Sociability is a trait that applies to the ecology and behavior of a species and not to individual organisms. Social species are genetically inclined to group together and follow a particular set of rules defining interactions between individuals. Humans can be considered a social species because we tend to live in communities instead of segregating ourselves as individuals and dispersing to unoccupied territory. In many species, a family unit, meaning parents and their immediate dependent young, groups together and follows particular guidelines of interaction. However, this does not qualify as a society. A society must be composed of more individuals than are contained in a family unit. Even in typically antisocial species, individuals may temporarily unite to bear and raise young before re-dispersing.
Sociability in animals must be either permanent or semi-permanent, unlike family units. The species must also divide social responsibilities among individuals within the group. For example, one group of individuals, whether determined by age, gender, or body shape, must consistently perform a particular function. This requirement disqualifies animals that are merely non-aggressive with one another, but that do not partake in the formality of social structure.
A rigorous definition of an animal society is: a group of animals belonging to the same species, and consisting of individuals beyond those in a family unit, who perform specific tasks, spend distinctly more time together, and interact much more within the group than with members of the same species outside of that group. A social animal is defined as any animal species that typically forms into societies.
Many ecologists are concerned with social behaviors, which are any behaviors specifically directed towards other members of the society. These can include cooperative, selfish, hurtful, or helpful behaviors. The sum of these behaviors determines the character of the society, such as its size and location, and the responsibilities of different societal members. For example, walruses live in coastal arctic regions, within herds containing up to several hundred individuals. Males within a herd are known to attack one another over disputes pertaining to female choice, territory, or food; however, entire herds have been known to come to the defense of a single member when placed in danger. Plants, fungi, and single-celled organisms are not considered social because their interactions are strictly dictated by physical and chemical needs. Thus they cannot behave, and without social behaviors a society is impossible.
Why Species Form Societies
The tendency of a species to form into societies is considered to be caused by the influence of natural selection . During the process of evolution, individual animals of a particular species that formed into societies were more likely to survive than those animals that remained isolated from one another. This pressure to be social could be genetic in origin, meaning that high levels of kinship cause extended families to protect one another. It could also be an environmental pressure—a particularly harsh environment may force animals to depend on each other for greater support. A certain species that is observed to be social today may no longer be under selective pressure to form societies, but retains the trait of sociability for genetic reasons.
Some species may be social in certain environments and solitary in others. This can be linked to food abundance. For example, if food is readily available, individuals can group together for greater protection from predators. If food is scarce, they will remain as individuals so as to avoid the responsibility of supporting weaker individuals. This illustrates how societies can be both cooperative and competitive.
They are cooperative because the animals within a society must share their space and resources in order to ensure the survival of as many members as possible. However, they are competitive because each individual animal within a society is primarily concerned with its own survival. In the case of ample food, a strong animal may enlist the help of weaker animals. The stronger animal benefits from the extra labor and shares the surplus with the weaker animals who may not have survived on their own. This is cooperative behavior.
If the same society is deprived of food, however, the stronger animal no longer benefits from sharing its scarce supplies. It separates from the weaker animals, which are now forced to fend for themselves. This is competitive behavior. In less extreme situations, members of a healthy, normal society express both types of behavior at different times.
Degrees of Sociability
Different species of animals may be social in greater or lesser degrees. The degree of sociability is loosely determined by the likelihood of forming a society, the degree of variation in societal size, the specificity of division of labor, the interdependence of members for survival, and possible phenotypic specializations, difference of body shape, related to societal rank. A non-social species may form into societies in extreme or abnormal situations. This is a common occurrence for captive populations of territorial animals that normally have large ranges in which they remain isolated. In zoos and research laboratories, these animals are artificially forced to live in close proximity to one another. They may display signs of cooperation that herald societal behavior, but this is most likely forced interaction that would not occur in the wild.
The Sizes of Societies
The relative sizes of societies within the same species is a clue to understanding their degree of complexity. If members of the same species located in similar environments always form societies of approximately the same number of individuals, then that species may have complex societal laws dictating group size. Often, maximum group size is determined by local predation on the social species, when regional predators are better able to locate large groups than small groups of prey.
