Chimpanzees Steal Less From Their Friends and Expect Them to Do the Same

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 in anti-social behavior remains underexplored. Here, we use food theft as a test case to examine the influence of friendship on anti-social behavior – and emotional responses to it. We assigned 26 sanctuary-living chimpanzees to pairs of friends and non-friends based on 20 months of observational data. The pairs then participated in a stealing task, in which one chimpanzee (the “thief”) could pull a tray with pieces of pineapple, which the other was feeding on (the “victim”). Theft was low cost since the chimpanzees were separated by an inaccessible corridor. Each victim was assigned to one friend and one non-friend thief (46 pairs total) and each pair participated in one session of three trials. Chimpanzees stole less from their friends. When theft did occur, chimpanzees initially expressed more negative emotions in the 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 emotion in chimpanzee friendship.