Group size and the resolution of insider–outsider conflict in animal societies

Although social group size and stability are key areas of interest for studying the evolution and maintenance of animal societies, the evolution of group membership control and how that affects the resulting group size have not been fully explored. Here we develop a game-theoretical model that considers how social and ecological factors jointly affect the resolution of conflict over group size between current group members (insiders) and potential joiners (outsiders). Our model predicts that group size will more closely approach the optimum for insiders when the potential conflict between insiders and outsiders is large, as well as when the cost of engaging in social conflict is high. We also show that the joining effort, repelling effort, cost of selfishness and genetic relatedness have interacting effects on conflict resolution between insiders and outsiders and, thus, on expected group size and structure. Our model further predicts that the expected group size will increase as genetic relatedness between insiders and an outsider increases, assuming that the direct fitness cost for insiders to accept an outsider is relatively large or that the benefit to the outsider joining is small. Ultimately, our model synthesizes previous insider–outsider conflict models to generate a framework for understanding the evolution of both group membership control and the size and structure of the resulting social groups.