Even long-time friends can fall out over a small misunderstanding. To resolve conflict before turning away completely, someone has to reach out first. The same goes for the great apes closest to humans. Seemingly trivial physical contact, like holding hands and patting shoulders, was found to help maintain peace in the group.
A team led by Zanna Clay, a professor of psychology at Durham University in the United Kingdom, said on the 21st (local time) that in bonobo and chimpanzee groups, reassuring physical contact such as hugging and hand-holding helps manage social tension and maintain peace, according to the international journal Proceedings of the Royal Society B: Biological Sciences.
Chimpanzees and bonobos share 98.9% of their genes with humans. They are the closest relatives in the course of evolution. Humans diverged from a common ancestor with chimpanzees 6 million years ago and evolved separately, and 2.5 million years ago, bonobos split from chimpanzees. Scientists study chimpanzees and bonobos to understand where human social behavior originated.
◇Taking the risk of putting fingers in a partner's mouth
The Durham University team conducted research at the Lola ya Bonobo sanctuary in the Democratic Republic of Congo in Africa and at the Chimfunshi Wildlife Orphanage Trust in Zambia. They examined five semi-wild groups of bonobos and chimpanzees to see what they did before competing for food.
A total of 116 chimpanzees and bonobos received food from humans in a confined space. Before that, social tension rises. The researchers analyzed video filmed for five minutes before the feeding. Bonobos and chimpanzees eased tension and built bonds by holding hands, patting shoulders, or hugging.
In particular, just before competition over food, chimpanzees engaged in risky behavior by putting their fingers or hands into a partner's mouth in a hair-trigger situation. Chimpanzees are known for strong aggressiveness. The researchers said they signaled their own vulnerability to gain trust and reduce tension.
The effect of reaching out while taking a risk was clear. Individuals that engaged in more social physical contact later had more time to eat without being disturbed. Rather than engaging in a free-for-all mud-slinging fight over food, they built trust beforehand and gained greater benefits.
◇More contact among females in bonobos, males in chimpanzees
It was the same physical contact, but the targets differed by species. Among bonobos, the effect of such contact appeared most clearly between females. This aligns with the role of female alliances in bonobo society. Bonobo society is matriarchal. Although females are smaller than males, they cooperate to subdue stronger males and maintain power.
Chimpanzees showed the opposite pattern, with the effect most pronounced among males. Because chimpanzee society is the reverse of bonobos, with powerful males dominating in a patrilineal structure, bonds among males are crucial.
The effect of social contact was distinct in both species in groups with higher tolerance toward partners. Calming partners before eating and strengthening alliances contributed to maintaining a tolerant social atmosphere.
One chimpanzee group known for generosity toward partners even engaged in behavior akin to a train game. When one male hugged another male from behind, other males joined in sequence to form a long line. The researchers called this behavior the "reassurance train."
Clay said, "This study shows that physical contact regulating social relationships in tense, emotionally heightened moments has been around for a long time and plays a powerful role." The researchers explained that because such social contact behaviors appeared in both bonobos and chimpanzees, they can be seen as having evolved at least 6 million years ago from the common ancestor with humans.
Jake Brooker, the first author of the paper, said, "Social touch is a powerful communication tool that long predates language," adding, "Using social touch appears to be a trait humans share with their closest relatives."
References
Proceedings of the Royal Society B: Biological Sciences (2026), DOI: https://doi.org/10.1098/rspb.2025.3190