxygen and to prevent desiccation. Electric eels and some bagrid catfishes produce infertile trophic eggs to feed their free-swimming young.
A number of fishes practice alternative mating tactics that exploit certain behaviors to a strategic advantage. For example, in mating systems with paired spawning, the pair consists of a dominant or parental male and a female. Other, smaller, males in the vicinity, known as satellite males, mimic females both in color and behavior. The satellite males approach a mating pair and, if successful at "fooling" the male, will not be chased off. Then the intruder inserts itself into the dominant male-female courtship bout and attempts to fertilize at least some of the female's eggs. In other cases, smaller "sneaker" males use stealth to approach a mating pair, then quickly streak or dart into the pair's spawning bout as eggs are released to fertilize a portion of them. Among nesting fishes, both tactics have been observed in the bluegill sunfish (Lepomis macrochirus; Centrarchidae), whereas the latter has been seen in salmons.
In pelagic spawning fishes, smaller satellite males will streak into the water column to join a pair during their spawning ascent and fertilize part of the female's eggs. This has been observed in a number of groups, and variations on the theme occur. Two independent teams of researchers studying the reproductive behavior of the "haremic" Japanese sandperch (Parapercis snyderi; Pinguipedidae), observed repeated sneaking behavior by dominant males from neighboring mating groups instead of by satellite males. One team found that these males sneak fertilizations in neighboring groups after spawning with all the females in their own groups had been completed for the night. The other research group also observed this pattern, but found that dominant males temporarily abandoned courtship with their own females and carried out "sneak" fertilizations in a neighboring group in close proximity to their own location, when the opportunity presented itself. Males also spent considerable amounts of time and effort defending their groups from sneaking neighbors. The downside of these behaviors was that the opportunity to mate with their own females was lost and the females mated with other males while their males were busy.
Lizardfishes (Synodus dermatogenys; Synodontidae) have two strategies that depend upon local population size and sex ratio. If the population is relatively low, and the numbers of female and males are approximately equal, paired courtship and spawning occurs. If, however, the population size is larger and males outnumber females, then a different strategy prevails. In this case, males form floating leks at sunset to display to larger females as the latter move about the spawning site. One male may be more successful and joins the female as she rises into the water column to release her eggs. As the female and male ascend, however, they are joined by other males, who all contribute sperm toward the fertilization of the eggs. Females do appear to exercise control over the timing of the spawning and the composition of the group. Unlike females of many other pelagic spawning reef species, female lizardfishes may spawn more than once during an evening. As courtship continues, the female rises to spawn again and is joined by more than one male. If, however, only one male joins her, she will abort the spawning ascent, return to the bottom, and wait for courtship from the group of males to resume. Females seem to pursue this strategy of group spawning in order to assure complete fertilization of their egg masses. The male closest to her in an ascent, the one she chooses from the others as the most attractive, will likely fertilize a significant proportion of her eggs, but the contributions of the other males may promote both genetic diversity and completion of the job!
Fishes use a variety of behaviors while feeding to detect and capture prey, extract plant materials, or to sift sediments to extract objects of nutritional value. Planktivory by fishes occurs during the day and at night. During daylight, hovering planktivores such as fairy basslets (Serranidae: Anthiinae), some freshwater sunfishes, damselfishes, angelfishes, certain triggerfishes (Balistidae), and many other species, feed upon zooplankton or phytoplankton by plucking individual plankton out of the water column. Plucking is accomplished with specialized mouthparts or by rapid movements of the mouth. The mola (Molidae), and other species that feed upon macroplankton or mesoplankton in the water column, forage in a similar fashion. Garden eels (Congridae) and other burrow-dwelling planktivores emerge partially from their burrows to feed upon plankton that drift past in the current. Schools of fusiliers (Caesionidae) dart erratically in the water column as they grab and feed upon plankton they detect there. Alternately, fishes such as whale sharks (Rhyncodontidae), basking sharks (Cetorhinidae), manta rays (Mobulidae), herrings, anchovies (Engraulidae), scads (Decapterus spp.; Carangidae), and similar species open their large mouths and strain the plankton from the water column as they swim. Fishes with smaller mouths, such as reef herrings and flyingfishes (Exocoetidae), also strain plankton in the water column. At night, a new set of planktivores emerges from shelter to feed upon pelagic and demersal plankton in the water column. These include squirrelfishes and soldierfishes (Holocentridae), cardinalfishes (Apogonidae), bigeyes (Priacanthidae), along with a host of deep-dwelling species. These fishes use their large eyes and other well-developed senses to detect and feed upon plankton.
Herbivory in marine fishes is more pronounced in tropical than in temperate species. Among the tropical fishes, halfbeaks (Hemirhamphidae), sea chubs (Kyphosidae), damselfishes, parrotfishes (Scaridae), blennies (Blenniidae), rabbitfishes (Siganidae), and surgeonfishes (Acanthuridae), are the prominent groups. In freshwater systems, numerous species feed upon benthic algae, emergent plants, and even seeds and fruits from terrestrial plants. Some marine groups, such as the butterflyfishes, angelfishes, filefishes (Monacanthidae), and triggerfishes, include species that are omnivorous and feed upon benthic algae. In temperate marine waters, some members of the Sparidae (porgies), Kyphosidae, Aplodactylidae (sea carps), Odacidae (rock or weed whitings), and
Stichaeidae (pricklebacks) are more dependent upon plant life. Freshwater omnivorous species, such as the carp and its relatives (Cyprinidae), and a number of characins (Characidae), often include plant materials as a significant part of their diet.
Herbivores feed by grazing or browsing, and are capable of learning what species of plant are edible and what are toxic. Access to edible benthic algae, sea grasses, or other plant materials may be as simple as swimming into a given area and stopping to graze or browse. Parrotfishes feed on zooxanthellic algae contained in coral skeletons by using their specialized beaks scrape algae off rocks or dead corals. They also bite off chunks of the coralline skeleton as they graze. The parrotfish's pharyngeal teeth crush the chunk within the mouth cavity, swallow and extract the algae, expel the resulting coralline sand. Territorial species, such as certain damselfishes known as "farmer" fishes, actively maintain patches of desirable species of algae that they tend, defend, and feed upon. Other herbivores attempting to feed upon this patch (or patches of algae not tended, but within the territory of any herbivorous species) are turned away. Fishes defending the patches must be overwhelmed before other herbivores can gain access to this resource. One way to accomplish this is for the grazing or browsing species to form schools that can move into a territory and easily outnumber the defender. One form of this type of school, the heterospecific or mixed-species shoal or school, consists of fishes of a number of species (including nonherbivorous fishes) that move about the bottom. Membership in the school is temporary, and its members not only gain protection from schooling but, more importantly, are able to breech the defenses of a territorial herbivore to feed upon the algae it attempts to protect.
Feeding by herbivorous monospecific schools takes place both at night and during the day. For example, unicornfishes (Acanthuridae) form schools that graze on sea grass flats at night. Their efficiency at feeding upon sea grass and benthic algae there is analogous to a giving the flat a good haircut.
Predators rely heavily upon one or more sensory systems in their search for prey. Among benthic species, ambush predators such as scorpionfishes (Scorpaenidae), flatheads (Platycephalidae), and sculpins (Cottidae) use vision to detect passing prey. Scorpionfishes and many sculpins remain motionless until they detect prey and estimate the distance to it. The predator then engulfs the prey by rapidly opening its mouth and sucking it in. Flatheads grab the prey with a mouthful of teeth, and manipulate and swallow it. Other ambush predators, such as lizardfishes or hawkfishes, launch themselves into the water column or down to the substrate to grab prey. Freshwater pikes and pickerels (Esocidae), groupers, basses (Centrarchidae), the Murray cod (Percicthyidae), and the barramundi (Centropomidae) utilize structures, such as submerged trees and stumps or weed beds, to mask their presence as they ambush passing prey. Their acute vision allows various trout species in streams to identify and assess aquatic insects carried by surface and subsurface currents. At night, the enlarged eyes of many nocturnal predators allow them to detect, track, and attack prey on the bottom or in the water column.
Other sensory systems, such as taste, touch, and chemoreception, allow fishes that prey on benthic invertebrates and smaller fishes, such as catfishes (various families), goatfishes, threadfins (Polynemidae), freshwater eels (Anguillidae), and moray eels (Muraenidae), to detect prey buried just beneath the surface of the substrate. Similarly, electrical receptors known as ampullae of Lorenzini allow elasmobranches, such as sharks and rays, to detect minute electrical currents generated by prey buried beneath sand, rubble, or mud; locate their position; and feed upon them. Knifefishes (Gymnotiformes) of South America and the Mormyridae of Africa use other organs to generate weak electric fields that allow them to detect prey in murky water.
In the pelagic realm, vision, olfaction, touch, and sound detection are important sensory components of predatory behavior. Swiftly moving predators, such as tunas (Scombridae) and billfishes (Istiophoridae), rely upon keen eyesight to track and hunt their swiftly moving prey. Predators, especially sharks, also rely upon the smell of prey, and in particular, the smell given off by injured prey, to detect them. Low frequency sounds generated by the movement of prey, whether swimming or struggling, are detected by the predator's lateral line system. These vibrations are felt, rather than heard, by the lateral line receptors. Higher frequency sounds generated by prey are detected by the inner ear and direct it to the location of the prey.
In deepwater environments, visual capabilities may be reduced in favor of other senses in some species, but accentuated in others. Deepwater predators with large eyes can detect bioluminescent flashes generated by potential prey. Chemoreception, hearing, and lateral line senses are also important in prey detection in waters where darkness prevails and little or no light penetrates from above.
The pursuit of prey varies among predatory species. Ambush predators frequently employ camouflage and position themselves among rocks, corals, or marine plants to conceal themselves until they can detect and ambush prey. Some ambush predators use lures fashioned from modified fin rays or other body parts. These lures, which may resemble a small fish or invertebrate in shape, are waved about to attract the prey. This behavior, sometimes referred to as aggressive mimicry, occurs in a few shallow and deepwater predatory families of fishes. In deepwater species, the lure may be bioluminescent. Other predators, such as juvenile snappers (Lutjanidae) or hamlets (Serranidae), mimic nonthreatening species such as damselfishes to get closer to prey. Predators, such as trevallys (Carangidae), often rapidly patrol the bottom throughout the day and night in search of prey that cannot retreat to shelter quickly enough to escape being eaten. These predators may also attack schooling baitfish species in the water column singly, in pairs, groups, or schools. Barracudas (Sphyraenidae) rest motionless in the water column, then strike rapidly, sometimes over a distance of several meters, at an unwary fish in the water column or on the bottom. Schools of salmon, striped basses (Moronidae), amberjacks (Carangidae), bluefishes (Pomatomidae), and tuna (Scombridae) rapidly chase and slash schools of fleeing baitfishes, and may also herd them while attacking. Other species, such as some barracudas and lionfishes (Scorpaenidae), hunt in packs and often utilize structures, including reef and cave walls and even suspended fishnets, to act as barriers to aid in escape. At sunset, a pack of lionfishes assembles and gathers near a school of sweepers (Pempheridae) that is emerging for the night. Then the lionfishes extend their large pectoral fins and use them to push the sweepers into an increasingly small aggregation that is ultimately trapped between the pack of lionfishes and the reef wall. The predators then rapidly inhale the sweepers with their large mouths as they try to escape. Other predators, such as groupers and large soapfishes (Serranidae), take advantage of this behavior to ambush escaping sweepers.
Many species detect prey hidden in bottom sediments and then dig or sift them out. A number of these predators have specialized teeth, mouths, or gills that allow them to do this; others fan the sediments to expose the prey. Some fishes, such as certain wrasses (Labridae) and trevallys, allow other fishes, such as stingrays and goatfishes, to do the digging for them. These predators follow the bottom-foraging species and feed opportunistically upon whatever may be exposed. Barracudas have been observed hanging motionless in the water column above nests of large triggerfishes (Pseudobalistes spp.). The triggerfishes constantly tend these nests by rearranging the substrate and turning over rocks, and in doing so they expose or startle prey fishes or other organisms. When this happens, the barracudas quickly rush down to the exposed prey and grab it before the triggerfish can respond.
Pelagic deepwater species ascend the water column as night falls and may rise hundreds of meters toward the surface as they track their prey. Some species are following the similar movements of pelagic invertebrates upon which they feed. Others track these fishes as prey, and some species, such as the snake mackerels or oilfishes (Gempylidae), have feeding behaviors that appear to be similar to shallow water species, such as barracudas (Sphyraenidae) or wahoos (Scombridae).
A number of predators employ mechanisms or behaviors that allow them to stun or otherwise immobilize their prey before they eat it. For example, torpedo rays (Torpedinidae), electric eels (Electrophoridae), electric catfishes (Malapteruridae), and some stargazers (Uranoscopidae) discharge a strong electrical current that stuns passing prey. Some wrasses (Labridae) smash their invertebrate prey with or against rocks. Archerfishes (Toxotidae) stun and knock down insect prey resting on mangrove branches or emergent grasses above the water surface by shooting a stream of water "bullets" at them. Billfishes and the swordfish (Xiphiidae) utilize their bills to herd, stun, spear, or slash their prey before eating them. Hammerhead sharks use their unique cephalic lobes to pin down stingrays, a favored prey. Sharks and barracudas use sharp teeth to capture and cut prey into smaller pieces before they are eaten. Sharks also take bites out of larger prey, such as whales, without capturing them. Relatively diminutive fishes, such as the poison-fang and fang blennies also practice this behavior. These small predators hover in the water column and launch themselves at passing fishes to bite off a scale or small piece of flesh. Fishes in the genus Aspidontus mimic blue-streak cleaner wrasses (Labroides dimidiatus), approaching fishes that may be fooled into thinking they will be cleaned, but end up being bitten. Juveniles of larger predators, such as the leatherback, Scomberoides lysan (Carangidae), also engage in this behavior. Scale-eating or biting off small pieces of flesh is also practiced in tropical freshwater fishes of Africa and South America. Specialized genera of scale-eating African cichlids (Cichlidae) include Perridodus and Plecodus from Lake Tanganyika, and Corematodus and Genyochromis from Lake Malawi. Fin-eating is practiced by characoid fishes of the families Citharinidae (genera Ichthyoborus, Mesoborus, and Phago from Africa) and Characidae (subfamily Serrasalminae, the piranhas and their relatives; genera Catoprion and Serrasalmus from South America).
Cleanerfishes, including the cleaner wrasses Labroides and Labropsis (Labridae), cleaner gobies (Gobiosoma), and some butterflyfishes establish cleaning stations along coral or rocky reefs, where they attract and clean the "client" fishes that approach. The cleaners swim in a regular pattern of movements. The client fishes, responding to the swimming behavior and distinctive color pattern of the cleaner, as well as to the location of the cleaning station, assume a posing posture to signal that cleaning is required and may begin. The cleaners then forage along the client's body, feeding on parasitic copepods and other parasitic organisms, along with any damaged tissues. Cleaner wrasses and gobies also enter the mouths and gill cavities of their clients, including large predators like groupers and moray eels, without being preyed upon. This type of behavior is a mutualistic symbiosis, because both the cleaner and the client benefit. Cleaning behavior has also been observed in freshwater fishes, including some members of the family Cichlidae. A pelagic variation of this behavior is practiced by remoras or sharksuckers (Echeneidae). These fishes attach themselves to large predators, such as sharks, billfishes, turtles, and whales, and hitchhike as their hosts swim. In turn, the remoras feed upon parasitic copepods on the host, but will also take advantage of scraps left over from the host's feeding bouts. This behavior is a commensalistic symbiosis, in that the cleaner gains while the client neither benefits or loses out.
Parasitism by marine fishes is not a common strategy, but it does occur in a number of different species. Internal parasites include the pearlfishes (Carapidae) that live in the gut cavities of seacumbers and large starfishes, where they feed upon gut tissue. The eel-like Simonchelys parasitica (Synaphobranchidae) has been found burrowed into the flesh of various bottom-dwelling fishes, but has also been recorded from the heart of a mako shark, a fast-swimming pelagic species. In freshwater fishes, however, catfishes of the tropical Amazon family Trichomycteridae are parasitic. Most species attack gill tissues of larger fishes, but members of the genus Vandellia are known to parasitize the urethra of mammals, including humans, causing considerable harm. Lampreys (Petromyzontidae) are external parasites, which attach themselves to the skin of their prey and feed upon tissue and body fluids with their specialized mouths.
Fishes that feed upon detritus use their mouths to scoop up sediments, from which they extract detrital materials with their pharyngeal teeth and expel sediments through their gills. Scavengers feed upon dead and dying fishes or other organisms and play an ecological role. Fishes bite or peck at the body of the organism to remove chunks of flesh. Some fishes, such as the deepwater hagfishes (Myxinidae), are specialized to enter the body cavity of dead fishes to feed upon them internally.
Predator avoidance behavior
Marine fishes have evolved a number of mechanisms and behavioral strategies to avoid predation. These include color patterns and modifications of body structure to provide camouflage, mimicry, or warnings of toxicity. Color patterns that disrupt the fish's outline, reduce its contrast against background coloration, or allow it to blend into the background all provide camouflage and protection from predation. These same attributes also favor predators who wish to hide from their prey. Countershading (dark color dorsally and pale or white ventrally) and reverse countershading (pale or white dorsally and dark ventrally) obscure the fish when it is viewed from above or beneath. A silvery or mirrorlike coloration, as seen in herrings, tarpons (Megalopidae), ladyfishes (Elopidae), smelts (Osmeridae), carps and minnows, and mullets (Mugilidae), reflects light and confuses potential predators. Fishes that are relatively transparent, such as in the glassfishes (Channidae) and some cardinalfishes, are difficult to see, especially in low light conditions. Similarly, modifications to the skin, fin rays, or other portions of the body can convey similar benefits. For example, the sargassumfish (Antennariidae) has modifications, which, in conjunction with its greenish brown coloration and hovering behavior, allow it to resemble sargassum algae. Pipefishes and seahorses (Syngnathidae), and some filefishes, have similar adaptations. Adult leaffishes (Nandidae) and juveniles of a number of species, including spadefishes and batfishes (Ephippidae), combine a color pattern with behavior to resemble inedible objects such as leaves.
Warning coloration informs potential predators that a fish is (or is giving the impression of) being toxic and should be ignored. The juveniles of some species of sweetlips (Haemulidae) have color patterns and behaviors that allow them to mimic toxic nudibranches and flatworms. Other fishes with toxins in their skin or organs, such as various tobys or sharp-nose puffers (Canthigaster spp.; Tetraodontidae), have color patterns that advertise their toxicity. One species of filefish, Paraluteres prionurus, is nonpoisonous, yet is afforded protection from predation because it has a color pattern and behavior that exactly mimics the black saddled toby (Canthigaster valentini), and, to a lesser extent, the crowned toby (C. coronata).
Behavioral responses to perceived threats detected by senses such as vision, hearing, the lateral line, and smell may be rapid or subdued. These responses are used by solitary individuals but are accentuated in aggregations or schools. Many species react swiftly to the sight of an approaching predator or to the detection of noises or pressure waves generated by its approach. Flight avoidance, usually in the form of rapid or erratic swimming away from the predator, occurs in the water column. Some species, such as flyingfishes, halfbeaks, and needlefishes (Belonidae), leap above the surface of the water and may coast for a few to several meters in the air before entering it again. As a school or aggregation, reef herrings and other baitfishes leap repeatedly into the air as they flee approaching predators. Schooling behavior provides significant predation avoidance benefits because of the fact that there is safety in numbers. Most predators have to target a single individual to successfully prey upon it. The presence of many individuals within a school means that, with more prey to choose from, the chances of any one healthy individual of being preyed upon is reduced as the school takes flight.
Fishes with less energetic responses than the reef herrings, such as lionfishes and other scorpionfishes, merely erect pectoral and dorsal fins that have poison-tipped spines to detract predators. Other fishes with poisonous spines show a similar behavior. Adult pufferfishes (Tetraodontidae) and porcupinefishes (Diodontidae) inhale water and inflate their bodies to avoid predation by all but the largest predators. These fishes may also be poisonous or have spines that make ingesting them difficult. Other species, such as garden eels (Congridae), sanddivers (Trichonotidae), tube blennies, burrowing gobies, and triggerfishes duck into holes or burrows. Triggerfishes can "lock themselves in" their holes by extending their dorsal and anal fin spines. Similarly, soles (Soleidae), flounders (Bothidae), and their relatives bury themselves in the sand. At night, sleeping parrotfishes wedge themselves into the reef and secrete a somewhat gelatinous cocoon that allows it to detect predators by touch if the cocoon is violated.
Another behavior normally attributed to birds but also observed in some species of reef and freshwater fishes is mobbing. Mobbing serves to ward off an intruding predator by displaying to or attacking it in number, but also to warn conspecifics of the intruder. For example, the damselfish Pomacentrus albifasciatus maintains territories adjacent to conspecifics on reef flats. If a predator, such as a moray eel or scorpionfish, enters the area, those damselfishes closest to the intruder rise into the water column and display their erect fins as they dance in front of, behind, or along the flank of the intruder. These damselfishes are soon joined by others from the territorial matrix, and together they mob the intruder until it leaves the area. This damselfish also appears to be able to recognize which kind of danger the intruder may pose before mobbing it. If the intruder is a scorpionfish, the damselfishes avoid displaying near its head and focus their attacks along the posterior flank or tail, presumably because scorpionfishes have large mouths and are capable of extremely rapid feeding attacks. However, if the intruder is a moray eel, the damselfishes also mob in the region of its mouth because it is relatively smaller and the eel's feeding behavior is different.
Behavior, evolution, and conservation
The behavior patterns in fishes evolved over countless generations, largely in response to pressures from natural and sexual selection. These behavior patterns are distinct and measurable, and they are just as much characteristics or traits as those based upon morphology or biochemistry. Although general patterns among different, and often quite divergent, species exist, the applications of these patterns and their subtle differences are often unique. Similarity in patterns may be a function of convergent evolution, common descent among related groups, or an affinity among species. Differences in patterns may be the result of a lack of affinity or may be subtle changes in application within a group of closely related species. The study of phylogenetic relationships among species by the comparative method provides an understanding of the patterns of behavior observed, as well as the processes that underscore their evolution within species. This field of study, known as historical ecology, has great utility in ascertaining patterns of character development between and within species, and has predictive power in instances where information on a particular species within a group of related species is relatively lacking. This method also allows for testing hypotheses that may confirm the validity of the prediction.
The methods of historical ecology in the study of fish behavior have considerable utility as tools for predicting how fishes behave under exploitation, habitat destruction, or other problems addressed in the science of conservation biology. Conservation efforts, especially in highly diverse systems, are often stymied because of a lack of information on the biology and behavior of some fishes within those systems. As detailed studies of most groups are lacking, some considerable effort has been expended upon understanding the biology and behavior of a relatively few species within these systems. Generalizations are feasible for attempting to understand species that are less well studied. However, only the steady collection of data coupled with the predictive advantages of historical ecology allows scientists to understand the larger picture so they can convey to fishery managers the information necessary for the design and implementation of effective conservation and management plans.
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Terry J. Donaldson, PhD
Behavior consists of all an animal's reactions to messages received by the central nervous system from any of the several sensory systems of the body. Herpetologists have been most interested in behaviors that have clear functional significance, such as behaviors involved in obtaining food, avoiding danger, finding mates, thermoregulating, and moving between microniches at alternate times of the active season (e.g., migration from hibernacula to feeding grounds and vice versa, migration from feeding grounds to oviposition sites and vice versa, and moving between feeding grounds).
For many vertebrates, vision is the most vital of all the senses, followed generally by hearing. In most reptiles, however, chemical sensitivity is as vital as vision, or more so. As in other vertebrates, olfaction is mediated by the paired nasal organs, which open to the exterior through the nostrils. In reptiles, however, additional mediators of chemical sensitivity are the paired vomeronasal organs, the openings of which are at the anterior extremity of the roof of the mouth, close to the nasal organs. In general, the nasal organs are sensitive to airborne or volatile chemicals, whereas the vomeronasal organs are sensitive to substrate-borne or nonvolatile chemicals. Hearing air vibrations varies in importance across reptilian taxa, although there is a general sensitivity to lowfrequency vibrations propagated through the substrate. Touch and taste receptors exist in reptiles, and they play important roles in mediating certain behaviors, but considerable variation exists across species. Sensitivities to polarized light, infrared radiation, and geomagnetism are known to play significant roles in some taxa, but analysis of these sensory processes is in its infancy.
Behaviors guided by chemical senses
Chemical guidance of predatory behavior has been extensively studied in snakes and lizards. Many species exhibit attack and ingestive behaviors on presentation of chemical cues derived from prey. In these experiments, the chemicals typically are presented on cotton-tipped applicators in the absence of any visual or tactile cues associated with prey. Therefore we can be certain that chemicals alone are responsible for triggering the predatory actions. This does not mean that garter snakes (Thamnophis sirtalis), for example, pay no attention to other stimuli arising from prey (fish, salamanders, frogs, worms) under natural conditions. We know that cues such as movement are vital for attracting the snakes' interest. Yet if investigatory behavior does not bring the snake into contact with appropriate chemical cues, the snake is unlikely to bite the target. Visual cues may command attention and lead to careful inspection, whereas chemical cues trigger the final consummatory acts. Of considerable interest is that neonatal garter snakes exhibit such responses to certain chemical cues before having any experience with prey. This finding leads to the conclusion that responses to these cues are innate. Furthermore, neonatal garter snakes born in different parts of the geographic range respond most strongly to different prey extracts. This finding indicates that subpopulations of snakes have experienced differential selection based on variation in prey abundance. In general, the chemical cues to which snakes respond most strongly are those associated with prey that happen to be available in the snakes' habitat. Changes in prey populations are followed by changes in prey recognition mechanisms within the snake population.
Because reptiles possess nasal and vomeronasal chemosensory systems, herpetologists have been keen to learn the relative contributions of these two systems to predatory behavior. Experimenters have developed various techniques for blocking one or both of the systems to study predatory behavior. Garter snakes have been subjects of most of these investigations, although a few studies have involved western rattlesnakes (Crotalus viridis). The common result has been that prey recognition remains undisturbed when the nasal system is blocked but that this behavior almost disappears when the vomeronasal system is blocked. In some particularly elegant experiments, the blocks were reversible, and restoration of the vomeronasal system was followed by a return of the abilities to recognize and respond appropriately to prey. This body of research leaves no doubt about the importance of the vomeronasal organs.
The role of the nasal system and its interaction with the vomeronasal system remains a matter of speculation. Most investigators believe that the nasal system is extremely sensitive but not particularly discriminatory, whereas the reverse is true of the vomeronasal system. The nasal system is thought to serve an alerting function. It informs the snake that something of interest is nearby and sets the vomeronasal system in motion in the form of tongue flicking. Vomeronasal examination
can identify the molecules in question and activate the appropriate behaviors. In this view, nasal olfaction functions as vision or detection of vibrations does in that ambiguous stimulation of any of these senses can activate tongue flicking so that the animal can conduct a definitive examination. Unambiguous stimulation of any sense, usually by an approaching predator, typically activates immediate escape without the need for cross-modal verification. Detection of potential prey is frequently an ambiguous matter because of prey crypsis (camouflage) and consequent reduction in intensity of stimuli and requires the synergistic action of multiple sensory systems. In the latter context, the vomeronasal system performs the "gold standard" test, and the other senses invite the vomeronasal system to conduct the test.
The vomeronasal system plays an equally important role in the reproductive behavior of garter snakes. In experiments, males with impaired vomeronasal systems did not follow the trails of estrous females and did not attempt to court or copulate with such females when they encountered them. When the vomeronasal systems of the males were restored, normal sexual behavior reappeared, including the ability of the males to follow trails deposited by females. Male garter snakes
differentiate the trails of conspecific and heterospecific females only through chemical cues. Males can differentiate large and small (but reproductively mature) conspecific females with chemical information alone. In experiments, males preferred chemical cues from large females over those of smaller females, a fact probably correlated with the greater number of eggs produced by large females. In other words, mating with larger females produces greater fitness benefits for males than does mating with smaller females, and males reflect the effect of this selective pressure in their chemosensory preferences. When the length of females was controlled but mass was varied, males preferred the heavier females and did so when only chemical information was available. Male garter snakes apparently can use chemical information to discriminate females of varying nutritional conditions. This capability probably is associated with an effect of nutritional condition on quantity and quality of eggs produced by females.
Although few other reptile species have been studied in as much detail as have garter snakes, especially T. sirtalis, we know that male skinks can do some of the things that male garter snakes can do. In particular, male skinks can differentiate conspecific and heterospecific females with only chemical cues. Likewise, female skinks can differentiate chemical cues of conspecific and heterospecific males. It seems probable that male skinks also are able to differentiate conspecific females of varying size and condition, but these phenomena have not yet been tested. Nor has anyone tested whether female skinks or garter snakes can differentiate chemical cues derived from males of varying size or condition. We assume the vomeronasal systems of garter snakes and skinks are responsible for discrimination, but this has not yet been demonstrated in experiments that block the vomeronasal system while the olfactory system is unimpaired.
Chemical cues associated with predators are detected by various reptiles. The result is avoidance or other self-protection reactions. We cannot yet be certain that the vomeronasal system mediates these reactions because appropriate blocking experiments have not been conducted, but this is a reasonable hypothesis.
Some reptiles have been shown to discriminate between their own chemical cues and those of conspecifics. Some male reptiles can differentiate male conspecifics on the basis of chemical cues. Experiments along these lines have been done with rattlesnakes, desert iguanas, and sand swimming skinks. It is likely the abilities will be found to be widespread. A number of benefits can be derived from individual recognition, including the interesting social dynamic known as the "dear enemy" phenomenon. Common among territorial birds, the phenomenon consists of individual recognition by neighboring territorial males. These animals respect the boundaries between their territories and do not intrude on each other; thus each animal is allowed to relax the level of vigilance and aggressive behavior that would otherwise be devoted to boundary patrolling and defense. If one of the males is replaced with a new individual, the remaining neighbor exhibits an immediate elevation in vigilance and defense, revealing that he notices that his former dear enemy is no longer present. Eventually the original territory owner and the new neighbor enter a dear enemy relationship, so that each can save energy and avoid injury from fighting. The dear enemies enjoy mutual benefits by respecting each other's property rights. Because individual recognition is clearly an important component of this behavior, the dear enemy phenomenon is generally thought to be an advanced form of social interaction. This does not mean that it is found only in birds and mammals; the behavior has been shown to occur in salamanders and lizards. Whether it occurs in any species of snake, turtle, or crocodilian is conjectural at this time, although a chemosensory basis for the phenomenon clearly exists in at least some species.
