Male resource defence during intergroup aggression among tufted capuchin monkeys

In group-living species, conflicts between groups frequently involve multiple individuals participating in aggression towards the opposing group. Although defence is a collective action, individuals differ in their motivations. For males, the spatiotemporal distribution of resources should determine whether they defend food or mates. Although resource defence as a male mating strategy has been demonstrated primarily in unimale social systems, males in multimale groups generally directly defend access to females. I examined the factors affecting individual assessment of the costs and benefits of participation in aggressive intergroup encounters among tufted capuchin monkeys, Sapajus (Cebus apella) nigritus, at Iguazú National Park, Argentina. Through provisioning and playback experiments I show that male aggression during intergroup encounters in tufted capuchin monkeys is directly related to the presence of a high-quality food resource, whereas behaviours more consistent with direct mate defence were not apparent. Although males and females demonstrated concordant strategies, male responses were stronger than those of females, but no differences were observed between dominant and subordinate males in the presence of a defendable food resource. Resident females may be manipulating male-male competition within groups, using males as 'hired guns' to increase access to food resources, thereby increasing infant survivorship and decreasing interbirth intervals. In this population, intersexual conflict, which is often associated with infanticide and sexual coercion in nonhuman primates, has promoted cooperation between the sexes.