Differences between chimpanzees and bonobos in neural systems supporting social cognition

Abstract

Our two closest living primate relatives, chimpanzees (Pan troglodytes) and bonobos (Pan paniscus), exhibit significant behavioral differences despite belonging to the same genus and sharing a very recent common ancestor. Differences have been reported in multiple aspects of social behavior, including aggression, sex, play and cooperation. However, the neurobiological basis of these differences has only been minimally investigated and remains uncertain. Here, we present the first ever comparison of chimpanzee and bonobo brains using diffusion tensor imaging, supplemented with a voxel-wise analysis of T1-weighted images to specifically compare neural circuitry implicated in social cognition. We find that bonobos have more gray matter in brain regions involved in perceiving distress in both oneself and others, including the right dorsal amygdala and right anterior insula. Bonobos also have a larger pathway linking the amygdala with the ventral anterior cingulate cortex, a pathway implicated in both top-down control of aggressive impulses as well as bottom-up biases against harming others. We suggest that this neural system not only supports increased empathic sensitivity in bonobos, but also behaviors like sex and play that serve to dissipate tension, thereby limiting distress and anxiety to levels conducive with prosocial behavior.

Fig. 1

Horizontal section through FA color map in one chimpanzee (left) and one bonobo (right).

Fig. 2

Differences in local gray matter volume estimates between chimpanzees and bonobos based on analysis of T1-weighted images. The t-statistic image is thresholded at t > 4.0 and overlaid on the common species gray matter template. The figure shows three coronal sections through the gray matter template at different anterior–posterior levels. Regions in blue have higher local gray matter volume estimates in bonobos; regions in yellow have higher local gray matter volume estimates in chimpanzees. The Y-coordinate is relative to Y = 0 at the anterior commissure.

Fig. 3

Differences in Jacobian warp fields between chimpanzees and bonobos based on analysis of diffusion-weighted images. The t-statistic image is thresholded at t > 4.0 and overlaid on the common species FA template. The figure shows four coronal sections through the FA template at different anterior–posterior levels. Regions in blue are larger in bonobos (larger Jacobians from template to subject space) and regions in yellow are larger in chimpanzees. The Y-coordinate is relative to Y = 0 at the anterior commissure.

Fig. 4

Group tractography result from posterior ventral frontal ROI derived from the jacobian comparison shown in Figure 3. Tracts from all three bonobos and all three chimpanzees were registered to template space, binarized and summed. The resulting map shows voxels where at least three of six subjects had a possible connection.