Bonobos, an endangered species of primate in Central Africa, are much mellower than chimpanzees. (Photo courtesy of Vanessa Woods)

The striking absence of aggression among one species of ape — bonobos, sometimes called pygmy chimpanzees — may be hard-wired into their brains, a new study suggests.

Though closely related to chimps, the endangered bonobos of central Africa are much mellower.

“They are the only ape in our family that does not kill,” said Brian Hare, an assistant professor who studies chimpanzees and bonobos at Duke University.

In contrast, male chimpanzees have been documented dispatching infants sired by other males. They also stalk and kill outsider chimps, often by stomping the interloper to death.

The much more laid-back bonobos react to stress by sharing, playing and engaging in lots of sex.

“It’s not like they never have antagonistic interactions,” Hare said of the bonobos. “But it’s a joke compared to what you see in chimpanzees.”

Scientists studying apes as a window into human behavior are eager to tease apart these differences, asking how they arose in the 1 million to 2 million years of evolution that separate chimps and bonobos.

The new study, which gathered detailed brain images of both species, hints at an intriguing answer: The brains of bonobos may be wired to chill.

In the study, James Rilling of the Yerkes National Primate Research Center, part of Emory University in Atlanta, scanned the brains of 13 living and dead bonobos and chimps. One imaging method, used with the living animals, built pictures of gray matter, the large-scale structures of the brain. The second technique, used on the deceased animals, filled in lines of white matter, the neuronal wires connecting various brain regions.

By combining the images, “you can get at circuits in the brain, how the brain is organized,” Rilling said.

Compared with those of chimps, bonobo brains displayed bigger, more developed regions thought to be vital for feeling empathy, perceiving distress in others and feeling anxiety, Rilling said.

One of these structures, the right anterior insula, is crucial for generating empathy, as people with damage to this region notably lack the ability to perceive how others are feeling, Rilling said.

Even more notably, Rilling said, bonobo brains carry a thick connection between a flash point of emotion and a higher brain region that helps control impulses. This neuronal wire runs from the amygdala, a deep-seated and evolutionarily old emotional center that can spark aggression, to a region called the ventral anterior cingulate cortex, which is thought be important for suppressing impulsive behavior.

Chimp brains displayed a much thinner connection along this aggression-suppression pathway, meaning the channel carries less information.

The thicker connection in bonobos may explain why the animals are “better at regulating [aggressive] impulses and better at avoiding anti-social behavior,” said Rilling, who published the study in the journal Social Cognitive and Affective Neuroscience.

But “this pathway does not function very well in people who are psychopaths, people with conduct disorder, people who don’t take others’ feelings into consideration,” Rilling said.

Besides being less aggressive than apes, bonobos are notably more anxious. Experts say the urge to soothe this anxiety may be impulse that drives the animals to engage in frequent sex and rough-house play.

The scans highlight a possible source of this anxiety-and-release lifestyle: An enlarged stress-response center called the hypothalamus. This brain region triggers the release of stress hormones and is “strongly implicated” in response to fear and anxiety, Rilling noted.

One question prompted by the study: why the social brains of these two species diverged so dramatically. Emerging theories suggest that bonobos evolved on one side of the Congo River with great food abundance, whereas chimps evolved on the other side of the river where intense competition for food may have driven the apes to fight and claw their way into every fruit tree.

Hare, who was not involved in the study, said the new findings support a theory he is developing that bonobos are “self-domesticated.” Whereas aggressive chimps are akin to wild wolves, he said, bonobos act more like domesticated dogs.

“There’s been a really strong natural selection against aggression in bonobos,” Hare said. “It’s been a natural process.” That process, in turn, may have been driven by the differing environments in which the two species evolved.

Although the study offers tantalizing glimpses into the social evolution of the two species, Rilling acknowledged that the small number of brains he scanned means the results are preliminary. “We really need a larger sample,” he said. “But bonobo brains are hard to come by.”

Experts say fewer than 10,000 bonobos survive in the wild; an additional 84 live in U.S. zoos.