Chimpanzees form friendships largely based on trust, researchers from the Max Planck Institute for Evolutionary Anthropology revealed in a new study. It turns out that this form of social bonding evolved millions of years ago and dates back to at least the last common ancestor of chimps and humans.

"Humans largely trust only their friends with crucial resources or important secrets," researcher Jan Engelmann said in a news release. "In our study, we investigated whether chimpanzees show a comparable pattern and extend trust selectively toward those individuals they are closely bonded with. Our findings suggest that they do indeed, and thus that current characteristics of human friendships have a long evolutionary history and extend to primate social bonds."

Previous studies have found that chimpanzees engage in relationships that resemble human friendships, and in the latest study, researchers attempted to answer whether chimpanzees only granted favors to select individuals; for instance, those they trusted. To do this they observed 15 chimpanzees living at Sweetwaters Chimpanzee Sanctuary in Kenya over a five month-long period. Based on their friendly interactions, which included grooming and eating together, researchers identified each chimpanzee's closest "friend" and a "non-friend."

Next, researchers asked the chimps to play a modified version of what's known as the human trust game, in which two chimps stood on opposite sides of a contraption that held boxes of food. One of the boxes contained two pieces of banana, which chimps consider an OK snack, while the other box held three pieces of banana and three pieces of apple – a much better snack.

Each chimp had a choice: to take the less desirable snack for himself – no sharing – by pulling a "no-trust rope;" or to send the better snack to his friend – hoping he would share – by pulling a "trust rope."  The trust rope resulted in a win-win – but only if chimp one trusted chimp two enough to return the favor (sharing the meal). Each chimp played the game 12 times with his or her friend and a non-friend. (Scroll to read more...)

Researchers found chimps trust their BFFs to share and routinely pulled the trust rope. But that wasn't the case when chimps were forced to interact with mere acquaintances. In such cases they frequently opted to take the sure bet and dine alone.

All of this suggests human friendship is not as unique as previously thought.

"Human friendships do not represent an anomaly in the animal kingdom," Engelmann added in the institute's release. "Other animals, such as chimpanzees, form close and long-term emotional bonds with select individuals. These animal friendships show important parallels with close relationships in humans. One shared characteristic is the tendency to selectively trust friends in costly situations."

The study was recently published in the journal Current Biology

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