Perhaps the most intriguing chemosensory behaviors of reptiles involve venom. Venom immobilizes and kills prey and contains powerful enzymes that greatly facilitate digestion. In experiments, rattlesnakes given a choice between envenomated and nonenvenomated mice otherwise equal in size, age, and sex selected the envenomated prey more frequently than they did the nonenvenomated prey. This remained true when the mice were wrapped in dark nylon mesh that blocked visual or tactile cues; thus we can be certain that chemical cues mediated the selection. When rattlesnakes were given a choice between a trail deposited by an envenomated mouse and one deposited by a nonenvenomated mouse, the snakes reliably selected the former trail, a finding that again indicates chemical cues mediate the behavior.
To understand how sensory bias might contribute to a snake's fitness, it is necessary to understand that rattlesnakes and many other venomous species are ambushers that strike and release adult rodent prey. This style of predation avoids injuries that could occur if struggling rodents were held in the snake's jaws after the strike. Although the venom kills the prey, the process takes as long as several minutes, during which the rodent's teeth, claws, and guard hairs can inflict serious damage. Releasing the rodent after the envenomating strike minimizes the snake's risk of injury, but the snake risks losing the prey, which can travel several meters from the site of attack while the venom takes effect. Recovering the rodent carcass becomes difficult, partly because the immobilized rodent no longer emits motion cues to attract the snake's attention and partly because the rodent may be behind objects or in a burrow, so thermal information (i.e., infrared radiation detectable by pit organs) is obscured or blocked entirely. Recovery of the rodent carcass therefore is mediated by chemical cues detected by the vomeronasal system. A rattlesnake that has delivered a successful predatory strike is capable of following the rodent's chemical trail with exactitude, even though the same snake usually does not follow such a trail in the absence of a predatory strike. The strike is necessary to trigger chemosensory searching and trail-following behavior. This is true for rattlesnakes; cottonmouths (Agkistrodon piscivorus), however, follow trails effectively whether or not a predatory strike has been delivered before the snake encounters the trail. Gila monsters (Heloderma suspectum) and Australian sand goannas (or monitors) Varanus gouldii, behave as cottonmouths do. For these predators, chemical cues on the substrate are sufficient to induce trail following, and this behavior may be associated with a propensity to search for carrion. For rattlesnakes and various other vipers, chemical cues on the substrate usually are not sufficient, and a successful predatory strike is critical.
In the case of rattlesnakes, two trails always are present in the poststrike environment, one deposited as the prey wanders into striking range (the preenvenomation trail segment) and one deposited as the prey moves away from the site of attack (the postenvenomation trail segment). Following the wrong, or preenvenomation, segment could result in losing the prey, wasted energy, and vulnerability to predators. It is not surprising that rattlesnakes discriminate the two trails and select the postenvenomation segment. What is the chemical basis for this fascinating discrimination?
Venom is a complex material, containing well over 30 identifiable fractions, some of which are lethal to rodents, some not. The latter fractions are thought to be synergizers or amplifiers, not necessarily harmful in themselves but capable of increasing the damaging effects of other fractions. Finding the particular fractions responsible for the snake's discrimination of pre- and postenvenomation trail segments is almost impossible. One reasonable hypothesis is that proteolytic enzymes, major constituents of venom, are responsible. The idea is that such enzymes break down rodent protein and contribute to immobilization, death, and digestion of prey and that the snakes have evolved a perceptual sensitivity to these effects. This sensitivity facilitates the important task of locating and following the postenvenomation trail segment. In other words, proteolytic enzymes have an initial or primary function, and they have acquired a secondary one because of the snake's ability to perceive some of these primary chemical effects. Another hypothesis is that specialized components have been added to venom, not necessarily because of their
lethal effects but because of their perceptual, trail-enhancing effects. The term "trail marker substance" can be used in recognition of the idea that the venom component has acquired its perceptual role not secondarily but as its primary (and perhaps only) function.
Differentiating these hypotheses hinges on the fact that rattlesnake venom passed through gel filtration columns separates into fractions according to molecular weight and that proteolytic enzymes sort into several fractions while other fractions contain little or none of this material. Therefore it is possible to use these various fractions as experimental injectants. That is, mice given injections of each of these fractions suspended in distilled water can be paired with control (nonenvenomated) mice and presented to rattlesnakes. If proteolytic enzymes are the critical elements, then snakes ought to be able to discriminate mice injected with these enzymes from control mice. If such mice cannot be differentiated from controls, one implication would be that some other venom fraction is the critical one. The result in experiments with western diamondback rattlesnakes (Crotalus atrox) has been that proteolytic enzymes do not cause a mouse to be discriminated from a control but that another fraction (containing no proteolytic enzymes) has this effect. The chemical composition of this fraction has not yet been identified, but the indications are that the second hypothesis is correct. This work underscores the subtlety of the chemical cues used by reptiles. Reptiles can detect gross cues, such as those associated with rotting carrion, and they can use remarkably subtle cues in remarkably low concentrations.
Ambush predators must first select an appropriate site, a place which prey are likely to visit or to pass through. Several reptiles have been shown to make this selection on the basis of chemical cues deposited by prey that have recently moved through the site. Prairie rattlesnakes making their vernal migration would stop if they were to encounter fresh chemical cues derived from deer mice (Peromyscus maniculatus). Once the rattlesnake occupies an ambushing site, however, it usually waits until a successful predatory strike is delivered before paying any further attention to chemical trails. There are exceptions, of course: Some snakes engage in active foraging for burrows containing the neonates of various small mammals, and this search is undoubtedly guided by chemical cues. Thus strikes are not always necessary to activate chemosensory searching. It seems likely that reptiles of many species can use chemical cues to discriminate large versus small populations of prey and to differentiate sites currently occupied by prey from sites previously occupied but now abandoned. Evidence along these lines has been collected, but a great deal of research remains to be done on the topics of habitat selection and chemical ecology.
Although heavy reliance on the nasal and vomeronasal senses is characteristic of many species of reptiles, particularly the most advanced species, numerous species are less reliant on chemical cues than on visual information. Chameleons are examples not only in their use of visual cues in locating and capturing prey but also in their use of visual cues in social and reproductive behavior. Interspecific variation exists among reptiles in the modalities that mediate important behaviors. Therefore statements about the role of vomeronasal stimulation should not be generalized to all reptiles. The relationships between type of food, behaviors involved in acquiring food, and the sensory modalities used are a major area of herpetological investigation. Herbivorous lizards have been found to behave in surprising ways. These animals not only detect edible plants through chemical cues but also detect and reject other plants on the basis of the presence of defensive compounds. Herpetologists have made theoretically important discoveries regarding the chemical senses of reptiles, but we have probably only scratched the surface.
Behaviors guided by vision
Visual cues, usually in the form of prey movement, have long been known to attract a snake's attention and to elicit predatory responses in insectivorous lizards. Equally interesting is the role of visual information in social and reproductive behavior of lizards. Chameleons exhibit "emotional colors" that involve changes in brightness during agonistic interactions and culminate in victory or submission, each with a characteristic pattern. It is partly through these color changes that chameleons have acquired their protean reputation. Iguanian lizards execute a set of movements, including head bobs, pushups, and dewlap extensions, combined in particular sequences that are species specific not only with respect to sequence but also with respect to cadence. Females of sympatric species can discriminate conspecific from heterospecific males on the basis of these display properties. Males use the displays to advertise territories and to settle boundary disputes with other males.
Results of experiments involving presentation of models to territorial males have established that color patterns and postures contribute to agonistic and courtship activities. These behaviors have been found to be innate, and the development of the behaviors is not influenced by social stimulation. Some display elements of the green anole (Anolis carolinensis) are present immediately after hatching, whereas other elements emerge later in ontogeny. Even in the later stages, social stimulation appears unnecessary, because the behaviors usually emerge on schedule in animals that live in social settings or in experimental isolation. This characteristic represents a major difference between green anoles and birds and mammals, both of which typically depend on early social stimulation for the proper development of social signals. Because only the green anole has been studied extensively in this regard, we look forward to comparable projects with additional reptile species. Only when such work is completed will we know whether the developmental characteristics of the green anole are common among reptiles. The indications are that the green anole is a reasonable model for reptiles in general, and many herpetologists believe this to be the case, but more taxa must be analyzed.
Reptiles use visual cues as lures for prey. The alligator snapping turtle (Macrochelys temminckii) is probably the best known example because of the wriggling wormlike process attached to the floor of the turtle's mouth. The animal remains motionless with jaws agape while the "worm" attracts fish, which are engulfed by a profound oral snap. Several snakes (e.g., the sidewinder rattlesnake [Crotalus cerastes] and the massasauga [Sistrurus catenatus]) use their tails as lures, in some cases only during the juvenile stage of life. As the snakes grow, they experience an ontogenetic shift in prey preferences such that caudal luring for lizard or frog prey is abandoned in favor of predation on rodents. Ontogenetic shifts in prey preferences are not uncommon, and they are associated with parallel ontogenetic shifts in habitat preferences and even in diel activity patterns. Some snakes rely on caudal luring throughout life. The best example is the death adder (Acanthophis antarcticus), which has a highly specialized tail that strongly resembles a wormlike or grublike creature and is quite attractive to lizards.
Play, learning, and plasticity
Although play behavior occurs in a few species at least under captive conditions, most species do not exhibit this phenomenon. Play observed in reptiles has not been a social phenomenon. It has involved the deployment of foraging, feeding, or other behaviors in unusual, nonfunctional contexts, sometimes aimed at inanimate objects, sometimes at humans. The players have been adults, and their playful activities have been idiosyncratic rather than common among conspecifics.
Exhibition of play behavior as a social phenomenon is a major difference between reptiles and mammals, with birds positioned in between. The usual explanation is that most reptiles have been strongly selected for precocity. Neonates are miniature versions of adults and must function effectively as predators, as avoiders of predators, and as competitors. They have no leisure to acquire behavioral skills during playful interactions with peers or by observing parents.
Even the most precocial (capable of independent activity from birth) birds and mammals have opportunities to learn,
although this process may be accelerated, as it is in filial imprinting in precocial fowl and analogous social phenomena in some precocial mammals. There are important ontogenetic effects of early social stimulation in these birds and mammals. In the more altricial (immature or helpless at birth) species, similar effects occur over a broader span of time, play being an important context for social learning. In reptiles, by contrast, such effects are less common. The usual observation is that neonates are competent at biologically significant tasks the first time they encounter the task.
This does not mean that learning is unimportant in reptiles, only that it is a less conspicuous aspect of reptile ontogeny than is the case for birds and mammals. For example, several species of turtles develop strong preferences for the foods encountered after hatching. The turtles are capable of accepting many different types of prey, but the types available at the time the turtles hatch becomes favored prey as a consequence of an imprinting-like phenomenon that occurs during early feeding experiences. Because piscine prey species can fluctuate in abundance, even replacing each other owing to normal ecological events, hatchling turtles apparently are better off without strong, innate preferences but with the capacity to form preferences after sampling foods that happen to be present in the posthatch habitat. A strong innate preference for a prey type that happens not to be available could have disastrous consequences, whereas experientially induced preferences would simply adjust the predators to prey currently in the food web.
Only when the composition of the food web is predictable or reliable does it make sense for neonates to have innate preferences. This phenomenon is well known in garter snakes.
Neonates exhibit strong preferences, on the basis of chemical cues, for prey normally present in their habitats. Even here two points are of interest. First, results of experiments have shown that neonates typically exhibit several preferences; they do not prefer only a single prey species. If one type of preferred prey is not present, another will probably be available. Second, although they have strong innate preferences, neonates are capable of acquiring new preferences on the basis of early feeding experience. If none of the preferred prey are available, hungry neonates are flexible enough to adjust to new foods.
The flexibility of adult snakes and other adult reptiles has not been studied experimentally, but the husbandry experience of zoo professionals and that of many hobbyists indicates that some species readily adjust to captivity and to the foods normally provided there, whereas other species make this transition only with great difficulty. Most rodent-eating snakes typically do well in captivity, even when captured as adults. The same is true of fish-eating species, especially if rodents are facultatively present in the natural diet, such that the snakes can be switched to rodent prey in captivity. Snakes with highly specialized diets are typically difficult to keep in captivity, but this problem usually is associated with the fact that the keeper has difficulty obtaining the necessary prey in sufficient quantity.
Even notoriously difficult species can be kept in captivity, if specialized habitat factors and required foods can be provided. This judgment is based far less on scientific data than on accumulated experience of gifted, dedicated keepers. Our experience with the viper boas (genus Candoia) is consistent with this point of view. Species that take rodents adjust well to captivity, whereas species that take lizards are quite difficult to keep, unless appropriate lizard prey are available. When such prey can be offered, the snakes accept them and remain in good flesh. Lizard prey, however, are not regularly obtainable, and because the lizard-specializing snakes do not readily switch to rodents, the snakes generally become thin and vulnerable to infections that occur as a secondary effect of nutritional compromise. This outcome is a shame because lizard-eating viper boas have pleasant personalities, being tolerant of handling without offering to bite. They could be easily maintained in captivity if the food problem could be solved. Commercially produced rodent prey are widely available in the United States. The rodents are generally free of infection and relatively free of parasites, especially dangerous ones. Wild-caught lizards are sometimes infested with microbiotic and macrobiotic organisms that can readily colonize snakes with disastrous consequences. This is another reason that rodent-feeding snakes have an advantage in captivity.
Although rodent-feeding snakes usually are easy to keep in captivity, there are exceptions. One of the most famous is the eastern diamondback rattlesnake (Crotalus adamanteus). Neonates born in captivity do fairly well, but wild-caught adults rarely thrive, even under the best husbandry conditions. This fact is all the more interesting because western diamondbacks are no less aggressive and no less likely to remain aggressive in captivity, yet they generally do reasonably well, accepting prey readily and breeding with alacrity when given the opportunity under appropriate thermal conditions. Neonatal western diamondbacks born in captivity usually are calmer than wild-caught adults, but the latter usually manage to do well in captivity even with their notorious dispositions. Why wild-caught adult eastern diamondbacks and western diamondbacks differ in their tolerance of captivity remains a mystery that probably involves a difference in behavioral and emotional plasticity. Unraveling this mystery will not only shed light on the dynamics of these species but also contribute to the ability to manage many other species in captivity. Problems similar to those among wild-caught eastern diamond-back rattlesnakes have been reported in many other species. Providing appropriate habitat may be part of the solution. Many keepers report success with defensive individuals if hiding places are provided. To our knowledge, experimental tests have not been performed, and we see here an excellent example of the interface between behavioral research and the development of good husbandry techniques.
Behaviors guided by tactile cues
Tactile cues are important contributors to social and reproductive behaviors. Although chemical cues usually guide males to females, once the individuals meet, tactile information comes into play. If several males are simultaneously attracted to the same female, male combat is likely in some species. The winner is the male that eventually mates with the female. In some species, many males are present simultaneously, all competing for one female. This process is called "scramble competition" rather than male combat and leads to formation of the famous mating balls, which can contain dozens or even hundreds of males. Garter snakes (T. sirtalis) in some parts of their range form such balls.
Less chaotic mating arrangements are more common, a few males courting one female. In this circumstance, ritualized combat is likely to occur, as in some rattlesnake species. Two males engage each other in a pushing contest involving the anterior parts of their bodies, which usually are raised off the ground, each male pressing against the other. The winner is able to press his opponent down to the ground and hold him there for a few seconds or longer. Losers of such contests typically retreat and are sexually refractory for days afterward. Although the contest requires effort, it seems most unlikely that physical exhaustion can be the explanation for the loser's withdrawal and refractoriness. Instead, a neuroendocrine mechanism must exist by which losing suppresses subsequent aggressive and sexual activities, at least temporarily. This mechanism may benefit both winner and loser in that the winner mates with no further distraction and the loser manages to avoid serious damage. An interesting characteristic of male combat among vipers is that males usually do not bite each other during the ritualized wrestling activity. Because the snakes are not immune to their venom, the vipers could inflict serious damage through fang punctures and venom injection. By replacing such actions with ritualized wrestling, both males benefit. By withdrawing after defeat, the loser spares himself further domination as well as, perhaps, a biting attack by the winner.
Species in which male combat occurs typically exhibit sexual dimorphism in body size, males being larger than females. The idea is that combat creates a selective pressure favoring large males, because size correlates with winning versus losing. On the other hand, females should generally be selected for large body size because this factor determines clutch size. Within the limits of opposing selective pressures, females should generally be large. Males also are large, even larger than females, if male combat is important for mating success. If male combat does not occur in a species, males are smaller than females. These generalizations appear to hold over a broad range of species.
Sexual dimorphism in size is relatively common among reptiles, as is sexual dichromatism. In dichromatic species, males usually are more brightly colored than are females, a fact that correlates with the males' need to defend and advertise territories and to attract females. There are some spectacular cases among the spiny lizards (genus Sceloporus) in which both males and females have brilliant (but different) color patterns. These cases are not well understood, but they constitute an extreme form of dichromatism in which both sexes may be sacrificing crypsis to communicate sexual identity. It is possible, however, that future research will reveal that one or both of these patterns, although perceived as brilliant and conspicuous to human observers of preserved specimens, are nevertheless cryptic in the natural habitat.
Because male combat is a form of tactile stimulation, (1) delivery of this stimulation has become highly ritualized in some snakes and lizards, (2) the "meaning" of the stimulation appears to depend on differential neuroendocrine events in winners and losers, (3) delivery of the stimulation appears to inhibit tissue-damaging bites, at least for a while (it is possible that escalated aggression, including bites, could occur if the loser fails to retire), and (4) this form of tactile stimulation has created a selective pressure favoring large body size and increased ability to deliver the tactile information. Females benefit when they mate with males who have won combat encounters and are, by this measure, superior males. Sons of such males are presumed to inherit the size and strength of their fathers. Daughters also may benefit from genes favoring
large size. Although females can benefit by doing nothing, that is, by simply letting males fight and then mate with the winner, it has been reported that females sometimes add an element of their own. Females of several pitviper species raise their heads above the ground when a male approaches. This is the same behavior that combating males use at the start of their contests. The female may be making an aggressive gesture that has the effect of differentiating dominant and subordinate males. Males that have recently lost in combat are intimidated by this action on the part of a female, whereas males who have recently won in combat are not intimidated and proceed with courtship. Females can easily discriminate these two types of males on the basis of the males' behavior immediately after the female presents a combat intention display. This is the hypothesis currently held by a number of herpetologists who study the reproductive behavior of vipers.
Courtship and copulation in nonvenomous species have been studied in detail, and a useful terminology has been developed. Precourtship behaviors are those by which the potentially large distances between male and female are reduced. These behaviors usually involve the following of female pheromone trails by males. In some species, males may also use a head-raised posture to search for visual cues arising from females. An important role of visual cues has been established for ratsnakes (genus Elaphe) and is believed to exist in other genera as well. Males are attracted by the visual cues, and when the male is relatively near the female, chemical cues become readily available regarding the female's specific and sexual identity and her state of sexual readiness.
On making contact with a receptive conspecific female, the male initiates a series of behaviors typically grouped into three phases: tactile-chase, tactile-alignment, and intromission and coitus. At first the male exhibits a high rate of tongue flicking, during which he is apparently confirming the information he has previously obtained during the trail-following period. Direct contact with sexual pheromone along the female's dorsal surface likely intensifies the male's sexual motivation, leading him to rub his chin in a linear series of jerky movements along the female's back. A female might flee at this point, and the male usually follows, sometimes attempting to mount the female. This tactile-chase phase continues until the female remains stationary and allows the male to mount along part or all of her dorsum. The most conspicuous act usually is chin rubbing by the male along the female's back, advancing toward her anterior, moving posterior, and starting forward again. In the family Boidae, in which males have vestigial pelvic structures in the form of spurs, the tactile-chase phase of courtship entails spurring, which is rapid, oscillating movements of the spurs against the female's body. In some species of rattlesnakes, the male curls his tail around the base of the female's tail and gently massages her by moving his curl over the female's vent and down the length of her tail while repeatedly rubbing his chin along her dorsal, anterior surface. These behaviors may be analogous to the titillation movements by which male pond turtles stroke the heads and necks of their mates with long fore claws.
The tactile-alignment phase contains all of the foregoing behaviors and the juxtaposition of cloacal apertures, a posture that eventually allows copulation. The male and female achieve this important posture entirely with tactile sensation; there is no visual guidance. (Although there is ample evidence of integumentary tactile receptors in snakes and other reptiles, the term "hedonic receptors" is rare or absent in the herpetological literature. This term and various synonyms occur commonly in the mammalian literature, especially the literature on human sexuality, and we suspect it is applicable to the receptors mediating these cloacal alignments during sexual encounters between squamates.) On attaining juxtaposition of the cloacal apertures, the female may exhibit cloacal gaping, and the male may evert one of his hemipenes. At this point, tail-search copulatory attempts occur in which the tails of the partners are engaging each other but the animals' heads are not (i.e., the eyes and other anterior sense organs are not involved). Male garter snakes have been observed to rub their chins on one female while aligning with the cloaca of another female.
Coupling is the eventual outcome of tail-search copulatory attempts. In some colubrid genera, such as Elaphe, Lampropeltis, and Pituophis, it is common for a male to obtain a mouth grip on a female's neck just before penetration and to maintain this grip during coitus. This is a regular feature of mating among lizards (most of the terminology developed by snake researchers can be applied to lizards). One of the most interesting aspects of snake reproduction has to do with the duration of coitus, which ranges from a few minutes in many species to well over 20 hours in others (e.g., vipers and pitvipers). In garter snakes of several species (T. sirtalis, T. sauritus, T. butleri, and probably others), coitus is relatively brief, but a mucoprotein plug forms thereafter in the cloaca of the female that blocks penetration of the female by other males. In vipers and pitvipers, the tendency of the male to remain coupled for extended periods accomplishes the same result as the copulatory plug in garter snakes. On the other hand, the male garter snake is free within 15–30 minutes to seek additional copulation, whereas male vipers and pitvipers are unable to do so for 20–30 hours. Such variation may be associated with several ecological factors, such as the availability of sexually receptive females nearby. If availability is high, it makes sense to uncouple quickly and for the male to initiate courtship with new females. If, however, females are widely spaced and it is likely that several males will be simultaneously attracted to each receptive female, a successful male may benefit more by guarding the inseminated female than by leaving her while he goes on an uncertain hunt for a new female. The density of predators that might take advantage of snakes in copula is another ecological factor that could influence the duration of coitus. A high density of predators leads to selection for brief copulations. These ecological factors have not yet been studied systematically by herpetologists.
Little or no interaction occurs between male and female after coitus. With rare exception (male western diamondback rattlesnakes have been observed to remain with females for some days before and after copulation), there have been no scientific reports of long-term pair bonds between males and females in any species of reptile. Females of some species exhibit maternal behavior. Building of nests with attendance of eggs has been reported among king cobras (Ophiophagus hannah), Great Plains skinks (Eumeces obsoletus), and most crocodilians. Female turtles deposit their eggs in nests, sometimes with elaborate digging and refilling, but no species has been reported to attend the eggs or nest after oviposition and refilling of the nest cavity. Indian pythons (Python molurus) have been observed to brood their eggs using shivering movements to generate heat. Although parental (maternal) care of neonates is now well known among crocodilians, early reports of this behavior were doubted until modern researchers developed methods to observe these animals, especially American alligators (Alligator mississippiensis) in captivity as well as in the wild. The well-organized maternal behavior of crocodilians, the last of the ruling reptiles, has prompted some paleontologists to speculate about the presence of maternal care in dinosaurs. This topic remains controversial but is gradually being supported by an impressive array of fossil evidence and insightful argument.
New evidence regarding maternal behavior is likely to arise from the study of certain viviparous snakes. Several investigators have reported that female rattlesnakes remain with or very near their litters for several days after parturition. Initially this behavior was considered unimportant, perhaps the result of maternal exhaustion from the act of giving birth. Now it has been hypothesized that maternal attendance of neonates may be more than an artifact of exhaustion. Perhaps the fitness of the neonates is enhanced by the mother's presence, and this means her fitness may be enhanced. The general idea is that the mother's presence may discourage predators who would otherwise take a toll on the neonates. The mother's presence also may allow the neonates to become familiar with her odors and use them to find their way to the hibernaculum. That is, the neonates might locate the den by trailing familiar odors deposited by the mother. None of these ideas has been subjected to rigorous scientific testing, but such tests are likely to be performed.
Activity patterns and thermoregulation
Activity patterns influenced by seasonal, daily, or other rhythms probably have been the targets of more herpetological research than any other aspect of behavior except diet. This is partly a consequence of the development of miniaturized radiotelemetric equipment and partly the result of the simple fact that understanding any animal requires a sense of the actions it performs and when it performs them. Numerous investigators are interested in energy budgets and related phenomena, and activity patterns tell us about most of the energy costs sustained by animals. Feeding success tells us about energy gains. The construction of activity budgets is an important step in the calculation of energy budgets. Many herpetologists consider the study of activity patterns a necessary starting point, once the basic questions of taxonomic allocation are at least tentatively answered. It could be hypothesized that we could predict activity patterns and energy expenditures if we knew the resource needs of a species and the distribution of these resources in the habitat. Although this reasoning is sound, rarely do we know about all the resource needs of a species, and we know even less about their distribution, especially in the perception of the animals. Knowledge of resource needs is an important goal, but instead of using the information to predict activity patterns, we frequently use data on the patterns to infer resource needs and distributions.
The longest migration by any reptile is the oviposition movement of some female green seaturtles (Chelonia mydas) from their feeding grounds in the kelp forests off the coast of Brazil to the beaches of Ascension Island in the South Atlantic, about midway between Brazil and the west coast of Africa (approximately 1,600 mi [1,900 km] from Brazil). Other populations of green seaturtles and all other seaturtles make substantial annual movements to nesting beaches, although none of these migrations is as long as the swims of the subpopulation of green seaturtles using Ascension Island. The sensory basis of this remarkable migration involves geomagnetic sensitivity, chemical cues, and probably celestial cues. Even more remarkable is the movement of hatchlings from Ascension Island to the Sargasso Sea and later to the Brazil kelp beds, the turtles again using the same variety of sensory systems. Somewhat less spectacular migrations are known in other reptiles, especially snakes in temperate zones, where hibernation sites and feeding sites are separated by distances on the order of 1–7 mi (1.6–11.2 km). The snakes make an annual vernal migration to the feeding grounds and an autumnal migration back to the hibernacula. Sensory bases for these movements have not been studied sufficiently, but it seems clear that chemical cues are involved, and there is reason to suspect a role for geomagnetic, celestial (especially solar), and visible landmark cues.
Homing has been observed in a variety of reptiles that have returned to sites from which they were experimentally displaced. Alligators, several species of snakes and turtles, and a few species of lizards have been studied, but we know relatively little about the sensory basis of this behavior. Geo-magnetic cues were probably used by the alligators, chemical cues were certainly involved in several of the turtle studies, and solar cues were well documented in several of the snake studies. In most other cases, however, our knowledge has not advanced beyond the fact that homing or related behavior (e.g., y-axis orientation) occurred. Some studies have shown no evidence of return to a site occupied before displacement. This lack of evidence gives rise to interesting speculation that the motivation to return depends on the quality of the initial habitat and the quality of the habitat in which the test animals were released. An important implication is that the animals assess their habitats and use this information to decide whether to move toward "home" or to adopt a new home for themselves. If such behaviors occur, the cognitive mechanisms involved in habitat assessment will prove to be even more interesting than the sensory mechanisms mediating orientation and movement toward home. The literature on reptile homing behavior contains many anecdotes that have substantially influenced our collective thinking. For example, rattlesnakes have been removed from a human property (usually a backyard, field, or a common area in a community) and released a distance away (usually on the order of a mile or two [1.6–3.2 km]) only to return to its initial location within a few days. Another interesting case involved a black ratsnake (Elaphe obsoleta) removed from a farm several times but always returning within a few days. In cases of this sort, the snakes were not marked at the time of original capture, so it is not absolutely certain that the returning snake and the removed snake were one and the same. On the basis of size, coloration, and unique features such as scars, we can be reasonably confident, but the following case, described by the late Charles M. Bogert, urges caution.
Bogert was collecting reptiles in New Mexico and caught a striped whipsnake (Masticophis taeniatus) in a particular bush, which Bogert marked with a golf tee. The next day, while passing the same bush, Bogert caught a second striped whipsnake in it. Later on the second day, Bogert visited the bush again, and, sure enough, a third specimen of the species presented itself. There is no question that the snakes were three different individuals because Bogert caught and preserved each one. He was interested in what might have been special about the bush, but he was able to identify nothing. Perhaps this was pure chance, always a possibility with anecdotes. On the other hand, maybe the bush was in a prey-rich area or in a migration path or in a spot containing an ideal refuge from the sun or from predators. If any of these conditions existed, it would not be surprising that removal of an initial occupant might be followed by the arrival of another. If the first occupant had not been captured and kept out of the habitat, we might easily misinterpret the second and third snakes as being the initial one. Although anecdotal reports of homing should not be discounted, they should be regarded cautiously.
Closely related to homing is territoriality, the defense of a resource, usually in a particular place but sometimes mobile, as in the case of potential mates that might move a considerable distance but are nevertheless defended in each place they occupy. Numerous lizards exhibit unambiguous territoriality, defending feeding areas from conspecifics and sometimes from heterospecifics that compete for the same foods. Clever experiments have shown that adding food to territories results in shrinkage of the area defended, whereas depleting food from territories results in expansion of the area defended. Results of such studies leave no doubt that the territorial behavior of the resident lizard is sensitive to the quantity and quality of the food contained within the territory. The results of the studies also leave no doubt that the lizards assessed their food supplies and responded accordingly. Food, however, is not the only resource that lizards defend. Oviposition sites are important in some species, and refuge is important in others.
Crocodilians are known to behave in a territorial manner, especially mature females who defend their nests. Large males also appear to be territorial, particularly during the breeding season. Turtles and snakes present a far more ambiguous situation, except for the few species in which females attend their eggs or young. Even these cases are not clear examples of territoriality, because there is no evidence that the females defend their eggs or oviposition sites against conspecifics. It is possible that mothers defend their eggs or neonates only against heterospecific oviphages or predators, in which case the term territoriality may not be appropriate. Territoriality implies intraspecific interaction. At present we cannot state with certainty that any species of turtle or snake exhibits true territorial behavior. Anecdotal evidence exists for both groups, and behavioral phenomena among captive specimens also are suggestive. The possibility of territoriality among these reptiles should be considered, but caution should be exercised in interpretation of anecdotal evidence.
