Participation in group defence: proximate factors affecting male behaviour in wild white-faced capuchins

When two social groups of the same species confront each other, what factors influence whether individuals participate in the struggle for defence of their group and its resources? In white-faced capuchin monkeys, Cebus capucinus, adult males play an important role in the outcome of intergroup encounters by cooperating aggressively against their opponents. However, not all male capuchin monkeys consistently participate in intergroup aggression. Our study therefore aimed to determine which elements influence the participation of males in a dispute. Playback experiments simulating intergroup encounters, which tested adult males in various social contexts, highlight the importance of the presence of another individual. Male white-faced capuchins, C. capucinus, participated in intergroup encounters more frequently when tested in the presence of another adult male than when tested alone. Male subjects were more likely to respond aggressively to the playback stimulus if the other individual present also responded aggressively. However, the number of males residing in the subject’s group, relative to the number of males in the group from which the playback stimulus was recorded, did not seem to affect a male’s response.