Some species maintain equivalent societal populations even in the absence of heavy predation, and these can be considered more socially complex. Highly complex societies divide tasks among societal members in a very precise manner. For example, some individuals may be responsible for hunting, some for reproducing, some for maintaining the society's living space, and some for raising the young. The clearer the distinction between the duties of societal members, the more complex that society is. In the most complex societies, individuals of a particular caste or rank almost never switch roles within their society. An animal may exchange roles only in situations where a particular caste that performs a particular function is severely depleted in numbers and needs to be augmented by new members.
In less complex societies, animals may have shifting duties and responsibilities. In such a society, a female may reproduce one year, but care for young the following year, and hunt the next year. The roles of individuals within the society continually shift based on the needs of the individual versus the needs of the group.
Effects of Sociability
Occasionally, in species with a very high degree of task specificity, the phenotype , or set of body characteristics, of individuals is determined by their caste. This principle supercedes gender differences in body type. An animal's societal role may be so distinct that its body grows into a form that can best perform that role. Often, the role is easily determined by the appearance of the animal. This is a very rare phenomenon for vertebrates, although it is relatively common in insects such as honeybees and termites.
This phenotypic variation is often based on internal hormones , or external pheromones . Pheromones are hormones released into the environment that can alter the development, behavior, and appearance of individual animals. Pheromones have species-specific effects, meaning that they typically affect only animals within the same species. Type of food consumed, environmental conditions, and population ratios between the castes can affect the caste phenotype of individuals either directly or indirectly by altering the exposure of individuals to particular pheromones.
Sociability is a trait of many organisms, but two particular animals provide models of highly complex animal societies. Termites and naked mole rats come from entirely divergent evolutionary origins, yet they share a similar social organization. This raises the question for ecologists of why this social strategy is so effective that it arose multiple times in multiple branches of the evolutionary tree. Understanding the social interactions of these animals can contribute to the theoretical explanation of this organizational pattern.
The highest degree of sociability in animals is given its own classification term, eusocial . Eusocial animals must exhibit a reproductive division of labor, which means that many sterile individuals work to support those individuals capable of breeding. Furthermore, they must be organized into a system of discrete castes distinguishable by differences in skeletal or body characteristics, and these castes must contribute to care of the young. Finally, the generations must overlap in time, meaning that the young do not have time to mature before another group of young is produced. Termites and naked mole rats both conform to this definition.
Termites live in carefully constructed structures entirely shut off from the light, usually within dead trees or wood products. Their social structure is relatively flexible compared to that of ants: In an ant colony, the sizes and body forms of individuals are fixed and constant at maturity, but termites change the shapes of their bodies depending on their role in the colony, even as adults.
Termite colonies may contain from several hundred to several million individuals. New colonies are founded annually during a short season when an existing colony produces sexual winged members that leave the nest by the thousands in a marriage flight. After about two and a half hours, the termites drop to the ground and purposefully break off their wings. The male and female dig a tunnel into a damp log for approximately two days and then mate within two weeks. Termites undergo incomplete development, which means that they resemble miniature, immature adults at hatching, and never pass through a grub stage of development. When the brood is old enough, they assume complete responsibility for feeding and caring for their parents, which are now called the royal couple, or the king and queen. Both the king and queen grow much larger than other caste members, and the queen's abdomen swells with eggs until she becomes up to 20,000 times the size of other colony members.
Termites continuously exchange food and clean one another, probably to ensure cleanliness, but these behaviors also serve to cement the social interaction of the colony. Pheromones secreted by the royal couple and spread through saliva and food-sharing inhibit the development of more sexual forms in the colony, but the mechanisms underlying differentiation of other castes are unclear. They seem to develop when needed by the colony.
Termites are divided into more castes than most social insects, and the castes are far more varied. The basic division of labor includes the reproductive royal couple and sterile soldiers and workers. There are two possible types of soldiers, although any given species contains only one type. The mandibulate uses its large pincers to attack enemies, while the nasute excretes a sticky, poisonous substance from its elongated snout to immobilize enemies.