Another approach to the study of activity patterns of reptiles is to record the number of individuals seen or captured (usually in pitfall traps) during each month of the year. In temperate North America, such projects reveal several annual patterns: a single peak during the warmest months with sharply reduced activity before or after; a single peak during the warmest months but fairly broad activity period such that substantial numbers of individuals are active before and after the warmest months; and two peaks, one in spring and one in autumn. Some species, such as garter snakes, appear to remain active all year if the temperature remains high enough. Other species appear to be endogenously programmed to become inactive during winter months, even if the temperature is artificially elevated. Having maintained many rattlesnakes in captivity, we have found that in species such as the prairie rattlesnake and the western diamondback, some individuals remain active all year if the temperature is kept at 79°F (26°C) or higher. Other individuals "shut down" for several months under the same conditions, refusing food for one to three months each year.
Closely associated with the circannual studies of reptile activity are parallel studies of reproduction. Periods of high activity or capture success can correspond with periods of intense feeding behavior, but they can also correspond with periods of courtship and copulation, particularly because of the likelihood of capturing males active in searching for reproductively motivated females. Pregnant female prairie rattlesnakes, for example, are relatively inactive, perhaps even congregating in birthing rookeries, whereas nonpregnant females have made a vernal migration to hunting grounds. The females that migrate are the ones likely to develop ripe ova and to become sexually motivated. Consequently, they are the ones mature males pursue, making themselves vulnerable to traps or to capture by hand. Although a number of reproductive strategies are observed across the various families of reptiles, most of these strategies involve seasonally increased activity by one sex or the other. It is safe to conclude that circannual activity patterns are partly correlated with ovarian and testicular cycles or with courtship and copulatory seasons in taxa that exhibit a dissociation between reproductive behavior and genital physiology (i.e., some species breed at times of the year different from the times gametes are produced; in such cases, activity is influenced by the breeding season rather than the gamete-producing season).
In addition to circannual studies of reptile activity, there have been many studies of circadian rhythms, that is, changes in activity during the 24-hour period of a typical day in the animal's active season. Some species are strictly diurnal (active only during daylight), others are strictly nocturnal (active only at night), and other species are crepuscular (active at dawn and dusk). Juvenile kingsnakes (Lampropeltis getula floridana) exhibit crepuscular and nocturnal patterns, whereas adults are diurnal. The shift toward diurnal behavior occurs when the snakes are approximately 35 in (90 cm) snout-vent length and capable of defending themselves against a variety of predators, particularly birds, that are active during daylight. Nocturnal reptiles exhibit relative cessation of activity during periods of full moon, when ambient illumination at night favors detection by predators.
In some species, daily activity patterns shift with average daily temperature. Among plains garter snakes (Thamnophis radix) and western diamondback rattlesnakes, individuals are diurnal at the relatively low temperatures of early spring and late autumn, but during the hottest days of summer, these same individuals become nocturnal. During intermediate parts of the year, such as late spring and early summer, the snakes are crepuscular, because it is too hot during the day and too cold at night. For species with very broad geographical ranges, it is possible that individuals in northern latitudes are diurnal whereas individuals to the south are crepuscular or nocturnal, depending on thermal conditions.
Temperature is a primary modulator of many physiological processes influencing digestion, reproduction, and locomotion in reptiles. Herpetologists have focused on this factor more than any other. Each species has a lethal minimum and maximum temperature, below or above which life ceases immediately. Within this range, which might be 17.6–104°F (−8°C to +40°C), there is a second range from the critical thermal minimum (CTMin) to the critical thermal maximum (CTMax), which might be from 35.6°F to 89.6°F (2°C to 32°C). Survival below the CTMin or above the CTMax is possible for brief periods, longer below CTMinthan above CTMax, but death occurs relatively rapidly as the temperature converges with the respective lethal extremes. Yet another range is nested with the boundaries of CTMinto CTMax. This range runs from the voluntary minimum to the voluntary maximum, within which the reptile prefers to remain when choice (i.e., behavioral thermoregulation) is possible. This range defines the animal's normal or preferred activity range, and its midpoint corresponds to the animal's voluntary or preferential mean temperature. The precise temperatures corresponding to these boundaries shift during the course of the year, a phenomenon called acclimation. For example, in early spring, a reptile might prefer to keep its body temperature between 64°F and 82°F (18°C and 28°C), but later in the summer, the voluntary minimum and maximum might shift to 68°F (20°C) and 86°F (30°C), respectively. As this acclimation effect occurs, the lethal minimum, CTMin, lethal maximum, and CTMax all shift upward by 3.6–5.4°F (2–3°C).
One of the dominating necessities of any reptile is to keep its body temperature within the preferred range of its species. Because all extant reptiles are ectotherms, temperature regulation involves a large suite of behaviors, some obvious and some remarkably subtle. Reptiles also exhibit many anatomical and physiological adaptations. Among the more obvious thermoregulatory behaviors are locomotory activities by which the animals shuttle between sunlight and shade as necessary to keep their body temperatures within the optimal range. Basking is another behavior by which body surfaces are exposed to sunlight to increase core body temperature. Reptiles of some species can thermoregulate by altering the amount of surface area exposed to sunlight. They do so with inconspicuous alterations of posture, sometimes associated with color change. Large snakes are known to retain fecal material and to use it for storing heat. If a snake needs to reduce its heat load, one option is to eliminate the feces. Coiling the body and aggregating with other snakes can profoundly reduce the rate of heat loss, such that snakes in winter dens can have higher body temperature than would be indicated by the ambient climate. Because of the many behavioral and other devices they have for regulating body temperature, reptiles can maintain their temperatures within a surprisingly narrow range. The typical standard error of the mean of repeated measurements of an individual reptile's preferred body temperature is less than 1.8°F (1°C). These measurements were made under close to ideal conditions; nevertheless, such studies show how precise reptiles can be when heat sources and refuge are readily available and when no obstructions are placed in the way of locomotion. Although reptiles lack the endothermic mechanisms of birds and mammals and only a few species of reptiles engage in shivering thermogenesis, when solar or other heat sources are available during the active season, many reptiles can maintain high body temperatures with surprisingly little variability.
The temperature at which reptiles maintain their bodies during the active season can be influenced not only by acclimation but also by disease. Infection with pathogenic bacteria can cause the reptile to prefer a higher than usual temperature. This so-called behavioral fever kills the bacteria within a few days. This interesting phenomenon has been found to occur among most ectothermic vertebrates and is straightforwardly analogous to the endogenous fever response of endothermic vertebrates to pathogenic bacteria. Another thermal phenomenon is emotional fever. Lizards handled briefly by humans regulate their body temperature between one and two degrees higher than normal. This elevation may be related to a flight or escape response or to a metabolic response to the immunosuppressive effects of stress. The presence of food in the stomach also induces reptiles to elevate their body temperature and digest the food more rapidly and more thoroughly than would otherwise be the case. Pregnant females prefer higher temperatures, which facilitate gestation. As we discover these fascinating events, our appreciation of the precision of reptile behavior increases, as does our ability to care for these animals in captivity.
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David Chiszar, PhD
Hobart M. Smith, PhD
Americans for years have been embracing new technology to make life more convenient, and at the same time grew accustomed to cheap, abundant energy to power that technology. Each year more than $500 billion is spent in the United States for energy to perform work and provide heat, and the energy needed for creating and building businesses, home-making, getting around, purchasing goods, and seeking pleasure. The decision-making exhibited while engaging in these daily activities varies widely, which results in diverse energy use behavior.
THE LONG-TERM IMPACTS OF DECISIONS
Energy-use behavior cannot be looked at just in the here and now or in isolation. Americans developed a greater reliance on heat, light, and power than any other nation, primarily due to decisions made by industry, government, and individuals of earlier generations that largely established the patterns for behavior today. Collective choices made long ago have behavioral consequences today, just as collective choices made today will have behavioral consequences for many more years to come.
The high-energy-consumption culture evolved primarily as the automobile became affordable in the 1920s and 1930s, offering tremendous mobility and the possibility of distancing home from work, school, and pleasure. It was then accelerated by the post-World War II federal policies of funding a vast network of highways and offering subsidies (home mortgage and property tax deductions) to make home ownership more affordable. Government policymakers at the time never realized the energy consequence of these nonenergy policies. Highways to everywhere, and home ownership subsidies promoted flight from city centers to suburbia and a high-energy-consumption economy largely based on the automobile. With these policies in place, the energy use patterns established a trend of greater and greater consumption. Moreover, the many benefactors of these policies amassed significant power and influence in the political system, discouraging any reversal.
Another factor contributing to high-consumption behavior is the ever-growing affluence of the population. In the 1950s, U.S. consumers spent around 25 percent of disposable income for food, which fell to less than 10 percent by the 1990s. For many, this translates into less than three hours of wages to cover the family's weekly food bill. (For much of the developing world, it can take seven or eight hours of wages to feed a family of four for one day.) Moreover, six or seven hours of wages in the United States can cover the cost of a month's worth of gasoline for the cars, and electricity and natural gas for the home.
This growing affluence allows a greater percentage of disposable income to go toward purchases of energy-using technology and ever-more-elective applications, rather than the cost of energy to power the technology. For example, while energy costs had remained flat or risen only slightly from 1980 through 2000, the average cost of an automobile tripled, going from around $7,000 to $21,000. Furthermore, as the United States became more affluent, the distinction between want and need—elective and essential—has blurred. Technology that was rare or nonexistent in the 1960s became a want in the 1970s and 1980s, and evolved into what many consider a need or necessity in the 1990s. Most households are now dependent on all sorts of energy-hungry technology that did not even exist in the 1960s—appliances such as lawn trimmers, personal computers, and microwave ovens, to name a few. Greater affluence, and an economy geared toward high consumption, has resulted in U.S. per capita energy use growing to more than thirty times that of developing world nations, and more than twice that of Western Europe or Japan.
One of the biggest fears with massive consumption and the high energy lifestyle is the high level of immigration that has accompanied it. Although the U.S. birthrate has been slightly more than self-sustaining since the 1970s, a liberal immigration policy had added more than 30 million people to the population by 1995, and is likely to push the population to more than 400 million by 2050. Millions of people from around the world try to immigrate, legally or illegally, to enjoy the freedom and the opportunities of the United States. Because most immigrants, regardless of culture and country of origin, tend to quickly assume the U.S. norm of high-energy-use behavior that increases carbon emissions, exacerbates air pollution (through a need for more power plants and automobile use), and worsens the dependence on imported crude oil, overpopulation and poverty is as much a United States problem as a world problem.
LIFESTYLES AND ENERGY USE
Energy is the ability to do work. The work can be done by man or machine; the fuel to do the work can come from food or fuel. Humans often have a choice of whether or not to continue to develop technology that replaces the work of man as well as do other types of work that man is not capable of doing. Historically, the choice has been almost always to embrace technology, driving the steady increase in demand for energy.
There is often a mental disconnect between technology and the energy needed to make technology work. Except for a small percentage of the population, people are not much interested in energy itself, but only what it can do for them. That is because energy is largely invisible. The units of energy—a kilowatt hour of electricity or a cubic foot of natural gas—cannot be seen. Thus, energy is primarily thought of in financial terms: the dollar amount of the electric or natural gas bill, or the cost of refueling the car.
Energy behavior can be categorized loosely as conserving (frugal), efficient, or high-energy (Table 1). It is not always fair to make such clear-cut distinctions because many behavior patterns exist within each income level that are beyond easy classification. For example, a few cross-classifications could be rich and frugal, rich and efficient, poor and frugal, and poor and wasteful. Ironically, the poor are often the most wasteful and inefficient because they own older cars and live in leaky homes.
The distinction between high-use versus conserving or efficient is well-known, but the difference between energy-conserving behavior and energy-efficient behavior is a more subtle distinction. Energy is used to perform important tasks such as getting to work. A vehicle will use a given amount of British thermal units (Btus) per mile. Multiplying the Btus per mile by the number of miles traveled, results in the total Btus used. The goal of energy conservation is to reduce the number of Btus used. Energy-conserving behavior is not strictly about forgoing consumption. The conserving person still needs to get to work; he just wants to get there by using less energy, say, by substituting the train, bus or carpool for driving alone. On a per passenger basis, the difference between the two could be 100 to 160 miles per gallon (mpg) for carpooling or mass transit, compared to 15 to 40 mpg for driving alone. Remembering to turn off the lights when leaving a room, or turning down the thermostat to 60°F, or shoveling the walk instead of buying a snow blower, are other examples of energy-conserving behavior.
Energy-efficient behavior usually involves embracing all technology to do work, but picking the most energy-efficient products to do that work. The energy-efficient choice is to stay in the single passenger automobile, but reduce the Btus per mile by buying an efficient model that gets 40 mpg rather than
|Conserving (Frugal) Lifestyle||Efficiency Lifestyle||High Use Lifestyle|
|Food||Eating a healthy diet and only the required 2,500 to 3,000 calories.||Prone to overeating, but tries to keep a healthy diet.||Overeating and an overabundance of meat and empty calories, highly processed junk foods.|
|Exercise||Commuting by walking or riding a bike.||Membership in fitness club to overeat yet remain thin.||Very little or none. Likely to become obese.|
|Recreational Activities||Walking and bicycling.||Running, swimming and aerobics.||Golfing with a cart, personal watercraft, all-terrain vehicles, and snow-mobiling.|
|Hobbies||Participatory activities.||Participatory activities.||Watching television and attending sporting events.|
|Home||1,500 sq. ft. apartment or condominium in a city high-rise.||5,000 sq. ft. energy-efficient home in the suburbs.||5,000 sq. ft., nonenergy-efficient home in the suburbs.|
|Heating||High-efficiency furnace and water heater. Bundles up with a sweater to keep thermostat at 65 degrees.||High-efficiency furnace and water heater. Keeps thermostat at 72 degrees, but programs it to 60 degrees for overmight hours.||Low-efficiency furnace and water heater. Keeps thermostat at 78 degrees at all times.|
|Air Conditioning||Only a few rooms, and only when occupied.||Central air that is programmed to come on an hour before arriving home.||Central air all the time, regardless if anybody is home.|
|Appliances||With limited space, fewer and smaller; only the necessities. The necessities must be energy-efficient. Buys food supplies daily on the way home from work; no need for a large refrigerator and freezer.||Desires all technology, but wants the most energy-efficient products.||Desires all technology and does not put a priority on energy efficiency in making buying decisions. Huge refrigerator and freezer for the convenience of limiting food shopping to once a week.|
|Vehicles||None||Two energy-efficient models.||Three or four automobiles and a recreational vehicle.|
|Commute||3 miles by walking, bicycling, or mass transit.||40 miles in an energy-efficient automobile. If there are high-occupancy vehicle lanes, will make effort to carpool.||60 miles in a 14 mpg sports utility vehicle, and a disdain for mass transit, carpooling and high-occupancy vehicle lanes.|
|Schools||Children walk to nearby school.||Bus to local school.||Drives children to school.|
|Children's activities||Nearby so that they can walk or take public transportation.||Effort to carpool and make it convenient for drop-off and pick-up on the way to and from work.||Little regard for proximity. The baby sitter will do driving.|
|Shopping||The majority is done locally and daily; accessible by walking or mass transit.||Drives to nearby shopping. Realizes that the additional gasoline cost of driving negates much of the discount achieved by giant retailer shopping.||Willing to drive many additional miles to get to malls and giant retailers.|
|Recycling||Yes||Yes, if there is a financial incentive.||No, unless mandated. Throw-away mentality.|
one that gets 15 mpg, or replace a 75-watt incandescent light bulb with a 25 watt fluorescent tube. The amount of illumination is the same, but the amount of energy used is one-third less. In terms of total energy savings to a nation, energy conservation is more important than energy efficiency since there is less energy used when no light is on than when an efficient one is on.
A person exhibiting energy-conserving behavior is likely to exhibit energy-efficient behavior as well, but someone who exhibits energy-efficient behavior is not necessarily going to be into energy conservation. Energy conservation usually indicates a sacrifice in something, be it safety, aesthetics, comfort, convenience, or performance. To many energy-efficient people, this is an unacceptable compromise. Energy-efficient homeowners will take energy efficiency into account in making decisions about home lighting; yet, if they have a strong preference for art gallery quality lighting, they do not hesitate to spend more for electricity than a conserving neighbor whose only concern is a good reading light. Whereas energy-efficient behavior primarily entails an approach to making technology-buying decisions energy-conservation habits require a full-time, conscious effort to reduce the amount of technology used.
The reasons for the range of energy-use lifestyles, and the motivation behind energy decision-making, are varied and complex. Prior to the 1970s, there was little study of energy-use behavior. But energy scarcities and the growth of the environmental movement made the energy problem a social problem that economists, psychologists, sociologists, and anthropologists all began to address.
Economists primarily look at energy decisionmaking, like all other decision-making, as a function of price and utility: The individual is a rational utility maximizer who gathers and weighs all the relevant information to make cost-benefit evaluations to arrive at decisions. If an energy conservation or efficiency product proves beneficial, the purchase will be made. It is uncertain how much of the population realizes that often the greater initial out-of-pocket cost for energy-conservation measures and energy-efficient products will be made up by greater savings through the lifetime of the product. And even when aware of the life-cycle savings, the purchase of more energy-efficient products may be rejected because of an unacceptable compromise in safety, aesthetics, comfort, convenience, quality, or performance. For example, the five-passenger Mercedes might be preferred over the five-passenger Hyundai since the Hyundai's acceleration, reliability, and luxury shortcomings more than outweigh the benefits of its lower price and better fuel economy.
The current price of energy is a primary factor entering into decision-making, yet the expected future price of energy can be even more important since the product life cycles of most technology is over ten years (autos and appliances 14 or more years, real estate 30 or more years). After the Energy Crisis of 1973, people adopted energy-conserving and efficient behavior more because of the fear of where energy prices were headed than the price at that time. Almost all energy forecasters in the 1970s and early 1980s predicted that energy prices would soar and oil resources would soon disappear. National Geographic in 1981 predicted an increase to $80 per barrel by 1985—a quadrupling of gasoline prices. By the 1990s, when these forecasts turned out to be wildly erroneous, most people reverted back to their old lifestyles, and became much more skeptical of any proclaimed impending crisis.
Whereas the focus of economists is on price and utility, the focus of psychologists has been on attitudes and social norms. For example, the nonprice reasons for choosing an energy-conserving or efficient lifestyle have been for social conformity and compliance—a sense of patriotism, good citizenship, or because it was the environmentally friendly thing to do. The main theory is that attitudes determine behavior, and if attitudes can be changed, it is more likely that behavior will change too. However, there is also the opposite theory: Behavior causes attitudes to change. If persons assume an energy-conserving lifestyle, they will assume attitudes to support that action to avoid internal conflict or hypocrisy (Stern, 1992).
Another problem in studying attitudes and behavior toward energy is the pace of change. Even in times of energy shortage, attitudes and behavior tend to change slowly and are influenced as much by cultural and geographical factors as price. A family living in a 5,000 sq. ft. home can turn down the thermostat, yet still needs to heat a significant amount of space. And if the family lives 75 miles from work, they can switch to a more-fuel-efficient car, yet still must burn considerable gasoline commuting. Of course, this family can make a drastic lifestyle change and consume a fraction of the energy by moving into a 1,200 sq. ft. condominium a few blocks from work and school, but this choice is unlikely for many who have grown accustomed to the comfort, lifestyle and perceived safety of suburban life and feel it is well worth the inconvenience and higher energy costs.
THE COMMERCIAL SECTOR
Energy is a capital investment decision that is often neglected by corporations. When a corporation decides to build or rent office space, the energy needed to heat, air-condition, and illuminate the facility usually is not a top priority. In the 1990s, layout, ergonomics, aesthetics, corporate image, proximity of highways and mass transportation, and other productivity factors ranked much higher. Moreover, there remains skepticism about the ability of energy-efficient technologies and design strategies to simultaneously save energy and improve labor productivity. If the price of energy skyrocketed, lowering energy costs would quickly become a priority again. Many office facilities, not competitive on an energy-efficiency basis, could become obsolete well before their expected lifetime of 50 to 100 years.
THE INDUSTRIAL SECTOR
Lean production—the improvement of processes and operations—is something that all industrial executives encourage, and is largely why the U.S. industrial sector energy consumption dropped about 20 percent from 1973 to 1983. With much of energy efficiency gains already achieved, company executives of the 1990s found the payback from other investments more economically efficient, and consequently have put the emphasis elsewhere. A top priority in the 1990s was faster and cleaner processes that reduced manufacturing costs and reduced waste. If these processes also reduced energy use, it was an added bonus.
Besides operational energy use, corporate decisions have an impact throughout society. One reason that per capita energy consumption in North America is much higher than in the rest of the world was the decision of corporate leaders to expand and relocate away from city centers and to major beltway loops in the suburbs. This necessitated more trucking and a workforce reliant on the private automobile instead of mass transit. In an era of tremendous job insecurity, even the most energy-conserving person is hesitant to reside near work, or live without the mobility of the personal vehicle.
THE TRANSPORTATION SECTOR
Driving by personal vehicle is the most popular mode of transportation. And although there is a desire for a fuel-efficient automobile, fuel efficiency is a consideration well behind style, performance, comfort, durability, reliability, status, and safety. The weak demand for a 40 mpg automobile occurs for several reasons: It is not a status symbol (not stylistic), accelerates too slowly (smaller engine), cramps the driver and occupants (smaller interior), and often offers inadequate protection (too light) in case of an accident.
Another reason for weak demand is that fuel cost in 1999 were a much smaller fraction of the cost of owning and operating a vehicle than it was in 1975. While the average cost of owning and operating an automobile more than tripled from 1975 to 1999, the price of fuel increased only marginally, and average fuel economy improved from 15 to 27 miles per gallon. As fuel cost declines relative to all other costs of operating an automobile, the purchase of a fuel-efficient automobile increasingly becomes a more secondary financial consideration. Most motorists express a preference for greater size, luxury, and performance from the automobiles and trucks purchased, knowing that such attributes usually are detrimental to fuel economy.
The Private Auto Versus Public Transit
The freedom of private transportation is something most Americans have taken for granted and consider a necessity. Only a generation ago, an automobile was considered a luxury, and a generation before that, a rarity. The ascension of the automobile coincided with the decline in mass transit in the 1920s. This trend accelerated after World War II when American society decided to build a vast interstate highway system, emphasizing private automobiles at the expense of mass transit. It is not easy to undo the technological impetus of large infrastructure changes of this nature. According to the U.S. Census Bureau, by 1990, 73 percent of the population drove alone to work, up from 64 percent in 1980. In percentage terms, walking (3.9%), bicycling (0.4%), public transit (5.3%), and carpooling (13.4%) all declined between 1980 and 1990. For other travel (shopping, family business, and leisure), which was responsible for two-thirds of all travel, the share of collective modes and nonmotorized modes was even lower.
A major mass transit handicap is sprawling growth. Mass transit works most effectively in a hub and spoke manner, and the decentralization of urban areas makes it harder and harder to design effective mass transit. This mismatched segregation of home, work, and leisure—particularly in cities such as Houston, Atlanta, and Los Angeles—would require a long time to reverse.
Even where mass transit offers better speed and comparable comfort and convenience, the love of the automobile results in uneconomic decisions affecting energy consumption. For someone driving 30 miles to New York City, tolls, fuel, and parking can easily exceed $20 a day. In comparison, a monthly train pass for the same 30 mile trip runs less than $10 a day. This premium for the privilege of driving is even greater if the yearly $6,000 to $10,000 cost of leasing, insurance, and maintenance is included. Because of congestion, few can claim to drive for speed reasons. The roadway trip often takes more time than via mass transit. Getting more people out of single-passenger cars and into more-energy-efficient mass transit or car pools is going to take more than improving mass transit and the price of gasoline tripling. Americans, like most others in the industrial world, have an emotional attachment to the automobile.
Another reason for choosing the automobile is its development as a mobile office. Cellular phones, laptop computers, and satellite linkups to hand-held communication devices—all of which are getting smaller and smaller—have made it possible to be more productive from the roadways.
Aside from work, the automobile is central for shopping and pleasure. Most people who lived in the suburbs in the 1990s had grown up in the suburbs rather than in central cities. The automobile, and the mobility it affords its owner, is a central aspect of suburban life. Shopping centers and businesses, lured by cheaper land in the exurban areas, do not fear locating away from city centers because Americans show an eagerness to drive greater distances. The auto also caters to the needs of family life. Few suburban parents drive directly to and from work anymore. The growth in extracurricular sports and activities, for parents as well as children, requires the convenience of the automobile.
Higher incomes, higher automobile ownership, and a decline in the population and workplaces that can be served by mass transit has lead to the declining mass transit demand. Criticism of this shift toward the private automobile comes mainly because the individual driver receives the short-term benefits (privacy, comfort, speed, and convenience), while the negative social consequences (air pollution, traffic jams, and resource depletion) are shared by all. Moreover, if people drove less, and drove more-fuel-efficient vehicles, the positive national goal of less dependence on imported oil would be achieved.
The explosive growth in air travel from 1980 to 2000 occurred because deregulation reduced air fares, disposable incomes rose, and travelers desired to get places faster. People are willing to pay a large premium for speed by flying instead of driving. The premium is largely a reflection of the much greater energy costs of flying, yet it is not always a greater cost. On a passenger-miles-per-gallon basis, usually more energy can be conserved by flying a full aircraft rather than having each passenger drive solo to a given destination.
People make decisions to fly based on speed, the price of the flight, and an airline's on-time arrival. If these factors are the same, then secondary concerns such as comfort become a factor—fewer seats with more spacious seating, and more room to walk about. Since greater comfort means fewer paying passengers, the airlines' decision to cater to the desire for comfort will adversely affect fuel economy per passenger.
The widespread disfavor toward prop planes is another preference adversely affecting fuel economy. Prior to the late 1990s, the 30- to 50-seat plane market was dominated by the more-energy-efficient turboprop planes, yet regional airlines ordered jet engine replacements, citing strong customer preference. Airlines are turning to jets because people want to get places faster. Turboprop planes, which attain near-jet speeds and near-jet performance with less fuel consumption, are noisier and thought to be less safe, even though safety records do not warrant this belief.
The future demand for jet travel is very uncertain. Businesses have traditionally felt a need to travel for face-to-face meetings, but the new communications technology revolution might in the future make business travel less necessary. However, any dropoff in business travel is likely to be replaced by growth in leisure travel as more affluent Americans decide to take more but shorter vacations, and fly for more weekend getaways.
Individuals and companies make shipping decisions based on price, speed, and reliability. If reliability and speed are equivalent, the decision usually comes down to price. Because energy costs of shipping by air, railway, waterway, and truck vary tremendously, so does the price. Whereas commodities and basic materials are moved by rail, just-intime components and finished product are mainly moved by truck, accounting for over 80 percent of all freight revenue. Rail is cheaper and a much more energy-efficient means to move freight, and has been approaching trucking for speed; yet businesses continue to pay a premium for trucking because rail historically has been less reliable.
THE RESIDENTIAL SECTOR
The American home is widely perceived as a good investment that appreciates. Any additional improvement to the home is considered wise for two reasons: the enjoyment of the improvement, and greater profit when the time comes to sell. This belief, in part, explains the preference for bigger new homes with higher ceilings over the large stock of older, smaller homes built from 1950 to 1970. The average new home grew from 1,500 square feet in 1970 to over 2,200 by 1997, and the inclusion of central air conditioning grew from 34 to 82 percent. These new homes are usually farther from city centers, and indicates a general willingness to endure the inconvenience and higher energy cost of longer commutes. The continuing trend of bigger suburban homes farther from city centers is attributable to the affordable automobile, the expansive highway system, and the pride of owning one's own home. Whereas a generation ago a family of eight felt comfortable sharing 2,000 square feet of living space, the generation of the 1990s located twice as far from work so that they could afford twice the space for a family of four. But home size alone can be a poor indicator of energy use. Energy consumption can often vary by a factor of two or three for similar families living in identical homes (Socolow, 1978).
Many people approach home ownership investments with only two major concerns: how much down and how much a month. They may be tuned into the present energy costs of a new home, but usually give little thought to how much energy will cost 10 or 20 years into the future. If energy-efficient features push up how much down and how much a month beyond what the buyer can afford, buyers must forgo energy-efficient features. However, selecting energy efficient features for a new home does not necessarily have to increase the down payment and monthly mortgage payment.
Aesthetics is often more important than energy efficiency to home buyers. Energy efficiency that is economically efficient is welcomed if it does not come at the expense of aesthetics. Tightening up a home with better insulation and caulking is fine, but solar collectors on the roof are thought by many to be an unsightly addition to a home. Prior to the 1960s, buildings in the sunbelt were usually built with white roofs. But as air conditioning became widespread, the "cooler" white roof grew in disfavor. Darker roofing shingles were perceived to be more attractive, and did a better job of concealing dirt and mold. Aesthetics won out over energy conservation. In a city like Los Angeles, replacing a dark roof with a white roof can save more than $40 in air-conditioning bills for the hot summer months. Eventually, through education, the aesthetic benefits of a dark roof might be deemed less important than its energy wastefulness.
The energy consequences of bigger homes filled with more power-hungry technology is a need for more energy. Because of federal standards for appliances and heating and air conditioning equipment, the new, larger homes have only incrementally increased consumption. However, the desire for ever-bigger refrigerators and freezers, and the continued introduction of more plug-in appliances, many of which run on standby mode, promises to keep increasing the residential demand for electricity.
By the 1990s, after decades of extremely reliable service, most customers have come to expect electricity to be available when they want it and how they want it, and feel the utility has the obligation to supply the power. If the customer's preference is to keep the air conditioning on all day, it is the obligation of the utility to supply the power for this preference. Customers have become so accustomed and reliant on electricity that the 1990s consumer felt service without interruption was a right. This was far different than a few decades earlier. New York City residents tolerated the major blackout of 1965 and the inconveniences it caused, yet the minor blackout of 1999 was widely believed to be inexcusable and that Con Edison (the local utility) should compensate its customers for damages, and it did.
During prolonged heat waves which can cause blackouts and brownouts, electric utilities always request that customers limit energy consumption by raising thermostats and turning off the air conditioning when not home. It is uncertain if these requests are heeded, and if they are, it is uncertain whether conformity is out of altruism (help the utilities) or self-interest (save money).
