Like Humans, Chimpanzees Choose Friends Based on Similar Personalities

Many animals have close and stable friendships, rather like humans. Yet exactly how individuals make these bonds has remained unclear--until now. Scientists have discovered that chimpanzees actually choose their friendships based on similarity of personality, revealing a little bit more about how animals are able to create relationships.

In order to better understand the bonds that animals form with one another, the researchers measured chimpanzee personality in two zoos with behavioral experiments and years of observations of chimpanzee behavior. The researchers also logged which chimpanzee sat in body contact with whom most.

"This is a clear sign of friendship among chimpanzees," said Jorg Massen, one of the researchers, in a news release.

That wasn't all the scientists did, though. After recording which chimpanzees were friends, they then examined their personality types. More specifically, they tested whether chimps that sat together had similar or differing types.

"We found that, especially among unrelated friends, the most sociable and bold individuals preferred the company of other highly sociable and bold individuals, whereas shy and less sociable ones spent time with other similarly aloof and shy chimpanzees," said Massen in a news release.

So why would chimpanzees choose to have friends that are like them? The researchers have a theory, though they still need to test it. It's very possible that such a strong preference for self-like individuals is adaptive, because frequent cooperation becomes more reliable when both partners have similar behavioral tendencies and emotional states.

In fact, the findings strongly resemble the known "similarity effect" in humans. We tend to make friends with people who are equally extraverted, friendly and bold as ourselves. This, in turn, reveals a little bit more about the similarities between humans and chimps.

"It appears that what draws and keeps both chimpanzee and human friends together is similarity in gregariousness and boldness, suggesting that preference for self-like friends dates back to our last common ancestor," said Massen in a news release.

The findings are published in the journal Evolution and Human Behavior.