Don't pat yourself on the back for holding that door open for a stranger just yet. We may call it human decency, but it’s not just humans who have it.

Bonobos, the friendly hippies of the primate world, are willing to help strangers even if there’s nothing in it for them, Duke University researchers report November 7 in the journal Scientific Reports. This shows that humans aren’t unique in their kindness to strangers, and suggests that such behavior may have evolved among our closest relatives, depending on their social needs.

"We're trying to understand what's similar to humans, what's different to humans," says Jingzhi Tan, a co-author on the study.

A Tale of Two Tests

The researchers knew that bonobos would share food with strangers, and they wanted to see whether the apes would also help a stranger without getting any reward themselves.

With apes from the Lola ya Bonobo sanctuary in the Democratic Republic of Congo, the researchers placed pairs of bonobos that didn’t know one another in adjacent rooms separated by a fence and hoisted a piece of an apple over one room. If the bonobo in the other room climbed the fence, it could trigger the fruit to fall—where only the stranger could get it.

Without waiting for an incentive or cue, many of the bonobos in the first room dropped the fruit into the stranger's room. When the other room was empty, the bonobos were far less likely to drop the fruit, suggesting they were motivated to help the strangers.

"If you're helping, you have to help without being asked," Tan says. “Helping strangers must be altruistic. It must be unselfish."

The second part of the study tested how conscious this Good Samaritan response was by having a group of bonobos watch short videos of other bonobos. The test subjects knew some of the animals starring in the shows, but others were strangers from the Columbus Zoo.

The videos showed apes yawning or maintaining neutral expressions, and the researchers wanted to see if the test subjects would yawn in response—a sign of empathy—with their groupmates or with the unfamiliar apes. As it turns out, the stranger apes' yawns were just as contagious as those of their buddies.

Since Tan and anthropologist Brian Hare had already studied food-sharing in bonobos, Tan says they expected the bonobos to have friendly interactions with those they didn’t know.

"When the two groups meet, they will not be as aggressive as chimpanzees," Tan says. "It's actually quite common they would just peacefully interact with each other."

Family Issues

The bonobo's behavior contrasts with that of the chimpanzees, their more aggressive counterparts. The two great apes share more than 98 percent of their DNA with each other, and with humans.

Bonobos live in peaceful, matriarchal communities where they use complex vocal sounds to communicate with each other, and sex plays a large part in certain social situations. When they get stressed, bonobos tend to go for hugs rather than the jugular. Chimpanzees, on the other hand, are known for being aggressive and violent.

"This is stuff that you just wouldn't see in a chimpanzee society," Zanna Clay, a psychology professor at Durham University in England. "You wouldn't be able to run this experiment with chimpanzees because they're so hostile."

Tan says chimpanzees help only when requested to do so, and yawn contagiously only with members of their own group.

Since humans, chimps, and bonobos are so closely related, this study hints at shared ways of dealing with strangers among the three groups, depending on the social scene. Female bonobos leave their family group as they reach adulthood, so getting along with strangers is important.

"Warlike hostility toward outgroups is just one part of our evolutionary history. Humans have both capacities," Clay says. "We didn't get to where we are now by working alone."