Grooming reciprocity in wild male chimpanzees

Abstract

Understanding cooperation between unrelated individuals remains a central problem in animal behaviour; evolutionary mechanisms are debated, and the importance of reciprocity has been questioned. Biological market theory makes specific predictions about the occurrence of reciprocity in social groups; applied to the social grooming of mammals, it predicts reciprocity in the absence of other benefits for which grooming can be exchanged. Considerable effort has been made to test this grooming trade model in nonhuman primates; such studies show mixed results, but may be confounded by kin effects. We examined patterns of reciprocity within and across bouts, and tested predictions of the grooming trade model, among wild male chimpanzees, Pan troglodytes: a system with negligible kin effects. In accord with the model’s expectations, we found that some grooming was directed by lower- to higher-ranked individuals, and that, on average, higher-ranked individuals groomed more reciprocally. We found no support, however, for a prediction that more reciprocity should occur between individuals close in rank. For most dyads, reciprocity of effort occurred through unbalanced participation in grooming bouts, but reciprocity varied considerably between dyads and only a small proportion showed strongly reciprocal grooming. Despite this, each male had at least one reciprocal grooming relationship. In bouts where both individuals groomed, effort was matched through mutual grooming, not alternating roles. Our results provide mixed support for the current grooming trade, biological market model, and suggest that it needs to incorporate risks of currency inflation and cheating for species where reciprocity can be achieved through repeated dyadic interactions. 2010 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. Altruistic acts are behaviours that benefit the recipient at an apparent cost to the actor (Trivers 1971). The theory of reciprocal altruism holds that such acts can evolve by natural selection if the roles of actor and recipient are reciprocated such that both incur costs (albeit offset in time) and obtain net fitness benefits. Evidence for the role of this ‘direct reciprocity’ in shaping altruistic behaviour among animal species is limited, however (Clutton-Brock 2009). Originally modelled using the Prisoner’s Dilemma (Trivers 1971), increasingly sophisticated game-theoretical approaches have been developed to accommodate more realistic assumptions. In the Prisoner’s Dilemma, individuals are constrained into interacting with a particular partner, and have only a choice between cooperation or noncooperation, with the value of cooperation equal to both partners (Trivers 1971). In contrast, for animals living in social groups, choice of partners for social interactions is common (Wilkinson 1984; Noe et al. 1991; Emery 2004; Connor 2007;