Even When Stealing Is Effortless, Chimps Take Less From Their Friends

Chimpanzees given an easy chance to steal food took it 70 percent of the time when the victim was a stranger, but only 61 percent of the time when the victim was a close friend, according to a new study of chimps in Uganda. That nine-point gap points to friendship still carrying some weight, even when a free snack sits just one easy rope-pull away.

When a stranger acts selfishly, it barely registers. When a close friend does the same thing, it can sting like betrayal. Chimpanzees, it turns out, seem to respond in much the same way.

Researchers studying a group of chimpanzees at a sanctuary in Uganda designed a simple test, described in a new paper in the journal Evolution and Human Behavior. They gave each chimp an easy opportunity to steal food from either a friend or a chimp it wasn’t close with, just by pulling a rope. The question was whether friendship could override the urge to grab an easy meal. It sometimes did, and the emotional aftermath turned out to be just as telling as the theft itself.

Scientists already knew that chimpanzees treat friends better in plenty of ways. They groom them more, share more food with them, and comfort them after fights. What hadn’t been tested was whether friendship also holds a chimp back from doing something harmful when the chance falls right into its lap. This study fills that gap, and the results hint at how far back in evolutionary history loyalty and betrayal might go.

How Scientists Figured Out Which Chimpanzees Were Friends

Before any stealing could happen, researchers needed to know which chimps actually liked each other. A research assistant spent 800 hours watching 26 chimpanzees living at the Ngamba Island Chimpanzee Sanctuary in Uganda, tracking how often individual chimps groomed each other, sat close together, touched one another, or fed side by side. Those patterns were turned into a bonding score for every pair in the group, a method the research team had used in earlier chimpanzee studies.

Using that score, each chimp was paired with its closest buddy and, separately, with a chimp it wasn’t particularly close to. To keep the comparison fair, the non-friend partner matched the friend in sex and rank, so any difference in behavior could be traced to the friendship itself rather than size or status.

How The Chimpanzee Food-Stealing Experiment Worked

Once the pairs were set, the chimps took part in what researchers called a stealing task. One chimp, playing the role of “victim,” was given a tray of pineapple pieces and allowed to start eating. A second chimp, the “thief,” was let into a neighboring room and given the chance to pull a rope that would drag the tray away, taking the remaining pineapple for itself.

Each chimp went through this setup twice, once paired with a friend and once with a non-friend, three trials each time. Researchers filmed every session from multiple angles and recorded a range of behaviors tied to emotion, including screaming, scratching, whimpering, barking, and banging, sometimes described as a chimp version of a temper tantrum. Before the real trials began, every single thief pulled the rope to grab the pineapple when no victim was present at all, proving that stealing was the tempting, easy choice whenever it was available.

What The Chimpanzees Did When Given The Chance To Steal

Once real victims were in the picture, something shifted. Thieves pulled the tray and stole food in 65 percent of trials overall, but that number broke down differently depending on friendship. Theft happened in 61 percent of trials involving friends, compared to 70 percent of trials involving non-friends. Chimps who outranked their partner were also less likely to be stolen from at all, regardless of friendship.

Emotions told a more surprising story. When a friend stole from a victim, that victim’s negative reactions were strongest early on, then eased noticeably as more trials went by. Victims paired with non-friends didn’t show that same pattern. Their reaction stayed about the same across every trial, almost as if theft from a stranger never stopped feeling like a violation, while theft from a friend became something they adjusted to and let go of.

Thieves had their own emotional pattern. Chimps who stole from a non-friend screamed more and showed more negative behavior than chimps who stole from a friend. Researchers read this as a sign that chimps treated stealing from a stranger as a more hostile act than taking food from someone they were bonded with, even though the physical act was identical in both cases.

Why Chimpanzees Steal Less From Friends

Self-control may be the most telling part of the study. Chimpanzees are extremely food-motivated, and the setup made stealing almost effortless, a simple tug on a rope with no real risk of payback during the short trial window. Holding back from that temptation, even some of the time, because the other chimp happened to be a friend points to close bonds shaping behavior even when a free meal sits right there. Resisting that meal is a stiffer test than being generous when generosity costs nothing, though whether a chimp is consciously weighing the friendship against the snack, or something less deliberate is at work, is a question the study leaves open.

Emotional reactions round out the picture. Getting more upset with a friend at first, then easing off, fits the idea that chimps expect more from close companions and feel it more sharply when those expectations break. One possibility is that they also rein in their anger to keep a valuable relationship intact. Staying agitated when stealing from a stranger hints that chimps may treat those interactions as combative from the start.

None of this means chimpanzees experience friendship exactly the way people do, and the researchers note that some of these emotional patterns were unexpected and need more testing before anyone draws firm conclusions. But the core finding is hard to ignore: a chimp with an easy, low-cost chance to take food from a friend is less likely to take it, and when things go wrong between friends, the sting fades faster than it does between strangers. Loyalty, it turns out, may be measured not just in what chimps give each other, but in what they refuse to take.