Animal societies vary widely in their size, structure, and apparent complexity. Early work on social structure examined hierarchical organizations with 'dominant' and 'subordinate' members. For example, in a linear hierarchy, the dominance relation is transitive, that is, individual A dominates individual B who dominates individual C, and A also dominates individual C.
Dominant individuals gain disproportionate benefits from group membership, including more opportunities to reproduce. Early theoretical work on the structure of animal groups developed the idea of reproductive skew, where some individuals reproduce more than others. The most extreme cases of reproductive skew are to be found in eusocial societies, where reproduction is dominated by one or a few reproductive individuals, with the remainder of the society members being responsible for foraging, defense, nest-building, etc. Eusocial societies can be found in the Crustacea, Isoptera, Hymenoptera, and Vertebrata. The determination of reproductive conflicts in such societies involve both coercion and cooperation, and can be explained to a large degree by kin selection.
In the 1960s, Bill Hamilton developed the idea of kin selection to explain the extreme case of reproductive skew in social insect colonies, in which only one or a few females reproduce, and most individuals are sterile female workers. Kin selection solves the puzzle of how the trait of sterility can possibly persist in a population, since individuals that have it do not reproduce. Hamilton's solution draws on the peculiar genetic system of the Hymenoptera, the order of insects that includes the social bees, ants, and wasps. In this system, it is possible that females could be more closely related to their sisters than to their daughters. This would make it likely that genes associated with sterility could persist; if one individual has certain forms of a gene, closely related individuals are also likely to have them.
Models of reproductive skew have been categorized into two groups: (1) 'transactional models' which focus on group stability and how this constrains the division of reproduction, and (2) 'compromise models' which ignore group stability and treat reproductive skew as the outcome of a conflict among group members who individually have no absolute control over the final division of reproduction. Numerous studies have tested models of reproductive skew in birds, mammals, and social insects.
Parasites may also have played a role in determining the structure of societies. Early empirical work showed that living in larger groups resulted in a higher ectoparasite load for cliff swallows. Such costs of parasitism led to the suggestion that the structure of animal groups, including behavioral interactions, should be shaped by the threat of parasites. While some social responses to parasitism are known (e.g., in termites, grooming is used to remove dangerous fungal spores) and recent empirical work has demonstrated the association between patterns of parasite spread and social organization, we know little about how important parasites have been in the evolution of social behavior within societies.
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