Biologically speaking, female and male bonobos have a weird relationship. First, there's the sex. It's the females who decide when and with whom they mate. They easily parry unwanted sexual advances—and the males know better than to force the issue. Second, there's the food. It's the females who usually control high-value, sharable resources—a fresh kill, say. They feed while sitting on the ground, unthreatened, while males hover in tree branches waiting for their turn.
This freedom enjoyed by females might sound normal by our standards, but according to Martin Surbeck from Harvard University, it's "totally bizarre for an animal like a bonobo." Bonobo males are larger and stronger than females, which gives them the physical upper hand to attack, force matings, and monopolize food. Like almost all other social mammals with larger males, bonobo societies should be dominated by males. And yet, bonobo females famously maintain a high social status compared to their larger male counterparts. Until now, though, nobody knew how this paradoxical dynamic was possible at all.
"There were competing ideas for how," says MPI-AB's Barbara Fruth who has led the LuiKotale bonobo research station for 30 years, "none of which had ever been tested in wild bonobos living in the jungles in which they evolved."
Female solidarity as a tool for power
Now, a study by Surbeck and Fruth has delivered the first empirical evidence from wild bonobos explaining the rare phenomenon: females maintain power by forming alliances with other females. The study found that females outranked males when they formed gangs, which the authors named "coalitions." In the vast majority of coalitions—85% of those observed—females collectively targeted males, forcing them into submission and shaping the group's dominance hierarchy.
"To our knowledge, this is the first evidence that female solidarity can invert the male-biased power structure that is typical of many mammal societies," says Surbeck, the study's first author. "It's exciting to find that females can actively elevate their social status by supporting each other."
A window to wild bonobos
An international team of researchers compiled 30 years of data from six wild bonobo communities across three field sites in the Democratic Republic of the Congo, which is the only country where bonobos live in the wild. The dataset included observations of 1,786 conflicts between males and females. The researchers analyzed the outcome of these conflicts—of which 1,099 were won by females—together with a range of social and demographic data. By doing so, they unearthed clues as to what influenced "female power" which they defined as all the factors that tip the outcome of a conflict. "You can win a conflict by being stronger, by having friends to back you up, or by having something that someone wants and cannot take by force," says Surbeck the first author.
The team had some early hunches as to where the results would point. Surbeck was sure that female domination was driven by reproductive strategies, such as hidden ovulation, which prevent males from monopolizing mating opportunities. The result of coalition formation came as a surprise. Adult females are unrelated immigrants from different communities who did not grow up together, which makes their deep bonds and cooperation unexpected. Also, adds Surbeck who runs the Kokolopori bonobo research station: "You just don't see coalitions forming that much in the wild."
But when coalitions form, they make an impression. The first sign is screaming so unbearably loud "you have to block your ears," says Fruth. It's hard for scientists to know what triggers a coalition as they form within seconds of an event, such as if a male attempts to hurt young. The target male is followed through trees by screaming females who can sometimes cause fatal injuries. "It's a ferocious way to assert power," adds Fruth. "You know why these males don't try to overstep boundaries."
Not always "dominance"
But the wide-ranging study, which compared six bonobo communities, laid bare previously unknown nuance in the famed dominance of females. While females in the study won 61% of conflicts and outranked 70% of males on average, this dominance was "by no means the rule," says Fruth. Rather, female dominance varied in populations along a spectrum. "It's more accurate to say that in bonobo societies, females enjoy high status rather than unchallenged dominance," she says.
Female coalitions are just one mechanism likely to drive the empowerment of female bonobos, the authors say. Female reproductive autonomy almost certainly changes power relations between the sexes. The fertile window of females is hidden from males, who gain more by trying to stay near females than by aggressively coercing them to mate. Testing this and other ideas are topics of future research.
Deeper questions linger, but their answers might forever remain elusive. Says Fruth: "I'm still puzzled why, of all animals, bonobos were the ones to form female alliances. We might never know, but it gives me a glimmer of hope that females of our closest living relatives, in our evolutionary line, teamed up to take the reins of power alongside males."