Just as gasoline is the energy source for the automobile engine, food is the energy source for the human engine, yet it is not common for people to think of food in this way. It is more common to look at food as a means to satisfy a hunger craving. Even among those claiming to want to lose weight, 35 percent of men and 40 percent of women are not counting calories according to the American Medical Association.
The minimum average energy requirement for a sedentary adult to survive is around 2,000 kcal a day, which rises to 4,000 or more if the person is engaged in strenuous labor much of the day. Only a very small percentage of the U.S. population is involved in strenuous labor, yet much of the population consumes over 4,000 calories each day, resulting in an America that is 50 percent overweight and 20 percent obese.
Few people do manual labor anymore. Technology has modified behavior so that most people burn 2,500 to 3,000 calories a day, not 4,000 or more calories as was common up until the 1960s. The washboard was replaced by the washing machine; the manual push mower was replaced by the power mower; the snow shovel was replaced by the snow blower; the stairs were replaced by the elevator. Technology has made it easy to be inactive, yet few individuals will blame technology for their obesity.
The tremendous growth in health and fitness clubs since the 1960s caters to those Americans who would rather burn more calories than reduce calorie intake. By working out regularly, they can overindulge. However, only a small minority of the U.S. population exercises regularly; the majority overindulges and remains overweight.
If a sedentary person consumes more than 4,000 calories a day, obesity is likely to result; yet, among this group, a failure to match food energy input with activity energy output is seldom mentioned as the reason for obesity. Because much of the population cannot control the short-term pleasure of overindulging, and despite awareness of the long-term consequences to health and appearance, some politicians have proposed that health insurance, Medicare, and Medicaid should pay for surgical procedures (stomach staple) and diet pills (Redux, Fen-Phen) to combat what they call an "overeating addiction."
When given a choice among materials, determining the most energy-wise material seldom easy. Consider the energy needed to build an aluminum bicycle frame versus a steel tubular frame. The aluminum bicycle frame takes considerably more energy to build, and needs replacement more often. However, if the bicycle is meant to be a substitute for the automobile, the lighter aluminum frame material can dramatically lower the human power output needed to climb hills. The energy saved during the use of the aluminum frame will more than compensate for the greater energy needed to manufacture it.
The energy-wise choice of material for packaging—paper, plastic, tin, aluminum, or glass—can be equally tricky. Although it takes less energy to make glass and plastic bottles than aluminum cans, few buyers think of purchasing a favorite soda because it comes in glass or plastic rather than aluminum. Energy rarely enters into the buyer's decision. Price and quality concerns are foremost. Plastic wins out on price and glass on quality; rightly or wrongly, there is a perception that glass is better at not altering the taste of the product. Few consumers would be receptive to fine wines coming in plastic bottles or aluminum cans. Glass could someday compete with plastic at the lower end of the beverage market as well if the price of crude oil increased significantly. But glass must also overcome a higher freight cost (glass is much heavier than plastic).
The recycling of bottles and containers is almost universally viewed as an energy-wise and environmentally sound moral good—one of the best way to conserve energy, resources and landfill space. Usually this is true. However, few people realize that for some materials there are great energy savings, and for others very little. For instance, whereas the net energy savings to recycle aluminum cans is substantial, it can take nearly as much energy, and generate as much pollution and waste, to recycle tin cans as to produce new ones from raw materials.
A modest amount of recycling of metals occurs today. Much more can be done, and much more can be done to encourage energy-wise buying decisions as well (choosing products packaged with materials requiring less energy over those requiring more). Whether the American public is willing to invest more time and effort into recycling, and willing to alter product packaging choices solely for energy and environmental reasons, is highly questionable.
Energy-related behavior modification goes on continually. Advertisers market the latest household electrical technology to make life easier; tourism agencies promote exotic cruises and resorts; and governments mandate energy conservation and energy efficiency measures to lower carbon emissions and reduce energy imports. These efforts, either direct or indirect, subtle or overt, are all designed to modify behavior that affects energy consumption.
Governments have taken an active role to alter behavior, to conserve energy, and to use it more efficiently. The rationale is that lower energy consumption reduces the need for additional fossil fuel power plants and the need for less crude oil imports. Since the Energy Crisis of 1973, the U.S. government's behavior modification efforts have been many and can be grouped into five categories: information campaigns, feedback, reinforcement, punishment, and reward.
Information campaigns refer to the broad range of brochures, flyers, billboards/signs, workshops, and television and radio advertisements designed to encourage energy-conserving behavior. The success of an information campaign depends on the audience paying attention and taking the message seriously; moreover, the intended audience must trust the government source, and receive confirmation from friends and associates (Stern, 1992).
A lack of trust and confirmation is one of the reasons that the energy-related informational campaigns of governments and environmental groups have faired so poorly. Despite millions being spent in the 1990s to promote conservation, energy efficiency, and renewable energy to combat global warming, the sales of energy-guzzling vehicles skyrocketed as economy car sales declined, the number of vehicles and average miles per vehicle increased, and the average home size and the number of electric appliances in each home kept rising. In a 1997 New York Times poll, when asked to rank environmental issues, only 7 percent ranked global warming first (47 percent said air and water pollution), and a CNN poll in that same year found 24 percent of Americans concerned about global warming, down from 35 percent in 1989. Since earlier informational campaigns to change behavior (for example, the supposed energy supply crisis of the 1970s was projected to only get worse in the 1980s) turned out to be erroneous, the American public viewed the government's global warming campaign much more skeptically.
Feedback modification efforts are targeted information programs that address the lack of awareness of people about the consequences, the belief being that if people are aware or educated about the negative consequences of such behavior, it is more likely that they can be convinced to engage in behavior more beneficial to the environment. For example, if the public better understood that all forms of energy consumption come with unfavorable environmental side effects, people would be more likely to conserve and use energy more efficiently. Once made aware of the deleterious environmental consequences, people would be more likely, in theory, to buy a more fuel-efficient minivan than a gas-guzzling sports utility vehicle that marketing research shows customers rarely or ever take off-road.
Reinforcement efforts include rewarding people for engaging in energy- and environmentally-beneficial behavior. High-occupancy vehicle lanes reinforce carpooling because the driver and passengers benefit with a faster trip for the ride-sharing sacrifice, and the EPA-DOE Energy Star label affixed to energy-efficient products is a sign to others that the purchaser uses energy in an environmentally friendly way. Punishments that have been tried include a fine or premium for energy use that causes pollution: the taxation of gas-guzzling automobiles. Rewards tried include tax credits for more environmentally friendly renewable energy and conservation measures, the construction of carpool park-and-ride lots, and government-mandated employer bonuses for employees who do not drive and therefore do not take advantage of subsidized parking (probably the most effective behavior modification program.
It is very difficult to determine the effectiveness of efforts to change behavior. There are those who feel a need to adopt an energy-conserving or efficient lifestyle for appearance sake, yet will assume high-energy behavior when no one is watching (Bell et al., 1990). Moreover, no universally effective method has been found for getting people to reduce energy consumption, and there is great debate as to whether it is a policy worth pursuing. First, many policy changes entail encroachments on freedoms. Since the freedom to choose is a major right that most are very reluctant to give up, passing policy that curtails freedom is controversial. Second, there is disagreement about whether human beings are rational about energy use, and actually modify behavior for any reason except self-interest (say, altruism). Third, sometimes the cost of the program exceeds the value of the energy saved, or the cost-to-benefit ratio is unfavorable. Finally, nonenergy policy has a greater impact on energy consumption than any energy policy itself. Government subsidies for home ownership, the public funding of highways, and policies encouraging suburban sprawl were far more responsible for per capita energy consumption being much greater in the United States than Japan or Europe than was any energy policy.
From the corporate end, almost all of the behavior modification efforts directed at consumers are to get people to use more technology. It usually follows that greater technology use results in greater energy use.
To combat a corporate image of a single-mindedness toward greater consumer consumption, companies such as Dupont and 3M make great efforts at what they refer to as eco-efficiency—improving processes to reduce energy use, waste, and air pollution. The message to consumers is yes, we produce the technology you demand, but we do so in an energy-efficient way that is friendly to the environment. Dupont has been aggressively seeking to hold energy use flat, relying more on renewable sources such as wind and biomass, and is trying to reduce its 1990 levels of greenhouse gas emissions by 45 percent by 2001, and by 65 percent by 2010. The U.S. government encourages this behavior, and would like more corporations to no longer think of energy as just another cost, equal in importance with all other costs. Except for the EPA-administered punishments for pollution, most of the Federal effort is toward reinforcing and rewarding good corporate citizenship actions, not mandating them.
When questioned by electric utilities, a majority of residential customers show a willingness to consider paying a modest amount more per month for electricity powered from nonpolluting renewable energy sources, despite not knowing much about them. How many would actually choose to pay a premium for renewable energy is very uncertain. For those who are skeptical or ambivalent about the possibility of fossil fuel resources exhaustion, air pollution and global warming, the primary interest is the lowest price and best service. Early green-energy marketing efforts have shown promise in reaching those who are concerned about environmental issues. In 1999, Mountain Energy of Vermont signed up over 100,000 Pennsylvanians and Californians who will pay a 5 to 35 percent premium for electricity generation not involving nuclear power or coal. It is a surprisingly good start for Mountain Energy, who cannot actually get green power to the home, but instead must sell the concept—the green power the customer buys displaces the traditionally generated electricity in the area. Because many consumers buying the green energy are in a different region from where it is produced and therefore will not "receive" the cleaner air for which they paid a premium, it is uncertain whether green marketing programs will provide measurable environmental improvements. The result may be significant new renewable energy development, or it may just support the renewable base already in operation.
Because of the huge food surplus in the United States, attributed to the high productivity of mechanized agriculture, there is probably no area of the energy consumption picture where more behavior modification takes place than food. The huge agricultural surplus keeps food products cheap, and would increase tenfold if Americans cut meat consumption in half and ate grains instead. Humans get from grain-fattened cattle only about 5 percent of the food energy they could get by eating the grain the cattle are fed.
There are thousands of special-interest groups trying to modify consumer behavior to eat more, or eat higher up the food chain. This would not be possible without a huge food surplus. It has never been easier to overeat. Even for those who lack the time or motivation to cook, the fast-food industry and microwaveable meals have made food more convenient and widely available than ever before. At the other extreme is the $33 billion a year weight loss industry promoting products to lose the weight gained "eating down" the food surplus. In the middle is the U.S. Department of Agriculture (USDA) with an inherent conflict of interest: One arm of the organization promotes the consumption of food while another arm publishes the highly political and ubiquitous food pyramid of good eating.
Since Americans enjoy the cheap food supply made possible from a huge agricultural surplus, and there seems to be no desire to reject technology and go back to a reliance on manual labor, the food surplus problem of an overweight and obese America is likely to remain for years to come.
Easy access to inexpensive energy has come to be viewed as a basic right. The American public goes about its daily life largely optimistic, feeling that any fossil-fuel shortage will be alleviated by new breakthroughs in developing supplemental sources, and that new end-use technology will be developed that can be powered by these new energy sources. Energy conservation and energy efficiency are lifestyle options. However, if the day ever comes when energy conservation or an accelerated adoption of energy-efficient products will need to be mandated, the American public would be more likely to choose the energy efficiency route because it does not necessarily entail sacrifice.
See also: Air Travel; Bicycling; Capital Investment Decisions; Communications and Energy; Economically Efficient Energy Choices; Economic Growth and Energy Consumption; Freight Movement; Green Energy; Government Agencies; Industry and Business, Productivity and Energy Efficiency in; Materials; Propulsion; Traffic Flow Management.
Bell, P. A.; Baum, A.; Fisher, J. D.; and Greene, T. E. (1990). Environmental Psychology,3rd ed. Fort Worth, TX: Holt, Rinehart and Winston Inc.
DeSimone, L. D., and Popoff, F. (1997). Eco-Efficiency: The Business Link to Sustainable Development. Cambridge, MA: MIT Press.
Energy Information Administration. (1997). Household Vehicles Energy Consumption, 1994. Washington, DC: U.S. Department of Energy.
Engel, J. F.; Blackwell, R. D.; and Miniard, P. W. (1998). Consumer Behavior,8th ed. Hinsdale, IL: Dryden Press.
Farhar, B. C. (1994). "Trends: Public Opinion about Energy." Public Opinion Quarterly 58(4):603-632. Farhar, B. C., and Houston, A. (1996, October). "Willingness to Pay for Electricity from Renewable Energy." National Renewable Energy Laboratory, NREL/TP-461-20813.
Gardner, G. T., and Stern, P. C. (1996). Environmental Problems and Human Behavior. Englewood Cliffs, NJ: Allyn & Bacon.
Kempton, W. (1995). Environmental Values in American Culture. Cambridge, MA: MIT Press.
Kempton, W., and Neiman, M., eds. (1987). Energy Efficiency: Perspectives on Individual Behavior. Washington, DC: American Council for an Energy Efficient Economy.
Nye, D. E. (1998). Consuming Power: A Social History of American Energies. Cambridge, MA: MIT Press.
Smil, V. (1999). Energies: An Illustrated Guide to the Biosphere and Civilization. Cambridge, MA: MIT Press.
Socolow, R. H. (1978). "The Twin Rivers Program on Energy Conservation in Housing: Highlights and Conclusions." In Saving Energy in the Home. Cambridge, MA: Ballinger Publishing Company.
Stern, P. C. (1992). "What Psychology Knows About Energy Conservation." American Psychologist 47:1224-1232.
U.S. Census Bureau. (1992). Census data, 1980 and 1990, Journey-to-Work and Migration Statistics Branch, Population Division, Washington, DC.
Veitch, R.; Arkkelin, D.; and Arkkelin, R. (1998). Environmental Psychology: An Interdisciplinary Perspective. Englewood Cliffs, NJ: Prentice-Hall.
See also 28. ATTITUDES ; 279. MOODS ; 334. PSYCHOLOGY .
- aberrance, aberrancy
- the condition or state of being deviant or aberrant.
- 1. having a tendency towards, or being in a state of abnormality.
- 2. something that is abnormal.
- a person who is characterized as being in some way abnormal.
- impulsive, rash, or irresponsible actions or attitudes, especially in the sphere of public life. —adventurist, n . —adventuristic, adj.
- the attitudes and behavior of one who exaggerates dangers or always expects disaster. —alarmist , n.
- Obsolete, illogicality, unreasonableness. —alogic, alogical, adj.
- the taking on of masculine habits and occupations by women.
- the dress and conduct suitable to a pastoral existence, usually with reference to the idealized description of pastoral life in literature. —Arcadian, n., adj.
- 1. the state of having recently achieved wealth or position, especially by unscrupulous or unethical means.
- 2. behavior typical of arrivism. —arriviste, n., adj.
- a severe self-deprivation for ethical, religious, or intellectual ends. —ascetic, n., adj.
- 1. a sad and gloomy individual.
- 2. an irritable and bad-tempered person. —atrabilious, adj.
- the practice of striking poses, either to mask or to express personal feelings. —attitudinarian, n.
- the characteristics attributed to attorneys; slyness; unscrupulousness.
- an automatic or involuntary action. —automatist, n.
- Rare. an abnormal fear of being egotistical, of referring to oneself.
- babuism, babooism
- Derogatory. the practices of Hindus who had only a slight English education. From bābū, a Hindi title equivalent to Sir or Mr.
- showmanship or any activity taking advantage of people’s credulity or desire for sensational entertainment, as practiced by P.T. Barnum (1810-91).
- the characteristics of a bashaw, especially tyranny and imperiousness.
- attitudes or behavior typical of a beatnik or one who has rejected conventions of society.
- a debased brutality, the opposite of humane activity: “I have lost the immortal part of myself, and what remains is bestial.” (Othello). See also 364. SEX .
- strangeness or grotesqueness, especially strange or unconventional behavior.
- behavior typical of a blackguard, characterized by use of obscene language and by roguish actions. —blackguardery, n. —blackguardly, adj.
- the practice of individualistic, unconventional, and relaxed conduct, of ten in an artistic context, expressing disregard for or opposition to ordinary conventions. —bohemian, n., adj.
- conduct characteristic of a stupid person or clown. —boobyish, adj.
- a braggart’s usual activity; bragging. —braggartist, braggart, n.
- the practice of advocating or engaging in brutality. —brutalitarian, adj.
- the set of attributes that characterize a brute. —brutish, adj.
- the actions of a bully.
- the characteristics associated with one who advances his career even at the expense of his pride and dignity. —careerist, n.
- an addiction to ceremonies or ritualism, especially in social and other nonreligious contexts. —ceremonialist, n.
- the study of character, especially its development and its variations. —characterologist, n. —characterologic, characterological, adj.
- the quality of having characteristics of a fraud. —charlatanic, adj.
- a habit or custom; usual behavior.
- foolish conceit or vanity; behavior typical of a coxcomb.
- reckless or foolhardy behavior. Also called daredeviltry . —daredevil, n.
- Obsolete, raving or maniacal behavior, as that of a bacchanal.
- proper behavior; action that is seemly and in good taste. —decorous, adj.
- the attitudes or behavior of one who stubbornly holds on to something, as an outdated view, untenable position, etc. —die-hard, n., adj.
- an admiration of or interest in the arts, often used pejoratively to designate a shallow, undisciplined, or frivolous attraction. —dilettante, n., adj. —dilettantish, adj.
- 1. an action characterized as being donkeylike; foolishness.
- 2. the characteristic of being like a donkey. —donkeyish, adj.
- idleness or indolence as a habit or regular practice.
- the habit of being shabbily dressed. —dowdyish, adj.
- the habit of performing actions in a histrionic manner.
- a pedantic adherence to logically constructed rules.
- an action or behavior that deviates from the norm; unpredictability in behavior.
- 1. a deliberately conspicuous or exaggerated mode of behavior, intended to gain attention.
- 2. the abnormal practice of indecent exposure. —exhibitionist, n. —exhibitionistic, adj.
- faineance, faineancy
- laziness; the state of being idle. —faineant, adj.
- the quality of being a fairy or having fairylike characteristics.
- swaggering boastfulness; vainglorious speech or behavior. —fanfaron, n.
- spiritual or intellectual dissatisfaction combined with a desire for power or material advantage. After Johann Faust (c.1480-c.1538), German scholar portrayed by Marlowe and Goethe. —faustian, adj.
- Rare. evil attitudes and actions.
- flunkyism, flunkeyism
- 1. the quality or state of being a servant or toady.
- 2. behavior typical of flunkyism. —flunky, flunkey, n.
- the condition of adhering solely to set formulas. —formularistic, adj.
- behavior typical of an earlier time; old-fashioned or stuffy attitudes; fogyism.
- the condition of having brotherly qualities. —fraternalist, n. —fraternalistic, adj.
- the administrative duties of officials. —functionary, n., adj.
- the habit of using organized violence to achieve one’s ends. —gangster, n.
- boastful or bragging behavior. Also gasconadc .
- 1. inclined to laughter.
- 2. laugh-provoking in conduct or speech.
- the extremely obsequious behavior of a sycophant. —gnathonic, adj.
- gourmandism, gormandism
- 1. a strong penchant for good food; gourmetism; epicurism.
- 2. gluttony. —gourmand, gormand, n., adj.
- attitudes and actions modeled on the grandees, Iberian nobles of the highest rank.
- gypsyism, gipsyism
- the activities and style of living attributed to gypsies. —gypsy, gipsy, n. —gypsyish, gipsyish, adj.
- a performance involving Harlequin or other characters of the Commedia dell’Arte; hence, buffoonery or clownish behavior. Also harlequinery.
- the practice of retiring from society and living in solitude, based upon a variety of motives, including religious. Also called hermitry, hermitship. —hermitic, hermitical, adj.
- a tendency to theatrical or exaggerated action. Also histriconism. —histrionics, n. —histrionic, adj.
- the state of being a hobo or vagrant.
- a dedication to taking holidays.
- lawless behavior or conduct typical of a hooligan.
- horsyism, horseyism
- looking or acting in some way like a horse. —horsy, horsey, adj. —horsily, adv.
- any behavior attributed to the Hottentots, in particular, a kind of stammering or stuttering.
- ill-bred, boisterous, or tomboyish behavior in a woman. —hoyden, n. , —hoydenish, adj.
- 1. pretentious behavior or attitudes.
- 2. imposing or deceptive behavior. —humbug, humbugger, n.
- humoralism, humouralism
- an obsolete physiological explanation of health, disease, and behavior, asserting that the relative proportions of four elemental bodily fluids or humors (blood-sanguinity, phlegm-sluggishness, black bile-melancholy, and yellow bile-choler) determined a person’s physical and mental constitution. —humoral, humoural, adj.
- division of patriotic loyalties, ascribed by some to foreign-born citizens in the United States.
- an idiosyncrasy or personal mannerism or peculiarity.
- a mannerism, action, or form of behavior peculiar to one person or group. —idiosyncratic, idiosyncratical, adj.
- lack of shame or modesty.
- 1. indecorous, improper, or unseemly behavior.
- 2. an indecorous thing or action.
- the customs or traditions of Indians, especially American Indians. —Indianist, n.
- the quality of revolting against established authority. —insurrectionist, n., adj. —insurrectionary, adj.
- a tendency to irritability and sudden fits of anger. Also called irascibleness . —irascible, adj.
- the quality of having traits or characteristics like those of Samuel Johnson. —Johnsonian, n., adj.
- juvenilism, juvenility
- Often pejorative. a mode of action or thought characterized by apparent youthfulness. —juvenile, n., adj.
- the actions and characteristics of a landlord. —landlordly, adj.
- the state of being noisy, rowdy, or disorderly. —larrikin, adj., n.
- a tendency to unrestrained, often licentious or dissolute conduct. Also libertinage . —libertine, n., adj.
- the pursuit or adulation of celebrities. —lionize, v.
- 1. an inclination to dispute or disagree with others, esp. through civil suits.
- 2. argumentativeness. —litigious, adj.
- the customs and characteristics of London and of those who reside there. —Londonish, adj.
- macaronism, maccaronism
- a tendency to foppishness. —macaroni, maccaroni, n.
- behavior characteristic of a maenad or bacchante; raging or wild behavior in a woman. —maenadic, adj.
- 1. the state or quality of being a maid, a young or unmarried woman.
- 2 . behavior or attitude typical of maidism.
- a style of action, bearing, thought, or speech peculiar to an individual or a special group. See also 23. ART . —mannerist , n. —manneristic, adj.
- an emphasis on scrupulous attention to the details of methods and procedures in all areas of life. —martinet, n. —martinetish, adj.
- 1. a tendency in temperament to be mawkishly sentimental and tearfully emotional.
- 2. a degree of drunkenness characterized by mawkish emotionalism. —maudlin, adj.
- behavior typical of that portrayed in a melodrama, i.e., characterized by extremes of emotion.
- 1. the state or quality of having a lively, fickle, volatile, or erratic attitude or charaeter.
- 2 . an instance of such behavior. —mercurial, adj.
- 1. the state or quality of being a weak and ineffectual person.
- 2 . behavior or attitudes typical of a such a person.
- an intense (and sometimes injurious) tendency to mimicry.
- boastful and pretentious behavior; quackery or any actions typical of a mountebank. Also mountebankery .
- behavior characteristic of a boorish person.
- the principle or practice of mutual dependence as the condition of individual and social welfare. —mutualist, n.
- weak or insipid behavior or attitude. —namby-pamby, n., adj.
- a quality or trait distinctive of Negroes.
- conduct characteristic of a ninny, or silly fool. —ninnyish, adj.
- a rootless, nondomestic, and roving lifestyle. —nomadic, adj.
- the practice of going nude. —nudist, n., adj.
- the characteristics and customs of people situated in western regions, especially the Western Hemisphere, as western European countries and the United States. —Occidentalist, n.
- the condition of resembling an ogre in actions and characteristics. —ogreish, adj.
- the conscious policy and practice of taking selfish advantage of circumstances, with little regard for principles. —opportunist, n. —opportunistic, adj.
- the habits, qualities, and customs of Oriental peoples. —Orientalist, Orientality, n.
- mindless imitation. Also called parrotry .
- the adherence to an exclusive subject, interest, or topic. —particularist, n. —particularistic, adj.
- 1. behavior or attitudes typical of one who has recently acquired wealth or social position.
- 2 . the state or quality of being a parvenu or upstart. —parvenu, n., adj.
- 1 . the state of being a member of one of the original citizen families of ancient Rome.
- 2 . the state of being noble or high born. —patriciate, n.
- the quality of having common manners, character, or style. —plebeian, n., adj.
- a tendency to conduct expressing indifference, nonchalance, or lack of concern. —pococurante, pococurantist, n. —pococurante, adj.
- the characteristics associated with being a coward or wretch. Also called poltroonery . —poltroonish, adj.
- a penchant for meddlesomeness and officiousness. Also polypragmacy, polypragmaty . —polypragmatist, n. —polypragmatic, adj.
- one whose conduct is unchaste, licentious, or lewd.
- praxeology, praxiology
- the study of human behavior and conduct. —praxeological, adj.
- excessive fastidiousness or over-refinement in language or behavior.
- hasty or rash action, behavior, etc.; undue or ill-considered haste. —precipitant, adj.
- the strict adherence to correctness of behavior. —prigger, n. —priggish, adj.
- 1 . dissolute or immoral behavior.
- 2. reckless and extravagant spending or behavior. —profligate , adj.
- the actions and qualities of a protagonist. —protagonist , n.
- a tendency to peevish, petulant, or insolent conduct.
- psychagogics, psychagogy
- a method of affecting behavior by assisting in the choice of desirable life goals. —psychagogue , n.
- affected or impertinent behavior; conceit.
- a tendency to absurdly chivalric, visionary, or romantically impractical conduct. —quixotic, quixotical, adj.
- Rare. a tendency to railing and quibbling. —rabulistic, rabulous, adj.
- the condition of being reactionary or resistant to change. —reactionist , n., adj.
- the characteristics of a reporter.
- the qualities of a reunion or social gathering. —reunionist , n.
- the state of being revolutionary. —revolutionary, revolutionist, n. —revolutional, revolutionary, adj.
- the excessive adherence to a routine. —routinist , n.
- noisy, quarrelsome, or disorderly conduct or behavior. —rowdy , n., adj.
- behavior typical or characteristic of a brutal and violent person. —ruffian , n.
- diabolical behavior. —Satanist , n.
- the condition of having uncivilized or primitive qualities. —savagedom , n.
- the practices characteristic of a schoolboy. —schoolboyish , adj.
- the characteristics and behavior of a scoundrel. —scoundrelly , adj.
- a person who seeks solitude or removes himself from the society of others; a recluse.
- the quality of having sensation. —sensorial . adj.
- Archaic. an ecstatic devotion, especially religious.
- 1 . a person who delivers sermons.
- 2 . a person who adopts a preaching attitude.
- a tendency to whimsical conduct in accord with absurd theories from past ages. [Allusion to the actions of Walter, father of the hero in Sterne’s Tristram Shandy. ]
- a tendency to conduct marked by outbursts of strong emotion. —spasmodist , n. —spasmodic, spasmodical, adj.
- activity characteristic of the observance of Sunday as a holy day.
- the condition of having qualities or traits like those of a superman. —supermanly , adj.
- a love of luxury. [Allusion to Sybaris, a Greek colony in Italy not-ed for its luxury.] —sybarite, n. —sybaritic , adj.
- the practice of self-serving or servile flattery. Also called sycophancy . —sycophant , n. —sycophantic , adj.
- the condition of having coinciding emotions in two or more people.
- Obsolete. a form of teasing or harassment in which a hope of some good or benefit is instilled in the victim, only to be repeatedly dashed and the reward shown to be unattainable.
- hypocrisy. [Allusion to Molière’s hypocritical hero, Tartuffe.] Also called tartuffery.
- a tendency to actions marked by exaggerations in speech or behavior. Also called theatricism.
- the habit of extreme neatness.
- a personal despair leading to misanthropy. [Allusion to Shake-speare’s Timon of Athens. ]
- a fawning flattery, obsequiousness, or sycophancy. —toady , n. —toadyish , adj,
- formal or superficial compliance with a law, requirement, convention, etc.
- the conduct characteristic of a tomboy, a boyish girl. —tomboyish , adj.
- Turcism, Turkism
- Obsolete, the attitudes and actions of the Turks.
- the habit of giving opinions and advice on matters outside of one’s knowledge or competence. —ultracrepidarian , n., adj.
- 1 . the tendency to wander from place to place without a settled home; nomadism.
- 2 . the life of a tramp; vagrancy. Also called vagabondage. —vagabond , n., adj.
- the malicious destruction or defamation of public or private property. —vandal, vandalization, n. —vandalish , adj.
- the actions or thoughts of members of a vanguard, those at the forefront of a movement, fad, etc. —vanguardist , n.
- the affection for or emulation of Victorian tastes or thoughts.
- the actions characteristic of a Viking, i.e., savagery, rapaciousness, etc.
- the compulsion to seek sexual gratification by secretively looking at sexual objects or acts; the actions of a Peeping Tom. —voyeur , n. —voyeuristic , adj.
- Rare. the state or quality of being foxlike, especially crafty or cunning. —vulpine , adj.
- behavior or character typical of a vulture, especially in the figurative sense of being rapacious. —vulturous , adj.
- the quality of having the traits of a werewolf.
- a penchant for rowdyism. [Allusion to Swift’s characters in Gulliver’s Travels. ]
- 1 . the state or quality of being a yokel or country bumpkin.
- 2 . behavior, language, etc, typical of a yokel.
- the style of a zany or buffoon.
- zealotism, zealotry
- a tendency to undue or excessive zeal; fanaticism.
- 1. abnormal zeal.
- 2 . morbid jealousy. —zelotypic , adj.
Insect behavior can be understood as the result of the complex integrated actions of insects in response to changes to their external (e.g., light, temperature, humidity, other insects) and internal (e.g., the level of a particular hormone) environments. In broad terms, insects exhibit two basic kinds of behavior: innate and learned. Innate behavior, commonly referred to as instinct, is based on inherited properties of the nervous system, whereas learned behavior is acquired through interaction with the environment involving adaptive changes due to experience. Whether a particular behavior pattern is inherited or learned is not always easy to determine, because some inherited behaviors may be modified by experience.
Innate behavior is responsible for simple reflexes, such as extending the proboscis to feed or turning over when falling on the back; for orientation mechanisms, such as leaving an unsuitable environment (for example, water bugs flying away from a drying pond); and for appetitive behavior, such as going toward a potential prey (e.g., female mosquitoes approaching a source of carbon dioxide to get a blood meal). Learned behavior involves the acquisition and storage (memory) of environmental information and the effects that this stored information has on the behavior of the insect. An example of learned behavior is the building of cognitive maps in several wasps, bees, and ants, which use landmarks to establish specific foraging routes or to locate their nests.
