Abstract
Humans are thought to be unique in their ability to voluntarily help others even though it may sometimes incur substantial costs. However, there is growing number of studies for prosociality among non-human primates, especially in chimpanzees and capuchins, two species living in complex societies. Prosociality has often been described as a major factor that facilitates group living. Nonetheless, it has never been explored whether solitary living primates, such as orangutans, share those propensities or not. Here, we tested captive orangutans, using an apparatus which allows them to make either a prosocial (both the actor and recipient receive rewards) or a selfish choice (only the actor receives a reward). Two orangutans, who passed the criterion to become an actor, played the actor’s role, and 4 orangutans, including the 2 actors, participated as recipients. When the orangutans were tested without a recipient, to confirm if they understood the apparatus, they chose the double rewarding option significantly more often than expected by chance (87.3% on average) to get food rewards from both the recipient and the actor’s side. However, when they were paired, they did not show any significant prosocial bias towards potential recipients (51.3% on average). Our results suggest that these orangutans were not as sensitive to others’ welfare, compared to group living primates. This study is consistent with previous findings suggesting that a prosocial propensity is elicited by living in complex social environments.
