Elaborate experimentation reveals that for chimpanzees friendship is based on trust.

Biology – January 16th, 2016 – Meta
Pan troglodytes
Let’s play the trust game. You fall back and hope someone catches you. Would it matter if a friend or non-friend is waiting behind you? Jan Maxim Engelmann and Esther Herrmann from the Max Planck Institute for Evolutionary Anthropology put groups of chimpanzees to a test of trust. Their findings, “Chimpanzees Trust Their Friends,” appeared yesterday in Current Biology.

Trust is very important from an evolutionary standpoint for any species that relies heavily on cooperation. And trust by definition is risky, with the potential for being let down. Humans help mitigate some of this risk by forming close bonds.

“Little is known about the evolutionary origins of the human tendency to form close social bonds to overcome the trust problem,” mention the authors.

So they turned to one of our nearest relatives, chimpanzees (Pan troglodytes), to help find some clues.

To get started, they observed 15 chimpanzees at Sweetwaters Chimpanzee Sanctuary in Kenya. All but two have been with the group since the mid-to-late 1990s. Four researchers collected observational data from December 2014 to May 2015 on activities like grooming, co-feeding, physical contact and arms-length proximity.

Chimps by nature are social animals. However, from 352 hours of collected data, the researchers were able to determine close one-on-one relationships within the group; those chimps were considered friends.

Next, the chimps participated in a modified version of a trust game. In 12 trials with friends and 12 trials with non-friends, subjects were able to pull a no-trust rope which gave them immediate access to a less-preferred food or a trust rope which sent a preferred food that could then be shared to the counterpart. Like a human study on trust, this one allowed the subject the option to invest in his or her counterpart for a better, shared reward.

Chimpanzee trust game

This is a modified version of the human trust game, subjects had a choice between pulling the trust rope and the no-trust rope. Credit: Engelmann and Herrmann | Current Biology 2016

The results showed that chimpanzees trust their friends significantly more frequently than their non-friends.

“While the previous study revealed that chimpanzees show basic trust in members of their social group, the current study highlights the relevance of close social relationships to trusting attitudes,” say the authors.

The researchers also looked at reciprocated trust. Human relationships, especially friendships, can tolerate inequalities in trust and reciprocity. A second finding of the study was that both friends and non-friends were equally likely to reciprocate trust. This may be attributed to the “low cost” set up of the study, having a chimpanzee share a resource that it did not actually own. But the chimpanzees in this study did not seem to be concerned with immediate reciprocation among friends; even while the friends did not reciprocate more than non-friends.

According to the authors, it is possible that this trust may be recognized not as strategic but as “emotional trust.” “Trust within close relationships is not unique to humans,” they write. And when playing the trust game, it may turn out that each of us, human or primate, would prefer to have a friend waiting to catch our fall.

Kimberly Hatfield | Meta Staff Writer

Featured image: The Sweetwaters Chimpanzee Sanctuary is located within the Ol Pejeta Conservancy and is the only place in Kenya where this highly endangered species can be seen. There are currently 42 chimpanzees at the Sanctuary. Credit: Valentina Storti | Flickr