Socioecology, but not cognition, predicts male coalitions across primates

Humans form agonistic coalitions and alliances in many contexts, but this behavior is thought to be rare in other species. A prominent hypothesis states that coalitions may be under cognitive constraints, but this idea is debated and remains to be tested empirically. In this study, we evaluate the cognitive constraint hypothesis against 3 alternative hypotheses that stress the role of demography, substrate use, and resource competition, for the evolution of male coalitions. A comparative analysis of a unique data set of 86 multimale multifemale groups of 38 nonhuman primate species from all major radiations revealed no evolutionary association of male coalition frequency with cognitive capacity (as indexed by neocortex ratio and endocranial volume). The observed variation was best explained by demography and resource competition in that male coalitions were more likely to occur in species characterized by larger male groups and reduced levels of contest competition (after controlling for phylogeny). These findings suggest that constraints imposed by the socioecological setting, rather than cognition, explain best why some primate species evolved customary coalitionary behavior while others did not. This study presents the first empirical evidence against the long-standing view that cognitive abilities may impose a limit on the use of coalitions in animals.