Fitness benefits of coalitionary aggression in male chimpanzees
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Coalitionary aggression occurs when at least two individuals jointly direct aggression at one or more conspecific targets. Scientists have long argued that this common form of cooperation has positive fitness consequences. Nevertheless, despite evidence that social bond strength (which is thought to promote coalition formation) is correlated with fitness in primates, cetaceans, and ungulates, few studies have directly examined whether coalitionary aggression improves reproductive success. We tested the hypothesis that among free-ranging chimpanzees (Pan troglodytes schweinfurthii), participation in coalitionary aggression increases reproductive output. Using 14 years of genetic and behavioral data from Gombe National Park, Tanzania, we found that coalitionary aggression increased a male’s chances of (A) siring offspring, compared to other males of similar dominance rank, and (B) ascending in rank, a correlate of future reproductive output. Because male chimpanzees form coalitions with many others within a complex network, we used social network analysis to identify the types of connections correlated with these fitness benefits. The beneficiaries of coalitionary aggression were males with the highest “betweenness”—that is, those who tended to have coalition partners who themselves did not form coalitions with each other. This suggests that beyond simply recognizing third-party relationships, chimpanzees may use this knowledge to choose coalition partners. If so, this is a significant step forward in our knowledge of the adaptive value of social intelligence. Regardless of mechanism, however, this is the first evidence of genetic benefits of coalitionary aggression in this species, and therefore has important implications for understanding the evolution of cooperation.
KeywordsCoalition Chimpanzee Social network analysis Cooperation Paternity Dominance rank Social bonds
Data collection was funded by the Jane Goodall Institute. Digitization and analysis of behavioral data were supported by grants from the National Science Foundation (DBS-9021946, SBR-9319909, BCS-0452315, LTREB-1052693), the University of Minnesota, the Harris Steel Group, the Windibrow Foundation, the Jane Goodall Institute, the Carnegie Corporation, Minnesota Base Camp and Duke University. Genetic analyses were supported by grants from the National Institutes of Health (R01 AI50529, R01 AI58715, P30 AI 27767). We thank TANAPA, TAWIRI, and COSTECH for permission to work in Gombe National Park, the Gombe Stream Research Center staff for maintaining data collection, and Richard Wrangham, Michael Platt, Daniel Schmitt, Julie Horvath, David Watts, and two anonymous reviewers for comments on earlier versions of this manuscript.
This research complies with the current laws of Tanzania.
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