Female bonobos often bond together to fend off male aggression, and that too in patterns that defy the standard primate rule book, reports Natalie Angier

The female bonobo apes of the Wamba forest in the Democratic Republic of Congo had just finished breakfast and were preparing for a brief nap in the treetops, bending and crisscrossing leafy branches into comfortable day beds. But one of the females was in estrus, her rump exceptionally pink and swollen, and four males in the group were too excited to sleep. They took turns wildly swinging and jumping around the fertile female and her bunkmates, shaking the branches, appearing to display their erections and perforating the air with high-pitched screams and hoots.

Suddenly, three older, high-ranking female bonobos bolted up from below, a furious blur of black fur and swinging limbs and, together with the female in estrus, flew straight for the offending males. The males scattered. The females pursued them. Tree boughs bounced and cracked. Screams on all sides grew deafening.

Three of the males escaped, but the females cornered and grabbed the fourth one — the resident alpha male. He was healthy, muscular and about 18 pounds heavier than any of his captors. But no matter. The females bit into him as he howled and struggled to pull free.

Finally, “he dropped from the tree and ran away, and he didn’t appear again for about three weeks,” said Nahoko Tokuyama, of the Primate Research Institute at Kyoto University in Japan, who witnessed the encounter. When the male returned, he kept to himself. Nahoko noticed that the tip of one of his toes was gone. “Being hated by females,” she said in an email interview, “is a big matter for male bonobos.”

The toe-trimming incident was extreme but not unique. Describing results from their long-term field work in the September issue of Animal Behaviour, Nahoko and her colleague Takeshi Furuichi reported that the female bonobos of Wamba often banded together to fend off male aggression, and in patterns that defied the standard primate rule book.

Close-knit

Adult females responded to a broad range of male provocations — unwanted sexual overtures, food disputes, pushing, kicking, vocal threats, persistent pestiness — by forming coalitions of two or more females, who would then jointly take on their male tormentors. Remarkably, the female partners in a bonobo posse cooperated with one another despite lacking any ties of blood or even close friendship. As the so-called dispersing sex, female bonobos must leave their birthplaces before puberty and find another social set to join, which means that none of the adult females in a given bonobo community are kin.

Moreover, female bonobos rarely formed coalitions with their preferred girlfriends — the individuals they spent the most time with and groomed the most ardently. Instead, the researchers found, coalitions arose when a senior female would step in and take the side of a younger peer caught up in an escalating conflict with a resident male.

By delivering the formidable lustre of her social standing, as well as an extra pair of hands, the intervening senior pretty much guaranteed that the skirmish would break her way. The new results add depth and complexity to our emerging understanding of Pan paniscus, the enigmatic, lithe great ape with the dark licorice eyes, who lives only in the Democratic Republic of Congo and is seriously endangered. The bonobo is a sister species to the more widespread common chimpanzee, Pan troglodytes, and the two share equal footing as our nearest primate kin.

The latest research indicates that the nature of the bonobos’ sororal bonds shifts depending on circumstances, and that the most effective deterrent to male harassment may be a cross-generational pact. “I sometimes think that bonobos sit up late at night reading papers about primates, and then decide to do the opposite,” said Joan Silk, a primatologist. “They’re unusual in so many ways.”

The outstanding question for researchers is how the female solidarity routine started. Male chimpanzees remain in their natal home, so their male-male bonds are built on the standard evolutionary principle of kin selection. Why did female bonobos defy the norm and start cooperating with one another? And why don’t male bonobos forge alliances with other nearby males who are likely to be their brothers and cousins? Differing ecological conditions may have helped set the stage for the behavioral divergence. By this hypothesis, bonobos evolved in a region with a comparatively abundant and reliable food source, which meant that females could forage in view of one another without coming to blows.

The more time they spent foraging, the more affiliative they became, and soon they were applying their displays of mutual respect and tolerance to other tasks, like rebuffing male harassers. As for male bonobos, they may be subordinate themselves to females in cliques, and they may have no interest in hanging out with the guys. But they have a secret social weapon: their mothers. Male bonobos stay with their mothers for life, and as her status grows with age, so does his. Researchers suggested that the new work has implications for understanding human evolution and the future, especially for women.