Friend or foe
Chimpanzees keep track of other group members’ bonding partners and use this knowledge in conflict situations
To know who your opponents’ family and friends are can be of advantage in a conflict situation. Humans make predictions about other people’s social relationships frequently. Whether other animals also have the cognitive skills to track their group mates’ social relationships across time and beyond close kin has so far not been known. Researchers of the Max Planck Institute for Evolutionary Anthropology in Leipzig have now conducted playback experiments with wild-living chimpanzees from Budongo Forest in Uganda. Two hours after a subject attacked or had been attacked by an opponent, the researchers broadcast the recording of a third individual’s aggressive barks from a speaker near the subject. If the call provider was their opponent’s close buddy or kin, subjects looked longer and moved away more often from barks than if the call provider was not a bond partner. This shows that chimpanzees know who their group mates’ kin or non-kin bond partners are and that their behavior may have an impact on them.
Socially living animals cooperate with bonding partners to gain an advantage over others, for example in a fight, when it comes handy to predict who another group member’s potential supporters are. In chimpanzees, the presence of a bonding partner may affect the balance of power between competitors in an aggressive conflict, the outcome of which can increase rank and mating opportunities. Whether chimpanzees are able to keep track of other group member’s kin and non-kind bonding partners has now been subject of a study by Roman Wittig and colleagues of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. “We investigated whether chimpanzees could link two separate aggressive events involving two different opponents by the quality of the relationship between the two opponents, even if the two events were separated in time”, says Roman Wittig.
To this end, the researchers selected 16 wild-living chimpanzees from the Budongo forest in Uganda as subjects to take part in two playback experiments. As playback stimuli the researchers used chimpanzee vocalizations – aggressive barks and friendly calls – that they had previously recorded in the same group. About two hours after a subject had either attacked or been attacked by an opponent, the researchers simulated the arrival of a third individual that had not been present in the fight by broadcasting his or her aggressive barks in the vicinity of the subject. They videotaped the subject’s reaction and found that it differed depending on the call provider’s relationship to the opponent. “Subjects looked longer to the speaker and were more likely to move away from it, if confronted with aggressive barks of an opponent’s bond partner”, says Wittig. “When opponent and call provider were not bond partners, subjects were more likely to move towards the speaker or parallel to it”.
In a second playback experiment, the researchers broadcasted a friendly call of the former opponent’s bonding partner about 90 minutes after a fight had taken place. Then, another hour later, they broadcasted aggressive barks from the same individual. “Subjects were neither averse to the call provider per se, nor to their presence after fighting with their bond partner”, says Wittig. “They showed aversion only when the call provider was a bond partner of their former opponent and was aggressively motivated.”
The researchers conclude from the two experiments that the subjects remembered the original fight and linked it to the knowledge of their opponent’s close relationships. “Impressively, subjects expected aggression from an opponent’s bonding partner when hearing his or her barks hours after the fight had taken place”, says Wittig. “Chimpanzees are aware of the possible impact others’ behavior has on them and can do this by remembering and linking specific social interactions with different individuals across time.”