Grooming for tolerance? Two mechanisms of exchange in wild tufted capuchin monkeys

The strategies used by individuals to deploy altruistic behaviors have long captured research attention. Two general mechanisms can account for the decision-making process underpinning the deployment of altruistic behaviors among nonkin. The first mechanism, referred to here as “temporal relation between events,” corresponds to classical reciprocal altruism; as such, it is strictly within-dyad and has a strong temporal component. The second mechanism, labeled here as “partner choice based on benefits received,” relies on across-dyad comparisons. Although these 2 mechanisms are both theoretically plausible and are not mutually exclusive, very little is known about their relative importance in the deployment of altruistic behaviors. To partially fill this gap, we explored the occurrence of exchanges between grooming and tolerance by assessing the roles of both temporal relations between events and of partner choice based on benefits received in wild tufted capuchin monkeys (Cebus apella nigritus). Data on grooming and cofeeding on contestable resources (as a measure of tolerance) were collected both in natural contexts and during supplemental feeding on provisioning platforms. Overall, our results document reciprocal exchanges between grooming and tolerance over food resources and thus represent the first evidence in wild New World primates for partner choice based on benefits received. More interestingly, partner choice was found to drive the reciprocal exchange much more strongly than the temporal relation between events. Our findings suggest that the role of partner choice based on benefits received should be given much more attention in studies investigating the mechanisms underlying altruism among nonkin.