Soldiers only serve to defend the nest from enemies, and cannot feed themselves or perform any other function in the colony. Workers perform all of the construction tasks of the colony as well as feeding and cleaning the royal couple, eggs, and soldiers. Substitute sexual forms develop from juveniles when one or both of the royal couple dies. They battle amongst themselves until only one reproductively capable couple remains, and these two will develop into the new king and queen.
Like termites, every species of ant, most species of bees, and many species of wasp live in similarly-structured social colonies, with one reproductive pair and many sterile castes. This strategy has evolved independently many times in insects, which means that it must confer some general evolutionary advantage to the species. Perhaps by sequestering the only viable pair in the heart of a well-defended fortress, these insects are better able to ensure the safety of mating and survival.
Naked Mole Rat
One species of vertebrate, the naked mole rat, has also adapted a colonial social structure. Naked mole rats are more closely related to voles than to either moles or rats. They mainly inhabit Somalia, Ethiopia, and Kenya, in sandy soil burrows that extend from near-surface level to several meters below the ground. These are the only known cold-blooded vertebrates. Their body temperatures are always equivalent to ambient temperature except when they are highly active.
These animals also differ from other mammals in that they live in a complex social environment. There is one breeding female per colony, and she can be considered the equivalent of a queen in the termite colony. She is longer-bodied and heavier than other members. Unlike the termite example, there is no single king to mate with the lead female; instead, any male in the colony is free to mate with the lead female, but there is little aggression or competition between these males. This female gives birth to one to six pups per litter, and approximately two litters per year, and these animals have a life span of twenty to forty years.
Castes in the naked mole rats are partially determined by age and partially by body size. The leading female maintains her dominance in the colony through shoving and threatening non-breeders. All castes except for the breeding female engage in building, digging, and transporting food and soil, but the amount of work they perform is clearly delineated by their caste. Small-bodied juveniles are frequent workers, meaning that they carry the bulk of work activity of the colony. Slow-growing individuals remain frequent workers their entire lives, although larger, faster-growing individuals move into the infrequent workers caste, and eventually into the nonworkers caste.
The infrequent workers and nonworkers are most often colony-defenders. When the breeding female dies or is killed, the younger, smaller-bodied female caste members set upon one another. Their bodies begin to produce greater degrees of female hormones, and they fight until one dominant female remains alive. At this point, the colony recognizes their new breeding female, and colony life reestablishes itself.
Being social seems to confer many advantages to social animals, such as increased defense of breeding individuals, increased likelihood of survival, and increased stability of food stores and habitat. However, many animals are not social. An example is sloth bears, which hunt termites for food and come together only for the sake of reproduction. Explaining the reasons for the sloth bear's asocial behavior is not entirely possible without knowing its distinct evolutionary history. However, the bear's large body size and scarce food source may be reasons why colonial living is inappropriate for this species.
see also Behavior; Dominance Hierarchy; Sociality.
Rebecca M. Steinberg
Caro, Timothy M. Cheetahs of the Serengeti Plains: Group Living in an Asocial Species. Chicago: University of Chicago Press, 1994.
Gadagkar, Raghavendra. Survival Strategies: Cooperation and Conflict in Animal Societies. Cambridge, MA: Harvard University Press, 1997.
Wilson, Edward Osborne. Sociobiology: The New Synthesis, 25th anniversary ed. Cambridge, MA: Belknap Press of Harvard University Press, 2000.
As decomposers, termites are highly important members of their ecological communities. Termites feed on dead plant cell wall material, with their most prevalent form of food being wood. Termites themselves do not digest the cellulose; rather, the protozoans and bacteria that live symbiotically in the termites' gut break down the tough wood fibers. In some communities, termites can be responsible for recycling up to one third of the annual production of dead wood, thereby making many needed nutrients available to their environment.
Naked mole rats consume their own feces and that of other mole rats. They also roll their bodies in the feces to ensure that no nutrients are lost through fecal matter that has not been entirely digested, and to mark colony members with an identical signature scent.