Humans learn at an early age that sharing is a virtue, despite a common urge to hoard toys from preschool peers. We tend to think of this as a uniquely human ethos, elevating us above other, greedier animals. But as a new study highlights, the kind of selfless behaviors that help build our social networks may have evolved long before we did.

Sharing with strangers is not particularly common in the animal kingdom, especially when it comes to food. Even social animals like chimpanzees, which often share with fellow group members, exhibit an innate wariness of outsiders. And in a cutthroat world where only the fittest survive, being a miser seems to make evolutionary sense.

Nonetheless, a study published this week in the journal PLoS One demonstrates how deep the roots of generosity might really be. Anthropologists from Duke University conducted the research on wild-born bonobos, an endangered species of great ape that's closely related to chimpanzees — and to humans — yet whose relatively pacifist, amorous behavior has earned it the nickname "hippie chimp."

The researchers performed four experiments at a bonobo sanctuary in the Democratic Republic of Congo, where they recruited 14 apes that had been orphaned and rescued from the illegal wildlife trade. The goal was to learn whether, how and why a bonobo might voluntarily share food with other bonobos, including strangers as well as friends.

For the first experiment, each bonobo was placed in a room featuring "a pile of highly desirable food" (i.e., bananas) as well as two sliding doors that led to adjacent rooms. Behind each door was another bonobo, including one friend and one stranger. The test subject thus faced a choice: Eat all the bananas, or share the feast by opening one or both doors. The second experiment was almost exactly the same, except only one of the adjacent rooms contained a bonobo while the other was left empty.

Not only did 12 of the 14 bonobos share their food at least once — with a total sharing rate of 73 percent — but most decided to release the stranger rather than the friend. The stranger often then released the third bonobo, even though that meant splitting the food three ways and being outnumbered by two groupmates. And in the second experiment, the bonobos didn't bother with the door leading to an empty room, suggesting they hadn't been releasing other bonobos simply because they liked the act of opening a door.

But why did they release other bonobos, especially ones they didn't already know? To find out, the researchers changed things up for the final two experiments. In one variation, the test subject couldn't access the banana pile or the other bonobos, but it could pull a rope that would release another bonobo (either a friend or stranger), allowing that bonobo to eat the food. Nine out of 10 bonobos pulled the rope at least once, opting to help friends and strangers equally, even without a tangible benefit for themselves.

This goodwill began to crumble in the fourth experiment, though, when both bonobos could access the food if one released the other, but they were still kept separated from each other. That would mean sacrificing some food without any potential benefit of social interaction, and not a single bonobo took the bait. The apes were apparently willing to help others get food when nothing was at stake for them, but they felt less generous when sharing their own food didn't yield any social upshot.

So what does all this mean? For one thing, it adds to a growing body of research that suggests humans don't have a monopoly on morality. The anthropologist Frans de Waal has long reported on empathy and altruism in nonhuman primates, for example, and a recent study even linked altruism to specific brain cells in rhesus monkeys. The bonobos' willingness to share with strangers likely serves an evolutionary purpose by expanding their social networks, according to the Duke researchers, who speculate that being kind to strangers helped our ancestors develop "an expanded social network of unrelated individuals, which further enabled cumulative culture and cooperation." They now hope to learn more about this phenomenon by studying our closest relatives.

"Our results show that generosity toward strangers is not unique to humans," lead author Jingzhi Tan adds in a statement. "Like chimpanzees, our species would kill strangers; like bonobos, we could also be very nice to strangers. Our results highlight the importance of studying bonobos to fully understand the origins of such human behaviors."

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