Bonobos Are Not the Peaceful Apes We Thought They Were

Utrecht University

The story is a familiar one in popular science: chimpanzees are the violent ape, bonobos the gentle one, and together they represent the dual nature of humanity. It is a tidy narrative. According to a study of 22 zoo groups published in Science Advances, it is also substantially wrong.

Researchers at Utrecht University and the University of Antwerp compared rates of aggressive behavior in zoo-housed chimpanzees and bonobos and found no meaningful difference in the overall amount of aggression between the two species. Bonobos are not less aggressive than chimpanzees. They are aggressive differently.

Same amount, different patterns

The research team, led by Emile Bryon with colleagues Edwin van Leeuwen and Tom Roth at Utrecht and Jonas Torfs, Marcel Eens, and Nicky Staes at Antwerp, measured both non-contact aggression (charging, chasing) and contact aggression (wrestling, biting) across all 22 groups. Using advanced statistical methods and a large sample of both groups and individuals, they found that neither form of aggression differed meaningfully between the two species in overall magnitude.

What differed was the distribution. In chimpanzees, aggression comes primarily from males and is directed broadly at all group members, male and female alike. In bonobos, aggression comes from everyone, males and females, but it flows predominantly toward males. The males are on the receiving end.

This pattern reflects the fundamentally different social structures of the two species. In chimpanzee groups, males are dominant. In bonobo groups, females hold power, maintained through coalitions that allow smaller females to dominate larger males. Bonobo females appear to use aggression strategically, directing it at males rather than at each other, while resolving conflicts among themselves through other means, including the sociosexual behaviors bonobos are famous for.

The self-domestication hypothesis in trouble

The finding complicates a popular evolutionary framework called the self-domestication hypothesis. This theory proposes that bonobos evolved reduced aggression because their environment, south of the Congo River, offered more stable food resources and fewer competitors (no gorillas, less predation) compared to chimpanzee habitats to the north. In this less competitive setting, the theory goes, bonobo females could form coalitions, become dominant, and preferentially mate with less aggressive males, gradually selecting for docility over generations.

The same process has been proposed for human evolution, where selection against aggression may have enabled the complex social cooperation that defines our species.

But if bonobo males are not actually less aggressive than chimpanzee males, as this study and recent wild observations suggest, then the central prediction of the self-domestication hypothesis does not hold. Bonobos may have different social structures and conflict resolution strategies without having evolved to be inherently less aggressive.

Wild observations are catching up

The zoo findings align with emerging data from wild populations. While one recent field study found that wild chimpanzee males were more aggressive than bonobo males, a second study reported the opposite. A recent report documented lethal within-group conflicts in wild bonobos, something previously thought to be exclusive to chimpanzees. The picture from the field is becoming muddier, not clearer, and this zoo study, conducted under relatively controlled conditions, adds weight to the view that the two species overlap more than the popular narrative suggests.

Bryon noted that wild bonobo research is inherently limited by logistics. Bonobos live only in the Democratic Republic of Congo, a conflict zone that restricts research access. Most wild bonobo data comes from a few study sites observed over shorter time frames than the long-running chimpanzee field stations. The complete picture of bonobo aggression in the wild may simply not be available yet.

The value of zoo comparisons

Studying great apes in zoos offers a specific advantage for this kind of comparison: environmental conditions are relatively similar for both species. Food availability, habitat structure, group size, and exposure to predators are all roughly equalized. This means observed differences between species are more likely to reflect genuine biological differences rather than environmental responses. The study found substantial variation between groups of the same species, with bonobo groups exhibiting both the highest and lowest aggression levels observed, a finding that underscores the risk of drawing species-level conclusions from a small number of groups.

What this does not settle

Zoo conditions differ fundamentally from wild environments. The absence of intergroup conflict, predation, and resource competition in zoos removes the very pressures that may drive species-level differences in aggression in the wild. Chimpanzees are known for lethal intergroup warfare in the wild, a behavior that cannot manifest in captivity. Bonobos' more relaxed intergroup dynamics in the wild may also be suppressed or altered in zoo settings.

The study measured observable aggression, charging, chasing, wrestling, biting, but did not assess subtler forms of social control, intimidation, or psychological stress that may differ between species. Aggressive behavior counts may not capture the full picture of how social power operates in these complex societies.

The sample, while larger than any previous comparative study, still includes only 22 groups, and group size, composition, and housing conditions varied across zoos. Whether the findings generalize to different captive conditions or, more importantly, to wild populations remains an open question that the authors explicitly flag for future research.

What the study does clearly demonstrate is that the simple narrative, aggressive chimps versus peaceful bonobos, does not survive careful quantification. Both species are aggressive. The difference lies in who directs it where, a distinction that tells us as much about social structure as it does about temperament.