They have similar sized brains, they look alike enough for bonobos to have been called once "pygmy chimpanzees", and in fact they belong to the same genus (Pan) having diverged some 1.3 to 2 million years ago (or 1.7-2.6 Ma following the logical thread of a newer paper). Their differences are therefore perfectly comparable to those among Homo species (all but H. sapiens extinct by now).
For this reason and because they are very close relatives of us (we diverged some 8-10 Ma ago), the two Pan sp. species are a major reference for anthropology, not the least because they display so different psychologies and sociologies in spite of being so closely related: while chimpanzees are male-centric, hierarchical, violent and retain female sexual (and not just reproductive) cycles, bonobos are female-centric, cooperative, peaceful and joyful and have sex all the time. Bonobos are also empathic like us, while the empathy of chimpanzees is, if it exists at all, quite shallow.
Now a new paper, using non-invasive brain scan techniques, has managed to discern the differences in what we can well call central wiring in the brains of chimpanzees and bonobos:
James K. Rilling et al., Differences between chimpanzees and bonobos in neural systems supporting social cognition. Social Cognitive and Affective Neuroscience, 2011. Pay per view.
The paper is discussed at Science Daily.
|A look at chimpanzee (L) and bonobo (R) central network (from the press release)|
Interestingly (from the SD article):
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.