Reproductive Skew in Multimember Groups

Cooperative societies vary in the extent to which reproduction is skewed toward one or a few socially dominant animals. Many recent models attempt to explain this variation on the basis that a dominant who benefits from the presence of subordinates may offer them incentives, in the form of reproductive opportunities, to remain in the group. While most societies contain multiple members, however, these models have considered only the relationship between a dominant and a single subordinate or have assumed that all subordinates are identical. We develop an incentive-based evolutionary stable strategy model of reproductive skew in three-member groups, in which subordinates may vary in their opportunities for independent reproduction, their contribution to group productivity, and in their relatedness both to the dominant and to one another. Our model demonstrates that the conclusions of two-member models cannot all be generalized to larger groups. For example, relatedness among group members can influence whether or not the dominant does best to offer staying incentives to subordinates in a three-member, but not a two-member, group. Both the degree of skew and group stability depend on the relatedness between subordinates as well as on the relatedness of each to the dominant, and the incentives that each individual subordinate receives are influenced by the traits of the other. Whether such effects increase or decrease skew and group stability depends crucially on whether a third group member increases group productivity to a greater or lesser extent than the first.