Bonobos: the self-domesticated ape?

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The two apes above might look very similar to the untrained eye, but they belong to two very different species. The one on the right is a bonobo; the one on the left is a chimpanzee. They are very closely related but the bonobo is slimmer, with a smaller skull, shorter canines and tufts of lighter fur. There are psychological differences too. Bonobos spend more time having sex, and playing with one another. They’re less sensitive to stress. They’re more sensitive to social cues. And they are far less aggressive than chimps.

Many years back, a young researcher called Brian Hare was listening to the Harvard anthropologist Richard Wrangham expound on this bizarre constellation of traits. “He was talking about how bonobos are an evolutionary puzzle,” recalls Hare. “They have all these weird traits relative to chimps and we have no idea how to explain them.”

But Hare had an idea. “I said, ‘Oh that’s like the silver foxes!’ Richard turned around and said, ‘What silver foxes?’”

Hare meant these silver foxes.

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They were the work of Russian geneticist Dmitri Belyaev. In the 1950s, Belyaev managed to breed domesticated foxes in a startlingly short amount of time. He simply selected for the nicest individuals, breeding those who were least aggressive towards their human handlers. Twenty generations later, and foxes that would once have snarled at human handlers would wag their tails instead.

But it wasn’t just the foxes’ temperaments that changed. The domestication process also warped their bodies. They ears became floppier, their tails curlier, their canines shorter, and their skulls smaller. They developed white patches on their fur. Their physiology changed too: they became less sensitive to stress and more sensitive to social cues.

They developed a suite of features known as the “domestication syndrome”, which you can see in domesticated animals from dogs to guinea pigs. This set of traits, both physical and psychological, seem to appear as a package. And it’s the same set that Hare recognised in the bonobo.

Now, in a new review, Hare puts forward the hypothesis that bonobos are “self-domesticated” apes. By naturally selecting for a ‘nicer’, less competitive ape, evolution has forged an animal with the same cluster of traits that humans have pushed onto other species.

I’ve written about Hare’s idea at Scientific American, so head over there for the full story, including why and how exactly this would have happened. For now, I want to highlight two bits of the work.

First, a nice quote from Greger Larsen – a domestication researcher – who sums up why Hare’s idea is interesting: “People have been thinking about domestication as a human-centered thing: purposeful, directed, something we do to animals. But what Brian says is that this process, which we imbue with all this human-centric meaning, is something that takes place in nature. That’s super cool.”

Second, Hare was refreshingly candid about the fact that this is a hypothesis, and open to criticisms (and you’ll find some from Frans de Waal at the SciAm piece). He himself identified three to me. First, it’s not clear if the ancestor of chimps and bonobos was more chimp-like than bonobo-like. Second, he’s making educated guesses about how the self-domestication happened because we know very little about the bonobo’s environment, both current and ancient. And third, you’d ideally want to test the self-domestication hypothesis in other species too, rather than just bonobos and chimps.

Still, it’s a fascinating idea and one that will no doubt be tested in the future. As Hare says: “The goal of the paper is to generate a lot of enthusiasm and excitement about studying bonobos,” he says.  “Even scientists don’t even know they exist and that’s horrible.”

Reference: Hare, Wobber & Wrangham. 2011. The self-domestication hypothesis: evolution of bonobo psychology is due to selection against aggression. Animal Behaviour  http://dx.doi.org/10.1016/j.anbehav.2011.12.007

Images by Pierre Fidenci and Ikiwaner; silver fox from Cornell