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Cetacea (Whales, Dolphins, and Porpoises)

Cetacea

Family: Ganges and Indus Dolphins
Family: Baijis
Family: Franciscana Dolphins
Family: Botos
Family: Porpoises
Family: Dolphins
Family: Beaked Whales
Family: Sperm Whales
Family: Belugas and Narwhals
Family: Gray Whales
Family: Pygmy Right Whales
Family: Right Whales and Bowhead Whales
Family: Rorquals

(Whales, dolphins, and porpoises)

Class Mammalia

Order Cetacea

Number of families 14

Number of genera, species
40 genera; 86 species


Introduction

Linnaeus originally assigned the name Cete to the order of mammals consisting of whales, dolphins, and porpoises. The term is derived from the classical noun cetos, meaning a large sea creature. Linnaeus conceived Cete to be the sole member of the group Mutica, one of his three primary subdivisions of placental mammals. The term Cetacea is the plural of cetos and was coined by Brisson in 1762. The study of cetaceans has come to be known as cetology, those who practice it as cetologists.

The lines of demarcation between the living cetaceans and other orders of mammals are firmly drawn, and there is no ambiguity. Similarly, the two living suborders of Cetacea are unequivocally distinct from each other, but also monophyletic; that is, derived from a common ancestor. The Mysticeti, or baleen whales, and Odontoceti, or toothed whales, differ fundamentally in the ways that the bones of their skulls have become "telescoped." The mysticete skull features a large, bony, broad, and flat upper jaw, which thrusts back under the eye region. In contrast, the main bones of the odontocete upper jaw thrust back and upward over the eye sockets, extending across the front of the braincase. Mysticetes have baleen and no teeth as adults, and they have paired blowholes (nostrils). Odontocetes, in contrast, have teeth and no baleen (in some species, many or most of the teeth are unerupted and non-functional, however), and a single blowhole. A major additional factor in the anatomical divergence of the two groups is the development in odontocetes of a sophisticated echolocation system, which has required various unique anatomical specializations for producing, receiving, and processing sound. Mysticetes generally lack the enlarged facial muscles and nasal sacs that characterize odontocetes.

Below the level of suborder, many different approaches to classification have been proposed, involving varying numbers and combinations of infraorders, superfamilies, families, and subfamilies. For simplicity here and in what follows, only families, genera, and species are considered. The present-day consensus among cetologists is that there are four extant families, six genera, and at least 14 species of mysticetes, and ten families, 34 genera, and about 72 species of odontocetes. These numbers will inevitably change as larger samples become available and as more sophisticated analytical methods are applied. It is instructive that no less than five "new" species of cetaceans have been described over the past 15 years, including two mysticetes (Antarctic minke whale, Balaenoptera bonaerensis, and pygmy Bryde's whale, Balaenoptera edeni) and three odontocetes (pygmy beaked whale, Mesoplodon peruvianus, spade-toothed whale, Mesoplodon traversii, and Perrin's beaked whale, Mesoplodon perrini). Some of these represent the formal recognition and description of species long known to exist, but others are genuine discoveries. More of both types of developments are to be expected.

Vernacular uses of the terms whale, dolphin, and porpoise have always been complicated and, occasionally, confusing. All baleen-bearing cetaceans are considered whales, but any of the three terms can be applied to toothed cetaceans, depending upon a number of factors. Body size is a useful, but not definitive, basis for distinguishing whales from dolphins and porpoises. In general, cetaceans with adult lengths greater than about 9 ft (2.8 m) are called whales, but some "whales"(e.g., dwarf sperm and melon-headed; Kogia sima and Peponocephala electra, respectively) do not grow that large and some dolphins (e.g., Risso's and common bottlenosed; Grampus griseus and Tursiops truncatus, respectively) can grow larger. There is considerable overlap in body size between dolphins and porpoises as well. Strictly speaking, the term porpoise should be reserved for members of the family Phocoenidae, all of which are relatively small (maximum length less than 8

ft [2.5 m]) and have numerous small, spatulate (spade-shaped) teeth. The proclivity of seafarers and fishers to apply the term "porpoise" (singular and plural) to any small cetacean that they encounter has led to its rather loose application to marine dolphins by scientists as well. It is occasionally suggested that porpoises can be distinguished from dolphins by their lack of a pronounced beak (the elongated anterior portion of the skull that includes both the upper and lower jaw), but a number of dolphins are at least as blunt-headed as any porpoise. In fact, there is no strict definition of "dolphin," as the term is equally valid for species as diverse as the very long-beaked, bizarre-looking river dolphins (superfamily Platanistoidea), the round-headed "blackfish" (pilot, false killer, and pygmy killer whales; Globicephala spp., Pseudorca crassidens, and Feresa attenuata, respectively), and the archetypal bottlenosed and common dolphins (Tursiops spp. and Delphinus spp., respectively). One other variant that often finds its way into the popular lexicon is "great whales." In most contexts, those who use this term mean it to refer to all of the baleen whales plus the sperm whale (Physeter macrocephalus). In essence, the great whales are those that had great commercial value and therefore were seriously depleted by the whaling industry.

