Here you are!—Selective and active food sharing within and between groups in captive Sumatran orangutans (Pongo abelii)

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A variety of studies on food sharing elucidate both its ultimate and proximate functions in non-human primates, especially in Pan. For chimpanzees, food sharing serves as a means to strengthen social relationships. In contrast, little is known about food sharing in orangutans, since their semi-solitary lifestyle barely provides an opportunity to share food outside of the mother-offspring context. However, recent long-term studies suggest that social bonding might play a more important role for orangutans than previously assumed. In zoos, orangutans are often kept in groups and seem to cope with group living quite well. If captive orangutans use food sharing as a social tool, they are expected to share food frequently and selectively with close social partners and to engage frequently in active transfers. We provided three orangutan groups with monopolizable food and recorded all dyadic food-related interactions. For each dyad, we determined the relationship quality and tested whether it predicts food sharing. We found that, in support of our predictions, almost two thirds of interactions involving food resulted in sharing and that the probability for an individual to share food with a particular partner increased with the strength of their relationship. Exceeding our expectations, food sharing occurred even between individuals from two neighboring groups. Finally, a comparison with studies on captive chimpanzees revealed a significantly higher proportion of active transfers for orangutans suggesting species-specific sharing psychologies. Sharing of food is a universal prosocial behavior in humans. Recent research aims to elucidate its adaptive functions and proximate mechanisms by comparison with other species, especially non-human primates, in natural and captive settings. For bonobos and chimpanzees, our closest relatives, the quality of social relationships was revealed to be important for food sharing. In contrast, there is very limited knowledge on food sharing in orangutans, our most distant and semi-solitary living hominid relatives. This study provides the first systematic investigation of food-sharing patterns and the role of relationship quality in captive orangutans. The results demonstrate that group-living orangutans share frequently and selectively with close associates and even more actively than found for chimpanzees. These findings add further evidence supporting the hypothesis that social bonding played a role in the evolution of human prosocial behavior.