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Brain Struct Funct. 2018 Oct 10. doi: 10.1007/s00429-018-1751-9. [Epub ahead of print]

Comparison of bonobo and chimpanzee brain microstructure reveals differences in socio-emotional circuits.

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Department of Anthropology, Center for the Advanced Study of Human Paleobiology, The George Washington University, Washington, DC, 20052, USA.
Department of Anthropology, Center for the Advanced Study of Human Paleobiology, The George Washington University, Washington, DC, 20052, USA.
Comparative Pathology Lab-RARC, University of Wisconsin, Madison, WI, 53726, USA.
Department of Ecology, Evolution, and Organismal Biology, Kennesaw State University, Kennesaw, GA, 30144, USA.
Ape Cognition and Conservation Initiative, Des Moines, IA, 50320, USA.
Fishberg Department of Neuroscience and Friedman Brain Institute, Icahn School of Medicine at Mount Sinai, New York, NY, 10029, USA.
Neuroscience Institute and Language Research Center, Georgia State University, Atlanta, GA, 30322, USA.


Despite being closely related, bonobos and chimpanzees exhibit several behavioral differences. For instance, studies indicate that chimpanzees are more aggressive, territorial, and risk-taking, while bonobos exhibit greater social tolerance and higher rates of socio-sexual interactions. To elucidate the potential neuroanatomical variation that accompanies these differences, we examined the microstructure of selected brain areas by quantifying the neuropil fraction, a measure of the relative tissue area occupied by structural elements of connectivity (e.g., dendrites, axons, and synapses) versus cell bodies. In bonobos and chimpanzees, we compared neuropil fractions in the nucleus accumbens (NAc; core and shell), amygdala (whole, accessory basal, basal, central and lateral nuclei), anterior cingulate cortex (ACC; dorsal and subgenual), anterior insular cortex (AIC), and primary motor cortex (M1). In the dorsal ACC and frontoinsular cortex (FI) we also quantified numbers of von Economo neurons (VENs), a unique subset of neurons thought to be involved in rapid information processing during social interactions. We predicted that the neuropil fraction and number of VENs in brain regions associated with socio-emotional processing would be higher in bonobos. In support of this hypothesis, we found that bonobos had significantly greater neuropil in the central and accessory basal nuclei of the amygdala, as well as layers V-VI of the subgenual ACC. However, we did not find a difference in the numbers of VENs between the two species. These findings support the conclusion that bonobo and chimpanzee brains differ in the anatomical organization of socio-emotional systems that may reflect species-specific variation in behavior.


Amygdala; Bonobo; Chimpanzee; Microstructure; Neuropil; Social cognition; Von Economo neurons


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