Evolution and systematics

Cetaceans are related to the hoofed mammals, or ungulates, and their ancestry is linked more or less closely to that of cows, horses, and hippopotamuses. Current thinking is that they are highly derived artiodactyls, with a particularly close evolutionary relationship to the hippos. The fossil record of cetacean ancestry dates back more than 50 million years to the early Eocene epoch. Most paleontologists agree that cetaceans arose from the Mesonychidae, an extinct family of primitive terrestrial mammals that inhabited North America, Europe, and Asia. Mesonychids can generally be described as cursorial (adapted for running) carrion feeders with large heads, powerful jaws, and five-toed feet with hoof-like claws. The transition from a wholly terrestrial to an amphibious existence is believed to have taken place initially in the Tethys Sea, a large, shallow, near-tropical seaway that extended from the present-day Mediterranean eastward to beyond the South Asian subcontinent. Most of the fossil evidence for this initial radiation of the stem or basal Cetacea, the extinct suborder Archaeoceti, has come from Eocene Tethys sediments in India, Pakistan, and Egypt, although some archaeocete material has also been found in Nigeria and Alabama (United States). The archaeocetes diversified between 45 and 53 million years ago (mya), and the group had spread into mid-temperate waters by 40 mya, toward the end of the middle Eocene. More than 35 different species have been identified for the interval 35–53 mya, during which time archaic cetaceans had expanded from riverine and near-shore habitats and become adapted to occupy oceanic settings as well. Their eyes and kidneys had probably become capable of tolerating different salt balances, they may have lost much of their hair and begun to acquire blubber for insulation and fat storage, their underwater hearing capability had become enhanced, and they had probably developed nasal plugs to close the nostrils when diving. Presumably, they had also begun to move their tails in an up-and-down, rather than side-to-side, fashion for more efficient swimming.

Archaeocetes exhibited many features typical of living cetaceans, including an elongate upper jaw with bony nostrils set back from the tip, a broad shelf of bone above the eye, anteroposteriorly aligned incisors, and an enlarged mandibular canal on the inner side of the lower jaw. They had a dense outer ear bone, or tympanic bulla, and later forms had an expanded basicranial air sinus similar to that of modern cetaceans. A major difference between archaeocetes and the more derived cetaceans is that the archaeocete skull was not telescoped; that is, it did not have overlapping bony elements. Most, and possibly all, archaeocetes had external hind limbs. In some instances at least, they probably used all four limbs for locomotion both in water and on land. Although they are often depicted as having sinuous, almost eel-like bodies, the basic skeletal structures of most archaeocetes would have supported bodies not much different in overall design to those of living cetaceans.

Five families of Archaeoceti are recognized: Pakicetidae, the amphibious earliest cetaceans; Ambulocetidae, the walking whales; Remingtonocetidae, the gavial-convergent cetaceans (the gavial is a long-snouted, freshwater, fish-eating crocodilian of the south Asian subcontinent); Protocetidae, the first pelagic cetaceans; and Basilosauridae, the so-called zeuglodonts, referring to their complex, many-cusped teeth (the Greek zugotos means yoked or joined, and odous, of course, tooth). The most primitive archaeocete identified to date was Nalacetus, known mainly from isolated teeth. Pakicetus, another small, very early archaeocete, had eyes on top of its head, drank only fresh water (confirmed from oxygen isotope ratios in its tooth enamel), and was predominantly wolf-or hyena-like in appearance. The other families of archaeocetes had been largely supplanted by the zeuglodonts during the late Eocene.

Probably the best-known zeuglodont was Basilosaurus, or the "king lizard" (from the Greek basileus for king and sauros for lizard). This animal could be almost 70 ft (21 m) long and weighed at least 11,000 lb (5,000 kg). Its small head in relation to the long body made it appear truly serpentine. The front appendages had been modified into short, broad paddles, but were still hinged at the elbow; and the rear appendages had atrophied to nothing more than stumps. Basilosaurids may have had dorsal fins and horizontal tail flukes, and they were likely hairless, or nearly so. In short, Basilosaurus was well along the path to becoming what cetologists now think of as a whale.

The archaeocetes are replaced in the fossil record by odontocetes and mysticetes beginning in the Oligocene, about 38 mya. By approximately the middle of that epoch, the archaeocetes appear to have died out completely. The oldest known cetacean in the mysticete clade is Llanocetus denticrenatus, found in late Eocene rocks on the Antarctic Peninsula. This species' most characteristic feature was its series of lobed, widely spaced teeth, which were somewhat reminiscent of the teeth of the crabeater seal (Lobodon carcinophagus). Like the crabeater seal, L. denticrenatus was probably a filter feeder on krill-like invertebrates or possibly small schooling fish. At least four families of tooth-bearing mysticetes have been described from the Oligocene (24–38 mya). The transition leading to rudimentary baleen plates in the spaces between teeth probably occurred about 30 mya with the emergence of the Cetotheriidae, or primitive baleen-bearing mysticetes. It is a slight misconception to say that the presence of teeth is a diagnostic feature of Odontoceti, the so-called toothed whales, because all archaeocetes and some of the primitive fossil mysticetes also had teeth. Further, all of the modern baleen-bearing mysticetes have teeth in the early fetal stages of their development.

