an autodidact meets a dilettante…

sexual dimorphism – a difference on average, but massive individual variation

Men are bigger than women, slightly. That’s how things evolved. It’s called sexual dimorphism. It happens with many species, the genders are different in size, shape, coloration, whatever. With humans there’s a size difference, and something of a shape difference, in breasts and hips, but really these aren’t significant. Compare, say, the deep-water triplewort seadevil, a type of anglerfish, in which the female is around 30 cms long, and the male a little over a centimetre. The difference in mass would be too embarrassing to relate.

Among our primate cousins the greatest sexual dimorphism, in size as well as other features, is found in the mandrills, with the male being two to three times the size of the females. In some gorillas there’s a substantial size difference too in favour of the males, and in fact in all of the primate species the male has a size advantage. But size isn’t everything, and the bigger doesn’t have to always dominate.

Female bonobos are smaller than the males, even more so than in humans, yet they enjoy a higher social status than in any other primate society, probably including humans, though it’s hard to compare, since humanity’s many societies vary considerably on the roles and status of women. So how have females attained this exalted status within one of the most highly socialised primate species?

Bonobos and chimpanzees are equally our closest living relatives. It isn’t clear when exactly they separated from each other, but some experts claim it may have been less than a million years ago. Enough time for them to become quite distinct physically, according to the ethologist Franz De Waal. Bonobos are more gracile with longer limbs and a smaller head, and they have a distinctive hairstyle, with a neat parting down the middle. They’re also more easily individuated by their facial features, being in this sense more like humans. And there are also major differences in their social behaviour. Male chimps are dominant in the troupe, often brutally so, whereas bonobo society is less clearly hierarchical, and considerably less violent overall. De Waal, one of the world’s foremost experts on both primates, became interested in bonobos primarily through studies on aggression. He noted that sometimes, after a violent clash, two chimps would come together to hug and kiss. Being interested in such apparent reconciliations and their implications, he decided to look at reconciling behaviours in other primates. What he discovered in bonobos (at San Diego Zoo, which in 1983 housed the world’s largest captive colony) was rather ‘shocking’; their social life was profoundly mediated by sex. Not that he was the first to discover this; other primatologists had written about it, noting also that bonobo sex was far more human-like than chimp sex, but their observations were obscurely worded and not well disseminated. There are other aspects of the physical nature of sexual relations in bonobos that favour females, such as female sexual receptivity, indicated by swelling and a reddening of the genital area, which pertains for a much longer period than in chimps. Female bonobos, like humans and unlike other primates, are sexually receptive more or less all the time.

This isn’t to say that bonobos are oversexed, whatever that may mean. Sexual relations are far from constant, they are casual, sporadic and quickly done with. Often they’re associated with finding food, and it seems likely that sexual relations are used to reconcile tensions related to food availability and other potential causes of conflict.

So how does this use of sex relate to the status of females in bonobo society. I’ll explore this further in the next post.

bonobo relations – more than just sex