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Sarah F. Brosnan and Redouan Bshary

Despite the fact that most models of cooperation assume equal outcomes between individuals, in real life it is likely rare that this is the case. Does it make a difference for our understanding of the evolution of cooperation? Following a taxonomy of cooperation concepts that focuses on costs and benefits, we explore this question by considering the degree to which inequity aversion may provide one mechanism to stabilize cooperation. We suggest a key role for inequity aversion in some contexts in both biological markets and direct reciprocity, and highlight the potentially unique role of positive inequity aversion for human reputation games. Nevertheless, a key challenge is to determine how different animal species perceive the payoff structure of their interactions, how they see their interaction with their partners, and the degree to which simpler mechanisms, like contrast effects or the associative learning seen in optimal foraging, may produce similar outcomes.

Sarah F. Brosnan, Catherine F. Talbot, Jennifer L. Essler, Kelly Leverett, Timothy Flemming, Patrick Dougall, Carla Heyler and Paul J. Zak

Recent evidence indicates that oxytocin plays an important role in promoting prosocial behaviour amongst humans and other species. We tested whether oxytocin affected cooperation and food-sharing in capuchin monkeys, a highly cooperative New World primate. Subjects received either 2IU oxytocin or an inert adjuvent intranasally prior to each session. Oxytocin influenced food sharing in capuchins in ways we did not anticipate. Recipients were less likely to passively acquire food from possessors when either individual had received OT than in the control, and also spent less time in proximity to their partner. Passive food sharing requires proximity, and oxytocin decreased the capuchins’ typical congregating behaviour, apparently resulting in decreased sharing. We propose that the likely mechanism for increased social distance is the known anxiolytic effect of oxytocin. Our results indicate a need to consider how oxytocin affects the context of interactions and interacts with modes of sociality unique to each species.