Direct and generalized reciprocity in the cooperative exchanges of free living Rhesus macaques on Cayo Santiago

Abstract

Direct reciprocity is the most cited explanation for the evolution of cooperation between unrelated individuals. Yet, few studies have examined generalized reciprocity as an alternative mechanism, despite generalized reciprocity’s minimal cognitive demands and capacity to explain the same patterns of cooperation as direct reciprocity. Studies of direct reciprocity tend to explore reciprocal exchange across either individual interactions or long periods of time, rarely studying both at once. In addition, most studies of reciprocity only account for maternal relatedness between cooperative partners, failing to rule out the effect of kin selection through paternal relatedness. In this thesis we investigated the role of direct and generalized reciprocity in female rhesus macaque cooperative exchanges. We modified a classic playback experiment and explored the effect of both recent and repeated grooming interactions on females’ willingness to provide coalitionary support to other adult females. We conducted this study using the rhesus macaques at the Cayo Santiago field station, where maternal and paternal relatedness is known for all study subjects. We found no evidence of direct or generalized reciprocity in the exchange of grooming for coalitionary support. Our results contradict the findings of similar playback experiments but are consistent with correlational studies in species that are despotic like rhesus macaques. The despotic nature of our study system, combined with our finding that females’ willingness to provide support declined as the number of infants in their matriline increased, could suggest that grooming is not a sufficient price to pay to acquire coalitionary support from unrelated rhesus macaques. We propose by-product mutualism and kin-selection may play a larger role than has been previously appreciated in decisions regarding who to support in agnostic encounters, and we highlight the importance of investigating multiple cooperative mechanisms when studying of the evolution of cooperative behaviours.