Insects show diverse and remarkable modes of reproduction. Parthenogenesis, the development from an unfertilized egg, is common among many species of aphids and orthopteroid insects, such as walking sticks. In these species, the males are unknown or rare, and females maintain the population by cloning themselves. Some moths and one beetle practice gynogenesis, in which females and males copulate but the sperm is used only to activate the development of the eggs, not to fertilize them. However, most insects reproduce sexually, males producing sperm that unite with eggs developed within females. There are some unusual types of sexual reproduction found in insects. A few scale insects (coccids) are hermaphrodites, producing both sperm and eggs and fertilizing themselves. All female ants, wasps, and bees display sexual parthenogenesis, in which unfertilized gametes generate only males with half the normal number of chromosomes. Numerous insects alternate between sexual and asexual reproduction at different stages of their life cycle.
Sexual behavior involves the location of a potential mate, usually followed by courtship, mating, and oviposition (egg laying). Males exhibit a drive to secure mates, which leads to competition for access to females. They typically are more colorful than females and can have showy structures to attract the females' attention, for example, large horns on the head of the atlas beetle, Chalcosoma caucasus; elongated mandibles in the Chilean stag beetle, Chaisognathus granti; and a pair of capitate setae on the head of the Mediterranean fruit fly, Ceratitis capitata. Females generally have duller colors and are larger in size. This phenomenon is called sexual dimorphism. An extreme case of sexual dimorphism is found in scale insects—females lack wings and are sessile (remain immobile and attached to the substrate), with reduced legs, whereas males look like normal winged insects. Females can choose among many potential partners, and their preferences are expected to raise their genetic success and, in turn, exert pressure on males favoring traits desirable by females. This is known as sexual selection.
Insect mating systems can be classified into three basic types: polygyny, polyandry, and monogamy. Polygyny results when some males copulate with more than one female in a breeding season, polyandry is when one female mates with more than one male, and monogamy refers to the male and female's having a single partner per breeding season. The most common form of mating in insects (and in other animals too) is polygyny. This is probably due to the vast supply of sperm the male possesses for fertilizing females, whereas each female has a relatively small number of eggs. Females mate simply to acquire enough gametes to fertilize their eggs, and one mating is usually sufficient.
Location of a potential mate
Insects use various communication strategies to locate mates, including mechanical, visual, and chemical tactics. Several families of insects use acoustic signals to locate mates. Calling sounds can be generated simply by the wings as an indirect result of flight, as in mature mosquito females and other flies, or by rubbing together parts of the body—a process called stridulation. Grasshoppers, crickets, cicadas, some bark
beetles, and water boatmen stridulate to call for mates. In Magicicada, the males sing in chorus, and this aggregation song is responsible for assembling both females and males. Grasshoppers rub the hind legs against a ridge in the forewing, causing the wing to vibrate. In cicadas, males have an area of thin cuticle in the abdomen, called a tymbal, underlined by several air sacs that amplify the click produced when a muscle pulls the tymbal in; the calls consist of a rapid succession of clicks. Some beetles produce mechanical signals by banging the head or abdomen against the ground to attract females, and water striders and some water beetles generate waves in the water to communicate with their potential mates.
Visual signals can be passive, as when a variety of colors are transmitted in a single distinctive message using body surfaces as signal generators, or active, as when body parts are moved in a variety of positions, thus creating a rapid sequence of signals. With the exception of bioluminescence in fireflies (beetles belonging to the families Lampyridae, Phengodidae, and Elateridae), visual signals are restricted to diurnal use. In the case of several swarming insects, such as various flies, mayflies, and caddisflies, individual males in a swarm attract females' attention by fluttering up several meters and then dropping down, reflecting the light with their wings. Firefly males usually fly around the habitat emitting flashing lights that vary from species to species with respect to the color, rate, length, and intensity of flash pulse. Females, perched on plants or rocks, return the message. In some species, females glow continuously or respond only to continuously glowing males, whereas in others only the male produces the light signals.
Olfactory signals to locate mates are widespread among insects; several moths, some flies, bumblebees, harvester ants, boll weevils, scorpion flies, and some bark beetles, among others, use them. Males, females, or both produce attractant molecules, called sex pheromones, from specialized glands. Pheromones are released into the air and dispersed by the wind, or, as in some types of territorial species, they can be used to scent mark a plant in the territory. Antenna receptors are able to detect just a few molecules of the pheromone in the air, allowing an insect to follow the trail leading to the opposite sex even when the insect is located at considerable distance from the source.
Some plants mimic the shape and color of certain insects to attract them to their flowers and to use them as pollinators. A well-known example is that of bees and certain orchids that resemble bee females and even produce bee pheromones. Pheromones also are produced synthetically and used as lures to trap or control the reproduction of some pest insects, such as fruit flies (Bactrocera dorsalis and Ceratitis capitata), yellow jackets (Vespula species), and gypsy moths (Lymantria dispar).
Many of the cues that act in mate location serve as releasers of mating or courtship behavior as well, leading eventually to copulation. In all insects, fertilization of eggs takes place inside the female reproductive ducts. In primitive insects, such as springtails, there is no mating; the male deposits sperm in packets called spermatophores and scatters them on the ground. Competition among males takes the form of males eating the spermatophores of other males. The female must locate the spermatophore to fertilize her eggs. In silverfishes, the male guides the female to his spermatophore by building a net of silken threads converging on it.
During courtship, escape and attack responses are momentarily inhibited. Insects display an amazing variety of mating patterns. Some are simple and consist in the coming together and copulation of the male and female; others involve elaborate courtship patterns. For example, in some Panorpa scorpion flies, a male offers a nuptial gift in the form of prey food to a female and copulates with her while she eats.
In most dragonflies and damselflies, females and males mate several times. A male can contribute to a substantial percentage of the progeny if he is the first one to grab and inseminate the female. In some species a female does not mate more than once in a particular oviposition episode, so that the male that is able to grasp her and mate with her before she oviposits is the most likely to fertilize the eggs that are laid. In numerous dragonfly and damselfly species, males are territorial, guarding a suitable oviposition site from other males. It has been found that in some species the male penis is used not only for insemination but also to remove sperm deposited by previous males from the female sperm storage organ, ensuring the fertilization of the eggs with his own sperm. This is known as sperm precedence or sperm competition. There are several mechanisms that help prevent the female from copulating with other males. For example, dung flies, dragonflies, and damselflies guard or protect the female after copulation, love bugs copulate for a prolonged period of time, male honeybees detach the genitalia and leave it inserted in the female genital opening after mating, and vinegar flies of the genus Drosophila transfer a chemical substance that makes the female unreceptive to other mates.
Insects are unique in the possession of an ovipositor, a specialized organ on the female abdominal tip used to lay her eggs, which usually consists of three pairs of plates. This device allows the female to deposit eggs in safer places, in crevices or inside plant tissues or other substrates. Ovipositors
Some insects simply drop their eggs at the oviposition site, as is the case with some dragonflies, which can be seen touching the water surface with the tip of the abdomen at the ponds where they breed. Others paste them to a certain substrate. In certain mayflies, the gravid female drops into the water, where her abdomen breaks, setting her eggs free. Other aquatic insects, such as aquatic beetles and damselflies, insert their eggs into the mud or place them in the leaves or stems of plants growing on the margins of ponds or streams; still others, like some mosquitoes, make egg rafts.
Once the eggs have been laid, most parent insects simply leave. Among cockroaches and praying mantids, the female secretes a cover around the eggs that provides protection against predators and dehydration; some cockroach females carry this egg case until the larvae emerge. Female mealybugs, scale insects, and some beetles protect their eggs by shielding them with their bodies, other beetles may carry them beneath their bodies, and some carabid beetles construct depressions in the soil for the eggs and clean them regularly until they hatch, to prevent fungi from growing on them. In the case of the chrysomelid beetle, Acromis sparsa, the female stays with her offspring until they reach adulthood; the larvae remain aggregated, feeding on the same leaf, so that the mother can shield them with her body in case of danger. Earwig females oviposit in burrows in the ground and guard the eggs until they hatch.
Insect males are mainly polygynous, and for this reason it is not advantageous for them to invest effort in parental care. There are some exceptions. The male of the scarab beetle, Lethrus apterus, helps the female construct a burrow and gather leaves to provision the brood cells. In the carrion beetle Necrophorus the male assists the female in regurgitating liquefied carrion to feed their offspring, which reside in a nest of carrion. Males of certain bark beetle and sphecid wasp species stay close to the nests of their mates and repel parasites and rival females that try to enter the burrows. The best-known examples of paternal care in insects are found among the water bugs of the family Belostomatidae. In the genera Abedus and Belostoma the female lays her eggs on the back of the male after mating. The male takes exclusive care of the progeny; he ventilates the eggs, prevents fungus from growing on them, and assists in the emergence of the larvae. In Lethocerus, another genus of the same family, the female lays eggs on a stick or plant stem at the level of the water surface, and the male is stationed close by, to guard them against predators.
A digger wasp female builds one or several burrows in the ground and provides the eggs with a certain type of prey, usually spiders or other insects; she then inserts an egg into each prey item and closes the burrow. The prey is alive but paralyzed with a substance injected together with the egg. When the wasp maggot emerges, it eats the prey from the inside out. In some species, the female keeps bringing prey to the larva until it pupates.
Some insects are surrounded by their food from the time of hatching, as a result of the oviposition habits of the parent. In the case of many insects that feed on plants, the mother places the eggs on the host plants or, in scavenger or parasitic insects, on suitable detritus or hosts. Among social insects, the larvae are incapable of searching for their own food, and the workers are in charge of feeding them. Most insects, however, must search for their food.
Insects feed on an almost endless variety of food and in many different ways. About half of insects are herbivores, feeding on plants. The most common plant hosts are flowering plants, but some insects feed on ferns, fungi, and algae too. Certain insects are polyphagous and show no preference for any particular host plant. An example is the desert locust, Schistocerca gregaria, which, when migrating, feeds on all the plants it finds along its way. Other insects are oligophagous, specializing in certain groups of plants, for example, Pieris butterflies, which feed on Cruciferae and other plants with mustard oils. Still others are monophagous, feeding on a single plant species, for example, the weevil Scyphophorus yuccae, which feeds only on Yucca.
Variation also exists as to the part of the plant that is consumed. Soil mealy bugs, wireworms, cicada larvae, and white grubs, among others, specialize on roots; bark beetles, carpenter ants, and termites on woody parts; psyllids, aphids, leafhoppers, leaf-mining larvae of moths, flies, beetles, and sawflies on leaves; and bees, wasps, beetles, butterflies, and moths on nectar and pollen of flowers. Some herbivorous insects inject a chemical into the plant that induces it to grow abnormally and form a gall. The feeding of the insect usually stimulates the formation of a gall, though in some cases it is initiated by oviposition. A plant gall may have (e.g., galls of psyllids, aphids, and scales) or lack (e.g., galls of moths, beetles, flies, wasps) an opening to the outside.
A few insects are specialized in the production of "fungus gardens." This peculiar habit is found in some ants, termites, and ambrosia beetles. These fungus-growing insects nest in the ground, where they excavate a complex system of galleries and chambers. They cut up leaves and take them to special chambers in the nest, where they are chewed up and seeded with a fungus, which they tend and eat.
An insect can be attracted to the food source from a distance by visual or olfactory clues, but the final selection occurs when the insect is in direct contact with the host. Physical characteristics of the plant, olfaction, and contact chemoreception play a part in this process. For example, in a leafhopper the first attraction to a plant is through color; thus, they land on host and non-host plants of similar color. The leafhopper finally determines the identity of the host plant by touching the surface with its proboscis or by inserting the proboscis into the plant for a short distance. Chemical substances characteristic of the plant then are detected, and the leafhopper stops feeding activities if the plant is not an appropriate host and keeps feeding if it is one.
Carnivorous insects feed on other animals, which are mainly other insects. There are two general sorts of carnivores: predators and parasites. Predator insects are active and seek their prey, usually consisting of one or more smaller insects per meal, whereas parasites live in or on the body of their hosts during at least part of their life cycle, take successive meals from the much larger host, and typically do not kill the host or do so only gradually.
Damselflies and dragonflies, both during larval and adult life; tiger, ground, and ladybird beetles; lacewings and some true bugs; robber flies and larvae of syrphid flies are examples of predators. Most predators actively forage for their prey, but some are ambush hunters. Antlion larvae, for example, dig a pit with sloping sides in the sand and bury themselves
at the bottom with only the head exposed. When an ant or other small insect walks on the margin of the pit, the antlion provokes a landslide, throwing sand with its head, which causes the insect to roll down to its open mandibles. Larvae of tiger beetles live in a vertical burrow in the ground; when an insect comes within range, the larva extrudes about half of the body and grabs the prey with its long, sickle-shaped mandibles. Larvae of caddis flies weave a net of silken threads with which they capture small organisms in the water. Praying mantids and water bugs sit and wait for their prey to come their way; when the prey is close enough, it is taken and held by the fast extension of the raptorial forelegs. In a similar way, some dragonfly larvae wait motionless and strike at passing prey, extending the labial mask with incredible speed and accuracy.
Most hunters have large eyes, since only visual stimuli are fast enough to allow them a rapid reaction to a moving prey. After the capture, recognition is mainly tactile. In predaceous forms with subterranean habits and poorly developed eyes, the localization of prey is largely olfactory. Once they capture a prey, many predators restrain the prey's movements
by mechanical strength and tear it to pieces with the mandibles. Predators with sucking mouthparts, such as true bugs, robber flies, and some lacewing and beetle larvae, inject salivary secretions that paralyze and kill the prey and then suck the liquefied organs, discarding the empty cuticular shell.
There are parasitic insects on vertebrates and on other insects and arthropods. Some are ectoparasites, living on the outside of their hosts, and others are endoparasites, living inside the host's body. Some parasites feed on only one host species and others on a group of related host species; still others have a wide range of hosts. Lice are an example of ectoparasites of vertebrates; both larva and adult are completely dependent on the host, on whom they spend their entire life cycle. Fleas, which also are ectoparasites on vertebrates, are less dependent on the host; they frequently change hosts and spend some time away from the host as adults. The larval stage is not spent on the host's body. Mosquitoes and some other blood-sucking insects, such as "no-seeums," bed bugs, and assassin bugs, feed from the host only for brief periods; often it is only the females, which require a blood meal to produce eggs.
Ectoparasites of other insects, such as certain "no-seeums," suck blood from the wing veins of lacewings and dragonflies. In many insects the first larval stage is active, whereas the older larvae are parasitic or fixed predators of a specific host. For example, larvae of meloid beetles, called "triangulins," wait in flowers that the parent bee may visit. The active triangulins climb onto the bee, attaching themselves to the bee's hairs with their claws, and are carried in this way to the bee's nest, where they become internal parasites of the bee's eggs or larvae.
Several larvae of flies are endoparasites of vertebrates, invading open sores, alimentary ducts, and nasal cavities; this phenomenon is called myasis. Gasterophilus bot flies lay their eggs on the fur coats of horses. When the horse licks its hair, the larvae hatch and affix themselves to the horse's tongue. They then pass into the digestive canal and attach themselves to the stomach or intestinal wall, causing ulcers. The fly Oestrus ovis places its larvae on the nostrils of sheep, where they crawl and enter the frontal sinus, causing vertigo. Cordylobia anthropophaga maggots produce ulcerated ridges in the skin of humans, dogs, and mice.
Most endoparasitic insects of other insects differ from endoparasites of vertebrates in that they reach a size equal to that of the host and eventually kill it: they are called parasitoids. Most parasitoids live and feed as larvae inside the host and become free-living adults after killing the host. Tachinid and sarcophagid fly larvae attack grasshoppers, caterpillars, true bugs, and wasp larvae, but most parasitoids are found among the ichneumonoid, chalcidoid, and proctrotrupoid wasps. Some parasitoids are very specific to their hosts— restricted to a single or a few species of insects. Parasitoids often regulate numbers of pest insects and are therefore important components of biological control programs.
Some insects are detritivores, feeding on decaying materials, such as carrion, leaf litter, and dung, and are important in the progressive breakdown of organic matter into their basic components to be returned to the soil, where they become available for plants. They also remove unhealthy and obnoxious materials from the landscape. Bow flies, carrion beetles, and skin beetles feed on dead animal tissue, skin, feathers, fur, and hooves and are of great importance in the removal of carrion from the environment. Dung beetles cut and shape vertebrate dung into balls that serve as a food source and a brood chamber for a single larva. Dermestid skin beetles feed on tissue and skin from vertebrate bones. Silverfish feed on dry organic debris and have a taste for paper, especially that containing starch or glue. Cockroaches and other insects are another example of scavengers that feed on dead plants and animals. Wood-boring beetles, termites, carpenter ants, and other wood feeders are important agents in facilitating the conversion of fallen trees and logs to soil.
Insects use many means of defense; these can be passive, where an insect relies on its appearance or location, or active, where an insect tries to escape, threatens a predator, or attacks it with chemical weapons. The habitat of numerous insects by itself provides them with a defense mechanism; insects that burrow into plant tissues or in the soil or live under rocks gain protection against predators. Some insects build a protective case or shelter that they carry around. Caddisfly larvae cement sand grains, small twigs and leaves, or other materials together to form the case inside which they live. Some chrysomelid beetles attach their feces to their backs to form a protective shield. Larvae of froghoppers use the excess fluid from the sap they suck to form a mass of bubbles that hides them from their enemies, and larvae of some psyllids construct a protective cover (a "lerp") of a sweet, crystallized substance called honeydew, under which they live.
Several beetles "play dead" when disturbed, dropping to the ground, folding up their legs, and remaining motionless for a while. Some insects blend with their backgrounds by closely resembling leaves, twigs, flowers, thorns, or bark. They are so well camouflaged in their normal surroundings that they become invisible to their predators in what is known as crypsis. In the forests of Papua New Guinea some weevils have modified wing covers that favor the growing of "miniature gardens" of mosses, lichens, algae, and fungi on their backs, which they carry around. The physical similarity with the surroundings is enhanced by the behavior of cryptic insects: inchworms freeze, holding their bodies upright like a twig, grasshoppers orient their bodies as if they were leaves, moths and walking sticks colored like bark remain immobile on their host trees.
On the contrary, the color patterns of several insects are striking in their attempts to intimidate predators or deflect their attention to parts of the body that are least vulnerable. This aposematic, or warning, coloration is found in several butterflies, grasshoppers, lanternflies, praying mantids, walking sticks, true bugs, and homopterans, among others. Some have brightly colored spots on the abdomen or hind wings that are hidden while the insect is at rest and are exposed suddenly when the animal is threatened. These flash colors may cause enemies to become startled, at least for a moment, allowing the insect to find a new hiding place and cover its conspicuous spots, rendering them invisible to the eyes of the predator. Other insects have a pair of eyespots on the upper surface of a pair of wings. If they are disturbed, they fully expose the wings with the eyespots. In some butterflies, this display is accompanied by a hissing or clicking sound produced by rubbing wing veins against each other and against the body. This behavior may elicit an escape response in birds, or else the attacks of birds may be directed at the eyespots and not at other, more vulnerable parts of the body. Other types of color advertisement, usually reds, yellows and blacks, are related to the palatability of the insect; predators associate a particular color with a bad flavor and learn to avoid insects displaying that color. Thus, the black and yellow of bees and wasps are associated with a sting and the red on a black or green background of certain butterflies is connected to a disagreeable taste. Several caterpillars show a striking color combination of yellow, orange, and green, usually associated with the presence of irritant hairs.
Some insects resemble or mimic other insects or even vertebrates. Predators learn to shun distasteful insects with striking colors, and certain insects take advantage of this behavior and avoid being eaten by displaying the warning color pattern of a distasteful or dangerous organism. A few noctuid moths move their legs in the manner of a bristly spider. Many beetles and true bugs mimic wasps not only in color but also in behavior, holding their wings upright, waving their legs and antennae, and bending the tips of their abdomens upward like wasps. The anterior part of the body of some swallowtail caterpillars is enlarged and painted with two eyespots, resembling the head of a snake. If the caterpillar is threatened, it will evert a scent gland that, besides producing an unpleasant odor, looks like the bifid tongue of a snake.
This kind of mimesis, where a harmless insect mimics a dangerous organism, is called Batesian mimetism. For example,
the South American butterfly Episcada salvinia rufocincta feeds on plants of the family Solanaceae and incorporates toxic substances (alkaloids) from its food plants, which make it distasteful to predators. Paraphlebia zoe is a damselfly that frequents the same forest clearings where the butterfly flies. Some males of the damselfly imitate the flight of the butterfly and have a white spot in the same position on the wings. Other males in the same population do not have any spots on the wings and appear invisible when they fly. There is also a form of Müllerian mimetism, in which an unpalatable or poisonous insect resembles another distasteful or harmful organism; predators learn to avoid only one color pattern, resulting in advantage to both mimic and model.
Many insects display an escape reaction when threatened, by flying, jumping, running, or diving. Certain noctuid moths detect the ultrasonic sounds produced by foraging bats and perform evasive actions by turning around, flying in zigzag patterns, or dropping to the ground and remaining motionless.
Chemical warfare provides defense for several types of insects. Numerous insects have repugnatorial glands, which secret noxious substances. Some of these secretions also act as alarm pheromones, warning other insects about the proximity of danger. Many cockroaches, stink bugs, beetles, ants, and walking sticks are capable of forcibly spraying an odoriferous secretion, sometimes for several feet. Caterpillars of swallowtail butterflies evert repugnatorial glands located behind the head, liberating a secretion that is effective against ants. The venomous secretions associated with the sting of wasps and bees and the bite of predatory true bugs also can be considered chemical defenses. Coccinelid, chrysomelid, lycid, lampyrid, and meloid beetles discharge blood when threatened, which is called reflex bleeding. In meloid beetles this blood contains substances that produce blisters or sores when it comes in contact with the skin of vertebrates.
Aphids, mealybugs, psyllids, whiteflies, and scale insects have developed a different defense strategy, in which they recruit ants as personal bodyguards. These insects produce a sweet secretion called "honeydew" that is attractive to ants. Thus, ants guard and tend colonies of aphids and aggressively attack any organism trying to feed on them.
Most species of insects disperse at some time during their life cycles in an attempt to populate new areas, though only a few have migratory mass movements similar to those of birds. In those insects with mass migrations, the movements usually are one way; one generation makes the trip in, and the following generation makes the return trip. Migration is accomplished mainly by flight, typically following the prevailing wind currents.
The best-known example of migrating insects is probably that of the monarch butterfly Danaus plexippus. After one or two generations in Canada and the northern United States, a combination of factors in the autumn, probably a shortening photoperiod and a drop in temperature, induces a generation of monarchs not to develop gonads and to migrate south to wintering places in California, Mexico, and Florida, where they congregate in great numbers on certain kinds of trees. In the spring they begin their journey back north, laying eggs along the way before dying. The subsequent generation completes the flight back, and the flight south of 2,000 mi (3,200 km) or more is repeated the following fall.
Swarms of migratory locusts have been known since biblical times. The migratory locust Schistocerca gregaria is known throughout the world for its mass migrations in which hundreds to billions of locusts swarm and advance, eating all vegetation in their sight. This species has two different phases. In its solitary phase the specimens are sedentary, selective in their food choices, and colored pale green or reddish, whereas in the migratory phase the specimens are gregarious, devour any kind of plant, and display contrasting colors. A young larva in either phase can be switch to the other one.
Not all migrating insects fly. Army ants (Eciton hamatum) migrate on the ground; army worms of the North American moth Pseudaletia unipuncta march onward, devouring every green thing in their path; and maggots of the mourning gnat Neosciara crawl in snakelike processions glued to each other in a slimy secretion, searching for an appropriate place for pupation in the forests of Europe.
Factors initiating migrations are not yet understood fully, but an onset of adverse environmental factors, such as crowding, lack of food, and short days, probably plays a part in the generation of endocrine changes leading to migration. For example, a sudden increase in population numbers may induce the production of winged forms in aphids that normally produce wingless forms, and this is correlated with the activation of certain hormone-producing organs.
Evans, Arthur V., and Charles L. Bellamy. An Inordinate Fondness for Beetles. Berkeley: University of California Press, 2000.
Holldobler, B., and E. O. Wilson. Journey to the Ants: A Story of Scientific Exploration. Cambridge, MA: Belknap of Harvard University Press, 1995.
McGavin, George C. Bugs of the World. London: Blandford Press, 1993.
Preston-Mafham, K. Grasshoppers and Mantids of the World. London: Blandford Press, 1998.
Thornhill, Randy, and John Alcock. The Evolution of Insect Mating Systems. Cambridge, MA: Harvard University Press, 1983.
Animal Behavior Society, Indiana University. 2611 East 10th Street, no. 170, Bloomington, IN 47408-2603 United States. Phone: (812) 856-5541. Fax: (812) 856-5542. E-mail: [email protected] Web site: <http://www.animalbehavior.org/>
Journal of Insect Behavior [cited December 23, 2002]. <http://www.kluweronline.com/issn/0892-7553>.
Natalia von Ellenrieder, PhD
Behavioral characteristics of invertebrates
This chapter will familiarize you with issues and examples related to the behavior of invertebrates. The large number and sheer diversity of invertebrates requires a restriction in the types of behaviors (and species) that can be discussed. The behaviors selected were based in part on their importance to the survival of an individual organism. Since there is little known about the behavior of many of the lower invertebrate and deuterstome phyla, examples of insects and other protostomes were used to illustrate the various kinds of behavior mentioned.
All animals are metazoans and are characterized by being multicellular. The principles of behavior discovered in unicellular organisms are fundamentally the same in multicellular organisms. The similarities that exist between both forms of organisms are fascinating because the evolution of multicellularity has led organisms to develop a fantastic array of complex activities and modifiable behavior. Consider, for instance, the behavior of the marine sponge Sycon gelatinosum (phylum Porifera). During the larval stage it is a free-swimming animal that moves toward light at the water's surface. As it develops, it lives near the substrate and eventually becomes a sessile adult. The adult sponge is no longer a free-swimming animal and is entirely incapable of active movement. Reactive behavior is elicited from the adult sponge when individual cells are stimulated directly. The resulting responses are localized, slow, and uncoordinated. The inability to hunt food items such as bacteria, plankton, and detritus requires the sponge to develop specially designed feeding structures that bring food to it.
Contrast the poorly coordinated behavior of sponges with the more active behavior of animals in the phylum Cnidaria, which includes multicellular marine animals such as jellyfish, corals, sea anemones, and the freshwater Hydras. In cnidarians, cells performing the same function are grouped into tissue. The creation of tissue allows cnidarians to behave in more complex ways than sponges. Hydras, for example, coordinate their tentacles to grasp prey, contract their entire bodies in response to strong mechanical stimulation such as predatory attacks, and move a single tentacle in response to a non-threatening organism or passing shadow.
A great advance in behavior is seen in worms of the phylum Platyhelminthes. This phylum contains animals such as planarians, flukes, and tapeworms. In these animals we find the first evidence of characteristics critical for the development of complex behavior. These characteristics include bilateral symmetry, the appearance of a brain, polarized neurons, and definitive anterior and posterior ends—with the anterior end containing a head, and eyes. The advances present in this phylum make complex orientation possible. The first examples of complex learning are also present in flatworms. An example of this development is the flatworm's ability to to discriminate between two signals—one of which leads to a biologically relevant stimulus. This organism's ability to modify its behavior based on the possible consequences of an encounter in order to avoid dangerous situations moves an existing reflex into a new context. Although primitive types of behavior modification are possible in members of the phyla Porifera and Cnidaria, they are not as complex as those found in flatworms, nor are they retained for as as long as they are in worms.
The advances first seen in members of the phylum Platyhelminthes and elaborated by worms in the phylum Annelida (e.g., polychetes, earthworms, leeches) and reach their apex in members of the phyla Mollusca (e.g., snails, clams, squid, octopus) and Arthropoda (e.g., spiders, crabs, crayfish, lobsters, honey bees, wasps, ants). For example, the cephalopods' neural development, problem solving capability, sensory apparatus, and ability to modify behavior is unsurpassed among the invertebrates. Among the arthropods, social insects such as the honey bee and ant have astonishing examples of defensive, social, and learned behavior patterns. What is behavior, and who studies it? Behavior is not easy to define and various definitions exist. For example, physiologists might describe the "behavior of a neuron," but some comparative psychologists would find this objectionable. Generally speaking, behavior is defined as "what organisms do." Behavior is the action an animal takes in order to adjust, manipulate, and interact with its environment. Actions such as moving, grooming, and feeding can be referred to as maintenance behavior. Action that influences members of the same and/or different species can be called communication behavior. Behavior that is modifiable is known as learned behavior. In general, each of these three types of behavior defines or "orientates" the animal in space.
Various disciplines have contributed to the study of invertebrate behavior. These disciplines include comparative psychology, ethology, physiology, ecology, and entomology.
Scientists engaged in the study of behavior often do so from an interdisciplinary approach in which psychologists, ethologists, physiologists, and entomologists all work alongside each other. Comparative psychologists have a special interest in searching for similarities and differences in behavior.
Over the years various classification systems have been developed to describe orientation, and the terminology used within these systems is confusing. Popular systems in which orientation can be described are kinesis and taxis. Orientation behavior represents an example of the type of behavior referred to as maintenance behavior.
The simplest response through which invertebrates find a suitable location to live is referred to as kinesis. The response is not directed toward or away from a stimulus, but nevertheless places the animal in an optimum location. Changes in activity, rate of movement, and/or turning is non-directional and directly related to the intensity of the stimulus from moment to moment. Kinetic responses often occur when the source of the stimulus cannot be sensed at a distance. Several types of kinesis are recognized including: barokinesis, hygrokinesis, orthokinesis, photokinesis, thigmokinesis, and klinokinesis. Theories regarding kinesis are made more confusing when describing orientation using two kineses. For example, an animal that changes its rate of movement under illumination is said to exhibit "photo-orthokinesis." The words negative and positive also can be added to these terms in order to further adapt their meaning; an animal that is active under little or no illumination is said to exhibit "photo-negative kinesis."
Examples of kinesis:
Various classes of invertebrates react to pressure changes, including increased locomotion because of changes in barometric pressure. Larvae of the crab genus Carcinus swim toward the water's surface when pressure increases. Copepods, adult and larval polychaetes, and jellyfish medusae are other examples of animals that increase their activity in response to pressure changes.
