The 82nd Annual Meeting of the American Association of Physical Anthropologists (2013)


Intergroup aggression and within-group cohesion in wild chimpanzees

MARTIN N. MULLER1, ZARIN P. MACHANDA2, DREW K. ENIGK1 and RICHARD W. WRANGHAM2.

1Anthropology, University of New Mexico, 2Human Evolutionary Biology, Harvard University

Thursday 2:30-2:45, Ballroom B Add to calendar

Aggressive competition between groups has long been proposed as a major factor selecting for human cooperation. If cohesion between group members is important for success in intergroup competition, then increased levels of conflict should favor increased levels of cooperation. Although several theoretical models support this idea, there is little empirical work testing it directly. In this study, we used 7 years of data from the Kanyawara community in Kibale National Park, Uganda, to examine the impact of intergroup conflict on affiliative behavior among male chimpanzees. Chimpanzees represent an appropriate study species because males are territorial, cooperatively defending a feeding range against other groups. Kanyawara represents an ideal study site, because intercommunity interactions are strongly tied to the consumption of ripe fruit from a single tree species, Uvariopsis congensis. These synchronously fruiting trees produce high-quality fruits in large groves clustered along the boundary with a neighboring community. Consequently, more than 75% of intergroup interactions at Kanyawara occur when chimpanzees are eating Uvariopsis. If external threats lead to reduced competition within groups, and increased levels of social bonding, we hypothesized that during periods of Uvariopsis consumption: (1) adult males would show increased levels of proximity with other adult males, (2) rates of male-male grooming would increase, (3) males would spread their grooming more equitably among available male partners, and (4) both the rate and intensity of male-male aggression within the group would be reduced. Our results have implications for the idea that intergroup aggression has selected for extensive cooperation in humans.

This research was supported by the National Science Foundation (Grants 0416125 and 0849380), and the Leakey Foundation.

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