Reciprocity and interchange in the social relationships of wild male chimpanzees

Abstract

Social relationships in nonhuman primates result from investments that individuals make while pursuing fitness-maximizing strategies. These strategies sometimes include social exchange, either reciprocity (exchange of the same acts) or interchange (exchange of different acts). Individuals in many species may negotiate for services in biological markets, particularly grooming and agonistic support. They also may compete for access to valuable social partners. Abundant evidence for reciprocity in grooming and in support and for competition over partners exists, notably for females in some cercopithecines. However, evidence for interchange of grooming and support is scarcer, and apparent interchange may be a byproduct of correlations between grooming or support and some third variable (e.g. dominance rank). Chimpanzees have been prominent in discussions of social exchange, especially because male chimpanzees cooperate in many ways. Most analyses of interchange have used data on captive chimpanzees; these provide good evidence for reciprocity, but ambiguity with regard to interchange. Data on an unusually large chimpanzee community at Ngogo, Kibale National Park, Uganda, strongly support the argument that social exchange is prominent in social relationships among males. Males at Ngogo show reciprocity in grooming and support. They also interchange grooming given and support received, as well as grooming received and support given, independently of reciprocity in grooming and support and of correlations of support and grooming with dominance rank. However, most cooperation in contests with third parties took low risk forms (e.g. both participants outranked their opponent). In this, males at Ngogo resemble captive chimpanzees and female cercopithecines. Reciprocity and interchange in this context may be important in the maintenance of social bonds between males, and in attainment and maintenance of high dominance rank, but probably represent mutualism, not reciprocal altruism.