Increased locomotion in reaction to changes in humidity is referred to as hygrokinesis. Some species of nematoda are stimulated to move due to conditions of low humidity and are less active when there are high degrees of humidity. The increase in activity under dry conditions increases the chances of finding a suitable damp environment. Increasing locomotion based on fluctuations in humidity levels is important among terrestrial invertebrates (e.g., planaria) because, with the exception of insects, very few have developed methods of conserving significant amounts of water.
Increased locomotion resulting from changes in levels of light is called photokinesis. Flatworms (e.g., Dugesia dorotocephala, Dugesia tigrina) all increase their activity depending on the intensity of illumination that surrounds them. Other examples of organisms that exhibit photokinesis include gill and skin fluke larvae (monogenea), jellyfishes, and rotifers. Not all invertebrates respond to increases in illumination. When conducting studies on the effect of light on activity levels it is important to separate the role of illumination from the temperature increases produced by light.
This form of kinesis is defined as increased locomotion in response to changes in contact with the immediate physical environment. Some invertebrates are more active in open spaces than in closed spaces. Examples of closed spaces include cracks and crevices. For example, the contraction of longitudinal muscles in nematodes produces a whiplike undulatory motion that relies on environmental substrata for the body to push against; when they are placed in fluid without substrata they thrash around.
other forms of kinesis
Orthokinesis refers to changes in the speed or frequency of movement in reaction to changes in the intensity of a stimulus. Stimuli that produces a change in direction (such as turning) is known as klinokinesis. Movement influenced by gravity is known as geokinesis, and changes of movement caused by water currents is known as rheokinesis. Kinetic responses also can be in reaction to chemical and temperature stimuli (chemokinesis and thermokinesis, respectively). The oncomiricidia (larvae) of the Monogenea have been shown to change speed and direction in reaction to gravity, current, and light stimuli.
Taxis is a more complex response through which invertebrates find a suitable location to live. The response is directed toward or away from a stimulus to place the animal in an optimum location to inhabit. Changes in activity, rate of movement, and/or direction are related to the intensity of the stimulus gradient from moment to moment. Taxis differs from kinesis in that taxic responses allow invertebrates to engage in specific activity as opposed to general activity, relative to a stimulus source.
Taxis can be characterized by:
- Whether the animal moves toward or away from a stimulus.
- The way in which the animal moves.
- The complexity of the sensory structures used to detect the stimulus.
An invertebrate with a single visual receptor can determine the direction of a light source simply by moving the receptor (such as turning its head) and sampling the stimulus gradient produced by the light. If the animal is attracted to light, its receptor becomes more active the closer it moves toward the light source. The majority of invertebrates have at least two receptors; the second receptor allows the animal to make simultaneous comparisons from each side of its body from moment to moment as it moves through a stimulus gradient.
Several different types of taxis are recognized including, phototaxis, klinotaxis, phototropotaxis, and phototelotaxis. Moreover, movements toward the source of stimulation are called positive, and movements away from the source are called negative. For example, movements toward a source of light is called "positive phototaxis," while movement away from light is referred to as "negative phototaxis."
Examples of various form of taxis include:
An animal that moves toward (positive phototaxis) or away (negative phototaxis) from light is exhibiting phototaxis. Movement is parallel to the direction of light. Examples of animals that exhibit this type of behavior are jellyfishes, oncomiricidia (monogenea larvae), and some echinoderms (sea stars and sea urchins). Planarians are negatively phototoxic in that they seek less illuminated areas.
A change in directed movement based on successive comparisons of a stimulus is referred to as klinotaxis. The larvae of many flies, including the common house fly, Musca domestica, find the location of a light source by moving their head left and right in order to compare the relative intensity of a stimulus. Derivatives of this behavior also occur to many lower invertebrates, including planaria and echinoderms.
Animals that display phototropotaxis undergo movement toward a source of illumination based on a comparison of information gathered by their eyes. The animal orientates toward the direction of light (assuming it exhibits positive phototropotaxis) and moves in a direction that keeps the eyes equally stimulated. Phototropotaxis is best demonstrated experimentally in what is known as a "two-light experiment." In this design, an animal is placed between two light sources. Phototropotaxis is indicated if the animal follows a path between the two light sources thereby stimulating the photoreceptors equally. Phototropotaxis also can be detected by preventing visual information reaching one eye. This test can be done by removing or painting an eye. Phototropotaxis is indicated if the animal begins to engage in "circus movements." For example, when the honey bee (Apis mellifera) is blinded in one eye it will perform "circus movements" (sideways movements) toward a light source. Positive phototropotaxis can be detected if the animals continuously turns toward a light source; if an animal is negatively phototropotaxic it will continuously turn away from a light source (i.e., it will turn so that its blind side faces the light).
Movement directed toward one of two sources of illumination is known as phototelotaxis. This form of behavior is dependent upon the type of sophistication present in the visual receptors. If the eye is capable of forming an image that will allow the animal to identify the source of illumination, the animal can move directly toward the source without the need for comparing two sources of illumination. Phototelotaxis is found in invertebrates possessing compound eyes (arthropods such as insects and crustaceans) including hermit crabs, isopods, and mysid crustaceans. There are no known examples of this behavior among the lower invertebrate and invertebrate deuterostomes.
Geotaxis refers to movement along lines of gravitational force. As with all forms of taxic behavior, the direction of movement can be either positive or negative. Geotaxis is observed on surfaces (especially inclines), in water, air, sand, or mud. The most pronounced examples of geotaxis are found in invertebrates that live in sand or mud. Many examples of geotatic behavior occur in animals with statocysts, although there are examples where statocysts are not involved. The statocyst is a heavy object (statolith) located in a fluid-filled chamber used to detect gravitational forces. When the statocyst is moved, the statolith induces movement by activating various sensory and motor systems that return the animal back to its normal balance. Geotaxis is most readily studied in invertebrates by having an animal crawl on a vertical glass plate that is gently rotated or inclined. In order to test burrowing animals such as polychaetes the animal can be sandwiched
between two glass plates filled with sand, or on a rotating table or centrifuge. Examples of geotactic behavior can be found in medusa (e.g., Cotylorhiza tuberculata), planaria (e.g., Convoluta roscoffensis), and polychaetes (e.g., Arenicola grubei). There are many cases of invertebrates that exhibit geotatic behavior without statocysts, including Helix, Limax, the sea anemone Cerianthus, monogeneans, starfishes, and sea urchins.
Rheotactic behavior involves movement directed by water flow, and can be found in most classes of invertebrates that inhabit water. Examples of organisms that display rheotaxis include anemones, planarians, monogeneans, and many protostomes (gastropods, crustaceans, and both nymphs and larvae of insects).
other forms of taxis
Thigmotaxis is defined as movement when direction is determined by a stimulus making contact with an animal's body. Turbellarians are positively thigmotactic on their ventral sides, and negatively thigmotactic on their dorsal sides, which keeps their ventral side against the substrate. Movement influenced by air currents is known as Anemotaxis. Taxic responses also are created by chemical and temperature stimuli (referred to as chemotaxis and thermotaxis respectively).
Learned behavior is another class of behavior exhibited by invertebrates. The reasons for studying learning in invertebrates are varied and include gaining further knowledge of how biochemistry and physiology affect the process of learning, searching for similarities and differences within and between phyla, and using learning paradigms to explore applied and basic research questions (e.g., how pesticides influence the foraging behavior of the honey bee).
The term learning, like the term behavior, has several definitions. When reviewing studies of learning, the reader should be aware that definitions may vary from researcher to researcher. For example, a researcher may consider behavior controlled by its consequences (i.e., behavior that is rewarded or punished) as an example of operant behavior, while others believe that it depends upon the type of behavior being modified. Moreover, some believe that any association between stimuli represents an example of Pavlovian conditioning, while others believe that the "conditioned stimulus" must never elicit a trained response prior to the process of association.
We will define learning as a relatively permanent change in behavior potential that comes as a result of experience. This definition contains several important principles. First, learning is inferred from behavior. Second, learning is the result of experience. Third, temporary fluctuations are not considered learning; rather, the change in behavior identified as learned must persist as such behavior is appropriate. This definition excludes changes in behavior produced as the result of physical development, aging, fatigue, adaptation, or circadian rhythms. To better understand the process of learning in invertebrates, many behavioral scientists have divided the categories of learning into non-associative and associative.
This form of behavior modification involves an association developing from one event, as when the repeated presentation of a stimulus leads to an alteration of the frequency or speed of a response. Non-associative learning is considered to be the most basic of the learning processes and forms the building blocks of higher types of learning in metazoans. The organism does not learn to do anything new or better; rather the innate response to a situation or to a particular stimulus is modified. Many basic demonstrations of non-associative learning are available in scientific literature, but there is little sustained work on the many parameters that influence such learning (e.g., time between stimulus presentations, intensity of stimulation, number of repeated trials). There are two types of non-associative learning: habituation and sensitization.
Habituation refers to a reduction in the response elicited by a stimulus as it is repeated. For a decline in responsiveness to be considered an instance non-associative learning, it must be determined that any decline related to sensory and motor fatigue do not exert an influence.
Studies of habituation show that it has several characteristics, including the following:
- The more rapid the rate of stimulation is, the faster habituation occurs.
- The weaker the stimulus is, the faster habituation occurs.
- Habituation to one stimulus will produce habituation to similar stimuli.
- Withholding the stimulus for a long period of time will lead to the recovery of the response.
Sensitization refers to the augmentation of a response to a stimulus. In essence, it is the opposite of habituation and refers to an increase in the frequency or probability of a response. Studies of sensitization show that this process has several defining characteristics, including the following:
- The stronger the stimulus is, the greater the probability that sensitization will be produced.
- Sensitization to one stimulus will produce sensitization to similar stimuli.
- Repeated presentations of the sensitizing stimulus tend to diminish its effect.
A form of behavior modification involving the association of two or more events, such as between two stimuli, or between a stimulus and a response is referred to as associative learning. This form of learning allows a participant to aqcuire the ability to perform a new task, or improve on their ability to perform a task. Associative learning differs from non-associative learning by the number and kind of events that are learned and how the events are learned. Another difference between the two forms of learning is that non-associative learning is considered to be a more fundamental mechanism for behavior modification than those mechanisms present in associative learning; examples of these differences can easily be found in the animal kingdom. Habituation and sensitization are present in all invertebrates, but classical and instrumental conditioning seems to occur first in flatworms (phylum Platyhelminthes). In addition, the available evidence suggests that the behavioral and cellular mechanisms uncovered for non-associative learning may serve as building blocks for the type of complex behavior characteristic of associative learning. The term associative learning is reserved for a wide variety of classical, instrumental, and operant procedures in which responses are associated with stimuli, consequences, and other responses.
Classical conditioning refers to the modification of behavior in which an originally neutral stimulus—known as a conditioned stimulus (CS)—is paired with a second stimulus that elicits a particular response—known as the unconditioned stimulus (US). The response which the US elicits is known as the unconditioned response (UR). A participant exposed to repeated pairings of the CS and the US will often respond to the originally neutral stimulus as it did to the US. Studies of classical conditioning show that it has several characteristics, including the following:
- The more intense the CS is, the greater the effectiveness of the training.
- The more intense the US is, the greater the effectiveness of the training.
- The shorter the interval is between the CS and the US, the greater the effectiveness of the training.
- The more pairings there are of the CS and the US, the greater the effectiveness of the training.
- When the US no longer follows the CS, the conditioned response gradually becomes weaker over time and eventually stops occurring.
- When a conditioned response has been established to a particular CS, stimuli similar to the CS may elicit the response.
instrumental and operant conditioning
Instrumental and operant conditioning refer to the modification of behavior involving an organism's responses and the consequences of those responses. In order to gain further understanding of this concept it may be helpful to conceptualize an operant and instrumental conditioning experiment as a classical conditioning experiment in which the sequence of stimuli and reward is controlled by the behavior of the participant. Studies of instrumental and operant conditioning show that they have several characteristics, including the following:
- The greater the amount and quality of the reward, the faster the acquisition is.
- The greater the interval of time between response and reward, the slower the acquisition.
- The greater the motivation, the more vigorous the response.
- When reward no longer follows the response, the response gradually becomes weaker over time and eventually stops occurring.
Non-associative and/or associative learning has been demonstrated in all the invertebrates in which it has been investigated, including planarians and many protostomes (polychaetes, earthworms, leeches, water fleas, acorn barnacles, crabs, crayfish, lobsters, cockroaches, fruit flies, ants, honey bees, pond snails, freshwater snails, land snails, slug, sea hare, and octopus). While there is no general agreement, most behavioral scientists familiar with the literature would suggest that the most sophisticated examples of learning occur in several of the protostome taxa (crustaceans, social insects, gastropod mollusks, and cephalopods). Many of the organisms in these groups can solve complex and simple discrimination tasks, learn to use an existing reflex in a new context, and learn to control their behavior by the consequences of their actions.
Defensive behavior represents a class of behavior referred to as communication behavior. Metazoans must defend themselves against an impressive array of predators. To survive against an attack, various strategies have evolved. These
strategies include active mimicry, flash and startle displays, and chemical/physical defense.
Physical and chemical defense
A common behavior exhibited by invertebrates in response to danger is the adoption of a threatening posture. For example, when the long-spined sea urchin (Diadema sp.) is threatened it will point its spines toward the predator or threat. Aquatic organisms found in the order Decapoda, such as the cuttlefish and squid, defend themselves by discharging an ink that temporarily disorientates the predator, allowing the organisms time to escape. Some decapods (such as the octopus) in the order Octopoda have a similar ink defense system. At least one case has been observed in which Octopus vulgaris was recorded holding stones in its tentacles as a defensive shield against a moray eel. When sea cucumbers are threatened they expel their intestines to confuse a predator and allow them to escape.
In general, organisms during early ontogenetic development approach low-intensity stimulation and withdraw from high-intensity stimulation (e.g., light intensity). Sessile invertebrates like anemones, corals, and tunicates will contract or withdraw to protect their most vulnerable body parts. Mobile invertebrates can usually escape an aggressor's high intensity stimulation by engaging in kinesis and/or taxis such as crawling, swimming, flying, or jumping. Such behavior is easily observed in the cephalopod Onychoteuthis (protostome) popularly known as the "flying squid." The flying squid can escape aggressors by emitting strong water bursts from its mantle, which propels the animal into the air where finlike structures allow it to glide for a brief period of time. Most other nonsessile invertebrates—like flatworms, echinoderms, and arrow worms—crawl away to hide under a rock, or change direction and swim away to escape predatory stimuli.
There are various forms of mimicry and only a few can be mentioned here. Some of the more well-known invertebrates that engage in mimicry are butterflies. The species Zeltus amasa maximianus (Lycaenidae) presents a "false head" to aggressors that is made more attractive to the predator by the motion of its wings. False-head mimicry requires not only morphological adaptations but also an ability to engage in behavior patterns that force the predator to focus its attention on the false structure. By presenting a predator with a convincing false target the probability of surviving an attack is increased. A similar strategy is also common in caterpillars. Species of Lirimiris (Notodontidae) actually inflate a head-like sac that is found posteriorly. The resulting fictitious appendage draws the attention of the predator away from the actual head and toward the comparatively tough posterior end. An interesting version of false-head mimicry exists in crab spiders (Phrynarachne spp.), and longhorn beetles (Aethomerus spp.), which both mimic the appearance of bird feces, and the Anaea butterfly caterpillar (Nymphalidae), which mimics the appearance of dried leaf tips. Mimicry and false mimicry—where animals mimic another animal—are not well developed among the invertebrate taxa.
startle displays and flash coloration
When some invertebrates are stimulated by an aggressor they quickly modify their posture in an attempt to make it appear larger, and at the same time their body will quickly present a "flash" of color. This type of behavior has evolved mostly in protostomes. However, many combjellies and jellyfish can produce flashes of bioluminescent light that deters or confuses predators.
Migration is a second example of communication behavior. Migratory behavior refers to the movement of entire populations. For invertebrates such movement can range from one or two meters to hundreds of meters. Some well known examples of migratory behavior can be found among insects such as the monarch butterfly, Danaus plexippus (Danaidae) and the locust Schistocerca gregaria (Tettigoniidae).
During migration, activities such as foraging for food and engaging in mate selection are reduced or suspended altogether. The separation of movement from vegetative activities such as feeding, defense, and reproduction is one criterion used to determine if migratory behavior is occurring.
Migratory behavior is usually confined to animals living in temporary habitats. The ability to leave a particular habitat is important for those animals that feed on vegetation or plankton that is seasonal or limited, and that live in unstable environments. Leaving aversive conditions related to crowding or food shortages is one hypothesis that explains migratory behavior in invertebrates. Examples of lower invertebrate migrations are few. While it is known that several species of jellyfish often congregate in groups of thousands, the mechanism that brings them together is largely unknown.
Many species that do not or are not capable of migration (i.e., sessile forms) may encyst, or produce encysted formations or eggs that can withstand seasonal variation in food and other environment conditions. Many sponges, flatworms, rotifers, nematodes, and gastrotrichs can produce resistant eggs or other forms that are capable of withstanding temporary environmental fluctuation.
Abramson, C. I. Invertebrate Learning: A Laboratory Manual and Source Book. Washington, DC: American Psychological Association, 1990.
——. A Primer of Invertebrate Learning: The Behavioral Perspective. Washington, DC: American Psychological Association, 1994.
Abramson, C. I., and I. S. Aquino. A Scanning Electron Microscopy Atlas of the Africanized Honey Bee (Apis mellifera L.): Photographs for the General Public. Campina Grande, PB, Brazil: Arte Express, 2002.
Abramson, C. I., Z. P. Shuranova, and Y. M. Burmistrov, eds. Russian Contributions to Invertebrate Behavior. Westport, CT: Praeger, 1996.
Brusca, R. C., and G. J. Brusca. Invertebrates. 2nd ed. Sunderland, MA: Sinauer Associates, 2003.
Fraenkel, G. S., and D. L. Gunn. The Orientation of Animals: Kineses, Taxes, and Compass Reactions. New York: Dover Publications, Inc, 1961.
Lutz, P. E. Invertebrate Zoology. Menlo Park, CA: Benjamin/Cummings Publishing Company, Inc, 1986.
Matthews, R. W., and J. R. Matthews. Insect Behavior. New York: John Wiley and Sons, 1978.
Preston-Mafham, R., and K. Preston-Mafham. The Encyclopedia of Land Invertebrate Behavior. Cambridge, MA: The MIT Press, 1993.
Romoser, W. S., and J. G. Stoffolano, Jr. The Science of Entomology. 3rd ed. Dubuque, IA: W. C. Brown Publishers, 1994.
Charles I. Abramson, PhD
Dennis A. Thoney, PhD
The large number and sheer diversity of protostomes necessitates a restriction on the kinds of behaviors (and species) that can be discussed in this chapter. The behaviors highlighted here are based in part on their importance to the survival of an individual organism.
Protostomes are some of the most morphologically complex, ecologically diverse, and behaviorally versatile organisms in the animal kingdom. They consist of more than one million species divided into approximately 20 phyla. Major representative phyla include the Platyhelminthes (flukes, planarians, and tapeworms), Nematoda (roundworms), Mollusca (chitons, clams, mussels, nautiluses, octopods, oysters, snails, slugs, squids, and tusk shells), Annelida (bristleworms, earthworms, leeches, sandworms, and tubeworms), and Arthropoda (ants, centipedes, cockroaches, crabs, crayfish, lobsters, millipedes, scorpions, spiders, and ticks).
When considering what a protostome is, it is important to note that the answer appears to be changing with the accumulation of new information. Evidence from studies of morphology and the fossil record generally support the view that animals in the phyla Annelida, Mollusca, and Arthropoda are indeed protostomes. However, new data based on rRNA analysis suggests that some animals in the Pseudocoelomate phyla (gastrotrichs, rotifers, and roundworms) and in the Acoelomate phyla (flukes, planarians, tapeworms, and ciliated worms) are also protostomes.
Despite the impressive diversity of organisms in this group, the vast majority of protostomes share certain basic characteristics of embryonic development. Indeed, the very name protostomes means "first mouth" and nicely illustrates the common characteristic that the initial opening to the digestive tract in the embryo develops into a mouth. Additional protostome characteristics include an embryonic stage known in the literature as mosaic development. Mosaic development produces a series of cell divisions (cleavage patterns) in which the fate of individual cells following the first cell division is fixed (determinate cleavage), while subsequent cell divisions are arranged spirally (spiral cleavage). Moreover, in protostomes the origin of the mesoderm (the germ layer producing such structures as the heart, muscles, and circulatory organs) is created from both the ectoderm (the germ layer producing the skin or integument, nervous system, mouth and anal canals) and endoderm (the germ layer producing the linings of the digestive tract and related glands) in a region known as the 4d cell. Protostomes also have an internal body cavity situated between the digestive tract and the body wall known as the coelom. Two cylindrical masses constructed from mesodermal cells split and the resulting cavities enlarge and combine to form an internal body cavity (coelom) that is surrounded on all sides by mesoderm cells (schizocely).
All protostomes must engage in activities that lead to survival and reproduction. The honey bee and ant, for example, must find and digest food and protect the colony. The planarian and crab must also meet nutritional requirements, re-produce, and defend themselves, but do so often in an aquatic environment. An earthworm is faced with similar problems of survival, but usually solves them underground. Protostomes that fly, swim, or burrow are all faced with the same set of problems. The solutions to these problems represent an interaction of environment and morphology, and here lies the differences in what is called behavior.
The word "behavior" is ambiguous. A physiologist, for example, may be comfortable describing the "behavior of a neuron," while a behavioral scientist might find this objectionable. Moreover, among behavioral scientists there are often discrepancies in the definition of behavior. John B. Watson, who popularized an early form of "behaviorism" in the early twentieth century, once defined behavior as muscle contractions and glandular secretions. Other behavioral scientists such as B. F. Skinner have used several definitions of behavior, including "the movement of an organism in space in relation either to its point of origin or to some other object." Many of these definitions give a novice the impression that, for behavioral scientists, the subject matter consists of bodily movements and mechanical responses. Many define behavior not as movement of an organism (which is the proper study of kinesiology), but as an act. Defining behavior in terms of actions captures the notion that behavior has consequences, in other words, scientists are primarily interested in what an organism "does." By defining behavior in terms of actions and consequences, the focus of a behavioral analysis is not on the individual movements that constitute a behavior (as important as this is), but what the behavior "accomplishes." For instance, how an organism acts in a social situation, responds to threats, or captures food is
intimately related to its body plan. Protostomes have a symmetrical body plan (e.g., planarians, earthworms, lobsters, or ants). One of the more interesting body plans is radial symmetry. Animals with radial symmetry have no front or back and take the general form of a cylinder (e.g., sea stars, and sea anemones) with various body parts connected to a main axis. Such animals have feeding structures and sensory systems that interact with their environment in all directions. Such a body plan is most common among animals that are permanently attached to a substrate (e.g., sea anemones) or drifting in the open seas (e.g., jellyfishes).
Another type of symmetrical body plan found in protostomes is bilateral symmetry. Invertebrates with bilateral symmetry (e.g., planarians, earthworms, crustaceans, insects, and spiders) have a definite front and back, left and right, and backside and underside orientation. Animals with such a body plan generally can control their locomotion, unlike sessile or drifting species (radial symmetry). The front end (anterior) contains an assortment of feeding and sensory structures, often encapsulated in a head (cephalization) that confronts the environment first. Moreover, the underside (ventral surface) typically contains structures necessary for locomotion, and the backside (dorsal) becomes specialized for protection.
Feeding behavior consists of several different types of acts associated with discovery, palatability, and ingestion. The expression of feeding behavior is a combination of evolutionary and environmental pressures. Depending on the species, protostomes consume an infinite variety of food ranging from microscopic organisms, vegetable matter, and other protostomes; some even grow their own food. Despite the large and varied number of protostomes, some generalizations can be found. First, the strategies for finding food can be reduced to those organisms that find food by living on it, foraging for it, waiting for it to pass by, growing it, and having other organisms provide it. Second, the mechanisms associated with feeding can be reduced to those that singly and/or in combination feed by suspension, deposition, macroherbivory and predation.
Crustaceans such as daphnids, brine shrimp, copepods, and ostracods (i.e., those not considered in the class Malacostraca) are excellent examples of filter- or suspension-feeding protostomates. Suspension feeders obtain food by either moving through the water or by remaining stationary. In both cases, bacteria, plankton, and detritus flow through specially designed feeding structures.
Interestingly, because of the large amount of energy required to continuously filter water, there are relatively few protostomes that actually use continuous filtration. A less expensive and also the most commonly employed strategy are to develop specialized filter mechanisms that contain a "sticky" substance such as mucus. An example of this is found in some species of tube-dwelling polychetes that direct water through their burrows and trap food particles in mucus. The mucus is then rolled up into a "food pellet" and manipulated by ciliary action to the mouth where it is consumed.
Well-known examples of protostomes that obtain food from mud and terrestrial soils include most earthworms and some snails. Direct deposit feeders extract microscopic plant matter and other nourishment by swallowing sediment. Such feeders can be either burrowing or selective. In contrast to burrowing feeders such as earthworms, selective feeders obtain food from the upper layers of sediment.
Macroherbivory feeders obtain food by consuming macroscopic plants. One of the best protostome examples of plant feeders is the order Orthoptera (crickets, locust, and grasshoppers). Members of this order have developed specialized mouthparts and muscle structures to bite and chew. The African Copiphorinae, for example, uses its large jaws to open seeds. Biting and chewing mouthparts are also seen in beetles and many orders of insects. Two other types of mouth-parts common to macroherbivory feeders are sucking and piercing. Sucking mouthparts enable insects such as butterflies and honey bees to gather nectar, pollen, and other liquids. Protostomes such as cicadas feed by drawing blood or plant juices. The leaf cutter ants (Atta cephalotes) are interesting example of macroherbivory feeders. These ants cut leaves and flowers and transport them to their nests where they are used to grow a fungus that is their main food source. A related feeding behavior is also found in termites (Isoptera). Termites of the species Longipeditermes longipes forage for detritus such as rotting leaves that become a culture medium for the fungi on which they feed.
Arguably the most sophisticated protostome feeders are those that obtain food by hunting, which requires the animal to locate, pursue, and handle prey. Most invertebrates locate prey by chemoreception; others use vision, tactile, or vibration, or some combination thereof. Predators can be classified as stalkers, lurkers, sessile opportunists, or grazers.
Planarians (Platyhelminthes) are an excellent example of animals that obtain food through hunting. The vast majority of planarians are carnivorous. They are active and efficient hunters because of their mobility and sensory systems. They feed on many different invertebrates, including rotifers, nematodes, and other planarians, and have several different methods of capture. One of the most common methods is to wrap their body around a prey item and secure it with mucus. An interesting example of this behavior can be found in terrestrial planarians. The terrestrial planarian Microplana termitophaga feeds on termites by living near termite mound ventilation shafts. The planarian stretches itself into the shaft and waves its head until a termite comes in contact, at which time the termite becomes stuck on the mucus produced by the worm. An interesting note is that it is not generally agreed upon that Platyhelminthes are protostomes.
Another interesting method that protostomes use to stalk prey can be found in members of the phylum Onychophora. These are wormlike animals that some scholars believe bridge the gap between annelids and arthropods. The velvet worm Macroperipatus torquatus forages nocturnally on crickets and other selected invertebrates and approaches its prey undetected by utilizing slow movements. When the potential prey is recognized as an item to be consumed, the worm attacks it by enmeshing the organism in a glue-like substance squirted from the oral cavity.
Perhaps the most well-known examples of hunting protostomes are the spiders in the phylum Arachnida. Members of the family Lycosidae, colloquially known as wolf spiders, can hunt by day, although some species hunt at night. Some wolf spiders pounce on prey from their burrow, while others actively leave the burrow on hunting trips. The jumping spiders of the family Salticidae and some lynx spiders of the family Oxyopidae also hunt for prey. Once found, the spiders can leap upon it from distances as much as 40 times their body length. Other examples of hunting behavior can be found in the metallic hunting wasp Chlorion lobatum, which specializes in capturing crickets, and in the army ant Eciton burchelli, which forms large colonies and searches for prey on the forest floor.
Protostomes must defend themselves against an impressive array of predators. To survive against an attack, various strategies have evolved. These strategies include active mimicry, flash and startle displays, and chemical/physical defense.
physical and chemical defense
A common behavior exhibited by protostomes in response to danger is adopting a threatening posture. When, for example, a specimen of Brachypelma smithi (mygalomorph spider) is threatened outside its burrow, it reacts by making itself appear larger by shifting weight onto the rear legs while simultaneously raising the front legs and exposing the fangs. Another physical defense mechanism of many species of mygalomorphs is that they use the fine sharp hairs that cover them to pierce their predators. This is not only painful, but may be toxic (these hairs can pierce human skin to a depth of 0.078 in [2 mm]). A spider can release these hairs by rubbing the hind legs against the abdomen. In addition to making themselves appear larger and covered with sharp and, in some cases, toxic hairs, they can also squirt a liquid from their anus.
A novel form of defensive behavior in spiders is found in females and immature males of the black widow (Latrodectus hesperus). When threatened, the black widow emits strands of silk and manipulates the silk to cover its vulnerable abdomen and, sometimes, the aggressor. Especially interesting is the defensive behavior of the cerambycid beetles (genus Hammaticherus) that use spine-like appendages on their antennae to whip their aggressor. Equally fascinating is the behavior of arctiid moths that produce a series of clicks when detecting the sound made by hunting bats.
A well-known active defense system is found in social insects such as honey bees, termites, and ants. The latter two organisms actually maintain a caste of "soldiers" for colony defense, as do several species of aphids (Colophina clematis, C. monstrifica, C. arma). When threatened, these organisms attack by injecting venom into the aggressor and can use their powerful mandibles to incapacitate. In aquatic organisms such as those found in the order Decapoda, cuttlefish and squid defend themselves not only by an ability to escape, but also by discharging ink that temporarily disorientates the aggressor. Some decapods in the order Octopoda, which includes the octopus, have a similar ink defense system. At least one case has been observed in which Octopus vulgaris was recorded actually holding stones in its tentacles as a defensive shield against a moray eel.
In general, organisms during early ontogenetic development approach low-intensity stimulation and withdraw from high-intensity stimulation. Protostomes can always escape high-intensity stimulation offered by an aggressor by crawling, swimming, flying, or jumping. Such behavior is easily observed in grasshoppers and the decapod Onychoteuthis, popularly known as the "flying squid." The flying squid can escape aggressors by emitting strong water bursts from its mantle to propel the animal into the air where finlike structures allow it to glide for a brief period of time. Fleeing is not always effective. The katydid, Ancistrocerus inflictus, does not confront aggressors by an active defense system such as that found in spiders, ants, honey bees, and termites. Rather, Ancistrocerus may be found living near the nests of several wasp species. It is these wasps that provide protection for the katydid.
