Babies, bonobos, and brains

I’ve written fairly often here about the behaviour of our nearest primate relatives, typically with a view to deflating the idea that there’s something special about the human animal. Of course, there is something special about us. But my contention has always been that our specialness is much more a matter of degree than of kind.

That is, to pick just one felicitously phrased example, I believe that we are specially creative, but not the product of special creation. Our superior mental abilities are extreme versions of similar or analogous abilities in other creatures; these abilities are not one-off gifts from a benevolent creative force, natural or supernatural.

I’m back to this topic thanks to the conjunction of three sources: a book, a journal study, and a popular science article. Although these sources are quite independent, taken together they highlight a number of connected points about primate mental development. And this set of overlapping sources adds yet another layer to the arguments that (1) evolutionary biology is the key to any deep understanding of human nature and behaviour and (2) our proudest achievements are extensions of the skills of other creatures.

The book is Chip Walter’s Last Ape Standing, which I recently reviewed on my other blog. Walter argues that we are lifelong neonates. We are born too soon, for if our large brains were any larger at birth, we would get stuck on our way out into the world. Our immature brains make us extraordinarily plastic learners in our first few years of life, and Walter writes that its our retention of at least part of that plasticity that makes us so open to new experiences and so creative.

What has this got to do with bonobos? Unlike other chimpanzees, bonobos are unusually, actually quite remarkably, social and non-aggressive. Of all animal societies, theirs looks the most like ours, featuring co-operation, sharing, and inclusion. (OK, so that’s how our societies are supposed to work, not how they do work, but that’s another topic altogether.)

The journal article is “Bonobos Exhibit Delayed Development of Social Behavior and Cognition Relative to Chimpanzees,” available here as a PDF. In this study, juvenile and adult bonobos and chimpanzees were observed during food sharing and competition tests. Even the youngest chimpanzees demonstrated a higher level of food competition than did the bonobos.

Most relevant to the current topic is that the behavioural differences seem to correspond to developmental delays in the brains of the bonobos. The researchers controlled for other factors like motivation and comprehension, concluding that

The association in bonobos of juvenile levels of tolerance, delayed development of social inhibition, and a pedomorphic cranium suggests that common developmental mechanisms might be responsible for the retention of juvenile traits into adulthood.

Is increased sociability and sharing in bonobos a result of the same kind of process that Walter believes generates human cognitive flexibility? Does an “infantile” brain make for a better primate, of whatever species?

The third source is a popular science article, “Bonobos Will Share With Strangers Before Acquaintances,” published on January 2nd by ScienceDaily. The article does not deal directly with persistent neoteny. Instead, it reports a study that shows a particular, peculiar feature of bonobo sharing behaviour.

“It seems kind of crazy to us, but bonobos prefer to share with strangers,” said Brian Hare, a professor of evolutionary anthropology at Duke University. “They’re trying to extend their social network.” And they apparently value that more than maintaining the friendships they already have.

Test subjects had learned how to open a door to either or both of two adjoining chambers. When subjects entered the enclosure with food, they could eat it all, or they could let in one or both of their companions and share. In the test, one companion was a bonobo known the subject, and the other was a bonobo previously seen only at a distance.

Nine of the 14 animals who went through this test released the stranger first. Two preferred their groupmates. Three showed no particular preference in repeated trials. The third animal was often let in on the treat as well, but more often it was the stranger, not the test subject, who opened the door for them.


Two other versions of the test were run, clarifying that it’s the social contact that motivates the bonobos to share.

Chimpanzees “fail” all of the tests, never choosing to share, and fighting over the food when with another animal.

The bonobo tests are not direct assessments of brain development, but combining these results with the information from the two other sources easily leads to the speculation that developmental “delay” in bonobos contributes to their extreme sociability, in a way similar to the idea that human neoteny contributes to our extreme flexibility and creativity.

It’s one more support for the idea that the way that shared primate characteristics are tweaked or emphasized by natural selection is a key to the development of our most prominent — and admirable — traits.

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