Martin Surbeck remembers the episode vividly. He was in the Democratic Republic of Congo’s LuiKotale rain forest, watching a group of bonobos, African apes that are closely related to chimpanzees. Two of them—Uma, a female, and Apollo, a young, low-ranking male—were trying to have sex. Camillo, the highest-ranked male in the group, caught wind of their liaison and tried to come between them. But Hanna, Apollo’s mother, rushed in and furiously chased Camillo away, allowing her son and his mate to copulate in peace.
This was just one of the many memorable incidents that Surbeck and his colleagues at the Max Planck Institute for Evolutionary Anthropology have observed over 16 years of watching bonobos. At first, before getting to know the individuals involved, Surbeck was surprised. “It’s not typical female behavior,” he says, in that female apes often stop unwanted males from mating with them, but very rarely police the mating attempts of other couples. He only worked out what was happening by collecting the bonobos’ poop, and sending the samples off to colleagues who sequenced the DNA within. Their analysis confirmed how the different bonobos were related, and clearly showed that mothers were repeatedly and actively improving their sons’ sex lives.
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.
Regardless, the broad difference between bonobos and chimps is both important and expected, says Michelle Rodrigues at the University of Illinois at Urbana–Champaign. Researchers already knew that a male bonobo’s rank depends on his mother’s, but it wasn’t clear if that was due to Mom’s social support, or her genes. After all, the same beneficial genes could account for both her success and his. “But if that was the case, we would see a similar pattern in chimps,” Rodrigues says. That we don’t, even though the two species are so genetically similar, “makes a compelling case” that it’s the differences in their social lives that matter.
Bonobos aren’t the only nonhuman animal group in which mothers clearly assist their adult sons. Orca (killer whale) mothers lead their sons to hot pots of salmon, and sons seemingly depend on that knowledge. If a mother orca dies, her sons’s odds of also dying within the next year can rise by up to 14 times.
This might explain why killer whales are among the few wild animals whose females go through menopause. Along with humans, short-finned pilot whales, narwhals, and belugas, orcas stop reproducing well before the end of their life. Perhaps they have evolved to, at some point, switch from having their own offspring to ensuring that their existing children survive long enough to bear grandchildren.
If that’s the case, one might expect bonobo females to also go through menopause—and there’s no evidence that they do. But Surbeck thinks that might be because no one has looked hard enough. “We’ve seen that certain females like Marta [Camillo’s mother] don’t reproduce for a long time,” he says. “We need to do a broader study to address this question properly.”