UC Santa Barbara

Are chimpanzees able to inhibit anti-social behavior towards their friends? And how do they react when a friend acts anti-socially towards them? Past work has demonstrated that chimpanzees are selectively prosocial towards their friends: they groom them more, help them more, and share more food with them. However, the influence of friendship on anti-social behavior remains underexplored. Here, we use food theft as a test case to examine (1) the influence of friendship on anti-social behavior and (2) emotional responses to anti-social behavior. We assigned 26 chimpanzees to pairs of friends (friend condition) and non-friends (non-friend condition) based on 800 h of observations of naturally occurring interactions. The pairs then participated in a stealing task, in which one chimpanzee (the “thief”) could pull a tray with pieces of pineapple, on which the other was feeding (the “victim”). Friends and non-friends were matched on sex and dominance rank. We found that chimpanzees stole less from their friends. When theft did occur, chimpanzees tended to initially express more negative emotions in the friend condition than in the non-friend condition, but then attenuated this response with increasing trial number. Thieves, meanwhile, expressed more negative emotions in the non-friend condition, suggesting that they treated interactions with non-friends as more hostile. These results speak to the complex role of friendship in regulating anti-social behavior and emotional responses to it.