In recent years, scientists have shown that chimpanzees favor certain peers by picking whom they prefer to groom and selectively sharing food. The behavior offers a glimpse into humanity’s prosocial evolutionary past.
But a question has lingered: How might a chimpanzee friendship affect antisocial acts?
In a new study that uses fruit as the loot, UC Berkeley researchers found that chimps steal less from their friends, and that friendship status affected the emotional expression of both victim and thieves. Closely bonded chimps swiped pineapple pieces from one another less frequently than those with weaker relationships. Additionally, victims were quicker to calm down when the thief was a friend, and thieves showed less hostility when stealing from a bonded pal than from a non-friend.
The new research deepens our understanding of the emotional underpinnings of our social lives, said Oded Ritov , the paper’s first author and a Berkeley psychology Ph.D. student.
“Collaborative relationships are important for our well-being, health and survival,” said Ritov, who works with Berkeley’s Social Origins Lab . “Increasingly, we’re understanding that this isn’t something that’s unique to us, but that other animals have friends as well.”
The paper was published online this month and will appear in the November issue of the journal Evolution and Human Behavior.
Scientists are sometimes hesitant to describe an apparently close relationship between chimps as akin to human friendship, arguing that we can’t grasp their cognitive and emotional complexity. To identify those who showed signs of a close relationship, researchers observed 26 chimps for more than 800 hours at the Ngamba Island Chimpanzee Sanctuary in Uganda. The team documented which chimps appeared to be closely bonded — like friends — by taking note of their grooming patterns, daily interactions and how regularly they were within arm’s reach of others.
Then they set up two rooms. One contained a “victim” chimp with three trays baited with pineapple. Researchers attached a rope to the trays and led it into the other room, where a second chimp — the thief — could either leave the rope alone or yank it to steal the pineapple.
Chimps stole in 70% of trials with non-friends, compared with 61% of trials with friends. While 9 percentage points may not seem large, each chimp experienced both friend and non-friend conditions, a methodological technique that leads to a higher degree of confidence that the difference is meaningful and not just random chance.
“Ours is the first study to show that they’re able to hold back antisocial behavior with their friends,” Ritov said.
Researchers also measured how the victims responded to being stolen from by evaluating their stomping, fist-pounding and screeching. Victims were initially angrier when friends stole from them but quickly moderated their feelings compared with when non-friends stole from them.
The thieves’ behavior also changed. When non-friends were the thieves, the thieves expressed negative emotions for much longer, suggesting they viewed the interaction as more hostile.
“This suggests that there may be mutual expectations in chimps from both sides of the friendship, which makes it seem more like the complex friendships we see in humans,” Ritov said.
Jan Engelmann , an associate professor of psychology and principal investigator with the Social Origins Lab, said previous work has shown relationship-specific intentions in nonhuman primates. But human friendships are about more than special intentions, he added — they’re about stronger expectations.
“We expect more of our friends,” said Engelmann, a senior author of the paper. “These findings really deepen our understanding of the evolutionary roots of friendships. This is, to my knowledge, the first evidence showing that also chimpanzees expect and maybe demand special treatment from their friends. These kinds of social expectations might form the basis of human moral principles.”
The paper adds to Ritov’s work that focuses on the evolution of human behavior and how morals and norms are part of our core being, traceable to other animals. He has recently studied whether nonhuman animals have a sense of fairness and is broadly interested in how emotions unfold across cultures and species.
“We think that these expectations might be the basic building blocks of the larger-scale cooperation we see in humans,” Ritov 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.”
Evolution and Human Behavior
Observational study
Animals
Chimpanzees steal less from their friends and expect them to do the same
1-Sep-2026