Michelle Brown: University of California, Santa Barbara

Group?level competition has important effects on individual fitness and is thought to be a crucial force in the evolution of complex social systems. However, such conflicts represent a collective action problem: if all group members share the spoils of battle but only a few incur the costs of fighting, why do participators tolerate free?riders and continue to provide a collective good? In humans, this problem is surmounted by punishing defectors and rewarding participants, but these mechanisms have not been observed in other group-living species. In a large-scale study of redtail monkeys (Cercopithecus ascanius) at the Ngogo site in Kibale National Park, Uganda, I test the underlying assumption that a collective action problem is an inherent feature of intergroup conflicts. In particular, I ask whether participants and defectors experience differing costs and similar benefits during conflicts, measured as changes in energy balance. My research indicates that individual motivations fluctuate over time, but individuals can nonetheless be categorized as either conditional participants or conditional defectors.