By Will Dunham
WASHINGTON, March 13 (Reuters) - Bonobos for years have had the reputation of being the cool hippies of the simian world, with a "make love, not war" ethos, in contrast to their belligerent cousins, the chimpanzees. But is this reputation valid? A new study suggests it is not.
Researchers tabulated aggressive behavior such as charging, hitting, biting, slapping, kicking and trampling among 22 groups of bonobos and chimpanzees - the two species that are the closest genetic relatives of humans - at 16 European zoos. The statistics showed no difference between bonobos and chimpanzees in the rate of aggressive behavior - contact or noncontact.
But there was a striking difference in the targeting of the aggressive behavior, driven by dynamics specific to each species.
Among chimpanzees, aggressive behavior primarily came from males, and was directed at both males and females. But female bonobos were found to be far more aggressive than female chimpanzees. Among bonobos, aggressive behavior came from both females and males, and was primarily directed at males.
"Regarding the dominance system, chimpanzees are patriarchal. Males associate with one another - in conflict within the group and against other communities of chimpanzees - and struggle for dominance with one another," said Emile Bryon, a doctoral student in animal behavior and cognition at Utrecht University in the Netherlands and lead author of the study published this week in the journal Science Advances.
"They also express aggression against females, in disputes over access for resources or in coercing them for copulation," Bryon said.
Dominance is reversed in bonobos.
"Bonobos are matriarchal. Females are gregarious and associate with one another. They also compete for resources although do not express aggression against one another," Bryon said. "Male bonobos can benefit from the rank of their mothers in accessing resources, but express less aggression against females than chimpanzees do. Males still express aggression against one another in a similar magnitude as chimpanzees."
While there were differences in levels of aggressive behavior within groups of both species, bonobos presented both the most aggressive and least aggressive groups observed.
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"The origins for the perspective of the 'hippie' bonobos is rooted in multiple elements. Unlike chimpanzees, they entertain mostly pacific relationships with neighboring groups, they never kill members of their own group or rarely do so - one instance reported in 2025 - and they mediate social tensions with socio-sexual behaviors," Bryon said.
"Our results suggest that despite these contrasts, aggression is still a relevant part of the bonobo social life, although it is distributed differently than in chimpanzees," Bryon said.
The socio-sexual conduct that Bryon cited includes such behaviors as copulation and genital rubbing involving various kinds of sex and age combinations including male-female, female-female as well as older and younger.
The study involved nine groups of chimpanzees spanning 101 individuals and 13 groups of bonobos spanning 88 individuals in zoos in Belgium, England, France, Germany, Italy, the Netherlands and Spain. While the findings provide insight into the two species, Bryon said it is difficult to assess whether behavioral differences may exist among wild populations.
Wild chimpanzees, for instance, have been observed engaging in deadly war-like conflicts with other groups of chimpanzees, with no such observations involving bonobos, a species that is less numerous and harder to observe.
The scientific name for bonobos is Pan paniscus. They look very much like chimpanzees, whose scientific name is Pan troglodytes, but have a more slender build. In the wild, chimpanzees inhabit 21 countries across equatorial Africa. Bonobos live only in the Democratic Republic of Congo.
The two species are equally related to humans, sharing approximately 98.8% of our genome. Their evolutionary lineage split from the lineage that led to humans roughly 7 million years ago.
"When it comes to aggression in our closest living relatives, the peaceful and warmonger binary was puzzling, and further called into question whether our evolutionary path most resembled chimpanzees or bonobos," Bryon said.
"While aggression may always exist, in one form or another, the way it is distributed is greatly fashioned by who holds power over whom," Bryon said. "Although no one-to-one inferences should be made - after all, our social systems are too different from those of chimpanzees or bonobos - this discussion is ultimately very informative when it comes to understanding the social fabric that makes or breaks the expressions of aggression in our species and beyond."
(Reporting by Will Dunham, Editing by Rosalba O'Brien)
