Economic profitability of social predation among wild chimpanzees: individual variation promotes cooperation

Social predation (hunting in groups) presents a collective action problem. If nonhunters can obtain meat following a kill, it is unclear why an individual would choose to incur hunting costs. We explored this question using long-term data on chimpanzees from Kanyawara (Kibale National Park, Uganda) to test why hunting probability increases with chimpanzee party size. Social predation was not simply a function of the additive probability of intrinsic individual hunting rates. The potential for sharing meat with preferred social partners or sexually receptive females did not increase hunting probability, and there was no evidence of collaboration. Instead, individual variation in hunting motivation was critical for predicting hunting. Two 'impact' males had persistently high hunting rates, and hunts rarely occurred in their absence. When present, these males acted as hunting 'catalysts'. We argue that the profitability of social predation for chimpanzees differs from social carnivores, for which energy maximization is the expected goal. Since meat contains valuable micronutrients that complement a predominantly plant-based diet, obtaining even a small amount of meat is likely to be beneficial to a chimpanzee. We found that the probability that a male obtained meat increased with the number of hunters, and thus social predation was economically profitable. However, in the largest parties, hunters and nonhunters were equally likely to obtain meat, suggesting that there was incentive to refrain from hunting. Together, our results show that the catalytic action of impact males promotes cooperative hunting to a degree, but the collective action problem persists in large parties.