In a controlled task at a Ugandan sanctuary, chimpanzees took pineapple less often from partners classed as friends than from non-friends: theft occurred in 61% of friend-condition trials and 70% of non-friend-condition trials. The 2026 study also found that victims’ and thieves’ emotional behavior varied with the relationship. These results describe the study’s sample and setup, not a rule about all chimpanzees or their behavior in the wild.
What did the chimpanzee study find?
The study, “Chimpanzees steal less from their friends and expect them to do the same,” by Oded Ritov and colleagues, reported lower theft rates when the thief and victim had been classified as friends. The researchers recorded theft in 61% of friend-condition trials, compared with 70% of non-friend-condition trials. Those percentages are proportions of trials in this experiment, not estimates of how often chimpanzees steal in general.
The study appeared online on September 1, 2026, in Evolution and Human Behavior (DOI: 10.1016/j.evolhumbehav.2026.106948). The journal’s indexed record lists it in Volume 47, Issue 6, scheduled for November 2026; the September date is its early-online publication date.
How was friendship defined and tested?
Relationships were based on observed social behavior
The researchers studied 26 chimpanzees at Ngamba Island Chimpanzee Sanctuary in Uganda. They used 800 hours of observations of naturally occurring interactions—including grooming, co-feeding, touch and proximity—to identify stronger and weaker social relationships. “Friend” was an operational category based on those observed interactions; it does not mean the researchers established that chimpanzees experience relationships exactly as humans do.
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The task created an opportunity to take food
In the experiment, a chimpanzee designated as the “thief” could pull a rope to take a tray of pineapple from a “victim” who had started feeding. Each victim took part in both a friend and a non-friend condition. The researchers matched friend and non-friend pairs on sex and dominance rank, allowing them to compare relationship conditions while accounting for those characteristics.
How did victims and thieves respond emotionally?
Observed negative emotion differed by both relationship condition and role. Victims initially showed more negative emotion when a friend took their food, but moderated their response over additional trials. Thieves showed more negative emotion in the non-friend condition, a pattern the researchers interpreted as consistent with those interactions feeling more hostile.
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The two findings are not contradictory: the victim and thief occupied different positions in the interaction, and their emotional behavior followed different patterns. The results indicate that relationship strength was associated with behavior in this task; they do not identify a single emotional response shared by both participants.
Do the results prove chimpanzees have a concept of friendship or fairness?
No. The experiment supports the narrower conclusion that relationship strength affected behavior in this food-taking task. The authors discuss social expectations and ways of protecting relationships as possible interpretations, but the study does not prove that chimpanzees consciously represent friendship, fairness or a human-like moral rule.
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A University of California Berkeley-attributed release, republished by Phys.org, quoted first author Oded Ritov saying, “Ours is the first study to show that they’re able to hold back antisocial behavior with their friends.” Senior author Jan Engelmann said, “We expect more of our friends.” Engelmann also described the results as evidence that chimpanzees “also expect and maybe demand special treatment from their friends,” adding that such expectations “might form the basis of human moral principles.” These are interpretations attributed to the release, not proof that the experiment established those mental processes.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What the study does—and does not—show
- It shows: In this sample and rope-and-pineapple task, theft was less frequent in the friend condition than in the non-friend condition, and observed negative emotion varied by relationship condition and role.
- It does not show: That chimpanzees everywhere steal less from friends, that the rates reflect natural theft, or that the animals follow a conscious moral rule.
- Its setting matters: The participants lived at Ngamba Island Chimpanzee Sanctuary in Uganda; the study was not conducted on wild chimpanzees.
For the published study, see Ritov et al. in Evolution and Human Behavior. The public summary and quoted comments appeared in the Berkeley-attributed release republished by Phys.org.
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