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Sociality is a genetically determined social behavior that dictates the social structure of particular groups of animals. Manifestations of sociality include living in close proximity, dividing tasks and responsibilities, and traveling together. Insects express sociality more frequently and to greater extremes than do most other animals, and as a result these animals have formed the archetype of all social animals.

Social insects live in every terrestrial ecosystem and form an important part of many food webs. Eusocial (extremely social) animals live in collective societal units referred to as hives or colonies. As adults eusocial animals serve their society in narrowly defined capacities know as "castes." Some examples of a caste are the worker, soldier, nurse, or reproductively capable king or queen.

In highly social animals all animals excepting the reproductive pair are sterile. In some animals the juvenile's role is predetermined, and it matures into the appropriate body size and shape. However, in other animals the caste of the mature adult is not decided until the animal has matured. In the latter case the individual's body size and abilities, and sometimes its age, will subsequently determine its caste position. There is evidence that some dinosaurs had complex social structures that caused them to herd and nest together. Although only one genus of vertebrate (the naked mole rat), is eusocial, many mammals, birds, reptiles, amphibians, and fish show social behaviors to a lesser degree.

Several theories have arisen to explain the evolution and conferred benefits of eusociality. One of these is "kin selection", suggested by W. D. Hamilton in his 1964 paper, "The Genetical Theory of Social Behavior." This theory suggests that social animals are often genetically very similar to each other. If one accepts that the evolutionary goal of a species is to ensure the survival of its DNA, then in animals with identical genomes , the survival and reproduction of individuals lose importance.

For example, if animal A and animal B have very different DNA, then each will desire to pass on its hereditary code to offspring because each code is unique. However, if A and B have very similar genomes, only one of them needs to reproduce for both of their genetic codes to be carried on to future generations. If the entire colony shares similar DNA, only two members of the colony are required to reproduce so as to pass on their genetic codes. This frees the remaining colony members from reproductive drive and allows them time and incentive to protect, feed, and house the reproductive couple. Thus the kinship of the animals ties them together.

The theory of kin selection is related to the idea of "inclusive fitness," which relates how a trait can pass from generation to generation directly, from parent to offspring, or indirectly through the help provided by individuals who possess the same trait as the parent. Wasps, bees, and ants have a genetic system in which all individuals are at least 50 percent identical, and this genetic similarity makes them a good candidate for kin selection and thus sociality.

see also Behavior; Dominance Hierarchy; Social Animals.

Rebecca M. Steinberg


Bourke, Andrew F. G., and Nigel R. Franks. Social Evolution in Ants. Princeton, NJ: Princeton University Press, 1995.

Frank, Steven A. Foundations of Social Evolution. Princeton, NJ: Princeton University Press, 1998.

Hamilton, W. D. "The Genetical Evolution of Social Behavior." Journal of Theoretical Biology 7 (1964): 1-52.