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Chimps form friendships based on trust: The banana-sharing test

Trust-based relationships may offer chimpanzees a number of evolutionary benefits, according to new research.

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    Two chimps walk together at Chimp Haven in Keithville, La., in 2013.
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A new study suggests that chimps, like humans, get by with a little help from their friends.

Over five months, researchers observed 15 chimps living at the Sweetwaters Chimpanzee Sanctuary in Kenya. They concluded that chimpanzees form close friendships, built on trust.  The study, led by Jan Engelmann and Esther Herrmann, both of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, appeared Thursday in the journal Current Biology.

“We were very surprised by our findings since some researchers depict chimpanzee social life as dominated by conflict, competition, and dominance – whereas our research suggests that chimpanzees are able to form friendships that are based on trust,” Dr. Engelmann says.

To demonstrate their hypothesis, researchers identified “friend” and “non-friend” pairs based on interactions such as mutual grooming and eating. Then Engelmann and Dr. Herrmann adapted a human trust game for their primate subjects.

In this game, each chimpanzee can either pull a “trust rope” or a “no-trust rope.” The no-trust rope yields immediate access to food that the chimp doesn't particularly like. But if the chimp pulls the trust rope, a box of high-quality food – chimpanzee favorites like apples and bananas – moves to its partner. The partner eats half, but is then faced with a decision. A “trustworthy” chimp will send the other half of the food back to its partner, while an “untrustworthy” chimp will keep the food for itself.

“The no-trust option is safe, but contains low-quality food,” Engelmann says. “The trust option, on the other hand, is risky but potentially has high-payoffs. The trust option is [also] risky because the partner might not send the food back, leaving the chimpanzee empty-handed.”

Engelmann and colleagues found that while chimpanzees were relatively unwilling to extend trust towards their non-friends, they were significantly more likely to share with friends. So friendship, like many supposedly human concepts, may be deeply rooted in evolutionary history.

“Research with other primates, baboons, has shown that friendships entail important evolutionary benefits,” Engelmann says. “Individuals with friends live longer, have more children, and [have] lower stress-levels. I can imagine that the same benefits apply to chimpanzees.”

Some non-primates appear to form friendships too. Dolphins and elephants, for example, have been observed maintaining these close social bonds. In these cases, the benefits are likely the same – increased chances of survival and more offspring.

Scientists are learning more and more that non-parental and non-sexual partnerships are somewhat common in the animal world. But there are still complex behaviors associated with friendship that have yet to be explored in nonhumans.

“We want to further investigate the similarities between chimpanzee and human friendships,” Engelmann says. “For example, do chimpanzees also preferentially help and share with their friends?”

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