The research suggests that the urge to develop close friendships has been hardwired into human DNA for millions of years.

Like humans, many animals have close and stable friendships. However, until now, it has been unclear what makes particular individuals bond.

Cognitive Biologists of the University of Vienna, Austria, and the University of Zurich, Switzerland, explored the question and found that chimpanzees choose their friendships based on similarity of personality.

Researchers Jorg Massen and Sonja Koski together measured chimpanzee personality in two zoos with behavioural experiments and years of observations of chimpanzee behaviour.

They also carefully logged which chimpanzee sat in body contact with whom most.

"This is a clear sign of friendship among chimpanzees," said Massen.

Subsequently, the researchers tested, if those chimpanzees who sit together frequently have similar or different personality 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," Massen said.

The researchers argue that such a strong preference for self-like individuals is probably adaptive because frequent cooperation becomes more reliable when both partners have similar behavioural tendencies and emotional states.

This finding strongly resembles the known "similarity effect" in humans: We tend to make friends with people who are equally extraverted, friendly and bold as ourselves.

"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.

The study was published in the journal ‘Evolution and Human Behaviour’.

(Agencies)

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