They are nicer to strangers than their own friends and family.

Finbarr O'Reilly/Reuters

PROBLEM: Being "prosocial," or putting yourself out there and meeting people outside of your immediate group of friends, isn't common in non-human animals. Chimps, for example, are extremely aggressive toward strangers, and will even kill their neighbors. But bonobos are much more tolerant, to the point of being go-out-of-their-way friendly, which, in the wild, is rare. Just how willing are they to make friends? Enough to share their food (which, among all animals, is huge)?

METHODOLOGY: One by one, 14 bonobos were placed in central enclosure, with a feast laid out before them. Two bonobos were locked in cages on either side of them. One was part of their in-group, and the other was a bonobo who they knew of, but to whom they had never been formally introduced. The bonobo could decide whether to keep all the food to himself, or to let one or both of the others in to share.

In other variations of this basic set-up, researchers at Duke tested to see whether withholding the motivations of social contact and recognition would affect the bonobos' sharing behavior.

RESULTS: Nine out of the 14 bonobos unlocked the stranger's cage and invited him in before their groupmate. 

Most often, the third bonobo would eventually be allowed to share the food as well, but it would often be the stranger who, having been given access to the central enclosure, would then go let them in -- even though that meant he would then be outnumbered by two members of the same group.

If the bonobos didn't have the opportunity to socialize with the stranger, nine out of ten would still help them get food -- but only when they could do so without sacrificing any of their own food. When they'd have to share, meaning they'd end up with less food than they otherwise would have, they horded it all for themselves.

CONCLUSION: Bonobos are more willing to share with strangers than their own groupmates, especially when doing so will result in social rewards.

IMPLICATIONS: Willingness to bond with strangers transcends things like language and social norms, and it's not unique to humans.

Of course, that bonobos are only self-sacrificing on the behalf of strangers when it means they can expand their social network means they don't understand altruism the way humans, who will share anonymously with strangers, do. Although kind to outsiders, that kindness comes from an inherently selfish place. But they're still better than chimps, who, as the authors point out, never even try to get to know someone outside of their group.

Among the new bonobo friends, in contrast, there was plenty of hugging, grooming, and tickling of one another's genitals. The researchers emphasize this never would have happened with chimps. Aggressive, xenophobic chimps.

The full study, "Bonobos Share With Strangers," is published in the journal PLoS ONE.