Odontocetes also radiated rapidly and widely during the Oligocene, by the end of which there were more than 13 families and 50 species of cetaceans in the world's oceans. This diversity was probably driven by changes in foraging opportunities related to breakup of the southern supercontinent of Gondwana, opening of the Southern Ocean, and the consequent polar cooling and sharpening of latitudinal temperature gradients. Several of the early odontocete lineages failed to survive beyond the Miocene (5–23 mya). The shark-toothed dolphins (Squalodontidae), with their sharp, triangular, serrated teeth, were likely active carnivores, while the very long-beaked Eurhinodelphinidae, with their overhanging upper jaws and many small, conical teeth, were more like the dolphins that cetologists know today. Both of these groups had vanished from the fossil record, and others had dwindled to mere remnants, by the end of the Miocene.

The cetotheres radiated further during the Miocene (5–23 mya), with more than 20 genera in which the blowholes were positioned about as far back on the top of the head as they

are in living mysticetes. Also, by the early Miocene, the two main branches of cetotheres were evident, one leading to the modern right whales (Balaenidae) and the other to the rorquals (Balaenopteridae) and gray whale (Eschrichtiidae). Gray whales do not appear in the fossil record until only about 100,000 years ago, and their ancestry is therefore particularly problematic. For their part, the odontocetes also experienced a major Miocene radiation. Beaked whale (Ziphiidae) fossils are common in marine sediments worldwide by 5–10 mya, and these include animals belonging to the modern genus Mesoplodon. Sperm whales in the family Physeteridae, similar in some important ways to the living species, were present by 22 mya.

Dolphins and porpoises as cetologists know them today also emerged in the Miocene, perhaps about 12 mya. The large, speciose odontocete family Delphinidae is one of the least resolved of the 14 extant cetacean families. In spite of fairly blatant external morphological differences among genera within the family, such as the globe-headed (pilot whales) versus long-beaked (common dolphins) dichotomy, the family's validity is supported by several lines of evidence. For example,

intergeneric hybrids have been observed for many delphinids both in captivity and in the wild, and all 17 included genera share the same basic skull architecture. Most of the morphological diversification within the family is related to body size and foraging structures such as rostral length and width, and the number, size, and form of the teeth. A recent phylogenetic analysis of the delphinids based on full cytochrome b gene sequences has revealed that certain of the genera may represent artificial assemblages of species and that extensive revision is needed at both the genus and sub-family levels.

One of the more high-profile and controversial issues in cetacean systematics that has arisen in recent years is the contention by some molecular biologists that sperm whales are more closely related to the baleen whales than to other odontocetes. However, this view has been refuted, contradicting as it does a host of morphological, paleontological, and even some other molecular evidence confirming that the odontocetes are a monophyletic group. As one expert summarized it, the proposed split linking sperm whales with mysticetes "would require morphological convergences and reversals of a magnitude that defies credibility."

Physical characteristics

The absolute range in body size is vastly greater for cetaceans than for any other mammalian order, from scarcely 5 ft (1.5 m) in length and 120 lb (55 kg) in weight for some dolphin and porpoise species to at least 110 ft (33 m) in length and 400,000 lb (180,000 kg) in weight for the Antarctic blue whale (Balaenoptera musculus). There is also considerable variation in morphology. Several species completely lack a dorsal fin (right whales and right whale dolphins, Balaenidae and Lissodelphis spp., respectively), others have only a hump or ridge (gray whale and Ganges river dolphin, Eschrichtius robustus and Platanista gangetica, respectively), and still others have a tall, prominent, even outsized dorsal fin (male killer whales and spectacled porpoises, Orcinus orca and Phocoena dioptrica, respectively). The very long, flexible pectoral flippers of the humpback whale (Megaptera novaeangliae) are in stark contrast to the small, rounded flippers of beaked whales (Ziphiidae) that fit into molded depressions on the sides of the body, so-called "flipper pockets." A cetacean's dorsal fin, like its tail flukes, has no bony support. The stiffness of these structures comes from tough fibrous tissue and, in the case of the flukes, tendons. The flippers, in contrast, are modified front limbs and therefore contain a full complement of arm and hand bones, which, however, are greatly compressed in length.

