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.
Bibliographical noteFunding Information:
Long-term research at Kanyawara was supported by funding from NSF Grant 0416125 to RWW. I.C.G. was also partially supported by NSF Grant IIS-0431141. We thank the Uganda National Council for Science and Technology, the Uganda Wildlife Authority, and the Makerere University Biological Field Station for permission to conduct research within Kibale National Park. This project would not have been possible without the hard work and dedication of the field research team, especially Francis Mugurusi, Christopher Muruuli, Peter Tuhairwe, Christopher Katongole, and the late John Barwogeza and Donor Muhangyi, as well as field managers Michael Wilson, Martin Muller, Katherin Pieta, Carole Hooven, Kimberly Duffy and Emily Otali. Many thanks to Kate Burmon for tireless data entry, Claudio Tennie for stimulating discussion of the meat-scrap hypothesis, and to Melissa Emery Thompson, Katherine McAuliffe and two anonymous referees for comments on earlier versions of this manuscript.
- by-product mutualism
- cooperative hunting
- individual variation
- payoff currency