Whether it’s giving to charity or helping a stranger with directions, we often assist others even when there’s no benefit to us or our family members. Signs of such true altruism have been spotted in some animals, but have been difficult to pin down in our closest evolutionary relatives. Now, in a pair of studies, researchers show that chimpanzees will give up a treat in order to help out an unrelated chimp, and that chimps in the wild go out on risky patrols in order to protect even nonkin at home. The work may give clues to how such cooperation—the foundation of human civilization—evolved in humans.
“Both studies provide powerful evidence for forms of cooperation in our closest relatives that have been difficult to demonstrate in other animals besides humans,” says Brian Hare, an evolutionary anthropologist at Duke University in Durham, North Carolina, who was not involved with the research.
In the first study, psychologists Martin Schmelz and Sebastian Grüneisen at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, trained six chimps at the Leipzig Zoo to play a sharing game. Each chimp was paired with a partner who was given a choice of four ropes to pull, each with a different outcome: give just herself a banana pellet; give just the subject a pellet; give both of them pellets; or forgo her turn and let her partner make the decision instead.
Unbeknownst to these partner chimpanzees, the chimp that always started the game—a female named Tai—was trained to always choose the last option, giving up her turn. From the partner’s point of view, this was a risky choice, Grüneisen says, as Tai risked losing out entirely on the banana pellets. Over dozens of trials, after Tai gave up her turn, the six partners pulled the rope that rewarded both themselves and Tai with a treat 75% of the time, indicating they valued her risking her own treats to help them.
But the researchers also wanted to see whether the subjects were willing to give up some of their own reward to repay Tai for her perceived kindness. “That kind of reciprocity is often claimed to be a landmark of human cooperation, and we wanted to see how far we could push it with the chimps,” Grüneisen says.
The team repeated the experiment, except this time when Tai passed the turn to the subjects, the subjects had the option of either giving themselves four banana pellets and Tai none, or giving both themselves and Tai only three banana pellets. The subjects chose the sacrifice option 44% of the time, compared with 17% of the time when the experimenters, not Tai, made the initial decision. This suggests that the chimps frequently felt compelled to reward Tai for her perceived unselfishness, even at their own expense, the researchers report today in the Proceedings of the National Academy of Sciences (PNAS).
“We were very surprised to get that finding,” Grüneisen says. “This psychological dimension to chimps’ decision-making, taking into account how much a partner risked to help them, is novel.”
The second study, also published today in PNAS, looked at what motivates male chimps to risk life and limb on patrol missions. Male chimps in the wild often team up and silently stalk the group’s boundaries single-file, sniffing for intruders. These can be costly excursions: About a third of the time, they meet chimps from a rival group, and occasionally the encounters turn bloody. So patrolling chimps risk injury or even death.
According to classic behavioral theories, chimps should put themselves in such peril only if they have offspring or close maternal relatives in the group. Yet, after analyzing behavior and relationship data from 3750 male chimps in Ngogo, Uganda, collected over the past 20 years, researchers learned that although that was true for most chimps, more than a quarter of the patrollers had no close relations in the group. What’s more, males who didn’t join these all-male patrols didn’t appear to face any repercussions, says the study’s lead author, anthropologist Kevin Langergraber from Arizona State University in Tempe. So, it was a bit surprising that so many chimps risked it.
He and his colleagues suggest that a theory known as group augmentation best explains these findings. This theory posits that by patrolling to protect the group’s food supply and expand its territory, the entire group becomes more attractive to females and improves each individual male’s chances of reproducing.
Anne Pusey, another evolutionary anthropologist at Duke who is unaffiliated with the studies, agrees it’s a reasonable hypothesis. Protecting and expanding the group’s territory, she says, would “secure or increase the space and food supply for resident females, as well as future immigrant females, with whom [the males] will eventually mate and have a chance of siring offspring.” More and healthier females means each individual male has a greater chance at producing offspring.
Langergraber adds that such behavior might serve as an evolutionary basis for human cooperation within huge, diverse communities. “One of the most unusual things about human cooperation is its large scale,” he says. “Hundreds or thousands of unrelated individuals can work together to build a canal, or send a human to the moon. Perhaps the mechanisms that allow collective action among chimpanzees served as building blocks for the subsequent evolution of even more sophisticated cooperation later in human evolution.”