Body streamlining is obviously an essential feature of the cetacean form. The eyes are on the sides of the head and the blowhole, or blowholes, are on top. The paired blowholes on all living mysticetes are positioned in approximately the same place—at the back and in the center of the rostrum. The single blowhole of odontocetes can vary in both its appearance and placement, but in all species it is skewed to the left of the midline, thereby reflecting the sinistral skew of the underlying cranium. A sperm whale's blowhole is a deep slit at the very front of the top of the head, which makes its blow cant forward and to the left, allowing an observer to identify the species at a considerable distance. In most dolphins, the blowhole is much farther back on the head, approximately even with the eyes, and it appears as a round hole. However, the blowhole of the Ganges river dolphin is a longitudinal slit well back on the top of the head. Another extraordinary feature of this species is its vestigial eyes, which are tiny and effectively non-functional. Cetaceans have no external ear appendages, and all reproductive and excretory organs are concealed within the body. Both males and females have a navel, genital slit, and anus along the ventral midline, and females normally have, in addition, a small mammary slit on each side of the genital slit. Two small, rudimentary pelvic bones embedded in muscle are the only vestiges of hind limbs.

Cetaceans have compensated for their lack of fur or hair by acquiring an adipose-rich hypodermis, a dense endodermal layer of fat, called "blubber," which functions not only as extremely efficient insulation (a core body temperature of about 98.6°F [37°C] is maintained regardless of ambient conditions), but also as an energy depot. They also have a highly developed counter-current heat exchange system, with arteries completely surrounded by bundles of veins. This system is configured so that heat loss and retention are controlled largely through blood flow to the flippers, flukes, and dorsal fin, none of which has a thick layer of insulative blubber.

Distribution

Cetaceans inhabit all marine waters throughout the world, as well as several large rivers and associated freshwater systems in Asia and South America. Their distribution is limited at the poles only by solid ice coverage. Land, ice massifs, and more subtle features such as depth and temperature gradients, current boundaries, and zones of low productivity constitute the biogeographical barriers that separate species and populations. Competitive interactions have probably also helped to shape the global pattern of cetacean distribution. It is worth emphasizing that cetaceans even occur in all large semi-enclosed seas and gulfs, such as the Black, Red, Baltic, and Japan Seas, the Arabian Gulf, and Hudson Bay.

It is important to recognize that human activities have played a major role in determining the present-day global distribution of cetaceans. Although human actions are not known to have exterminated any cetacean species entirely, they have at least reduced certain species to levels at which they no longer play a significant role in the ecosystem. For example, bowhead whales (Balaena mysticetus) were conspicuous members of the marine fauna of the eastern Atlantic Arctic (Greenland and Barents Seas) before European commercial whalers arrived at the end of the sixteenth century. By the early twentieth century, only scattered individual bowheads remained. Gray whales were present in the North Atlantic Ocean until at least as recently as the seventeenth century but have been extinct there for more than 150 years and now occur only in the North Pacific Ocean. The disappearance of river dolphins from large segments of their range in the Indian subcontinent, Southeast Asia, and China is a well-documented result of deliberate killing, incidental mortality in fishing gear, and dam construction. Moreover, in the Antarctic and no doubt elsewhere, the severe depletion of blue, fin (Balaenoptera physalus), and humpback whales have probably changed the species composition and relative abundance of other high-order consumers. Although difficult to test, the hypothesis that minke whales (as well as crabeater seals and perhaps even some seabirds) increased and expanded their range as the larger krill-consuming whales were eliminated is at least plausible. Some scientists have also argued that sei whales (Balaenoptera borealis), as copepod specialists, were given a competitive advantage and thus proliferated in temperate regions as the numbers of copepod-eating right whales (Eubalaena spp.) were decimated. Again, this hypothesis is all but impossible to prove or disprove.

Generally speaking, human agency has not been responsible for the introduction of cetaceans into new areas of distribution; that is, made them into "alien invaders." However, a few relevant incidents have been documented. It was recently reported that one or more Indo-Pacific humpback dolphins (Sousa chinensis) had breached the Suez Canal, moving from the Red Sea into the Mediterranean Sea—a transoceanic switch facilitated by canal construction. On a few occasions, captive bottlenosed dolphins that originated in one ocean basin have escaped or been released into another basin, opening the possibility that an invasive species or genetic variant

could become established accidentally. Thus far, there has been no report of movement through the Panama Canal by a cetacean, but manatees (Trichechus spp.) have negotiated this route from the Atlantic to the Pacific during the last few decades of the twentieth century and into the early years of the twenty-first century.

Habitat

Three living families of cetaceans, Lipotidae, Iniidae, and Platanistidae, consist of dolphins that are obligate inhabitants of freshwater environments. The Iniidae, in particular, exhibit a remarkable ability to survive, indeed flourish, in habitat that seems unlikely for a cetacean. Amazon River dolphins, or botos (Inia geoffrensis), occupy both the large, turbid, "white-water" rivers and the "black-water" streams and lake systems of Amazonia and Orinoquia, seasonally entering the flooded rainforest to forage among roots and vines. Some platanistids in the upper reaches of the Ganges River system live in relatively cool, clear, fast-flowing streams, while their relatives downriver occupy the wide, brown, slower-flowing channels

of the Gangetic plain. All river dolphins tend to be most abundant in counter-current eddies, where prey is more easily available and less energy is needed to maintain position.

