First, we use voxel-based morphometry (VBM) to show differences between local gray matter architecture in the two species. Here, for the first time, we examine interspecies differences using two approaches that have the potential to reveal detailed brain-wide differences both in gray matter regions, and in the white matter circuitry that connects these regions. Although the sample size was too small to conclusively demonstrate significant species differences, there was a suggestion that bonobos had a larger lateral nucleus after correcting for brain size ( Barger et al., 2007). Finally, a histological study compared the volume of the amygdaloid complex, including the basolateral division specifically, in a sample of great apes that included three chimpanzees and two bonobos. Chimpanzees were found to have a larger cerebellum than bonobos after adjusting for brain size, and bonobos had greater leftward asymmetries in the striatum and motor hand area compared with chimpanzees ( Hopkins et al., 2009). Another study used the same MRI scans from living bonobos, supplemented with additional post-mortem scans, to compare the volume of seven brain regions in chimpanzees and bonobos. A later study based on a sub-set of these MRI scans showed that the dorsal sector of the frontal lobe, as contrasted with the orbital and mesial sectors, occupies a larger proportion of the frontal lobe in chimpanzees than in bonobos ( Schenker et al., 2005). A comparative in vivo MRI study found no significant differences in brain size between the two species ( n = 4 bonobos and n = 6 chimpanzees) after controlling for body size ( Rilling and Insel, 1999). The neurobiological bases of these species differences in behavior have been only minimally investigated, largely due to the very limited availability of bonobo brains. Despite their close phylogenetic relationship, chimpanzees and bonobos exhibit noteworthy differences in temperament and behavior, with bonobos exhibiting less severe aggression ( Goodall, 1986 Kano, 1992 Wrangham and Peterson, 1996 Wrangham, 1999 Parish and de Waal, 2000), more adult play ( Palagi, 2006), greater variety and frequency of sexual behaviors ( de Waal, 1987, 1995 Parish, 1996), greater stress reactivity ( Wobber et al., 2010b), greater social tolerance ( Hare et al., 2007 Hare and Kwetuenda, 2010) and behaviors that may be indicative of greater empathy ( de Waal, 1997) compared with chimpanzees. 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.Ĭhimpanzee, bonobo, brain, social cognition INTRODUCTIONĬhimpanzees ( Pan troglodytes) and bonobos ( Pan paniscus) are our closest living primate relatives, having diverged from a common ancestor with humans ∼6 million years ago ( Goodman et al., 1990), and from each other just 1–2 million years ago ( Becquet et al., 2007 Hey, 2010). 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 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. 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.
However, the neurobiological basis of these differences has only been minimally investigated and remains uncertain. Differences have been reported in multiple aspects of social behavior, including aggression, sex, play and cooperation. 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.
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