Ecological and social influences on the hunting behaviour of wild chimpanzees, Pan troglodytes schweinfurthii IAN C. GILBY * , LYNN E. EBERLY †, LILIAN PINTEA * & ANNE E. PUSEY * *The Jane Goodall Institute’s Center for Primate Studies, Department of Ecology, Evolution and Behavior, University of Minnesota yDivision of Biostatistics, School of Public Health, University of Minnesota (Received 29 September 2005; initial acceptance 2 November 2005; final acceptance 4 January 2006; published online 6 June 2006; MS. number: A10257) There has been considerable discussion of the factors that influence the hunting behaviour of male chimpanzees. Explanations invoking social benefits hinge upon the potential for males to share meat with sexually receptive females in exchange for mating (‘meat for sex’), or to share meat with other males in exchange for social support (‘male social bonding’). Ecological factors may also affect hunting: chimpanzees may hunt more frequently (1) in response to food shortages (‘nutrient shortfall’); (2) when energy reserves are high (‘nutrient surplus’); (3) in habitat types with good visibility and increased prey vulnerability; and/or (4) when ecological factors favour cooperative hunting. We used 25 years of data on chimpanzees in Gombe National Park, Tanzania, to examine the relative importance of social and ecological factors in the decision to hunt red colobus monkeys, Colobus badius. The presence of sexually receptive females was associated with a significant decrease in hunting probability, suggesting that males face a trade-off between hunting and mating (‘meat or sex’ rather than ‘meat for sex’). Hunting by specific males did not vary with adult male party size, providing evidence against the male social-bonding hypoth- esis. After controlling for the effects of party size, diet quality was not associated with the probability of hunting or hunting successfully. Hunts were more likely to occur and to succeed in woodland and semi- deciduous forest than in evergreen forest, emphasizing the importance of visibility and prey mobility. Finally, per capita meat availability decreased with adult male party size, suggesting that hunting was not cooperative. These results provide evidence against social explanations for hunting in favour of more simple ecological alternatives. Ó 2006 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. Capture of mobile prey provides nutritional benefits, but even for specialized carnivores, can be costly in terms of energy expenditure and risk of injury (Packer & Ruttan 1988). Studies of hunting in a variety of species show that animals weigh these benefits and costs in hunting de- cisions. Ecological factors, particularly pursuit costs, are central to hunting studies (Creel & Creel 1995; Creel 1997; Packer & Caro 1997). African lions, Panthera leo, hunt more in areas where prey are most vulnerable (Hopcraft et al. 2005), have better hunting success in long grass and on dark nights (Funston et al. 2001), and cooperate to capture large ungulates when the mutual benefits outweigh the costs (Grinnell et al. 1995). Simi- larly, wolves, Canis lupis, preferentially kill white-tailed deer, Odocoileus virginianus, in areas with increased stalk- ing cover (Kunkel & Pletscher 2001). Wild chimpanzees have a diet composed chiefly of fruit and leaves, but they also capture and eat a variety of mammals, birds and other vertebrates (Wrangham 1977; Nishida et al. 1979; Goodall 1986), preying most frequently upon red colobus monkeys, Colobus badius (Uehara 1997; Mitani & Watts 2001). Recent studies of the factors that influence chimpanzee hunting decisions and success have empha- sized the potential social benefits of hunting. Selective meat sharing may yield increased mating opportunities and/or aid in the maintenance of cooperative intrasexual relationships, thus providing added incentives for hunt- ing. However, ecological factors are also likely to be Correspondence and present address: I. C. Gilby, Department of Anthropology, Harvard University, 11 Divinity Ave, Cambridge, MA 02138, U.S.A. (email: gilby@fas.harvard.edu). L. E. Eberly is at the Division of Biostatistics, School of Public Health, University of Minnesota, Minneapolis, MN 55455, U.S.A. L. Pintea is now at The Jane Goodall Institute Africa Programs, 4245 N. Fairfax Drive, Arlington, VA 22203, U.S.A. A. E. Pusey is at the Jane Goodall Institute’s Center for Primate Studies, Department of Ecology, Evolution and Behavior, University of Minnesota, St Paul, MN 55108, U.S.A. 169 0003–3472/06/$30.00/0 Ó 2006 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. ANIMAL BEHAVIOUR, 2006, 72, 169–180 doi:10.1016/j.anbehav.2006.01.013