Fingers in mouths: what ape hugs reveal about the ancient politics of trust
In the animal kingdom, few gestures signal trust as vividly as a male chimpanzee sliding his finger into another male’s mouth. Chimpanzees are known to bite off fingers during fights. To place a digit voluntarily between the teeth of a potential rival is to say, with absolute certainty: I do not expect you to bite.
This gesture is one of several findings from a new study published in Proceedings of the Royal Society B (DOI: 10.1098/rspb.2025.3190) by researchers at Durham University. The study examined how bonobos and chimpanzees, humanity’s two closest living relatives, use physical touch to manage social tension before competitive situations. The results suggest that the impulse to reach out and touch someone, far from being a cultural invention, is part of an ancient primate peacekeeping toolkit that predates the human-chimp split by millions of years.
The feeding experiment
The researchers studied five groups of bonobos and chimpanzees at two sanctuaries, one in the Democratic Republic of Congo and one in Zambia. They designed a controlled competition scenario: a familiar call drew the apes to the edge of their enclosure, where the researchers lowered a trough of peanuts, a high-value resource that reliably triggers competitive behavior.
Before the food arrived, the team recorded all instances of physical contact among group members: embraces, hand-holding, grooming, and, in the case of the male chimpanzees, the finger-in-mouth gesture. They then measured how the apes behaved once the food was available, specifically, how much time they spent feeding together versus fighting over access.
The pattern was clear. Groups in which individuals had exchanged more reassuring contact in the minutes before the feeding event showed less aggression and more co-feeding once the food was present. Apes that hugged were more likely to eat side by side rather than chase each other away.
The effect was strongest in groups that were already more peaceful, suggesting that social touch reinforces existing bonds rather than papering over deep conflicts. Touch, in other words, is not a peace treaty. It is a maintenance ritual for relationships that are already functional.
Vulnerability as a strategy
The finger-in-mouth behavior in male chimpanzees deserves particular attention because it inverts the normal logic of dominance displays. Most primate social signals, chest-beating, teeth-baring, charging displays, are about communicating strength and readiness to fight. The finger-in-mouth gesture communicates the opposite: I am voluntarily putting myself in a position where you could hurt me, because I trust that you will not.
This is not abstract anthropology. Male chimpanzees do bite off each other’s fingers during serious conflicts. The gesture carries real, measurable risk. That it occurs specifically in the context of approaching a competitive feeding event suggests it functions as a bond-affirmation ritual, a way of saying “we are not rivals today” before the tension of competition introduces uncertainty.
The behavior has no exact human equivalent, but the underlying psychology is recognizable. Humans also use physical touch, a hand on the shoulder, an embrace, to reassure someone before a stressful interaction. The difference is that human reassuring touch is often directed at the person who is anxious, whereas the chimpanzee finger-in-mouth gesture seems directed at the person who might be a threat.
Two species, one toolkit
The study included both bonobos and chimpanzees because the two species, despite being equally closely related to humans, have very different social systems. Chimpanzee society is hierarchical and male-dominated; bonobo society is more egalitarian and female-bonded. If both species use touch to manage competition-related tension, the behavior likely predates their split from the common ancestor they share with humans, meaning it is at least 6 to 8 million years old.
The finding challenges the notion that primate conflict resolution is primarily about dominance and submission. “Reassuring social touch reduces competition-related conflict in both ape species,” the authors note. The study suggests that social bonds, not just power relations, shape how apes navigate moments of potential conflict.
It also raises a question that the study cannot answer: if physical touch is such an effective conflict-reduction tool, why does it not prevent all aggression? The answer, the data suggest, is that touch works best where bonds already exist. A hug cannot stop a fight between strangers. It can only reinforce the peace that is already there.
Reference: Durham et al., “Reassuring social touch reduces competition-related conflict in chimpanzees and bonobos,” Proceedings of the Royal Society B (2026). DOI: 10.1098/rspb.2025.3190.