In a pineapple-heist experiment at a Ugandan sanctuary, chimps held back from robbing close companions, and victims calmed faster when a friend was the thief.

We hold our friends to a higher standard. We expect them not to lie to us, not to take advantage and, when they do let us down, we tend to forgive them more readily than we would a stranger. A new study suggests chimpanzees may operate by similar rules.

Researchers at the University of California, Berkeley, found that chimpanzees stole less food from their friends than from other members of their group, and that friendship shaped how both thieves and victims reacted emotionally. The study was published in Evolution and Human Behavior.

“We expect more of our friends,” said Jan Engelmann, an associate professor of psychology at Berkeley and the study’s senior author. “These findings really deepen our understanding of the evolutionary roots of friendships.”

Do chimps have friends?

Scientists have often been wary of using the word friendship to describe relationships between animals, fearing it projects human feelings onto other species. But a growing body of research has shown that chimpanzees, along with many other social animals, form lasting, selective bonds. Chimps preferentially groom, share food with and support certain individuals, and some of those relationships endure for years.

“Collaborative relationships are important for our well-being, health and survival,” said first author Oded Ritov, a Ph.D. student in psychology at Berkeley. “Increasingly, we’re understanding that this isn’t something that’s unique to us, but that other animals have friends as well.”

Most research on animal friendship has focused on the positive side: who helps whom. Ritov and Engelmann wanted to know whether friendship also restrains bad behavior.

Engelmann has explored that territory before. In a 2016 study, he and a colleague found that chimpanzees were more willing to trust friends than other group members in a game where trusting a partner carried a risk of losing food. The new work turns from trust to restraint.

The great pineapple heist

The researchers spent more than 800 hours observing 26 chimpanzees at the Ngamba Island Chimpanzee Sanctuary on Lake Victoria in Uganda. They identified friendships by tracking grooming, daily interactions and how often pairs of chimps stayed within arm’s reach of each other.

Ngamba Island, a forested island of about 40 hectares, has served as a sanctuary since 1998 for chimpanzees confiscated from illegal trade. Its residents live in a large social group, spending their days in the forest and returning to shelters at night, an arrangement that allows researchers to run voluntary experiments with animals that know one another well. Chimpanzees are classified as endangered by the International Union for Conservation of Nature.

Then they set up a heist. A chimp, the victim, sat in one room with three trays of pineapple attached to a rope. In a neighbouring room, a second chimp, the thief, could either leave the rope alone or pull it to steal the fruit. Each chimp faced the test with both friends and non-friends, a within-subject design that compares each animal with itself and strengthens confidence that any difference reflects the relationship rather than individual personality.

Chimps stole in 70 percent of trials when the victim was not a friend and in 61 percent when the victim was. The difference was modest. “Ours is the first study to show that they’re able to hold back antisocial behavior with their friends,” Ritov said.

Angry, then forgiving

The researchers also measured emotional reactions, recording stomping, fist-pounding and screeching. When a friend did the stealing, victims were angrier at first than when a non-friend did. But they calmed down more quickly.

Thieves showed a mirror-image pattern. When stealing from non-friends, they displayed negative emotion for longer, suggesting they treated the encounter as more hostile.

Taken together, the results suggest that friendship carries expectations on both sides. A friend’s betrayal provokes a sharper initial reaction, as if it violated an expectation, but the relationship also seems to cushion the conflict and speed reconciliation. “This suggests that there may be mutual expectations in chimps from both sides of the friendship,” Ritov said, “which makes it seem more like the complex friendships we see in humans.”

Engelmann was more pointed. “This is, to my knowledge, the first evidence showing that also chimpanzees expect and maybe demand special treatment from their friends,” he said.

Building blocks of morality

Chimpanzees are, with bonobos, our closest living relatives, sharing a common ancestor with humans roughly six to seven million years ago. Traits shared by humans and chimps may therefore offer clues to the social capacities of that ancestor.

Engelmann and Ritov see their findings as a possible window into the origins of human morality. “These kinds of social expectations might form the basis of human moral principles,” Engelmann said.

Ritov, whose research explores the evolution of norms and whether animals have a sense of fairness, suggested how that might have happened. “We think that these expectations might be the basic building blocks of the larger-scale cooperation we see in humans,” he said. “The expectations that chimpanzees have from their friends or close collaborative partners might have scaled up to more generalized expectations for how we should treat each other.”

The study has limits. It involved 26 sanctuary chimps, many of them orphans rescued from the bushmeat and pet trades, whose social lives may differ from those of wild chimps. The difference in theft rates was nine percentage points, and the release does not report statistical details. Still, the experiment offers rare evidence that friendship, in chimps as in people, is not only about what friends do for each other, but also about what they refrain from doing.

---

Notes

1. Ritov, O., et al. “Chimpanzees steal less from their friends and expect them to do the same.” Evolution and Human Behavior, published online September 2026 (November 2026 issue). ScienceDirect PII: S1090513826001273.
2. University of California – Berkeley. “Chimps plunder less pineapple from friends — and expect them to do the same.” EurekAlert!, 30 September 2026. https://www.eurekalert.org/news-releases/1146152.

Featured image: Photo by Antonio Friedemann on Pexels.com