Despite their contrasting reputations, bonobos and chimpanzees show similar levels of aggression in zoos

(Press-News.org) Chimpanzees have a reputation for being aggressive, while bonobos are often seen as their peaceful counterparts. This contrast has frequently been used to explain different sides of human nature. However, a new study by Utrecht University behavioural biologists Emile Bryon, Edwin van Leeuwen, Tom Roth and international colleagues shows that, in zoos, chimpanzees are not more aggressive than bonobos. The study was published today in the scientific journal Science Advances.

The team of researchers compared the occurrence of aggressive behaviours in 22 zoo-housed groups of chimpanzees and bonobos. While no difference was found in overall aggression, the team did find that the two sister species differ in how the aggression is distributed. Bryon: “In chimpanzees, aggression mainly comes from males and is directed at everyone. In bonobos, aggression comes from everyone but is mostly directed at males.”

Insights into aggression in chimpanzees and bonobos, our closest cousins, can shed light on the evolutionary origins of our own aggression. “Some see in them a reflection of our nature or how we ought to be,” says Bryon. “Aggressive and war-minded like chimpanzees, or peaceful like bonobos. The truth is that, evolutionarily speaking, we are equally related to both species. And the dichotomy between aggressive chimpanzees and peaceful bonobos might be less clear than previously thought.”

War and infanticide

The idea that chimpanzees are more aggressive than bonobos does however not come out of thin air. “In the wild, chimpanzees do engage in highly aggressive interactions, including war-like conflicts with other groups. There are also reports of infanticide, the intentional killing of infants, in chimpanzees. So far, these things have not been reported in bonobos,” says Bryon.

However, the longstanding idea that chimpanzees are inherently more aggressive has recently been challenged by observations from the wild. While one recent report found that chimpanzee males were indeed more aggressive than bonobo males, a second study reported the opposite, with bonobo males showing higher levels of aggression. Additionally, a recent report shows that conflicts within groups can be lethal in bonobos.

Variation between groups

In the new study, the team, including Jonas Torfs, Marcel Eens and Nicky Staes from the University of Antwerp, examined the rate of both non-contact aggression, such as running toward or chasing another individual, and contact aggression, including wrestling and biting. Using advanced statistical methods and data from a relatively large sample of groups and individuals, the researchers are confident that the two ape species did not differ meaningfully in the magnitude of either form of aggression. But they did find that aggression levels did vary substantially between groups of the same species, meaning that, within species, some groups had higher levels of aggression than others. Notably, bonobo groups exhibited both the highest and lowest levels of aggression observed.

“Bonobos live only in the Democratic Republic of Congo, a conflict zone, which has restricted opportunities to observe them in the wild,” says Bryon. “Most of what we know about bonobos in the wild, comes from only a few groups of bonobos observed on a shorter time frame compared to chimpanzees. So perhaps we do not yet have the complete picture of the extent of aggression in bonobos.”

Different environments

To try to make evolutionary sense of the presumed less aggressive nature of bonobos, researchers have often turned to the “self-domestication hypothesis”. In the wild, chimpanzees and bonobos live in different environments: chimpanzees live north of the Congo River, while bonobos are found south of that river. Compared to bonobo habitats, food resources are less evenly distributed in chimpanzee habitats, leading to greater competition for food both within and between groups. Chimpanzees also compete with gorillas, which are absent from the areas where the bonobos live, and face higher levels of predation.

Self-domestication?

The more stable, less competitive and less dangerous environments may have allowed bonobos to evolve their supposedly less aggressive nature. “These conditions are thought to encourage associations between females. Unlike chimpanzees, where males are dominant, in bonobo societies the females are dominant. Bonobo females can dominate larger males because they form coalitions. According to the “self-domestication hypothesis”, bonobos may have become less aggressive because the dominant females preferred less aggressive males, leading to a selection for friendliness.”

The process of self-domestication has also been suggested to have shaped human evolution, where selection against aggression may have helped increase sociality and shape our complex society. Bryon: “However, neither our results nor the most recent findings from the wild do support the predictions from the self-domestication hypothesis in bonobos. Bonobo males are not markedly less aggressive than chimpanzee males.”

Males on the receiving end

While bonobo males are aggressive, they direct most of their aggression at members of their own sex. “The females are in charge,” Bryon explains. “So males cannot afford to be aggressive towards them. And females seem to find other ways to mediate conflicts amongst themselves, such as through increased sociosexual behaviours, which bonobos use to reduce tensions. Or rather than attacking each other, they divert their aggression towards the males.”

Future studies

Studying great apes in zoos allows researchers to compare behaviour under relatively similar environmental conditions. This means that external factors that influence aggression are similar for both species, and potential differences between species can thus be better identified.

“The overlap in aggressive behaviors we have observed in zoos provides valuable insight,” Bryon says. “Future studies in wild populations will help confirming whether they show the same patterns and differences in aggression, which could deepen our understanding of the origins of group-level differences in aggression.”

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