NEW YORK, NY.- In the early 1900s, primatologists noticed a group of apes in central Africa with a distinctly slender build; they called them “pygmy chimpanzees.” But as the years passed, it became clear that those animals, now known as bonobos, were profoundly different from chimpanzees.

Chimpanzee societies are dominated by males that kill other males, raid the territory of neighboring troops and defend their own ground with border patrols. Male chimpanzees also attack females to coerce them into mating, and sometimes they even kill infants. Among bonobos, in contrast, females are dominant. Males do not go on patrols, form alliances or kill other bonobos. And bonobos usually resolve their disputes with sex — lots of it.

Bonobos became famous for showing that nature didn’t always have to be red in tooth and claw. “Bonobos are an icon for peace and love, the world’s ‘hippie chimps,’” Sally Coxe, a conservationist, said in 2006.

But these sweeping claims were not based on much data. Because bonobos live in remote, swampy rainforests, it has been much more difficult to observe them in the wild than chimpanzees. More recent research has shown that bonobos live a more aggressive life than their reputation would suggest.

In a study based on thousands of hours of observations in the wild and published Friday, for example, researchers found that male bonobos commit acts of aggression nearly three times as often as male chimpanzees do.

“There is no ‘hippie ape,’” said Maud Mouginot, a biological anthropologist at Boston University who led the analysis.

As our closest living relatives, bonobos and chimpanzees can offer us clues about the roots of human behavior. We and the two species share a common ancestor that lived about 7 million years ago. About 5 million years later, bonobos split off from chimpanzees.

In 2012, a trio of Harvard University researchers proposed that bonobos evolved much like dogs did. Less aggressive wolves were not as likely to be killed by humans, which over time led to the emergence of dogs. In a similar fashion, the researchers argued, female bonobos preferred to mate with less aggressive males, giving birth to less aggressive offspring.

The researchers called their idea the self-domestication hypothesis. In later years, they speculated that humans may have undergone a self-domestication of their own.

Mouginot found the hypothesis intriguing, and decided to test it by tracking individual male chimpanzees and bonobos over several years.

In 2018, she traveled to Tanzania to observe chimpanzees. She would follow an individual male all day long, noting when it committed aggression — a push, a bite, a chase — against another male.

The next year, she went to Congo to watch bonobos; she used binoculars to follow them as they raced around in the forest canopy. “Most of the time, I’m seeing their butts,” she said.

Mouginot soon became perplexed, as she saw that male bonobos acted aggressively on a regular basis. Unlike male chimpanzees, who started their days in a mellow mood, the male bonobos seemed to wake up ready for a fight.

“I thought, where is the peaceful bonobo?” Mouginot said.

She and her colleagues trained field assistants, who made more observations throughout the pandemic. The new analysis, based on 9,300 hours of observations on 12 male bonobos and 14 male chimpanzees, found that bonobos committed aggressive acts 2.8 times as frequently as chimpanzees did.

“Those numbers are really big — I thought I messed something up,” Mouginot said. But she hadn’t.

Mouginot found that the frequent bonobo aggressions almost always involved a single male attacking another male. Chimpanzees, in contrast, often ganged up to attack a victim.

Brian Hare, an anthropologist at Duke University and one of the authors of the self-domestication hypothesis, said the study set a new standard for comparing aggression in bonobos and chimpanzees.

“It’s absolutely worth its weight in gold,” he said.

Mouginot speculated that male chimpanzees engage in one-on-one aggression less often because it poses bigger dangers: A victim of aggression may not want to go on a border patrol with the perpetrator, for example. Or he may bring back some of his own allies to wreak vengeance.

It may be easier for male bonobos to get away with aggression, Mouginot said, because in their female-dominated society, they don’t face the risks that come with male alliances. “I think that’s why we see more aggression in bonobos — because it’s less risky to act aggressively against other males,” Mouginot said.

In fact, male bonobos may benefit from attacking other males. Mouginot and her colleagues found that the apes that carried out the most aggressive acts were also the ones who mated most often.

Hare acknowledged that the study’s results mean that parts of the self-domestication hypothesis “clearly need refinement.” It may be important to consider the effect that different kinds of aggression have on a species, rather than lumping them altogether, he said.

Still, he argued that the differences between the two species remained significant. “Chimpanzees murder, and bonobos don’t,” he said.

This article originally appeared in The New York Times.