Two new studies suggest chimpanzees may display true altruism, helping unrelated apes even where there is no reward for themselves. While this is a common occurrence in human society, it's a much rarer behavior in the animal world, and the science behind whether or not our closest relatives genuinely display it is still debated.

The first study, published in the Proceedings of the National Academy of Sciences (PNAS), looked at captive chimps and how they responded when given the opportunity to give others a treat when they could in theory take it for themselves. The apes were trained in a game that involved two chimps and the pulling of one of four ropes. If the subject chimp pulled the first rope, they got a tasty banana pellet, the second one gave their partner a pellet, the third gave them both a treat, while the fourth allowed the chimp to skip their turn and give it instead to their partner.

However, the researchers gamed the experiment by always pairing chimps with another named Tai, who had been separately trained to go first and always pull the fourth rope, putting the control into the other ape's hands. They found that in 75 percent of the cases, the subject chimp – who now had to make a decision – chose to pull the third rope, seemingly recognizing Tai's risk of letting others decide the outcome and rewarding her for her behavior.

In a second experiment, the researchers changed the rules. Now when Tai gave up her turn, the subject chimps could either give themselves four pellets and Tai none, or give themselves three pellets and Tai one. They found that in 44 percent of the trials, the chimps chose to receive fewer treats in order to give Tai some, compared with 17 percent when the experimenters made the same decision. This, according to the researchers, is novel insight into chimpanzee psychology, with the chimps seemingly rewarding Tai for her generosity. 

In another study published this week in PNAS, researchers from Arizona State University looked at wild male chimpanzee behavior in Uganda. Specifically focusing on territorial patrolling, the team assessed 20 years’ worth of data on which males were going on patrols and which had offspring in the chimp community at the time.

This is different than simply defending their patch of forest, as they actively seek out chimps from other communities and engage in conflict. By killing off neighboring groups of chimps, they can increase the resources for their own. Yet despite being able to shirk patrolling, the researchers found that even when males did not have any offspring within the community, they would still go out on patrol. This was despite the fact that males who chose not to join the patrols faced no repercussions.

These two studies lend weight to the theory that chimpanzees do indeed show true altruistic tendencies, rather than only acting out of self interest.