Cooperation is often hailed as a key trait that separates humans from animals, but researchers said Monday that our closest relative, the chimpanzee, can learn to work as a team.

In fact, chimps prefer cooperation by a ratio of five to one over competition, and find ways to discourage peers from freeloading, said the study in the Proceedings of the National Academy of Sciences, a peer-reviewed US journal.

"Given the ratio of conflict to cooperation is quite similar in humans and chimpanzees, our study shows striking similarities across species and gives another insight into human evolution," said lead author Malini Suchak, who was a graduate student at the Yerkes Research Center in Atlanta, Georgia, at the time of the study.

She is now an assistant professor of animal behavior, ecology and conservation at Canisius College in Buffalo, New York.

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Previous studies that found chimps were unlikely to cooperate took place in strictly controlled lab settings.

So researchers at Yerkes National Primate Research Center tried instead to mimic the chimps' natural environment for this study, placing 11 of them in a grassy outdoor area near a rope-like apparatus they could tug to get treats and rewards.

But they had to work together to get the goods, either in groups of two or three.

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Chimps were allowed to choose their own partners.

Though they started out primarily competing against each other, they soon figured out that it was more advantageous to help each other.

Across 94 hour-long test sessions, researchers tallied 3,656 successful cooperative acts.

On the flip side, there were more than 600 competitive interactions, when chimps stole -- or tried to steal -- rewards without working to get them, pushed others out of the way or started fights.

Chimps sometimes overcame these forms of competition by "directly protesting against others," said the study.

Or, they refused to work in the presence of a freeloader, a strategy known as "avoidance," and which humans use, too.

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