Read: Bonobos just want everyone to get along

Sometimes, they did so without trying. Bonobos live in mostly matriarchal societies, where females both occupy the highest ranks and form the core of social groups. If sons stick close to their mother, they’re more likely to end up at the center of a community, where more females sit. “That creates more mating opportunities,” Surbeck says. “It’s not that the moms physically drag their sons over. It’s more like a social passport.”

But mothers frequently took matters into their own hands, too. As Hanna did, they would stop unrelated males from interfering with their sons’ sexual encounters. They’d interfere themselves, stopping unrelated males from mating with other females. They’d gang up with their sons to evict other males from trees with lots of females.

Surbeck thinks that the mothers use these strategies as a way of furthering their own genetic legacy. They can do this by having more children of their own, or by ensuring that their children give them more grandchildren. They have little influence over their daughters, because bonobo females tend to leave home to find their own communities. Males, however, stay with their birth group, and especially near their mother. Even in the best-case scenario, a male bonobo can easily go through life without reproducing, and without a mother’s presence, the odds of his having a kid are about one in 14. To increase the size of her own dynasty, a mother needs to ensure that her sons have the best sexual opportunities.

And that’s exactly what the team has now found: Males who still live with their mother were three times more likely to sire their own children than those whose mothers had gone.

The same wasn’t true for chimps. Chimps also live in groups where sons stay home and daughters emigrate, and many chimp mothers will support their sons. But that support, on average, doesn’t seem to affect their sons’ reproductive success. Surbeck thinks that’s because chimps live in strict patriarchies where all adult males outrank all females. In such a world, a mother’s ability to help her son might count for less. But in bonobo matriarchies, where females have power and leverage, it counts for a lot. “It’s a great study that highlights the significance of elevated female social power in this close cousin of ours,” says Zanna Clay at Durham University.

Cat Hobaiter at the University of St Andrews adds that chimp mothers might just have fewer chances to help their sons, because they’re less socially cohesive than bonobos. “A bonobo mother will probably see her son most days, while a chimpanzee mother might not see her son for weeks, or even months,” she says. But Hobaiter notes that some chimp subspecies, such as those in West Africa, are more cohesive—and in Surbeck’s study, it’s telling that West African chimps were the only group in which a mother’s support did benefit her sons.