There are various forms of mimicry. Some of the best-known protostomes that engage in mimicry are butterflies. The species Zeltus amasa maximianus (Lycaendae) increases its chances of surviving an attack by giving its enemy a choice of two heads—one of which is a decoy. By presenting a predator with a convincing false target, the probability of surviving an attack is increased. The decoy, or "false head," is created by morphological adaptations present on the wing
tips. False-head mimicry requires not only morphological adaptations, but also that the animal be able to engage in behavioral patterns that will focus a predator's attention on the decoy. One of the methods a butterfly might use to focus a predator's attention on their false head is to make their morphological adaptations seems more "attractive"; this process is accomplished by certain butterflies using the ribbon-like structures located near their wing tips. When the butterfly moves its wings the ribbons begin to resemble antennae, diverting attention away from the true head located on the opposite side of the butterfly.
A similar strategy is also common in caterpillars. In species of Lirimiris (Notodontidae), the animal actually inflates a head-like sac at its rear. The resulting fictitious appendage draws the attention of the predator away from the actual head to the comparatively tough rear end. Another version of the false head is found in crab spiders (Phrynarachne sp.), and longhorn beetles (Aethomerus sp.), whose mimicry resembles bird feces, and the Anaea butterfly caterpillar (Nymphalidae) that resembles dried leaf tips.
In contrast to false mimicry, some protostomes actually mimic the behavior of other species. Active mimicry is common in a wide range of invertebrates. An interesting example is found in Acyphoderes sexualis (Ceramybiid). This beetle mimics the behavior of two different animals, depending on the threat. When the beetle is touched, it resembles some species of ponerine ants (Formicidae), and when threatened in flight, the behavior changes to resemble polybiine wasps. Another example is tephritid flies (Rhagoletis zephyria). At least two genera emit behaviors that resemble salticid spiders—their main predator. Many other fascinating examples can be found, including wasps (Ropalidia sp.) that create nests resembling fruit, and assassin bugs (Hiranetis braconiformis) that reduce the probability of serving as a parasitic host by imitating the walking pattern of its impregnator, complete with fake ovipositor.
startle displays and flash coloration
When stimulated by an aggressor, some protostomes quickly modify their posture to make them appear larger and at the same time to quickly present a "flash" of color. The various postures and displays are characterized by their position, such as frontal displays and lateral displays. The colors associated with these displays are often effective forms of defense because aggressors learn to associate certain colors with results that may have occurred through prior interaction with the intended prey. For example, if the prey had exhibited a certain color to its attacker, and then the predator became sick after ingesting the prey, or the intended prey sprayed the aggressor with a disagreeable fluid its body produces, the predator learns to associate that outcome with the flash of color it had seen and will attempt to avoid repeating the situation. Rapid display of color is also effective because the display itself will often frighten aggressors away. An example of a flash display is found in the katydids (Neobarrettia vannifera). When disturbed, this animal quickly opens its wings to reveal a polka-dot pattern. A display resembling a large face awaits any aggressor who disturbs the peanut bug, Laternaria later-naria (Fulgoridae), and flag-legged insects (Coreidae) quickly wave a brightly colored leg that it can afford to lose.
The reasons for studying learning in protostomes are varied. Some scientists hope to exploit the nervous system of invertebrates in an effort to reveal the biochemistry and physiology of learning. Other scientists are interested in comparing invertebrates with vertebrates in a hunt for the similarities and differences in behavior. Still other scientists use learning paradigms to explore applied and basic research questions such as how pesticides influence honey bee foraging behavior and if learning is used in defensive and social behaviors.
A prerequisite for the study of learning is that be clearly defined and that the phenomena investigated as examples of learning be clearly defined. When reviewing studies of learning, the scientist should be aware that definitions vary from researcher to researcher. For example, a researcher may consider behavior controlled by its consequences (i.e., behavior that is rewarded or punished) as an example of operant behavior, while others believe that it depends upon the type of behavior being modified (either operant or instrumental learning). Moreover, some believe that any association between stimuli represents examples of Pavlovian conditioning, while others believe that the "conditioned stimulus" must never elicit the response to be trained prior to any subsequent association.
Here, learning is defined as a relatively permanent change in behavior potential as a result of experience. Several important principles of this definition include the following:
- Learning is inferred from behavior.
- Learning is the result of experience; this excludes changes in behavior produced as the result of physical development, aging, fatigue, adaptation, or circadian rhythms.
- Temporary fluctuations are not considered learning; rather, the change in behavior identified as learned must persist as such behavior is appropriate.
- More often than not, some experience with a situation is required for learning to occur.
To better understand the process of learning in protostomes, many behavioral scientists have divided the categories of learning into non-associative and associative.
This form of behavior modification involves the association of one event, as when the repeated presentation of a stimulus leads to an alteration of the frequency or speed of a response. Non-associative learning is considered to be the most basic of the learning processes and forms the building blocks of higher order types of learning in protostomes. The organism does not learn to do anything new or better; rather, the innate response to a situation or to a particular stimulus is modified. Many basic demonstrations of non-associative learning are available in the scientific literature, but there is little sustained work on the many parameters that influence such learning (i.e., time between stimulus presentations, intensity of stimulation, number of training trials).
There are basically two types of non-associative learning: habituation and sensitization. Habituation refers to the reduction in responding to a stimulus as it is repeated. For a decline in responsiveness to be considered a case of non-associative learning, it must be determined that any decline related to sensory and motor fatigue does not exert an influence. Studies of habituation show that it has several characteristics, including the following:
- The more rapid the rate of stimulation is, the faster the habituation is.
- The weaker the stimulus is, the faster the habituation is.
- Habituation to one stimulus will produce habituation to similar stimuli.
- Withholding the stimulus for a long period of time will lead to the recovery of the response.
Sensitization refers to the augmentation of a response to a stimulus. In essence, it is the opposite of habituation, and refers to an increase in the frequency or probability of a response. Studies of sensitization show that it has several characteristics, including the following:
- The stronger the stimulus is, the greater the probability that sensitization will be produced.
- Sensitization to one stimulus will produce sensitization to similar stimuli.
- Repeated presentations of the sensitizing stimulus tend to diminish its effect.
This is a form of behavior modification involving the association of two or more events such as between two stimuli or between a stimulus and a response. In associative learning, the participant does learn to do something new or better. Associative learning differs from non-associative learning by the number and kind of events that are learned and how the events are learned. Another difference between the two forms of learning is that non-associative learning is considered to be a more fundamental mechanism for behavior modification than those mechanisms in associative learning. This is easily seen in the animal kingdom. Habituation and sensitization are present in all animal groups, but classical and operant conditioning is not. In addition, the available evidence suggests that the behavioral and cellular mechanisms uncovered for nonassociative learning may serve as the building blocks for the type of complex behavior characteristic of associative learning. The term associative learning is reserved for a wide variety of classical, instrumental, and operant procedures in which responses are associated with stimuli, consequences, and other responses.
Classical conditioning refers to the modification of behavior in which an originally neutral stimulus—known as a conditioned stimulus (CS)—is paired with a second stimulus that elicits a particular response—known as the unconditioned stimulus (US). The response that the US elicits is known as the unconditioned response (UR). A participant exposed to repeated pairings of the CS and the US will often respond to the originally neutral stimulus as it did to the US. Studies of classical conditioning show that it has several characteristics, including the following:
- In general, the more intense the CS is, the greater the effectiveness of the training.
- In general, the more intense the US is, the greater the effectiveness of the training.
- In general, the shorter the interval is between the CS and the US, the greater the effectiveness of the training.
- In general, the more pairings there are of the CS and the US, the greater the effectiveness of the training.
- When the US no longer follows the CS, the conditioned response gradually becomes weaker over time and eventually stops occurring.
- When a conditioned response has been established to a particular CS, stimuli similar to the CS may elicit the response.
Instrumental and operant conditioning refer to the modification of behavior involving an organism's responses and the consequences of those responses. It may be helpful to conceptualize an operant and instrumental conditioning experiment as a classical conditioning experiment in which the sequence of stimuli and reward is controlled by the behavior of the participant.
Studies of instrumental and operant conditioning show that they have several characteristics, including the following:
- In general, the greater the amount and quality of the reward are, the faster the acquisition is.
- In general, the greater the interval of time is between response and reward, the slower the acquisition is.
- In general, the greater the motivation is, the more vigorous the response is.
- In general, when reward no longer follows the response, the response gradually becomes weaker over time and eventually stops occurring.
Non-associative and/or associative learning has been demonstrated in all the protostomes in which it has been investigated, including planarians (for some scientists, turbellarians are not considered protostomes), polychaetes, earthworms, leeches, water fleas, acorn barnacles, crabs, crayfish, lobsters, cockroaches, fruit flies, ants, honey bees, pond snails, freshwater snails, land snails, slugs, sea hares, and octopuses. While there is no general agreement, most behavioral scientists familiar with the literature would suggest that the most sophisticated examples of learning occur in Crustacea, social insects, gastropod mollusks, and cephalopods. Many of the organisms in these groups can solve complex and simple discrimination tasks, learn to use an existing reflex in a new context, and learn to control their behavior by the consequences of their actions.
Abramson, C. I. Invertebrate Learning: A Laboratory Manual and Source Book. Washington, DC: American Psychological Association, 1990.
——. A Primer of Invertebrate Learning: The Behavioral Perspective. Washington, DC: American Psychological Association, 1994.
Abramson, C. I., and I. S. Aqunio. A Scanning Electron Microscopy Atlas of the Africanized honey bee (Apis mellifera): Photographs for the General Public. Campina Grande, Brazil: Arte Express, 2002.
Abramson, C. I., Z. P. Shuranova, and Y. M. Burmistrov, eds. Contributions to Invertebrate Behavior. (In Russian.) Westport, CT: Praeger, 1996.
Brusca, R. C., and G. J. Brusca. Invertebrates. Sunderland, MA: Sinauer Associates, Inc., 1990.
Lutz, P. E. Invertebrate Zoology. Menlo Park, CA: Benjamin/Cummings Publishing Company, Inc., 1986.
Preston-Mafham, R., and K. Preston-Mafham. The Encyclopedia of Land Invertebrate Behavior. Cambridge, MA: The MIT Press, 1993.
"The Infography." Fields of Knowledge. [August 2, 2003.] <http://www.fieldsofknowledge.com/>.
Charles I. Abramson, PhD
In biology the term behavior refers to the means by which living things respond to their environments. At first glance, this might seem to encompass only animal behavior, but, in fact, plants display observable behavior patterns as well. One of the principal manifestations of plant behavior is tropism, a response to a stimulus that acts in a particular direction, thus encouraging growth either toward or away from that stimulus. Behavior in plants is primarily a matter of response to stimuli, which may be any one of a variety of influences that derive either from inside or outside the organism. Response to stimuli is automatic, and even humans are capable of making these types of programmed responses. In most cases, behaviors in organisms are designed to ensure their survival. Such is the case, for instance, with the complex of behaviors known as territoriality, whereby animals defend what they perceive to be their own.
HOW IT WORKS
Stimulus and Response
A stimulus is any phenomenon that directly influences the activity or growth of a living organism. Phenomenon, meaning any observable fact or event, is a broad term and appropriately so, since stimuli can be of so many varieties. Chemicals, heat, light, pressure, and gravity all can serve as stimuli, as indeed can any environmental change. Nor are environmental changes limited to the organism's external environment. In some cases its internal environment can act as a stimulus, as when an animal reaches the age of courtship and mating and responds automatically to changes in its body.
All creatures, even humans, are capable of automatic responses to stimuli. When a person inhales dust, pepper, or something to which he or she is allergic, a sneeze follows. The person may suppress the sneeze (which is not a good practice, since it puts a strain on blood vessels in the head), but this does not stop the body from responding automatically to the irritating stimulus by initiating a sneeze. Similarly, plants respond automatically to light and other stimuli in a range of behaviors known collectively as tropisms, which we explore later in this essay.
INNATE AND LEARNED BEHAVIOR.
Not all responses to stimuli are automatic, however. Certainly not all behavior on the part of higher animals is automatic, though, as we have noted, even humans are capable of some automatic responses. In general, behavior can be categorized as either innate (inborn) or learned, but the distinction is frequently unclear. In many cases it is safe to say that behavior present at birth is innate, but this does not mean that behavior that manifests later in life is learned. (Later in this essay we look at an example of this behavior as it relates to chickens and pecking.)
Behavior is considered innate when it is present and complete without any experience whereby it was learned. At the age of about four weeks, human babies, even blind ones, smile spontaneously at a pleasing stimulus. Like all innate behavior, babies' smiling is stereotyped, or always the same, and therefore quite predictable. Plants, protista (single-cell organisms), and animals that lack a well-developed nervous system rely on innate behavior. Higher animals, on the other hand, use both innate and learned behavior. A fish is born knowing how to swim, whereas a human or a giraffe must learn how to walk.
Ethology is the study of animal behavior, including its mechanisms and evolution. The science dates back to the British naturalist Charles Darwin (1809-1882), who applied it in his research concerning evolution by means of natural selection (see Evolution). Darwin presented many examples to illustrate the fact that, in addition to other characteristics of an organism, such as its morphologic features or shape, behavior is an adaptation to environmental demands and can increase the chances of species survival.
The true foundations of ethology, however, lie in the work of two men during the period between 1930 and 1950: the Austrian zoologist Konrad Lorenz (1903-1989) and the Dutch ethologist Nikolaas Tinbergen (1907-1988). Together with the Austrian zoologist Karl von Frisch (1886-1982), most noted for his study of bee communication and sensory perception, the two men shared the 1973 Nobel Prize in physiology or medicine.
Lorenz and Tinbergen, who together are credited as founders of scientific ethology, contributed individually to the discipline and, during the mid-twentieth century, worked together on a theory that animals develop formalized, rigid sequences of action in response to specific stimuli. According to Lorenz and Tinbergen, animals show fixed-action patterns (FAPs) of behavior which are strong responses to particular stimuli. Later in this essay, we look at examples of FAPs in action. In addition, Lorenz put forward the highly influential theory of imprinting, discussed briefly in this essay and in more detail elsewhere (see Instinct and Learning).
Behaviorism and Conditioning
The development of ethology by Lorenz and Tin-bergen occurred against the backdrop of the rise of the behaviorist school in the realms of philosophy, psychology, and the biological sciences. This school of thought had its roots in the late nineteenth century, with the writings of a number of philosophers and psychologists as well as practical scientists, such as the Russian physiolo-gist Ivan Pavlov (1849-1936). Pavlov showed that an animal can be trained to respond to a particular stimulus even when that stimulus is removed, so long as the stimulus has been associated with a secondary one.
Pavlov began his now famous set of experiments by placing powdered meat in a dog's mouth and observing that saliva flowed into the mouth as a reflex reaction to the introduction of the meat. He then began ringing a bell before he gave the dog its food. After doing this several times, he discovered that the dog salivated merely at the sound of the bell. Many experiments of this type demonstrated that an innate behavior can be modified, and thus was born the scientific concept of conditioning, or learning by association with particular stimuli.
The variety of conditioning applied by Pavlov, known as classical conditioning, calls for pairing a stimulus that elicits a specific response with one that does not, until the second stimulus elicits a response like the first. Classical conditioning is contrasted with operant conditioning, which involves administering or withholding reinforcements (that is, rewards) based on the performance of a targeted response.
During operant conditioning, a random behavior is rewarded and subsequently retained by an animal. According to operant conditioning theory, if we want to train a dog to sit on command, all we have to do is wait until the dog sits and then say, "Sit," and give the dog a biscuit. After a few repetitions, the dog will sit on command because the reward apparently reinforces the behavior and fosters its repetition.
Human parents apply operant conditioning when they admonish their offspring with such phrases as "You can't watch TV until you've cleaned your room." Likewise, young chimpanzees learn through a form of operant conditioning. By observing their parents, young chimps learn how to strip a twig and then use it to pick up termites (a tasty treat to a chimpanzee) from rotten logs. Their behavior thus is rewarded, an example of the way that operant conditioning enables animals to add new, noninherited forms of behavior to their range of skills.
Though the theory of operant conditioning goes back to the work of the American psychologist Edward L. Thorndike (1874-1949), by far its most famous proponent was another American psychologist, B. F. Skinner (1904-1990). In applying operant conditioning to human beings, Skinner and his followers took the theory to extremes, maintaining that humans have no ideas of their own, only conditioned responses to stimuli. Love, courage, faith, and all the other emotions and attitudes that people hold in high esteem are, according to this school of thought, simply a matter of learned responses, rather like a parrot making human-like sounds to earn treats. This extreme form of behaviorism is no longer held in high regard within the scientific or medical communities.
Behavior in Plants
As noted earlier, the term behavior would seem at first glance to apply only to animals and not to plants. Certainly the majority of attention in behavioral studies, outside the realm of humans, is devoted to ethology, but plants are not without their observable behavioral characteristics. These features primarily manifest in the form of tropism, a response to a stimulus that acts in a particular direction, thus encouraging growth either toward or away from that stimulus. Tropism primarily affects members of the plant kingdom, though it has been observed in algae and fungi as well.
Though the word tropism itself may be unfamiliar to most people, the phenomenon itself is not. There are plenty of opportunities in daily life to observe the response of plants to energy, substances, or forms of stimulation. For example, perhaps you have noticed the way that trees or flowers grow toward sunlight, even bending in their growth if it is necessary to reach the energy source. Similarly, plants in a parched region are likely to develop roots directed laterally toward a water source.
Among the various forms of tropism are phototropism (response to light), geotropism (response to gravity), chemotropism (response to particular chemical substances), hydrotropism (response to water), thigmotropism (response to mechanical stimulation), traumatropism (response to wounds), and galvanotropism or electrotropism (response to electric current). Most of these types involve growth toward a stimulus, a phenomenon known as positive growth, or orthotropism. Plants tend to grow toward light or water, for instance. On the other hand, some kinds of stimuli tend to evoke diatropism, or growth away from the stimulus. Such is bound to be the case, for instance, with traumatropism and electrotropism.
Tropism, along with movement due to changes in water content, is one of the two principal forms of innate behavior on the part of plants. In general, stems and leaves experience positive phototropism, as they grow in the direction of a light source, the Sun. At the same time, roots exhibit positive gravitropism, or growth toward the gravitational force of Earth, as well as positive hydrotropism, since they grow toward water sources below ground. On the other hand, a plant may move in a specific way regardless of the direction of the stimulus. Such movements are temporary, reversible, and result from changes in the water pressure inside the plant.
An excellent example of an innate animal behavior, and one in which humans also take part, is the reflex. A reflex is a simple, inborn, automatic response to a stimulus by a part of an organism's body. The simplest model of reflex action involves a receptor and sensory neuron and an effector organ. Such a mechanism is at work, for instance, when certain varieties of coelenterate (a phylum that includes jellyfish) withdraw their tentacles.
More complex reflexes require processing interneurons between the sensory and motor neurons as well as specialized receptors. These neurons send signals across the body, or to various parts of the body, as, for example, when food in the mouth stimulates the salivary glands to produce saliva or when a hand is pulled away rapidly from a hot object.
Reflexes help animals respond quickly to a stimulus, thus protecting them from harm. By contrast, learned behavior results from experience and enables animals to adjust to new situations. If an animal exhibits a behavior at birth, it is a near certainty that it is innate and not learned. Sometimes later in life, however, a behavior may appear to be learned when, in fact, it is a form of innate behavior that has undergone improvement as the organism matures.
For example, chickens become more adept at pecking as they get older, but this does not mean that pecking is a learned behavior; on the contrary, it is innate. The improvement in pecking aim is not the result of learning and correction of errors but rather is due to a natural maturing of muscles and eyes and the coordination between them.
In studying fixed-action patterns of behavior, or FAPs, Lorenz and Tinbergen observed numerous interesting phenomena. Male stickleback fish, for example, recognize potential competition—other breeding stickleback males—by the red stripe on their underside and thus engage in the FAP of attacking anything red on sight. Tinbergen discovered that jealous stickleback males were so attuned to the red stripe that they tried to attack passing British mail trucks, which were red, when they could see them through the glass of their tanks. Tinbergen termed the red stripe a behavioral releaser, or a simple stimulus that brings about a FAP.
Once a FAP is initiated, it continues to completion even if circumstances change. If an egg rolls out of a goose's nest, the goose stretches her neck until the underside of her bill touches the egg. Then she rolls the egg back to the nest. If someone takes the egg away while she is reaching for it, the goose goes through the motions anyway, even without an egg. Not all animal behavior is quite so predictable, however. In contrast to FAPs are complex programmed behavior patterns, which comprise several steps and are much more complicated. Birds making nests or beavers building dams are examples of complex programmed behavior.
As we noted earlier, Lorenz initiated the study of a learning pattern that came to be known as imprinting. Witnessed frequently in birds, imprinting is the learning of a behavior at a critical period early in life, such that the behavior becomes permanent. The very young bird or other organism is like wet concrete, into which any pattern can be etched; once the concrete has dried, the pattern is set.
Newly hatched geese are able to walk. This is something they learn the moment they are hatched, and they do so by following their parents. But how, Lorenz wondered, do young geese distinguish their parents from all other objects in their environments? He discovered that if he removed the parents from view the first day after the goslings hatched and if he walked in front of the young geese at that point, they would follow him. This tactic did not work if he waited until the third day after hatching, however.
Lorenz concluded that during a critical period following birth, the goslings follow their parents' movement and learn enough about their parents to recognize them. But since he also had determined that young geese follow any moving object, he reasoned that they first identify their parents by their movement, which acts as a releaser for parental imprinting. (Imprinting is discussed further in Instinct and Learning.)
Much of an animal's behavior (this is true of the human animal as well) takes place in interaction with others. This interaction may include rudimentary forms of communication, such as bee dances, studied by Lorenz and Tinbergen's colleague Frisch. As he showed in perhaps the most important research of his career, bees communicate information about food supplies, including their direction and the distance to them, by means of two different varieties of "dance," or rhythmic movement. One is a circling dance, which informs the other bees that food is near (about 250 ft., or 75 m, from the hive), and the other is a wagging dance, which conveys the fact that food is farther away.
There are numerous other forms of communication using one or more sense organs. Birds hear each other sing, a dog sees and hears the spit and hiss of a cornered cat, and ants lay down scent signals, or pheromones, to mark a trail that leads to food. This is only one level of interactive behavior, however. Quite a different variety of interaction is courtship, discussed in Reproduction. Other forms of interactive behavior include the establishment of an animal's territory, a subject we discuss at the conclusion of this essay.
LIFE IN COMMUNITIES.
Interactive behavior comes into play when animals live in close proximity to one another. Certainly there are benefits to group life for those species that practice it: the group helps protect individuals from predators and, through cooperation and division of labor, ensures that all are fed and sheltered. In order to be workable, however, a society must have a hierarchy. Thus, in a situation quite removed from the human ideals of freedom and democracy, insect and animal societies are ones in which every creature knows its place and sticks to it.
Bees, ants, and termites live in complex communities in which some individuals are responsible for finding food, others defend the colony, and still others watch over the offspring. In such a highly organized society, a dominance hierarchy or ranking system helps preserve peace and discipline. Chickens, for example, have a pecking order from the most dominant to the most submissive. Each individual knows its place in the order and does not challenge individuals of higher rank. This, again, is quite unlike humans, who at least occasionally step out of line and challenge bullies; by contrast, that never happens with chickens (fittingly enough).
Almost everyone has seen a dog "mark its territory" by urinating on a patch of ground or has watched a cat arch its back in fury at an intruder to what it perceives as its territory. In so doing, these household pets are participating in a form of behavior that cuts across the entire animal kingdom: territoriality, or the behavior by which an animal lays claim to and defends an area against others of its species and occasionally against members of other species as well.
The physical size of the territory defended is extremely varied. It might be only slightly larger than the animal itself or it might be the size of a small United States county. The population of the territory might consist of the animal itself, the animal and its mate, an entire family, or an entire herd or swarm. Time is another variable: some animals maintain a particular territory year-round, using it as an ongoing source of food and shelter. Others establish a territory only at certain times of the year, when they need to do so for the purposes of attracting a mate, breeding, or raising a family.
Territorial behavior offers several advantages to the territorial animal. An animal that has a "home ground" can react quickly to dangerous situations without having to seek hiding places or defensible ground. By placing potential competitors at spaced intervals, territoriality also prevents the depletion of an area's natural resources and may even slow down the spread of disease. Furthermore, territorial behavior exposes weaker animals (which are unable to defend their territory) to attacks by predators and thus assists the process of natural selection in building a stronger, healthier population.
EXAMPLES OF TERRITORIES.
A territory established only for a single night, for the sole purpose of providing the animal or animals with a place to rest, is known as a roost. Even within the roost, there may be a battle for territory, since not all spots are created equal. Because roosting spots near the interior are the safest, they are the most highly prized.
Another type of specialized territory is the lek, used by various bird and mammal species during the breeding season. Leks are the "singles bars" of the animal world: here animals engage in behavior known as lekking, in which they display their breeding ability in the hope of attracting a mate. Not surprisingly, leks are among the most strongly defended of all territories, since holding a good lek increases the chances of attracting a mate. Like the singles-only communities that they mimic, leks are no place for families: generally of little use for feeding or bringing up young, the lek usually is abandoned by the animal once it attracts a mate or mates.
An animal has to be prepared to defend its territory by fighting off invaders, but naturally it is preferable to avoid actual fighting if a mere display of strength will suffice. Fighting, after all, uses up energy and can result in injury or even death. Instead, animals rely on various threats, through vocalizations, smells, or visual displays.
The songs of birds, the drumming of woodpeckers, and the loud calls of monkeys may seem innocuous to humans, but they are all warnings that carry for long distances, advertising to potential intruders that someone else's territory is being approached. As noted earlier, many animals, such as dogs, rely on smells to mark their territories, spraying urine, leaving droppings, or rubbing scent glands around the territories' borders. Thus, an approaching animal will be warned off the territory without ever encountering the territory's defender. Or, if the invader is unfortunate enough to have trespassed on a skunk's territory, it may get a big blast of scent when it is too late to retreat.
Suppose an animal ignores these warnings, or suppose, for one reason or another, that two animals meet nose to nose at the boundaries of their respective territories. Usually there follows a threatening visual display, often involving exaggeration of the animals' sizes by the fluffing up of feathers or fur. The animals may show off their weapons, whether claws or fangs or other devices. Or the two creatures may go through all the motions of fighting without ever actually touching, a behavior known as ritual fighting.
The degree to which a creature engages in these displays of bravado helps define its territory. If the creature perceives that it is at the center of its own territory and is being attacked on home ground, it will go into as threatening a mode as it can muster. If, on the other hand, the animal is at the edge of its territorial boundaries, it will be much more halfhearted in its efforts at intimidation. As with humans, few animals want to fight when there is nothing really at stake. Also like humans, animals many times may seem to be spoiling for a fight without actually fighting, such that when a fight does break out, it is an aberration. This typically happens only in overcrowded conditions, when resources are scarce—again, not unlike the situation with humans.
Late in his career, Lorenz devoted himself to studying human fighting behavior. In Das sogenannte Böse (On Aggression, 1963), he maintained that fighting and warlike behavior are innate to human beings but that they can be unlearned through a process whereby humans' basic needs are met in less violent ways. Just as fighting in animal communities has its benefits, Lorenz maintained, inasmuch as it helps keep competitors separated and enables the larger group to hold on to territory, so fighting among humans might be directed toward more useful means. As discussed in Biological Communities, it is possible that sports and business competition in the human community provides a more peaceful outlet for warlike instincts.
WHERE TO LEARN MORE
Animal Behavior Resources on the Internet. Nebraska Behavioral Biology Group (Web site). <http://cricket.unl.edu/Internet.html>.
Applied Ethology (Web site). <http://www.usask.ca/wcvm/herdmed/applied-ethology/>.
Dugatkin, Lee Alan. Cheating Monkeys and Citizen Bees: The Nature of Cooperation in Animals and Humans. New York: Free Press, 1999.
Ethology: Animal Behavior (Web site). <http://www.nua-tech.com/paddy/ethology.shtml>.
"Growth Movements, Turgor Movements, and Circadian Rhythmics." Department of Biology, University of Hamburg (Germany) (Web site). <http://www.biologie.uni-hamburg.de/b-online/e32/32c.htm>.
Hart, J. W. Light and Plant Growth. Boston: Unwin Hyman, 1988.
Immelmann, Klaus, and Colin Beer. A Dictionary of Ethology. Cambridge, MA: Harvard University Press, 1989.
Tropisms (Web site). <http://www.ultranet.com/~jkimball/BiologyPages/T/Tropisms.html>.
Learning by association with particular stimuli. There are two varieties of conditioning: classical conditioning, which involves pairing a stimulus that elicits a specific response with one that does not until the second stimulus elicits a response like the first, and operant conditioning, which involves administering or withholding reinforcements (i.e., rewards) based on the performance of a targeted response.
The study of animal behavior, including its mechanisms and evolution.
Fixed-action patterns of behavior, or strong responses on the part of an animal to particular stimuli.
The learning of a behavior at a critical period early in life, such that the behavior becomes permanent.
A term to describe behaviors that are present and complete within the individual and which require no experience to learn them. For example, fish have an innate ability to swim, whereas humans must learn how to walk.
The process whereby some organisms thrive and others perish, depending on their degree of adap tation to a particular environment.
An inborn, automatic response to a stimulus by a part of an organism's body.
Any phenomenon (for example, an environmental change) that directly influences the activity or growth of a living organism.
The behavior by which an animal lays claim to and defends an area against others of its species and occasionally against members of other species as well.
A response to a stimulus that acts in a particular direction, thus encouraging growth either toward or away from that stimulus.
Amphibians are not by nature especially social creatures. Most live solitary lives, and even when they form temporary aggregations, they tend to ignore one another. Some tadpoles form large schools that protect them from predators or enhance feeding, and some exhibit a preference for aggregating with closely related individuals. There is little evidence of such cooperative behavior in adult amphibians. Most social inter-actions are competitive, and most competition is related to acquisition of mates. Sometimes such competition is relatively benign, with males scrambling for access to females, but in some species, males fight violently for individual females or for territories that contain resources that are attractive to females.
