Chimpanzees Expect Better Behavior From Their Friends. Sound Familiar?

Friendship comes with certain expectations. We expect the people closest to us to be considerate, trustworthy and perhaps a little more forgiving than everyone else. When a stranger behaves badly, it’s annoying. When a friend does the same thing, it can feel personal.

Apparently, chimpanzees understand something about that distinction, too.

Researchers at the University of California, Berkeley, studying 26 chimpanzees at a sanctuary in Uganda found that the animals were less likely to steal food from closely bonded companions than from chimpanzees with whom they had weaker relationships. They also reacted differently when friends violated that apparent understanding.

The findings, published in Evolution and Human Behavior, offer an intriguing possibility: Some of the unwritten rules of friendship may have roots that predate human society.

A friendship test involving pineapple

The researchers spent more than 800 hours observing chimpanzees at the Ngamba Island Chimpanzee Sanctuary, documenting their grooming habits, everyday interactions and proximity to one another. Those observations helped establish which animals appeared to have particularly close relationships.

They then devised a relatively simple experiment involving two adjoining rooms, three trays of pineapple and a rope. One chimpanzee occupied a room with the pineapple, while another could pull the rope from an adjoining room and take the food.

The chimpanzees took the pineapple in 70% of trials involving non-friends, compared with 61% of trials involving friends.

That nine-percentage-point difference may seem modest, particularly considering that the animals were still willing to steal from their friends more than half the time. But the researchers tested individual chimpanzees under both conditions, allowing them to compare behavior across different relationships.

The result suggests that chimpanzees exercise at least some restraint toward animals with whom they share stronger bonds. Their friendships apparently have rules, although pineapple can still be very persuasive.

Betrayal feels different when it comes from a friend

The emotional responses were arguably more revealing than the theft itself. Researchers observed behaviors including stomping, fist-pounding and screeching when food was stolen. Chimpanzees whose friends took their pineapple initially reacted more strongly, but they also calmed down more quickly than those robbed by less familiar animals.

The thieves behaved differently, too. When stealing from non-friends, they displayed negative emotions for longer, suggesting a more hostile interaction.

This combination resembles a familiar human contradiction. We may feel especially hurt when a friend behaves badly because we expected better, yet the history of that relationship can also make it easier to move past the offense.

Jan Engelmann, a UC Berkeley psychology professor and senior author of the study, explained that human friendships involve more than simply treating particular people differently. They also involve expectations about how those people should treat us. The findings suggest chimpanzees may possess an early version of that same social understanding.

Where friendship becomes something more

Scientists have known for years that chimpanzees form strong social bonds, groom preferred companions and selectively share resources. This research adds another dimension by examining how those relationships influence behavior that works against another animal’s interests.

Being especially generous toward a friend is one thing. Restraining yourself from taking something you want because of that friendship suggests a different kind of social consideration.

The researchers propose that expectations within close relationships may represent evolutionary building blocks for broader forms of cooperation and social norms.

It would be too much to conclude that chimpanzees possess a human understanding of loyalty, betrayal or morality. The experiment cannot establish exactly what the animals were thinking. What it does demonstrate is that the same action can carry different social consequences depending on the relationship between the individuals involved.

The complicated rules of being friends

Perhaps the most entertaining part of the research is that the chimpanzees apparently expected better behavior from their friends while remaining perfectly willing to steal from those friends themselves.

That contradiction should sound familiar to anyone who has navigated a complicated friendship. Human relationships are rarely governed by perfectly consistent rules. People expect consideration while occasionally behaving selfishly, become upset over minor offenses and forgive behavior from close friends that they would never tolerate from strangers.

What makes friendship meaningful is not an absence of conflict, but the additional expectations that come with the relationship. The Berkeley findings suggest some of that complexity may be much older than we imagined.

For all the sophistication humans bring to friendships, shared histories, promises, loyalty and elaborate discussions about boundaries, we may be working with social instincts that existed long before anyone could explain them.

Sometimes friendship means respecting another person’s needs. Sometimes it means forgiving an offense. And sometimes, apparently, it means knowing perfectly well that you shouldn’t steal your friend’s pineapple and doing it anyway.

Source & reporting note

UC Berkeley News, “Chimps plunder less pineapple from friends — and expect them to do the same,” September 30, 2026. Research by Oded Ritov, Jan Engelmann and colleagues, published online in Evolution and Human Behavior. https://news.berkeley.edu/2026/09/30/chimps-plunder-less-pineapple-from-friends-and-expect-them-to-do-the-same/