Some delphinids (e.g., the tucuxi, Sotalia fluviatilis, and Irrawaddy dolphin, Orcaella brevirostris) and one species of porpoise (the finless porpoise, Neophocaena phocaenoides) are called "facultative" freshwater cetaceans because they have populations that live not only far up rivers and in freshwater lake systems, but also in marine coastal waters. Some of the other coastal small cetaceans, notably the humpback dolphins and the franciscana (Sousa spp. and Pontoporia blainvillei, respectively), tend to exist in greatest densities in portions of coastline with high volumes of continental runoff, that is, in and near large river mouths. Such areas are typically very productive.

Numerous cetacean species are best characterized as inhabitants of the continental shelf, and they are found mainly inside the 660 ft (200 m) depth contour. Among these, several of the great whales are strongly migratory, going from winter calving and breeding grounds in tropical waters to high-latitude feeding grounds in summer. Gray whales, for example, congregate in warm, shallow lagoons along the Pacific coast of Mexico's Baja California peninsula in winter, and many then travel close along the western North American coast for 4,600–6,200 mi (7,500–10,000 km) to shallow feeding grounds in the Bering and Chukchi Seas, only to return south again by approximately the same route to Mexico during the following autumn. Humpback whales are also long-distance migrators, congregating on shallow banks and reefs in tropical latitudes to give birth, nurse their young, and breed in winter, and moving to productive subpolar and polar waters to feed in summer. Some humpbacks cover 10,000 mi (16,000 km) in their annual round-trip migration. Unlike gray whales, they often strike out across expanses of deep water to get from one segment of habitat to another.

Still other cetacean species are pelagic, or "blue-water," animals, living along the steep contours of continental slopes, near the edges of offshore banks and seamounts, or in canyon areas where sharp depth gradients create beneficial foraging conditions. Some pelagic species forage in the deep scattering layer, a complex of organisms that migrate vertically in the water column, approaching to within about 650 ft (200 m) of the surface at night and descending to depths of 1,000 ft (300 m) during the day. Dolphins that are not especially deep divers take advantage of this phenomenon by resting and socializing during the day and foraging at night. The spinner dolphin (Stenella longirostris), for example, is one of the most widespread warm-water species of cetaceans. Many spinner populations centered on offshore islands or atolls move inshore to bays or reef-fringed lagoons during the day, then offshore at night to feed.

Behavior

The behavior of cetaceans, like so many other aspects of this diverse order, spans a wide range of characteristics. When at the surface, porpoises, beaked whales, and pygmy and dwarf sperm whales (Kogia breviceps and K. sima, respectively) are cryptic and undemonstrative. In contrast, some dolphin species are energetic and conspicuous, leaping high above the surface, spinning, somersaulting, and churning the water. Bow-riding species charm seafarers as they race toward a fast-moving boat and "hitch a ride" in the pressure wave. Some species live in small groups of 10 or fewer individuals and can be considered almost solitary, while others are among the most gregarious mammals. At both extremes, however, it is important to consider that appearances may not reveal the entire story. Given the fact that most cetacean communication is acoustic, not visual, it is possible that individuals and small groups maintain contact over large distances. Thus, the level of social integration may be much greater than an apparently "scattered" pattern of distribution implies. In this regard, the low-frequency calls of blue and fin whales can be heard at distances of hundreds of miles when entrained in deep sound channels.

Remarkably, even many of the earliest odontocetes appear to have been capable of echolocation; that is, able to use sound echoes for detection and navigation as a supplement to, or substitute for, vision. High-frequency clicks produced by the movement of recycled air within the diverticula, sacs, and valves of the nasal passages are projected into the environment via the melon (the lump of fatty tissue that forms an odontocete's "forehead"). These sounds reflect off objects and bounce back. The echoes are transmitted to the ears via the side of the face and pass through the thin wall of the mandible before reaching the ear region. The ear bones, isolated

in fat bodies, receive a given sound at different times, thus facilitating directional hearing. Although proven experimentally for only a few species, it is likely that all odontocetes echolocate. Mysticetes, in contrast, do not echolocate, although it has been speculated that bowhead whales may "read" the undersurface of sea ice, and thus assess the dimensions of a floe, for example, by listening to the reverberations of their calls. This would be a crude form of "echo-sensing." Besides their echolocation clicks, many odontocetes produce high-frequency whistles that are used to communicate. Some mysticetes produce patterned sequences of sounds that constitute "song" in a technical sense, and that are believed to function as sexual advertisement during the mating season.

The social structure of several odontocete species has been studied in detail. Killer whales, for example, have a society centered on matrilineal groups that coalesce to form pods of up to about 60 individuals. Pods are organized into clans, which are collections of pods with similar vocal dialects. Sperm whale social structure has been likened to that of elephants, with adult males roving between stable matrilineal pods on the tropical breeding grounds and becoming essentially solitary while on their high-latitude feeding grounds. Bottlenose dolphins live in fission-fusion societies in which group composition changes frequently as individuals join and leave. Nevertheless, calves stay with their mothers for several years, and in some areas males establish pair bonds that last for decades. The social systems of baleen whales are generally thought to be less complex and structured than those of toothed cetaceans.

