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Among most apes, males seem to dominate the social hierarchy—but for one of our closest living relatives, it’s a different story. In their social circles, female bonobos call the shots. They exhibit unique control over mating and get the first pick of food, while males wait for their turn. But researchers haven’t been entirely sure how the females earn their high status against their bulkier, brawnier male bonobo peers.
Some scientists have hypothesized that female bonobos dominate males by forming coalitions, but this theory has been tough to prove. Others have proposed that the female bonobos’ elusive sexual cycle gives them the upper hand. Or that a self-reinforcing effect is at work: Individuals are more likely to win a conflict when they have won one before, and more likely to lose if they have lost. Played out over generations this could create a population-level dynamic.
Recently, a team of scientists from the Max Planck Institute in Germany, Harvard University, and other institutions analyzed three decades of bonobo observations from the wild and found that they support only the coalitions hypothesis: When the female bonobos under study formed more frequent coalitions, they tended to win more conflicts and attain higher rank compared to males. The findings were published in a study in Communications Biology.
Bonobo mothers tend to guide their sons’ social lives, including mating.
“We see this really beautiful relationship” in the data during the years that females form more coalitions in specific communities, says study author Leveda Cheng, an evolutionary anthropologist at Harvard. Cheng and her colleagues also found that 85 percent of female coalition aggression was directed at males. Sometimes this aggression seemed to serve as retaliation for male aggression against mature females or against offspring. Other times, the triggers for aggression were difficult to identify.
When female bonobos team up against males, things can get ugly: In the field, researchers have observed females chasing, kicking, and even injuring males. Once, a female bonobo at the Stuttgart Zoo in Germany was suspected of nearly biting a male bonobo’s penis in half. At the Kokolopori Bonobo Reserve in the Democratic Republic of the Congo, Cheng saw 15 cases of physical injuries among males caused by female or mixed-sex coalitions. Some injured males were spotted avoiding members of their community for weeks afterwards.
Any researcher who has worked with female bonobos in the wild is familiar with these female groups and their crucial societal role, says Zanna Clay, a comparative psychologist who specializes in primatology at Durham University in England. But the new study provides the first comprehensive set of evidence for the idea. “This study really systematically tests using these large datasets across a long time period,” says Clay, who was not involved in the new research. “It’s an important confirmation of what people have long suspected.”
Cheng’s team crunched data spanning 30 years’ worth of observations at six wild bonobo communities in the Democratic Republic of the Congo—the only country where the species still lives outside of captivity. Observation periods ranged between two and six years for any given community. With their rich data set, the scientists modeled several hypotheses to explain female bonobo supremacy, including the advantages conferred to female bonobos’ by their reproductive systems.
An abundance of food may have allowed female bonobos to start sharing meals, the seeds of future coalitions.
The females’ fertility window is difficult to decipher for the males, which makes it more advantageous for them to hang around and let the females choose when to mate, than to force them to mate with aggression. When many primates ovulate, the skin around their genitals swells—but the timing of bonobos’ ovulation can vary widely, and it may not coincide with peak sexual swelling. This drives the males bananas. The recent paper didn’t find strong evidence for this reproductive trick as a key driver of female power, but Clay says it deserves further investigation.
Plenty of questions remain to better understand some of our closest living relatives. For one, it still isn’t clear why bonobos’ sexual politics vastly differ from those of most other non-human primates. Chimpanzees, with whom bonobos share 99.6 percent of their DNA, maintain a male-driven hierarchy. And yet, female chimps do seem to display a degree of social power and form strong bonds when observed in sanctuaries, says Jake Brooker, a primatologist at Durham University who didn’t work on the recent study. But he says it’s not quite as pronounced as what researchers have seen in bonobos.
These differences could be traced back to primate evolution, Cheng says. Chimps and bonobos are thought to have diverged nearly 2 million years ago, when they were split by the formation of the Congo River. In their new territory, bonobos perhaps no longer had to compete with other apes for food. This abundance may have allowed females to start sharing meals, which could have led to the development of tight-knit coalitions. Cheng says more modeling is needed to test this theory.
But why don’t bonobo males band together to fight back against females? One possible explanation: Bonobo mothers tend to guide their sons’ social lives, including mating—some even stand guard while their sons mate. This may keep them in line, Cheng says, and mums likely shape relationships throughout bonobo societies. Chimpanzee mothers play a less significant role in the lives of their sons, by comparison.
Clay and Brooker say they hope that researchers take a closer look at the individual behavior of bonobos and other primates to grasp the variety of behaviors within social groups. These nuances could help explain what made us human—but Brooker says these insights will only be possible if we protect the habitat of wild bonobos.
Variation is paramount to social groups, even human ones, Brooker says. “No two human communities are the same in how much cooperation we might show—or how much hostility we might have towards out-groups. 
Lead image: Edwin Butter / Shutterstock
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