Space, time, and group size: a model of constraints on primate social foraging

Group size is a fundamental aspect of the adaptation of a social animal to its environment. There have thus been numerous attempts to infer optimal group sizes based on both empirical studies of specific taxa and theoretical models that aim to establish generalities. One such generality is that larger groups experience greater within-group foraging competition because of either direct interference by other group members or the rapid exploitation of resources. The ecological constraints hypothesis proposes that increases in group size will increase intragroup feeding competition, thus forcing individuals to visit more patches and to cover greater areas than they would in smaller groups. This paper provides a simple formal model of this constraint, suggesting that two types of constraint limit group sizes in this scenario. First, a constraint on time available to reach an energetic threshold may limit groups to sizes that allow each individual forager to attain that threshold. Second, the balance of foraging costs to benefits may curtail group size even in the absence of a temporal constraint. Both these constraints can be obviated by the adoption of fission–fusion social systems in which a group divides into subgroups during foraging. This latter finding is discussed in relation to primate social systems with an emphasis on time budget approaches.