Although it is widely assumed that whales are "gentle giants," there is considerable evidence of aggressive behavior in some species. Quite apart from the fact that killer whales regularly kill and eat mammalian prey, male Indo-Pacific bottlenosed dolphins (Tursiops aduncus) form coalitions to fight with other males and aggressively herd females; common bottlenosed dolphins occasionally kill harbor porpoises (Phocoena phocoena) for reasons not readily apparent; adult male beaked whales and narwhals (Monodon monoceros) engage in combat that results in extensive body scarring; and male humpback whales, while competing for access to an adult female on the breeding grounds, may engage in bouts of slashing and scraping that result in bleeding or abrasion of a competitor's head knobs and dorsal fin.

The diving abilities of cetaceans vary in relation to their ecology, distribution, and diet. Sperm whales can dive to depths in excess of 6,080 ft (1,853 m). Both they and bottle-nosed whales (Hyperoodon spp.) can remain submerged for well over an hour at a time, and they are known to feed near the bottom in very deep water. Mysticetes generally do not dive as deep, or for as long, although some are capable of staying down for half an hour or longer.

Feeding ecology and diet

Cetaceans are generally regarded as apex predators, and even the baleen whales, which in many respects feed more like grazers than predators, are positioned relatively high on the trophic pyramid. With their specialized feeding apparatus, the baleen whales are all filter feeders although their actual strategies for collecting prey vary. The balaenids and the sei whale are skim feeders, meaning that they tend to swim steadily through the water, mouth open, allowing prey organisms (usually zoo-plankton) to be continuously filtered against the mat of baleen fringes on the inside of the mouth. At the end of a feeding run, the whale uses its massive tongue to sweep the food into the throat. It then resumes the food-gathering process. Balaenopterids other than the sei whale are gulp feeders, meaning that they take large volumes of seawater into the mouth, normally causing substantial distention of the throat (ventral grooves), then close the mouth and squeeze the water out through the baleen, trapping the prey inside the mouth and swallowing it. Skim feeders tend to have supple, finely fringed baleen, while gulp feeders have stiffer, coarser baleen. The diets of baleen whales range from the stenophagous habits of the blue whale, a krill (euphausiid) specialist, to the more euryphagous habits of the minke, humpback, and fin whales, which take zooplankton, schooling fish, and occasionally even squid.

Toothed cetaceans also prey upon a very broad spectrum of organisms that includes fish of many sizes, from small (herring, capelin, sand lance) to medium (cod, salmon, halibut) to large (sharks and tuna), cephalopods (especially squid but also cuttlefish and octopus), shrimp, and crabs. Killer whales are the only cetaceans known to prey upon warm-blooded animals on a regular basis. Their diet can include everything from seabirds and sea turtles to seals, sea lions, sea otters, and fellow cetaceans. While the baleen whales often consume thousands or even millions of animals in a single feeding bout, odontocetes mainly catch one creature at a time. Those species with reduced dentition, notably most of the beaked whales (Ziphiidae), Risso's dolphin, the pilot whales, and narwhal, probably use suction to capture their prey, which are mostly squid. For the most part, prey is swallowed whole, although groups of rough-toothed dolphins (Steno bredanensis), for example, have been seen tearing chunks from large fish that they had apparently captured cooperatively. Killer whales obviously must bite pieces of flesh from their larger prey. In fact, when they kill a baleen whale, they typically consume the tongue, lips, and throat region first. One odontocete species, the boto, has differentiated dentition. Its rear teeth are flanged and molar-like, presumably so that hard-bodied prey such as armored catfish can be crushed before swallowing.

Reproductive biology

The reproductive and excretory organs are all concealed within the body. The male's retractile penis, similar anatomically to that of the bull, contains a great deal of tough, fibrous tissue. Erections apparently result at least in part from the elasticity of that tissue, which comes into play when the retractor muscles relax. The elongated testes lie within the abdominal cavity just behind the kidneys, rather than in an external scrotum. Female reproductive anatomy is basically similar to that of most other mammals, with the two ovaries in the same position as the male's testes. The ovaries of odontocetes are elongated and somewhat egg-shaped, while those of mysticetes are much more irregular in shape, studded with rounded protuberances. A unique aspect of cetacean reproductive anatomy is that the corpora albicantia; that is, the degenerated corpora lutea that follow ovulation remain evident throughout a female's life. This means that the ovaries provide a complete and permanent record of the animal's reproductive history, allowing scientists to count the number of times that ovulation (but not necessarily pregnancy) has occurred.

The reproductive strategies of cetaceans are generally typical of K-selected species; that is, ones that grow slowly, have relatively few offspring, live for a long time, and exhibit substantial parental involvement in the rearing of young. Even the harbor porpoise and franciscana, two of the fastest-maturing species, take at least several years to achieve sexual maturity, and they give birth to only one calf per year when in their prime. Some of the longer-lived social odontocetes take at least 10 years to mature, and they give birth at intervals of at least three years. The gestation period of sperm whales is 14–16 months, and although the calf may begin taking solid food before the end of its first year, it may continue to be suckled for at least five more years. The reproductive parameters of most odontocetes fall between those of the harbor porpoise and the sperm whale. Baleen whales generally mature before 10 years of age, have a gestation period of 10–14 months, a lactation period of six months to one year, and give birth at intervals of two to five years. Most species are migratory to a greater or lesser extent, and give birth and breed during the winter months in relatively low latitudes.

