Are human beings the only animals that will help other members of their species out, even if they don't know them? Not according to Jingzhi Tan, a postdoctoral associate in evolutionary anthropology at Duke University. Working with associate professor Brian Hare, he has observed wild bonobos being nice to bonobos that they don't know.

The scientists had already ascertained that bonobos will share food with strangers, in a previous study. A new study, however, was recently carried out at the Lola ya Bonobo sanctuary in the Democratic Republic of Congo.

In one experiment, 16 different bonobos were led into one of two adjacent rooms separated by a fence. In the other room (the one they weren't in, and couldn't get into) an apple hung from a rope. They couldn't reach that apple, but they could see it. They could also climb the fence and release a peg that was holding the rope in place, causing the apple to fall to the floor of the other room.

As it turned out, they were four times more likely to release the apple when an unfamiliar bonobo was in the other room – meaning it could grab the fallen apple for itself – as opposed to when the room was empty. They would do so without first being asked for help (via gestures) by the stranger.

In another experiment, 21 bonobos watched videos both of group members that they knew, and of unfamiliar bonobos from Columbus Zoo in the US. In both cases, when the apes in the videos yawned, the apes watching the videos would yawn too. This reportedly demonstrates a basic form of empathy known as "emotional contagion." Humans, of course, display similar behaviour when it comes to watching other people yawn.

So, why do the bonobos bother? According to Tan, the impulse to be nice to strangers likely evolves in species where the benefits of bonding with others outweigh the costs.

"All relationships start between two strangers," he says. "You meet a stranger, but you may meet them again, and this individual could become your future friend or ally. You want to be nice to someone who's going to be important for you."

A paper on the research was recently published in the journal Scientific Reports.