Stable group size in cooperative breeders: the role of inheritance and reproductive skew

Recent studies of reproductive skew have revealed great variationin the distribution of direct fitness among group members, yetthere have been surprisingly few attempts to explore the consequencesof such variation for stable group size, and none that takeinto account the future benefits of group membership to nonbreeders.This means that the existing theory is not suited to explainthe group size of most cooperatively breeding vertebrates andprimitively social insects in which group membership involvessubstantial future benefits. Here we model the group size ofsuch species as social queues in which nonbreeders can inherita breeding position if they outlive those ahead of them in thequeue. We demonstrate, however, that the results can be generalizedto systems in which inheritance occurs via scramble competition,rather than via a strict queue. The model predicts that stablegroup size will depend on the number of breeding positions inthe group and the mortality rates of breeders and nonbreeders,but not on the distribution of reproduction among the pool ofbreeders. This is because deaths occur at random, so that eachindividual has the same chance of surviving to reach each breedingposition. We tested a specific prediction of the model usingdata on ovarian development in the paper wasp, Polistes dominulus.We found a positive correlation between group size and the proportionof females with fully developed eggs, as predicted. Our resultsclarify the interaction between the dominance structure andsize of animal groups and add to the growing recognition ofthe potential for inheritance as a major determinant of bothindividual behavior and group-level characteristics of animalsocieties.