Conservation

Cetacean conservation emerged during the late twentieth century as one of the world's most highly publicized environmental issues. International focus on the decimation of the stocks of great whales portrayed the human capacity for greed and wanton destruction of natural resources like few other issues could have. The collapse of blue and fin whale stocks in the Antarctic, following as it did the sequential destruction of the stocks of right, bowhead, humpback, and gray whales in other oceans, finally brought serious international regulation to the commercial whaling industry. Having closed the fish-eries for one species and stock after another, the International Whaling Commission (IWC) finally agreed in the 1980s to impose a global moratorium on commercial whaling, which remains in effect. Controversy continues, however, over Norway's ongoing commercial hunts for minke whales in the North Atlantic, and Japan's hunts for an expanding variety of

species in the western North Pacific and Antarctic. The hunts by Norway are legal because that country exercised its sovereign right to object to the moratorium in the first instance, and the Japanese hunts are justified through a loophole in the whaling convention that allows member states to issue national permits for "scientific" catches regardless of prohibitions in the IWC schedule.

The deliberate killing of whales, dolphins, and porpoises for meat and other products continues in many parts of the world, including Japan, where tens of thousands of small cetaceans are taken annually in addition to the "scientific" catch of minke and larger whales; the Faeroe Islands, where many hundreds of long-finned pilot whales and Atlantic white-sided dolphins (Globicephala melas and Lagenorhynchus acutus, respectively) are killed in most years; Greenland, where 160–180 minke whales and 10–15 fin whales are taken annually under the IWC's exemption for "aboriginal subsistence" whaling, as well as many hundreds of harbor porpoises, narwhals, and belugas (Delphinapterus leucas); and Canada, the United States (Alaska), and Russia (Chukotka), where thousands of belugas and narwhals, plus several hundred bowhead and gray whales, are killed each year in what are considered

traditional hunts for "subsistence." While it is true that the absolute scale of the killing of great whales has declined with regulation over the last few decades of the twentieth century and into the early years of the twenty-first century, serious problems remain as many of the hunts for small cetaceans are inadequately regulated to ensure sustainability or permit recovery from depletion.

During the past several decades of the twentieth century and into the early twenty-first century, incidental mortality in fishing gear (so-called bycatch), especially in large-mesh gillnets, has become of paramount importance as a threat factor for cetaceans. Some species, notably the Critically Endangered vaquita (Phocoena sinus) and baiji (Lipotes vexillifer), have been driven close to extinction, and numerous populations of other cetacean species have been greatly depleted, as a result of interactions with fisheries. Efforts to reduce the scale of incidental mortality have centered on development, testing, and mandatory use of acoustic pingers to deter the animals from approaching nets; time and area fishery closures; and establishment of protected areas where high-risk fishing is forbidden. Another threat factor for some populations, and particularly for the Endangered North Atlantic right whale (Eubalaena glacialis) population off the North American east coast, is mortality from collisions with ships. Thus far, mitigation measures have consisted of reconfiguring the ship channels in southeastern Canada to reduce traffic in areas where right whales congregate during summer, and implementation of early-warning systems in portions of the U.S. East Coast where right whales and heavy ship traffic overlap.

Several other factors are of increasing concern: underwater noise, chemical contaminants, and climate change. The possibility that whales are disturbed by industrial noise (e.g., seismic testing, ocean drilling for oil and gas) has been a source of concern for decades, but recent evidence suggests that under certain circumstances, high-energy artificial sounds can actually cause lethal injuries to beaked whales. Pollution of the world's waterways and oceans has become recognized as a serious threat to many forms of life. Cetaceans and other marine mammals are no exception. Because they store large amounts of fat in their bodies, they tend to accumulate very high levels of lipophilic contaminants such as the organochlorines (e.g., PCB, DDT). Interestingly, heavy doses of these toxic chemicals are transmitted to first-born calves through the placenta and milk, which means that this age-class within a cetacean population may be especially at risk. Finally, the rapid ongoing change in global climate is certain to have implications for cetaceans, as for other wildlife. Those species that live in high latitudes could be affected the most. Thinning of sea ice and melting of glaciers will certainly influence productivity and change the character of habitat in the Arctic and Antarctic. While some wild species could benefit, others are likely to be harmed.

Significance to humans

Cetaceans have been of great significance to humans for millennia, beginning when primitive coast-dwellers scavenged stranded carcasses for meat, blubber oil, and bone material. The flesh was eaten by people but also fed to domestic animals, most importantly sled dogs in the Arctic and Subarctic. Whale oil was burned to illuminate homes and footpaths, and in lamps to provide warmth. Bones of whales were used in the construction of dwellings and to manufacture tools and appliances. Baleen had many uses as well. Ironically, some early whalers in the Arctic fashioned sea anchors from woven baleen and attached them to harpoon lines to provide resistance for a harpooned whale trying to escape; they thus used a product obtained from one whale to help them capture another. Although the widespread, critical reliance upon whales for food, oil, and other products no longer applies, some aboriginal communities in the Arctic still consider whale hunting central to their identity and sustenance.

