Ecological and social influences on the hunting behaviour of wild chimpanzees (Pan troglodytes schweinfurthii)
Author(s):
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;
† Division of Biostatistics, School of Public Health, University of Minnesota
We used 25 years of data from Gombe National Park, Tanzania, to examine the factors affecting male chimpanzees’ decision to hunt red colobus monkeys. Our results provide evidence against social explanations for hunting in favour of more simple ecological alternatives. Upon encountering red colobus monkeys, male chimpanzees were less likely to hunt if sexually receptive female chimpanzees were present, indicating that they were not seeking meat to trade for sex. Instead, they may be forced to choose between hunting and mating. Males did not seek meat to trade with other males for social support, and hunting probability was not affected by diet quality. Hunts were more likely to occur, and to succeed, in woodland 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 males did not benefit nutritionally by hunting cooperatively.
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Correspondence: I.C. Gilby,
Dept. of Anthropology, Harvard University, 11 Divinity Ave, Cambridge, MA 02138, USA.
gilby@fas.harvard.edu
There has been considerable discussion of the factors that influence the hunting behaviour of male chimpanzees, Pan troglodytes. Explanations invoking social benefits hinge upon the potential for trading meat with sexually receptive females for mating (“meat-for-sex”) or with males in return for social support (“male social bonding”). Ecological factors may also affect hunting probability: 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, P. t. schweinfurthii, 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 tradeoff 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 hypothesis. 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.
