'Impact hunters' catalyse cooperative hunting in two wild chimpanzee communities.

Even when hunting in groups is mutually beneficial, it is unclear how communal hunts are initiated. If it is costly to be the only hunter, individuals should be reluctant to hunt unless others already are. We used 70 years of data from three communities to examine how male chimpanzees 'solve' this apparent collective action problem. The 'impact hunter' hypothesis proposes that group hunts are sometimes catalysed by certain individuals that hunt more readily than others. In two communities (Kasekela and Kanyawara), we identified a total of five males that exhibited high hunt participation rates for their age, and whose presence at an encounter with red colobus monkeys increased group hunting probability. Critically, these impact hunters were observed to hunt first more often than expected by chance. We argue that by hunting first, these males dilute prey defences and create opportunities for previously reluctant participants. This by-product mutualism can explain variation in group hunting rates within and between social groups. Hunting rates declined after the death of impact hunter FG in Kasekela and after impact hunter MS stopped hunting frequently in Kanyawara. There were no impact hunters in the third, smaller community (Mitumba), where, unlike the others, hunting probability increased with the number of females present at an encounter with prey.