The defining element of friendship is trust, finds a new study on chimpanzee pals.

This means friendship based on trust evolved millions of years ago, and dates back to at least the last common ancestor of chimps and humans. The findings are reported in the journal Current Biology.

"Humans largely trust only their friends with crucial resources or important secrets," co-author Jan Engelmann of the Max Planck Institute for Evolutionary Anthropology said in a press release.

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Engelmann continued, "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."

Prior research found that chimps have relationships that look a lot like friendships. For example, they will extend favors preferentially toward select individuals. To determine what those relationships were based on, Engelmann and co-author Esther Hermann observed the interactions of 15 chimpanzees living at Sweetwaters Chimpanzee Sanctuary in Kenya. The study took place over a 5-month period.

Based on friendly interactions among chimp pairs, including grooming and eating together, the researchers were able to identify each chimpanzee's closest "friend" as well as a "non-friend."

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Next, the chimps played a modified version of what is known as the human trust game. They played the game both with their friend and then again with their non-friend, repeating matches 12 times for each.

The setup consisted of a board positioned between the two players. A "no trust" rope as well as a "trust rope" connected to the board controlled what kind of food reward the players received. If a chimp pulled the no trust rope, he or she got immediate access to a not-so-great food item. If a chimp pulled the trust rope, the other player speedily received much more tempting treats that he or she could share with the rope puller.

Because of this win-win outcome with the trust rope, pulling it offered the best scenario for both players, but only if the first chimp trusted the other one enough to share the goodies.

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As predicted, the chimps showed greater trust when they were playing the game with a friend. The researchers explained that the "chimpanzees were significantly more likely to voluntarily place resources at the disposal of a partner, and thus to choose a risky but potentially high-payoff option, when they interacted with a friend as compared to a non-friend."

Chimps might not hand out Valentine's Day cards in February, but they do have friends, the study confirms. Chimp besties are, as suspected, bonded in a comparable way that human buddies are.

"Human friendships do not represent an anomaly in the animal kingdom," Engelmann said. "Other animals, such as chimpanzees, form close and long-term emotional bonds with select individuals."

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Next the researchers hope to find out if chimps are more likely to offer help to their friends. Such studies not only reveal more about our fellow primates, but they also provide insight on human evolution.