Modes of communication
Any type of social interaction between individuals involves an exchange of communication signals based on chemical, visual, acoustic, or tactile cues. The three major lineages of amphibians have undergone millions of years of independent evolution, and, not surprisingly, their modes of communication are different. Little is known about the communication and social behavior of caecilians. We do not know, for example, how males and females of any species of caecilian locate one another. Because most caecilians spend their lives underground, are entirely or nearly blind, and are not known to produce sounds, it is likely that chemical signals are used for mate location and courtship.
Chemical communication in salamanders
The ancestral mode of communication in salamanders appears to be chemical. Salamanders have a variety of specialized glands that produce chemical signals (pheromones) that convey messages of aggression or attraction to other individuals. The use of chemical signals in aggressive interactions has been studied best in the North American red-backed salamander (Plethodon cinereus). Both males and females defend feeding territories under logs and other cover objects (objects used for cover) outside the breeding season. Territory owners mark their territories with fecal pellets containing pheromones produced by glands near the cloaca. Other individuals avoid areas marked by territorial salamanders. During the breeding season, females apparently use the same chemical cues to assess the quality of potential mates. In laboratory experiments, females were more likely to enter territories of males marked with fecal pellets containing termites, a high-quality food, than those marked with pellets containing ants, a low-quality food.
For many other salamanders, chemical cues are used in the initial identification of potential mates as members of the same species. Studies of several closely related species in the terrestrial genus Plethodon have shown that males court only females of their own species and prefer both airborne and substrate-borne chemical cues from conspecific females to those of other species. Similar results have been obtained in studies of dusky salamanders in the genus Desmognathus. In both of these groups, hybridization (mating between species) is relatively common in areas where populations have diverged only recently, and behavioral experiments have shown that discrimination of chemical cues is most accurate in populations where hybridization does not occur.
Male salamanders also use chemical cues during courtship to increase the receptivity of females. Many salamanders have elaborate courtship behavior that involves the transfer of pheromones from the male to the female. The ancestral condition appears to be the production of pheromones by glands in the cloacal region. In mole salamanders (Ambystomatidae), such as the tiger salamander (Ambystoma tigrinum), the female follows the male in a tail-nudging walk with her snout pressed against the male's cloacal gland, presumably receiving some chemical stimulation from the male. More derived salamanders in the families Salamandridae and Plethodontidae have courtship glands on the chin or head. In the North American eastern newt (Notophthalmus viridescens), the male clasps the female around her neck with his hind legs and rubs the side of his head against her snout, transferring pheromones from glands on his cheeks. In large species of the genus Plethodon, such as the red-legged salamander (P. shermani), the male leads the female in a tail-straddling walk, with the female walking over the male's tail and resting her chin at the base of the tail. Periodically, the male turns and slaps the female's snout with a large padlike gland on his chin. This gland produces a protein-based pheromone that has been shown experimentally to increase the sexual receptivity of the female.
In the dusky salamanders of the genus Desmognathus, males often have enlarged front teeth that are used to transfer
pheromones from small glands at the tip of the chin. In most species, the male rakes his teeth across the skin of the female, dragging the chin gland across the wound to introduce the chemical secretions directly into the bloodstream. In two very small species, Desmognathus wrighti and D. aeneus, this somewhat violent form of courtship is carried a step further, and the male actually bites the female to deliver the pheromone into the bloodstream. Remarkably, this unusual form of courtship appears to have evolved independently in these two species (a phenomenon known as convergence), which are not closely related.
Visual communication in salamanders
Some salamanders also make use of visual displays during courtship, often in conjunction with pheromone delivery. This form of communication is best developed in the aquatic Old World newts (Triturus). Males do not clasp females but display near them. Triturus males have wide tail fins and crests extending over most of the back, and the fins, crests, and sides of the body are marked with bright colors and dark spots and blotches. A male uses his tail fin to waft pheromones produced in cloacal glands toward the female, but this display probably provides visual stimulation as well. In the largest species of newts, including the great crested newt (Triturus cristatus) and the marbled newt (T. marmoratus), components of courtship involved in chemical signaling are reduced, while visual displays have become elaborated, with the male exhibiting his bright coloration in broadside displays to the female.
Chemical communication in frogs and toads
Chemical communication is poorly developed in most frogs and toads, although there is evidence that males of some species emit chemical signals that are attractive to females. In dwarf African clawed frogs (Hymenochirus), males have glands behind their front legs that become greatly enlarged during the breeding season. Experimental studies have shown that females are attracted to water containing breeding males or
to extracts from the glands but not to males from which the glands have been removed surgically. The use of chemical signals by an aquatic frog such as Hymenochirus is not surprising, because pheromones are dispersed readily through water. More surprising is the finding that males of a terrestrial frog, the magnificent treefrog (Litoria splendida) from Australia, also produce a courtship pheromone, called splendipherin, that is attractive to females.
Visual communication in frogs and toads
Males of some species of frogs and toads have bright coloration that develops during the breeding season and probably serves as a visual signal to other males. In the North American green frog (Rana clamitans), breeding males have bright yellow throats that probably advertise ownership of territories to other males. Males of several species of frogs that breed in fast-running streams near noisy waterfalls have independently evolved foot-flagging displays, in which a hind foot is raised above the head or extended sideways, often displaying bright white or blue webbing between the toes. These displays are used both for territorial display to other males and to attract females. The displays provide a conspicuous visual signal in a noisy environment, where calls are difficult to hear. Very similar foot-flagging displays have evolved in frogs from Malaysia (Staurois latopalmatus, Ranidae), Brazil (Hylodes asper, Leptodactylidae), Venezuela (Hyla parviceps, Hylidae), and Australia (Taudactylus eungellensis, Myobatrachidae, and Litoria genimaculata, Hylidae). Some frogs also use postural displays to appear larger, often elevating the body during aggressive encounters with other males.
Acoustic communication in frogs and toads
Frogs and toads are unique among amphibians in having evolved elaborate acoustic signals that are used both in aggressive interactions with other males and to attract females. Indeed, frogs probably were the first vocal vertebrates, and their calls are a familiar sound to anyone who lives near a swamp or pond. Frog calls are produced by contractions of muscles in the trunk region that force air out of the lungs, through the vocal cords, and, in most species, into a thin vocal sac that expands to radiate sound to the surrounding air. Vocal sacs of some species are balloon-like structures in the throat region, whereas in other species they expand from slits in the sides of the head.
The muscles involved in call production differ from other muscles in the body, having specialized anatomical and physiological features that allow them to contract hundreds of times per hour for hours at a time without becoming exhausted. This type of sound production is energetically expensive, and some tree frogs' metabolic rates while calling are more than 25 times their resting metabolism. In such species as the North American spring peeper (Pseudacris crucifer), which call at high rates in cold weather, calling is supported by huge stores of fat that accumulate in the trunk muscles in the fall, before the frogs go into hibernation. Consequently, the length of time a male can remain in a chorus may depend on energy reserves that were accumulated months earlier. This, in turn, can affect a male's ability to acquire mates.
Most frogs have a repertoire of several kinds of calls. The most commonly heard are advertisement calls, which serve not only to attract females but also to communicate a male's ownership of a territory to other males. Experiments with many different species have shown that females are attracted only to the calls of their own species, and this ensures that females do not waste their reproductive effort on matings that cannot produce viable offspring. Often, a relatively simple feature of the call is sufficient for females to discriminate between members of their own species and those of closely related species.
For example, two species of North American gray tree-frogs are closely related and sometimes breed in the same ponds. One species, Hyla chrysoscelis, has a normal diploid complement of chromosomes, whereas the other, Hyla versicolor, has a double set of chromosomes (that is, it is a tetraploid animal) and evolved from Hyla chrysoscelis. These species look almost identical, and their calls have the same frequency structure (pitch). The calls consist of a series of repeated pulses of sound, but they differ in the rate at which pulses are produced. The pulse rate of Hyla chrysoscelis is about twice that of Hyla versicolor, and females readily approach males of their own species and reject males of the other species, even when the calls of the wrong species are much louder. The ability of females to find males of their own species in a noisy chorus of several kinds of frogs prevents wasted matings that would result in inviable hybrid offspring.
Many frogs also have courtship calls that are used in close-range interactions with females. In some species, the courtship call is simply a more rapidly repeated version of the advertisement call, which provides a better directional signal to females trying to locate males. In other species, a male gives a distinctly different call that is softer than the advertisement call, probably to avoid attracting nearby males that might attempt to intercept the approaching female. In some species, females even answer males with calls of their own. These calls invariably are very soft, because female frogs lack vocal sacs
to amplify their calls, but they probably allow the male and female to approach each other more efficiently. In some frogs, such as European midwife toads (Alytes), males and females call on the ground away from water and engage in duets as they approach one another. Similar duets have been recorded in African clawed frogs (Xenopus), which call entirely under-water and have a completely different mechanism of call production from other frogs. Their calls consist of a series of simple clicks, and females respond to males with clicks of their own. This calling probably enables males and females to find one another in the muddy pools where these frogs normally breed.
Most frogs also have aggressive calls that are used to challenge intruders into male territories and in actual fights with
other males. Usually, these calls are quite distinct from the advertisement call, but in some species, such as North American cricket frogs (Acris), aggressive calls grade into advertisement calls and differ mainly in the number and timing of repeated pulses. Some frogs have graded aggressive calls that vary in structure as a function of the intensity of the aggressive interaction. For example, in a tiny treefrog from Panama, Hyla ebraccata, males produce aggressive calls that are similar to advertisement calls, but call notes are much longer and have higher pulse rates. As males approach each other in fights, these calls become progressively longer, signaling an increase in aggressiveness.
Mating systems and sexual selection
Much of the exchange of communication signals in amphibians occurs during mate attraction and competition. As is the case for most animals, males tend to compete for access to females rather than the other way around. This is because males can fertilize the eggs of many females, so the availability of females limits male reproductive success. This situation results in intense competition among males for the available females. The exact nature of this competition depends on the length of time females are available and the degree to which they are aggregated in a limited area.
Many amphibians have explosive breeding periods that last only a few days. This is characteristic of many desert-dwelling amphibians, which rely on temporary rain pools for reproduction, and of many species that breed in temporary ponds in early spring. In both cases the breeding season is short, because it is critical for eggs to be laid quickly and larvae to develop and get out of the ponds before they dry up. These
conditions generally result in dense aggregations of males and females and lead to a mating system known as scramble competition. In North American spotted salamanders (Ambystoma maculatum), males gather in large numbers in early spring and engage in group courtship of females. Fertilization is internal and is accomplished by means of spermatophores, or sperm packets, deposited by males on the bottom of a pond. Males often interfere with the mating of other males by placing their spermatophores on top of those already deposited by other males. When a female picks up spermatophores with the lips of her cloaca, she is likely to get only that which is placed on top of the pile.
The European brown frog (Rana temporaria) and the similar North American wood frog (Rana sylvatica) both form "explosive" mating aggregations. Males search the pond for mates, grabbing anything that resembles a female frog. Often, several males pile onto a single female and struggle to be the one to fertilize her eggs. These mating balls can be dangerous to females, and many are crushed or drowned by the competing males. Similar scramble competition occurs in some African treefrogs (genus Chiromantis) that lay eggs in foamy masses on tree branches over temporary ponds. It also is characteristic of some Central and South American tree-frogs (Agalychnis and Phyllomedusa) that lay eggs in jelly masses over water. In these species more than one male sometimes remains on the back of the female when she lays her eggs, so more than one male can fertilize her eggs.
Mate searching and mate guarding
When breeding seasons are relatively long, the arrival of females is less predictable. In the case of many species of salamanders, males search for mates and court females individually. This is the mating system of many newts, including Triturus in Europe and Notophthalmus in North America. Males do not produce chemical signals that attract females from long distances but instead move about the pond bottom in search of suitable mates. Males of the genus Triturus court females but do not physically restrain them. In Notophthalmus and many other genera in the family Salamandridae, males clasp females during courtship. This is a form of mate guarding behavior that prevents other males from courting the same female. Because most frogs and toads use vocalizations to attract females, males usually do not search for mates, but some species engage in prolonged mate guarding. South American toads of the genus Atelopus sometimes remain in amplexus for weeks or months, presumably because females are encountered infrequently.
Leks and choruses
Among some of the larger European newts, such as the great crested newt (Triturus cristatus), males gather in groups and defend small territories, where they display to females. This mating system resembles the leks of many birds and mammals. A lek is a traditional display ground on which males gather to attract females. They defend territories used as display sites, but these territories do not contain resources that are attractive to females. Female choice in this type of mating system is based on behavioral or morphological characteristics of the males, and for this reason sexual dimorphism in size and coloration often is pronounced. Newts with lek mating systems are among the most sexually dimorphic of all salamanders.
Many frogs that gather in large choruses also have lek-like mating systems, with males defending a small space around a calling site that is used to attract mates; once a female arrives, however, she carries the male in amplexus to another site to lay eggs. Females use the rate at which males call or other aspects of their vocal displays to assess the quality of potential mates, usually choosing the ones with the most vigorous displays. In many species, however, it is simply persistence that pays off; males that spend the most time in a chorus tend to be the ones that mate most frequently. For many species time in the chorus probably is limited by energy reserves to support their vigorous calling.
Some amphibians attract females by defending resources, such as egg-laying sites, as territories; males with the most attractive territories obtain the most mates. This type of mating system is rare among salamanders, but does occur in North American hellbenders (Cryptobranchus allegeniensis) and closely related members of the same family, the Japanese and Chinese giant salamanders (Andrias). In these species, males defend cavities under rocks on the bottom of rivers as territories. Other males are excluded with biting and other aggressive behavior, but females are allowed to enter the territory to mate. Males with large cavities often mate with several females, which place their eggs in large groups under rocks. Some male frogs also defend egg-laying sites. This type of mating system is characteristic of North American bullfrogs (Rana catesbeiana) and green frogs (Rana clamitans), and males often fight violently for possession of choice territories. Males with the best territories may mate five or six times in a single breeding season and fertilize as many as 100,000 eggs, whereas males with poor-quality territories often do not mate at all.
In South America, males of the large treefrogs known as gladiator frogs, such as Hyla boans and Hyla faber, build mud nests at the edges of streams and defend them against other males. These frogs are equipped with sharp spines in the thumb region that are used to slash and stab other males in fights. Some males are seriously injured, but those with especially good nests are most likely to mate and produce offspring. Territorial males sometimes continue to guard their nests after eggs are laid, to prevent other males from destroying the egg masses.
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Kentwood D. Wells, PhD
When asking what is typically mammalian in behavior, we must first consider which adaptations and preconditions of a mammal normally shape its life and body. Mammals are warm-blooded, or endothermic, and their system of body temperature regulation through metabolism requires more energy than what is needed by ectotherms. Foraging is also an important aspect of behavior, and has to be considered as a decisive factor in shaping social systems. Also, mammals in general (and female mammals specifically) invest a lot more in terms of time, effort, energy, nutrition, and risk, into their offspring than most other vertebrates do. Again, this shapes social systems, in particular mating and rearing, but also puts severe demands on foraging strategies. Another characteristic is the highly developed brain, specifically in those areas that are necessary for behavioral plasticity and variability, such as the highly evolved forebrain and its hemispheres. This in turn allows the mammal to adapt to a diversity of ecological conditions, and also to form complex and individualized societies. In connection with the intensive and often long periods of infant care, not only by the parents but also other members of the group, this can lead, again, to highly variable and adaptable solutions to ecological as well as social problems and situations. In this chapter, we will cover two of those areas in which mammals are special: learning and behavioral plasticity, and social systems (which include mating and rearing as well as foraging and anti-predator systems). Each of these fields is currently the focus of scientific attention in many places, and by many different approaches. In order to fully understand any biological phenomenon, Tin-bergen in 1963 proposed to answer four questions, and only after getting satisfactory answers to all four can we presume that we have "explained" this phenomenon. They are:
- Where did it come from in evolution?
- What selective advantage does an individual get from having this particular trait (the so-called ultimate reasons)?
- How does it work (physiology, so-called causal mechanisms)?
- How does it develop in an individual's life (so-called ontogeny)?
We shall use these four questions to structure our discussion of mammalian behavior. In order to answer these questions, a combination of different scientific approaches is necessary. Thus, we will draw data from long-term field studies as well as from laboratory and zoo research, from experimental trials as well as from purely observational approaches, and will also need support from other biological disciplines such as endocrinology and molecular genetics. Behavior in itself is at the interface of genetics and ecology, and its under-standing is central also to questions of animal welfare, conservation biology, zoo management, and our relationship with pets and companion animals.
Learning in itself, of course, is by no means specific for mammals, or even higher animals. When asking the first Tin-bergen question, we then have to look for those areas of behavioral plasticity that distinguish mammals from their reptilian ancestors. So-called higher forms of learning, which require certain degrees of neural complexity, are (among others) spatial memory and cognitive mapping. Predators that follow prey, primate bands that follow certain routes between sleeping and foraging sites, caribou that migrate over long distances, and other mammals on the move often display an astonishing ability to cut corners, find shortcuts over ridges, circumvent deep parts in rivers after nightly rainfall, and still arrive at their destination without delay. Caribou that are delayed by late snowfall in spring even use these shortcuts to save time in migration. In all of these cases, some sort of "map" must be represented in the animals' nervous systems, and each element of the map must not only have an "address," but also a possibility to relate it to other elements. Another form of behavioral plasticity is called "problem-solving by insight." In typical cases, an animal is confronted by a situation it cannot immediately solve, such as bananas hanging too high to reach, or food hidden in a box. Problem-solving by insight requires that the animal first familiarize itself with the situation and then start to act in a goal-directed way (such as using a tool, elongating one stick with another one, or opening the lid of the box with a lever). Tool use has been described for mammals from at least six orders. A tool here is defined as a movable object that is not a fixed part of the animal's body, is being carried shortly before or during usage, and is positioned in an adequate way for its subsequent use. Following this definition, mongoose use tools to crack eggs, sea
otters carry stones as anvils, elephants use twigs to swat flies, primates throw stones and branches not only to defend themselves but also to detach fruit from trees, chimpanzees angle for termites, etc. Remarkably, more forms of tool use have been described from captive than free-ranging animals, and only in some apes do we have sufficient evidence for observational learning of tool use from the field.
Even though some of these higher forms of learning and cognition can be found in some birds as well, they are not yet in any case described from reptiles, and we can thus safely assume that the ability for them evolved somewhere in mammalian phylogeny. Thus, question number one seems at least partly answered.
What about selective advantage and survival value? It is of course easy to state that animals that learn better will be better able to cope with environmental challenges and will thus be more apt to survive. Hard evidence from carefully designed studies, however, is scarce. In several vole species of the genus Microtus, there is a clear correlation between spatial learning ability and ranging behavior: only in species where males have larger home ranges than females do males fare better in spatial learning (maze-running) tests. In food choice trials with rodents as well as ferrets and other carnivores, decision time was significantly shorter between novel, or new, foods for animals reared with a more variable diet. When an animal is quicker to reach a decision to eat something, it can eat more per given time, and the extra amount of nutrients certainly is an advantage. Feeding can also become more efficient when search-images have been developed, as demonstrated with hamsters and other rodents. Animals that learn about potentially dangerous predators, as ground squirrels do from hearing other colony members giving warning calls, are another example of learning with a direct survival value.
To address the third question, physiological correlates of learning are known for at least several learning phenomena: brain areas responsible for spatial learning are larger in males of those vole species whose spatial learning is better than females, but not in those without such a sex difference. The olfactory bulb in the brain of a young ferret during the critical period of olfactory food imprinting is larger than before or after this time. We also know that thyroxine, the hormone of the thyroid gland, is responsible for neurological changes during food imprinting in this species, and that oxytocin, a pituitary hormone, is necessary in the brain of monogamous animals to learn who their specific partner is during pair formation. Several areas in the limbic system of the brain, particularly the hippocampus, have been identified as being responsible for exploratory behavior and learning.
So, to address the fourth and last question, what data do we have about ontogenetic influences on behavioral plasticity?
When observing young mammals, play behavior is among the most obvious patterns performed regularly. There are many suggestions that during play, behavior is trained and general reactivity and adaptability is thus improved. Again, however, there are mostly plausibility arguments for this: "Because play occurs, and because it is costly in terms of time, energy, risk of injury, etc., it must have some positive effect. Otherwise, selection would have abolished it long since." Field studies of the same species under different conditions, with different amounts of juvenile play, often find less social cohesion in those individuals that played less. But this could also result from differences in other ecological conditions.
Nevertheless, we return to the question of learning and socialization in the discussion of social systems and social behavior. (There are, however, several studies on the influences of rearing condition and environmental factors on learning and problem-solving later in life.) From studies with laboratory rats and mice, we know, for example, that a well-structured environment, such as cages with climbing and hiding possibilities, is crucial to an animal's later ability to learn how to run through a maze, explore novel situations, climb over ropes, etc. The advantage of using laboratory rodents for these studies is that there are inbreeding strains that differ in learning ability. Thus we have "bright" and "dumb" mice, genetically speaking. However, rearing a "bright" mouse in a boring environment (standard lab cage) and a "dumb" mouse in an enriched, well-structured one leads to a near reversal of their genetic disposition—the "dumb" strain is now as good as, and
sometimes even better than, the "bright" one. Another approach to ontogenetic studies of learning and problem-solving was taken in studies with juvenile macaques and vervet monkeys. It was found that those monkeys who, as juveniles, were able to control their environment by deciding when to press a lever and get a food reward, later in life were more active in exploring and solving new situations that those that could press the same lever but received the same amount of food via random, computer-generated portions. Similar results are also described for domestic dog puppies raised in a challenging
environment. Even a mild social stress, such as handling them a few times during early pup life, increased their activity levels, exploration, social initiative, and other environmentally directed activities considerably.
Sexual reproduction in animals generally puts a heavier load on the female side. In mammals, however, this bias in cost of reproduction is far more extensive due to the period of gravidity (pregnancy) and the subsequent lactational period, both of which cannot be taken over by a male. Consider a female mouse suckling six young: shortly before weaning, each young has about half her weight. Thus, she has to nourish and support 400% of her body weight! There is an even higher evolutionary pressure on mammalian females in at least two aspects: females have to forage more intensively, and more effectively, in order to cover their energetic and nutritional
demands of reproduction. Secondly, as each young or litter forms a rather high proportion of her total lifetime reproduction, she is on heavy demand to select her potential mating partner. Male quality is thus very important, and female mate choice can be expected to be even more careful and elaborate than in other vertebrates.
Animal social systems are supposed to evolve in the context of providing each individual with a so-called "optimal compromise" regarding the demands of foraging, predator avoidance, reproduction, and sheltering. We have to accept the fact that a social group (or other social unit above the individual level) is not some sort of super-organism with its own demands and evolutionary history. Instead, each social unit is brought into existence simply and solely if it is catering to the demands of the individual members, and will remain stable only as long as all of its members do not have any option that, regarding this compromise, provides them with better conditions in total. This does not mean that the animals have to be aware of these choices and options. For natural selection to work, it is sufficient if they behave, based on at least some hereditary components of behavior, in the "correct" way, and their reward will likely be to have more, more viable, or otherwise advantageous young. This is the concept of Darwinian fitness—everyone has to put as many bearers of their own genetic heritage into the next generation, and the one with most young reared successfully into the next generation's gene pool is the fittest. What we as humans use to colloquially call fitness (as in going to a fitness studio) is, in the terminology of behavioral ecology, called resource-holding power (RHP), the possibility to defend resources such as a territory, a mate,
or food, and provide those resources to one's potential social or mating companions.
The diversity of mammalian social systems
Before approaching explanatory questions by means of Tinbergen's questions again, a brief attempt at categorization of social systems: in order to categorize the diversity of mammalian social systems, there are several variables that need to be described for each species. One is the degree of sociality. We find at least three types of social organization here: first are the solitary individuals that do not regularly have any social contact with conspecifics outside the narrow timespan of reproduction. Individuals of solitary species are commonly found alone in periods of both activity and inactivity. Examples are several species of shrews, small mustelids, and probably some other small carnivores. Next are the individuals of species with a dispersed social system that are also mostly found alone during their period of activity. They do, however, have a network of non-aggressive social relationships with neighbors (often closely related individuals) and may form sleeping groups in periods of inactivity. Examples are many prosimian species, several small possums, some wallabies and rat-kangaroos, but also brown bears, female northern white rhinos, female roe deer, and possibly many other species of ungulates formerly classified as solitary. Finally, gregarious or "social" species are those mostly found in groups, such as larger canids, zebras, or savanna-living bovids.
The second variable to consider is territorial defense. A territory is some area that is actively defended at least against members of the owner's age/sex class, where males at least do not tolerate other fully adult and reproductively active males. Territories thus cannot be "automatically" assumed as a
species' characteristic trait, from the fact that some individuals are solitary. Solitary species may well live in undefended, overlapping home-ranges, or even avoid each other actively without defending a territory, as can be seen in females of smaller cats as well as domestic cats in suburban areas (there are, however, also social feral cats). On the other hand, active defense of territories can also be found in truly social species, such as the European badger, the chimpanzee, larger canids, or the spotted hyena.
The third variable to describe mammalian social systems concerns the degree of overlap in the home range. This is, of course, something that can only be found in species with a dispersed or gregarious system. We can roughly distinguish four types here:
- Pairs are found, when one male and one female overlap in their range. This does not necessarily mean that they are found together, such as in gibbon pairs. So-called solitary ranging pairs such as tupaias, red fox, or some prosimians are a common type of mammalian social organization. Pair-living also is not necessarily connected with a monogamous reproductive system, because extra-pair copulations are not uncommon.
- Polygynous systems are those with one male and several females' ranges overlapping. This system is often called a "harem," or "uni-male group." Again, from looking purely at numbers of animals in the group, we cannot fully describe the structure. "Harems" may be kept together solely by the male's herding behavior, such as in hamadryas baboons, or they may stay together even in the male's absence, such as in plains or mountain zebra, even though the mares are not related to each other. Or, they may consist of a matriline, a clan of closely related females, such as in patas monkeys, forest guenons, or Eurasian wild boar.
- Polyandrous systems are those in which two or more males overlap with one female. This is found in some large canids, e.g. the African hunting dog, but is generally more common in birds than mammals.
- Multi-male/multi-female systems where more than one adult of both sexes overlap are typical for many diurnal primates, large bovids, lions, or small diurnal mongooses. In these, but also in polyandrous (rarely in polygynous) systems we cannot automatically assume that all adult members are reproductively active. Helpers, such as in canids or dwarf mongoose, can be fully adult but reproductively suppressed individuals. The degree of reproductive cooperation and suppression is thus the last variable to consider, again mostly for gregarious (or theoretically at least, disperse) species. There are very few truly eusocial species of mammals (the naked mole-rat and some other bathyergids), which means that reproductive suppression is irrevocable, leading to sterile worker castes and an overlap of several generations to be found. However, helpers can be found in many families (callitrichids, canids, marmots). These helpers normally are the young of previous years that remain within their parents' group and range, refrain from reproducing themselves, and help in rearing their parents' next offspring. The degree of helping often depends on the degree of relationship between helpers and the next litter (as demonstrated for the alpine marmot). Helping can be done by carrying them (callitrichids), feeding, guarding, and playing (canids), keeping the nest warm (marmots), or taking part in anti-predator vigilance or defense (dwarf mongoose).
Sociality in the framework of Tinbergen's questions
What do we know about phylogeny? It is not normally possible to find behavior in fossilized form, thus we have to take another, but also reliable approach, by comparing the phenomenon in question among as many living species as possible. When doing this with regard to social systems, the most basic one seems to be a sort of solitary or dispersed female system, foraging alone in undefended home ranges. This pattern can be found in members of so many different taxa that we may assume it to be one that their common ancestors probably shared. Taking males into account as well, we can assume that a system of dispersed polygyny, one male overlapping the ranges of several females, probably was the basic male-female system. From the basic female system, evolutionary
paths could have led either via territorial defense (then group defense and dispersed feeds) to social foraging in group territories, or without defense, via formation of ephemeral, and then later persistent groups.
What do we know about selective advantages of sociality? Behavioral ecology and sociobiology, those areas of behavioral biology that deal with this question, are among the most productive ones in about 20–30 years. Thus, only a few studies should be mentioned, to cover several aspects of this question. Anti-predator vigilance in the dwarf mongoose is, over a longer period of time, only guaranteed in groups of at least six adults; smaller groups sooner or later fell prey to raptors. Jackal pairs with one or more helpers had more success in rearing young—the energetic demand on parents for hunting and producing milk was significantly lower, and juvenile survival higher. Adult male sugar gliders that share dominance with an adult son have a higher proportion of time spent with young in the absence of the mother, which is helpful in defending as well as warning them. Pairs of klipspringer take turns in anti-predator vigilance, one feeding while the other watches out. Eastern gray kangaroos form larger groups in open areas and also during those times of the day when their main predator, the dingo, is likely to hunt. Lastly, survival of alpine marmots is higher when more young of the previous year hibernate together with their parents.
Physiological mechanisms that regulate mammalian social behavior are also currently subjects of intense studies. We already heard about the influence of oxytocin on development of social bonding, studies which have predominantly been conducted on the monogamous vole Microtus ochrogaster. Prolactin has been identified as the hormone of parental care, and, excitingly enough, is not only maternal but also elevated in helpers, such as subordinate individuals in canid packs that help to rear the alpha pair's young. Testosterone in both sexes is connected with status/dominance position. Remarkably enough, testosterone levels often follow, not precede, an increase in status such as after winning a fight. Cortisole, one of the stress-related glucocorticoids, actively lowers status-related behavior and makes an individual more submissive, particularly in contest-related aggressive situations. Stressful reactions to potentially harmful or frightening situations are lower, or absent, if the situation is encountered in the presence of one's bonding mate.
Finally, some data related to the fourth Tinbergen question, ontogeny. The importance of complete socialization has been demonstrated in countless studies. Guinea pig males that had been reared in an all-female group were unable to integrate themselves peacefully into new colonies at sexual maturity due to a lack of two important behaviors: they did not behave submissively towards adult males, and they courted any female (even firmly bonded ones) that they might meet. However, young males reared in the presence of an adult male performed "correctly" immediately after introduction, and thus were integrated without any stress or aggression. Feral cats reared in the presence of other cats (or people) apart from their mothers and litter-mates, and coyote pups raised in presence of adult helpers at the den, became more gregarious than those without these influences. Monkeys reared in isolation were unable to perform socio-sexual behavior correctly, if they did not get at least regular play sessions with other juveniles. Female monkeys without experience in baby care (prior to giving birth themselves) were less competent in handling their own infants.
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Udo Gansloßer, PhD