Atlanta, GA, USA. A comparative analysis of chimpanzee and bonobo brains, our two closest living primate relatives, shows neuroanatomical differences that may be responsible for divergent social behaviors.

The journal of Social Cognitive and Affective Neuroscience has now published the most comprehensive comparative analysis to date of the neural systems of chimpanzees and bonobos, yielding data that appears to match what we know about the human brain and behavior.


Chimpanzees and bonobos diverged from a common ancestor with humans about six million years ago, and from each other just one-to-two million years ago. Despite this relatively brief separation in evolutionary terms, the two species exhibit significant differences in social behavior. Compared with chimpanzees, bonobos are more anxious, less aggressive, more socially tolerant, more playful, more sexual and perhaps more empathic.



Chimpanzee and Bonobo. Neuroanatomical differences between the brain of the chimpanzee (left) and the bonobo (right) match what we know about the human brain and behavior. Image courtesy of James Rilling.

Click Pic for DetailsIt’s been a puzzle why our two closest living primate relatives, chimpanzees and bonobos, have such widely different social traits, despite belonging to the same genus. “Chimpanzees tend to resolve conflict by using aggression, while bonobos are more likely to use behavioral mechanisms like sex and play to diffuse tension,” says James Rilling, an anthropologist at Emory University.

“The social behaviors of the two species mirror individual differences within the human population. The neural circuitry that mediates anxiety, empathy and the inhibition of aggression in humans is better developed in bonobos than in chimpanzees.”

Well-developed versions of both behavior sets can be found side-by-side in humans. “By contributing to our basic understanding of how brain anatomy relates to social behavior, this study may provide clues to the brain dysfunction underlying human social behavioral disorders like psychopathy and autism,” Rilling says.

Rilling heads the Emory Laboratory for Darwinian Neuroscience, which uses non-invasive neuro-imaging technology to compare the neurobiology of humans and other primates. The anthropology department lab draws on resources of the Yerkes National Primate Research Center at Emory.

“In addition to exploring links between neuroanatomy and different social behaviors, we’re mapping the underlying biology for how species evolve and differentiate,” Rilling says.

A range of imaging and analytical techniques were used in the chimpanzee-bonobo study. Voxel-based morphometry compared the gray matter in standard structural scans of the brains. Diffusion tensor imaging (DTI) captured the white matter connections, to compare the fiber tracts that “wire” the brain.

The results showed that bonobos have more developed circuitry for key nodes within the limbic system, the so-called emotional part of the brain, including the amygdala, the hypothalamus and the anterior insula. The anterior insula and the amygdala are both implicated in human empathy.

“We also found that the pathway connecting the amygdala and the prefrontal cortex is larger in bonobos than chimpanzees,” Rilling says. “When our amygdala senses that our actions are causing someone else distress, we may use that pathway to adjust our behavior in a prosocial direction.”

Chimpanzees have better developed visual system pathways, according to the analysis. Previous research has suggested that those pathways are important for tool use, a skill which chimpanzees appear better at than bonobos.

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<style type="text/css"><!--.quote {width:400px; padding: 6px; border: solid 1px #456B8F; font: 11px helvetica, verdana, sans-serif; color: #222222; background-color: #ffffff}.quote a {font: 13px arial, serif; color: #003399; text-decoration: underline}.quote a:hover {color: #FF9900; }//--></style><div style="float:left;padding:5px;"><img src="../images/stories/ChimpBonoboDeWaal.jpg" width="125" alt=""/></div><div class="quote"><a href="../neuroscience/29567-clues-to-human-social-brain-found-in-chimps-and-bonobos" target="_blank">Clues to Human Social Brain Found in Chimps and Bonobos</a><br/>Tuesday, 05 April 2011<div align="right" style="width:400px"><p style="text-align:right;"><a href="/" target="_blank">TS-Si.org</a></p></div></div>

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