We’ll have more to say about the evolution of our very distinctive social organization as the blog carries on. But it may be informative to consider our closest relatives, chimpanzees and bonobos. The two species are closely related, having diverged only about 2 million years ago. They remain physically quite similar, and people didn’t even figure out that bonobos are a separate species until the twentieth century. There are some broad similarities in their social organization. Both species have fission-fusion societies, in which subgroups form and reform within a larger, more stable community. Both species have male philopatry: males spend their lives in the community they were born in, while females transfer out of their natal community to a new community when they reach sexual maturity. In both species, females commonly mate with many males over the course of an estrus cycle. But there are some important differences.
Jane Goodall began studying chimpanzees in the wild at Gombe National Park, Tanzania, in the 1960s. The early reports from Gombe captivated the world with stories of chimpanzee social life, tool use, and interactions with human observers. It was the 1960s, and chimpanzees – hairy, sexually promiscuous, grooving in the jungle – looked familiar: they were hippies.
The picture darkened a lot in the 1970s, when the community at Gombe split in two. Between 1974 and 1978, the two daughter communities were effectively in a state of war. Males from the larger of the two communities carried out a series of raids against the smaller, with raiding parties opportunistically picking off and killing isolated individuals, eventually eliminating all the males and some of the females. Subsequent studies of other chimpanzee populations have made it clear that this was not an isolated incident: intergroup warfare and group extinction are general features of chimpanzee life. Chimpanzees are still hairy, still sexually promiscuous, but they now look less like hippies and more like bikers. Really scary bikers.
Bonobos look like the real hippies. They are more peaceable. They show less violence between groups, with members of neighboring groups sometimes even feeding peacefully in proximity to one another, something unthinkable for chimps. There is also less within-community male-male violence among bonobos. Bonobo females play a major role in regulating and intervening in male-male competition, and may even be dominant to males. There are tensions within bonobo communities but these are often resolved by (non-reproductive) sexual activity. For example, females, who are generally not related to one another because they were born elsewhere, might be expected to find themselves fighting over food. Instead they settle potential feeding conflicts peaceably by “g-g (genital-genital) rubbing,” rubbing their sexual swellings together until they reach orgasm. Or do a pretty convincing job of faking it: “I’ll have what she’s having.”
However recent DNA tests have revealed an unexpected twist to the chimpanzee/bonobo comparison. In spite of the more peaceable nature of male bonobos compared to male chimps, it turns out that there is actually greater reproductive inequality among male bonobos and a stronger relationship between dominance rank and reproductive success! Dominant male bonobos are more successful than dominant male chimps in monopolizing reproduction. If bonobos still look like hippies, then they are the kind of hippies where a lot of free loving is going on, but the whole happening is run by and for the leader (backed up by his mom) and his groupies.