Abstract
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.
