Belligerent bonobos

Of all Jane Goodall’s discoveries about chimpanzees, perhaps none was more astonishing than that males sometimes form coalitions to make war-like attacks against rival groups, killing members of their own species.

But bonobos, which are just as closely related to humans, have been seen as nonaggressive types who settle their differences amicably.

Now, a study led by Maud Mouginot, a biological anthropologist who recently completed her doctorate with College of Biological Sciences Professor Michael Wilson, has revealed a subtler picture of bonobos that complicates the picture of how human aggression evolved. Comparing bonobos in Kokolopori, a community reserve in the Democratic Republic of the Congo, to chimpanzees in Gombe National Park, Tanzania, the researchers observed a three-fold higher rate of male-on-male aggression among the bonobos.

That difference held even when they analyzed only instances where the parties made physical contact.

“I was looking at them and thinking, ‘Where’s the peaceful bonobo in all this?’” said Mouginot after watching bonobo males bite, push and scream at each other.

The findings “support the view that contrary to male chimpanzees, whose reproductive success depends on strong coalitions, male bonobos have more individualistic reproductive strategies,” says the team, which, in addition to Mouginot, included Wilson and two other colleagues.

Reproduction reigns

Chimpanzee males use aggression and sexual coercion to improve their reproductive success. The males dominate the females, and male-on-female aggression is more common than the reverse.

This is consistent with the observation that male chimpanzees in a community rely on one another to defend their territory, making aggression among them more costly than aggression toward females.

But in bonobo society, the tables are turned.

“As expected, given that females commonly outrank males, we found that bonobos exhibited lower rates of male-female aggression and higher rates of female-male aggression than chimpanzees,” the researchers report.

Other researchers have suggested a “self-domestication” hypothesis to explain male bonobos’ relative lack of aggression toward females. It posits that bonobo males who behave less aggressively are more likely to leave offspring.

However, the new study found that in both species, it was the more aggressive males that were more likely to mate with fertile females.

Blurring the picture of evolution

Around seven to nine million years ago, the branch of the evolutionary tree containing the last common ancestor of humans, bonobos and chimpanzees split in two. One branch gave rise to humans, the other to the ancestor of bonobos and chimpanzees. Those latter two species split from each other around two million years ago.   

A big question in the study of human evolution concerns when traits, including aggression and lack of it, originated. For example, if either chimpanzees or bonobos—but not both—existed and shared a trait with humans, it would be tempting to make assumptions about how we evolved from our recent ancestors.

But when traits show different patterns in humanity’s two cousins, it’s harder to know, for example, which might have come from our common ancestor and which appeared later.

“In reality, our ancestor was probably really different from bonobos or chimpanzees or humans,” Wilson says.

The study, “Differences in expression of male aggression between wild bonobos and chimpanzees,” appears in the journal Current Biology.