Chimps vs. bonobos

Chapter 12 endnote 20, from How Emotions are Made: The Secret Life of the Brain by Lisa Feldman Barrett.
Some context is:

[Bonobos] are very social creatures, far more egalitarian and cooperative than common chimps. [...] it is possible that a bonobo brain, immersed in a language-rich environment, can learn the meaning of concrete words.

Most chimpanzees, like macaque monkeys, require food or other rewards from experimenters to perform even simple learning tasks. There are also several striking cases where bonobos appear to have spontaneously learned concepts without explicit rewards. Compared to chimps, bonobos have larger and more strongly connected nodes within parts of their interoceptive network, particularly one very important node in the default mode network, which some scientists believe translates into differential capacity for empathy. Chimps, on the other hand, show larger regions with in the control network, and in regions of the interoceptive network that overlap with the multimodal integration network, which is involved in orienting attention for the purposes of processing prediction error. Scientists take this to mean that bonobos are specialized for social interaction, whereas chimps are adapted to tool use and tracking moving objects. Frans de Waal, the noted primate researcher, describes bonobos as more affective, at times more warm and empathic, hugging and kissing to soothe one another (e.g., when another bonobo was bullied), and more nervous; chimps come off as more calculating in comparison.

These differences in affective engagement are sometimes explained as sex differences in leadership. Chimp troops are run by an alpha male, whereas bonobo groups are usually led by a dominant female. Chimpanzees even have higher prenatal testosterone levels than bonobos (and humans). This interpretation might contain some implicit gender stereotyping.

Some of the differences between chimp and bonobo behavior might be environmental. Both species live in large groups (usually more than 100 members) and eat fruit and herbs, traveling on the ground while sleeping and eating in trees. Bonobos, however, were relatively isolated for most of their existence, in a forested area with abundant food and little need to compete for it. Chimps, on the other hand, have had no such luxury. The psychologist Brian Hare proposes that bonobos therefore evolved more cooperative and flexible behavior for dealing with one another, and also have the capacity for greater changes in affect. This behavior is pretty consistent with their stronger interoceptive network connections. He has also argued that because chimps are adapted to circumstances involving competition, they might not require rewards to broaden their affective niche in such circumstances.

There is an old adage that people turn to food for comfort, and it appears to be equally true for our ape cousins. Humans who were insecurely attached at the age of two (could not be soothed when upset by their mothers) are more likely to develop obesity by age four and a half. Kanzi, for example, a male bonobo who was separated from his mother as an infant, may have had the same problem. He is famous for learning to use a lexigram with symbols to communicate. Less well-known is that Kanzi is attention seeking and became quite obese. We can’t know for sure if this was related to the early separation from his mother, but we can’t rule it out, either.