Male Bonobos, Close Human Relatives Long Thought to Be Peaceful, Are Actually Quite Aggressive, Study Suggests

The new research found bonobos were three times more likely than chimpanzees to commit an act of physical aggression

Two male bonobos, one older (left) and one younger (right), smile and hug
New research suggests that male bonobos exhibit aggressive behaviors such as chasing, charging, hitting and kicking more often than scientists thought. Anup Shah via Getty Images

Bonobos aren’t as mellow as scientists long suspected, new research finds, challenging previously held ideas about the “peaceful” primate species—and one of humans’ closest relatives.

In a new study published Friday in the journal Current Biology, scientists found that male bonobos are actually much more aggressive than assumed—even more so than the notoriously violent chimpanzees.

Chimpanzees and bonobos are humans’ closest living relatives today. The common ancestor of the three species lived about seven million years ago, before Homo sapiens separated from the pack. Five million years later, chimpanzees and bonobos split. Since then, evolving independently, the two creatures have long been considered behavioral opposites in the primate world.

It is well-documented that male chimpanzees exert dominance in their communities, defending territories against other tribes’ males and even displaying aggression toward females and infant chimps. On the other hand, bonobos have been thought of as peacekeepers, harboring a female-dominant society with rare, male-initiated violence. Food-sharing is commonplace, and instead of fighting, disputes among bonobos are usually solved with sex. As a result of their cooperative behavior, bonobos have been called “hippie” apes.

But unlike chimpanzees, bonobos are very difficult to study in the wild, because they make their homes in remote, hard-to-reach rainforests. Because of this, a team of scientists familiar with bonobos felt the species leads more nuanced lives than others give them credit for.

“It’s a species with such complex behavior that just limiting the species to being a hippie, for this study it’s not going to work,” Maud Mouginot, an anthropologist at Boston University and a lead author of the study, tells the Guardian’s Nicola Davis. “It’s just too simplistic.”

A young bonobo sits in a lush rainforest environment
A 1-year-old bonobo. Wild bonobos are found only in the Democratic Republic of the Congo. DBeaune via Wikipedia Commons CC BY-SA 4.0

In the Democratic Republic of the Congo’s Kokolopori Bonobo Reserve, the researchers observed bonobos for 2,047 hours. They were astonished by what they saw—and the surprises came right from the start. During the first week of observation, Mouginot saw two bonobos chasing each other in the trees.

“It was five in the morning, and the bonobos had just woken up. The field assistants said, ‘This is an aggression.’ And I was like, ‘Wait, where is the peaceful bonobo in this?’” Mouginot tells National Geographic’s Tim Vernimmen.

As the study period went on, the team recorded more and more instances of aggression between bonobos. They counted 521 instances of both physical (punching, kicking, biting) and non-contact (charging, chasing) aggression from 12 males of the species.

“I was so confused that I looked at each aggression one by one to make sure I had no duplicates,” Mouginot says to National Geographic.

For comparison, the researchers also studied 14 male chimpanzees, which were observed in Tanzania’s Gombe Stream National Park. Among this group, scientists tallied 654 aggressive acts across the same categories in about 7,300 hours. This means male bonobos were 2.8 times more likely than chimpanzees to commit an aggressive act, and three times more likely to commit physical-only aggression.

“[The study] is absolutely worth its weight in gold,” Brian Hare, an anthropologist at Duke University who was not involved in the research, tells the New York Times Carl Zimmer.

Three bonobos sit on a tree branch and groom each other.
Unlike in chimpanzee societies, male bonobos don't form alliances or coalitions—which lessens the chance of a group response to an act of aggression, researchers posit. DBeaune via Wikimedia Commons under CC BY-SA 4.0

Scientists have proposed a few different theories to explain this behavior. One posits that because male bonobos don’t form alliances or coalitions among themselves, acting aggressively toward one male won’t trigger a large-scale response from multiple apes, unlike what is seen in chimpanzee societies. This makes the fallout of an aggressive act potentially less severe.

The researchers also found that aggressive male bonobos had more offspring, suggesting the behavior makes them more desirable to females. Male bonobos did not tend to act aggressively toward females, while male chimpanzees did.

But other scientists say that further research should be done. Differences within bonobo and chimpanzee societies—the former being female-led, the latter being male-led—create different contexts for how and why the primates behave as they do, such as negotiating mating rights and hierarchies, Gisela Kaplan, an animal behavior researcher at the University of New England in Australia who was not involved in the research, tells New Scientist’s James Woodford.

Plus, conflating “aggressive” behavior across the two species may not necessarily be fair, especially because chimpanzees are known to kill, and bonobos are not.

“There’s more pointless violence in chimpanzees and humans than in other species like bonobos,” Kaplan tells New Scientist.

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