A chimpanzee with a taste for pineapple had to make a choice: grab the fruit or leave it for a friend. In a new study, published in ScienceDirect’s Evolution and Human Behavior journal, chimpanzees were less likely to steal pineapple from friends than from non-friends, suggesting that close social bonds can influence anti-social behaviour as well as cooperation. Researchers studied 26 chimpanzees at the Ngamba Island Chimpanzee Sanctuary in Uganda. They first observed the animals for 800 hours to identify their strongest social relationships, then tested whether those relationships affected their willingness to steal food. Friends got more restraint Researchers identified each chimpanzee’s closest social partner using measures including grooming, physical contact, co-feeding and proximity. They then matched each chimpanzee with a non-friend of the same sex and similar dominance rank. The animals took part in a pineapple-stealing task. One chimpanzee, the “victim,” received a tray with three pieces of pineapple. Another chimpanzee, the “thief,” could pull a rope attached to the tray and bring the remaining food within reach. Across 138 trials, the chimpanzees stole the tray 65 per cent of the time. Theft occurred in 61 per cent of trials involving friends and 70 per cent involving non-friends. The difference was statistically significant, indicating that the chimpanzees were less likely to steal from their friends. The animals’ interest in the food was clear during familiarization trials. When no victim was present, every thief pulled the rope and retrieved the pineapple. That means the lower theft rate in the friend condition occurred despite the food remaining available as a reward. Friendship also shaped emotional responses The researchers recorded several behaviours associated with negative emotional states, including scratching, banging, screaming, barking and whimpering. Among chimpanzees who were robbed, the emotional response changed over repeated trials depending on the relationship between the animals. Victims showed a decline in their combined negative-affect measure across trials when the thief was a friend. Negative affect refers to signs that an animal is experiencing an unpleasant or uncomfortable emotional state. There was no comparable change across trials when the thief was a non-friend. An exploratory analysis focusing on the period immediately surrounding the theft found a similar pattern, with the interaction also appearing in measures of screaming, whimpering and banging. The researchers caution that the results do not establish exactly why victims’ responses changed over repeated encounters. They note several possible explanations and say further research is needed to distinguish between them. The thieves responded differently, too The chimpanzees doing the stealing also showed different levels of negative affect, depending on who they were stealing from. Thieves displayed more negative affect when stealing from non-friends than when stealing from friends. The difference in the combined measure was statistically significant, and screaming was also higher in the non-friend condition. The researchers say the findings add to previous evidence that chimpanzee friendships are associated with behaviours such as helping, trust, consolation and food sharing. This study adds another piece to that picture: friendship was also associated with a lower likelihood of stealing and with differences in emotional responses during theft. The results suggest that chimpanzee friendships can affect both cooperative and anti-social behaviour. They also provide evidence that close relationships are relevant to how chimpanzees respond during potentially costly social interactions.
