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.
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.
One question prompted by the study: why the social brains of these two species diverged so dramatically. This brain region triggers the release of stress hormones and is “strongly implicated” in response to fear and anxiety, Rilling noted. The scans highlight a possible source of this anxiety-and-release lifestyle: An enlarged stress-response center called the hypothalamus. 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 thicker connection in bonobos may explain why the animals are “better at regulating impulses and better at avoiding anti-social behavior,” said Rilling, who published the study in the journal Social Cognitive and Affective Neuroscience.īut “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.īesides being less aggressive than apes, bonobos are notably more anxious. 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.Ĭhimp brains displayed a much thinner connection along this aggression-suppression pathway, meaning the channel carries less information. 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.Įven 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. The second technique, used on the deceased animals, filled in lines of white matter, the neuronal wires connecting various brain regions.īy combining the images, “you can get at circuits in the brain, how the brain is organized,” Rilling said.Ĭompared 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 imaging method, used with the living animals, built pictures of gray matter, the large-scale structures of the brain. 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. 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. 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. “But it’s a joke compared to what you see in chimpanzees.” “It’s not like they never have antagonistic interactions,” Hare said of the bonobos.
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