As early maritime communities in more temperate regions ventured into coastal waters and learned to capture cetaceans, they established markets to distribute and sell the oil and baleen (whalebone), giving rise to the global whaling industry, as mentioned earlier. The pursuit of whales was a motivating force in exploration and in the development of many remote regions. Whalers brought trade goods, diseases, firearms, and employment to the people they visited as they scoured the planet for their prey. They also enlisted crewmembers from island outposts like the Azores, Cape Verde Islands, and Hawaii, facilitating a diaspora of sorts. Even if unintended, the consequences of activities of whalers were often disastrous to local societies. An obvious example is the degree to which commercial whalers destroyed the stocks of whales, in some instances literally depriving indigenous people of an essential natural resource.

Whales and dolphins are popular, but high-maintenance and controversial, performers in captivity. Bottlenosed dolphins, belugas whales, and killer whales are the most common species in oceanaria, but numerous other species have been trained to perform as well. The captive display industry played a key role in raising awareness about these animals and in getting people to view them as both sentient and vulnerable. In fact, the killer whale's reputation was completely transformed once people had been exposed to several captive individuals. Ironically, oceanaria have now themselves become targets of protest by campaigners who view the keeping of cetaceans as unethical. Dolphins and small whales have also been the subjects of ex situ research of various kinds, including one program in Hawaii that focuses on developing ways for humans and dolphins to communicate with one another. Some success has been reported in efforts to treat autism by allowing patients to interact with captive dolphins, and luxury hotels in a number of tropical holiday destinations keep animals in sea pens and offer "swim-with-the-dolphin" options for guests. Finally, the U.S. Navy has, for decades, used trained dolphins and small toothed whales to locate and recover objects from the sea floor and participate in at-sea research of various kinds. There were reports during the 2003 invasion of Iraq that dolphins were being used by American forces to detect and help destroy mines in the Persian Gulf. The captive population of common bottlenosed dolphins in the United States is considered by some experts to be self-sustaining; that is, capable of replenishing itself without the need for more captures from the wild. In some respects, the domestication of this species may be at hand.


Resources

Books

Dizon, Andrew E., Susan J. Chivers, and William F. Perrin, eds. Molecular Genetics of Marine Mammals. Lawrence, KS: Society for Marine Mammalogy, 1997.

Evans, Peter G. H., and Juan Antonio Raga, eds. Marine Mammals: Biology and Conservation. New York: Kluwer Academic/Plenum, 2001.

Harrison, Richard, and M. M. Bryden, eds. Whales, Dolphins and Porpoises. New York: Facts on File, 1988.

Hoelzel, A. Rus, ed. Marine Mammal Biology: An Evolutionary Approach. Oxford, U.K.: Blackwell Science, 2002.

Mann, Janet, Richard C. Connor, Peter L. Tyack, and Hal Whitehead, eds. Cetacean Societies: Field Studies of Dolphins and Whales. Chicago: University of Chicago Press, 2000.

Perrin, William F., Bernd Würsig, and J. G. M. Thewissen, eds. Encyclopedia of Marine Mammals. San Diego: Academic Press, 2002.

Reeves, Randall R., Brent S. Stewart, Phillip J. Clapham, and James A. Powell. National Audubon Society Guide to Marine Mammals of the World. New York: Alfred A. Knopf, 2002.

Reeves, Randall R., Brian D. Smith, Enrique A. Crespo, and Giuseppe Notarbartolo di Sciara. Dolphins, Whales, and Porpoises: 2002–2010 Conservation Action Plan for the World's Cetaceans. Gland, Switzerland: International Union for Conservation of Nature and Natural Resources, 2003.

J. E., Reynolds, III, and Sentiel A. Rommel, eds. Biology of Marine Mammals. Washington, DC: Smithsonian Institution Press, 1999.

Rice, Dale W. Marine Mammals of the World: Systematics and Distribution. Lawrence, KS: Society for Marine Mammalogy, 1998.

Ridgway, Sam H., and Richard Harrison, eds. Handbook of Marine Mammals. Vol. 3, The Sirenians and Baleen Whales. London: Academic Press, 1985.

——. Handbook of Marine Mammals, Vol. 4, The Sirenians and Baleen Whales. London: Academic Press, 1985.

——. Handbook of Marine Mammals, Vol. 5, The Sirenians and Baleen Whales. London: Academic Press, 1985.

——. Handbook of Marine Mammals, Vol. 6, The Sirenians and Baleen Whales. London: Academic Press, 1985.

Twiss, John R. Jr., and Randall R. Reeves, eds. Conservation and Management of Marine Mammals. Washington, DC: Smithsonian Institution Press, 1999.

Randall Reeves, PhD

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