1Department of Anthropology, University of Minnesota, 2Department of Ecology, Evolution and Behavior, University of Minnesota, 3Department of Structural Biology, Stanford University, 4Gombe Stream Research Centre, the Jane Goodall Institute, 5Vaccine Research Center, National Institutes of Health, 6Departments of Medicine and Microbiology, University of Pennsylvania, 7Department of Evolutionary Anthropology, Duke University
Thursday 2:45-3:00, Ballroom B
For male chimpanzees, each additional male group member poses a trade-off in terms of reproductive benefits and costs. Groups with more males are more likely to win intergroup contests, and thus defend larger territories, providing more food for self, mates and offspring. At the same time, each additional male represents increased competition for mating opportunities. Here we use observational and genetic data to examine these tradeoffs in the neighboring Mitumba and Kasekela communities in Gombe National Park. Coalition size had important consequences: as the Kasekela males increased their numerical advantage, they spent a greater proportion their time ranging within historic Mitumba range (N=14 years; linear regression: R2=0.53; P=0.003). Despite this, two of the Mitumba males (Rudi and Edgar) reduced their coalition strength by killing their group’s other adult male, Vincent. During Vincent’s tenure as alpha male (1996-2004), he sired 4 of 6 known paternities, while Edgar and Rudi each obtained only one. After killing Vincent, only Rudi and Edgar sired offspring, obtaining 4 and 2 paternities, respectively. Both of the attacking males thus increased their proportion of paternities after killing Vincent – but suffered costs, including not only increased competition for territory, but also the death of an infant killed during an intergroup encounter. These findings underline the intensity of reproductive competition among male chimpanzees, and suggest that under certain conditions, chimpanzees may be more sensitive to the short-term gains obtained from eliminating within-group rivals than to the resulting long-term costs of reducing their coalition size for intercommunity competition.
Data collection was supported by the Jane Goodall Institute, construction of the long-term database was supported by grants from the NSF (DBS-9021946, SBR-9319909, BCS-0648481, BCS-0452315, IOS-LTREB-1052693), genetic analyses were supported by grants from the NIH (R01 AI50529, R01 AI58715, P30 AI 27767), and Wroblewski was supported by a Ruth L. Kirschstein National Research Service Award (NIH F32 AI085959-03).