From the brains of gentle bonobos, clues to what makes some animals aggressive

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.

Male chimpanzees have been documented dispatching infants sired by other males. They also stalk and kill outsider chimps. The much more laid-back bonobos react to stress by sharing, playing and engaging in lots of sex.

In the study, James Rilling of the Yerkes National Primate Research Center 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.

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. Even more notable, he said, is that bonobo brains carry a thick connection between the amygdala, a deep-seated emotional center that can spark aggression, and a higher brain region, the ventral anterior cingulate cortex, which helps control impulses.

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 antisocial behavior,” said Rilling, who published the study in the journal Social Cognitive and Affective Neuroscience.