Behavioral scientists of the German Primate Center cooperating with colleagues of the Universities of Portsmouth and Bogor have found out that crested macaques react faster if threatened by predators when a group member they share close social bonds with calls for help. In order to study this behavior, the scientists led by Antje Engelhardt, head of the junior research group Primate Sexual Selection, recorded recruiting calls of the monkeys. The macaques utter them, when predators like pythons are in sight.
By these calls they attract group members, which cooperatively drive the snake away. Hereafter the scientists replayed the recordings to different individuals of the group in order to document their reactions. Calls of "friends" as well as calls of less close group members were played to the macaques. When they heard calls of befriended monkeys, they reacted substantially faster than to the other calls. The study also sheds light on the evolution of social relationships in humans, the researchers conclude.
Hard to say if the scientists of the junior research group "Primate Sexual Selection" at the DPZ ever imagined they would have to mimic a snake to reach their scientific aims. They probably didn't. But only this way the researchers in Antje Engelhardt's group in the field station Tangkoko in Indonesia were able to study the effect of social bonds on how the macaques react to predators. Holding a life-size model of a python they hid behind a tree and subsequently showed the cardboard-snake to different macaques. Colleagues meanwhile recorded the calls with which the macaques reacted to the sight. Attracted by these "recruiting calls," members of the macaques' group usually join the caller to drive away the predator, in this case the fake snake.
The behavioral scientists tried to find out, whether macaques react differently to calls from socially close group members: The scientists replayed the recorded calls to different individuals of the group, once using the call of a socially close group member, once using the call of a more distant one.
Using video cameras the researchers then recorded the reaction and analyzing these found out: When a socially close macaque sounded the recruiting call, the others reacted more promptly than when the call came from a "non-friend."
"Our results show, that close social bonds which surpass kinship played an important role even before the evolution of the human species," Antje Engelhardt says. "Cooperative defense against predators seems to be one of the benefits within this context."
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