A young male bonobo is groomed by his mother in the Kokolopori Bonobo Reserve Martin Surbeck

If your mum gets too involved in your love life, spare a thought for bonobos. Females of these great apes, which are closely related to chimpanzees, help their sons with hook-ups, guard the young lovers while they mate, and even haul rival males off females mid-sex.

And it’s a strategy that works. Males whose mothers are in their group have three times the number of babies as those who don’t.

It’s a strategy that’s akin to the “grandmother hypothesis” in humans, which says that older women can boost their reproductive success by helping their daughters rear children rather than having more offspring of their own. “Female bonobos can increase their fitness even if they don’t reproduce any more — but not through daughters, it’s through their sons,” says Martin Surbeck of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.


Read more: Chimp evolution was shaped by sex with their bonobo relatives

Bonobos have an unusual social structure in that the top-ranking individuals are female, and sons usually stay with their mothers while young females leave to find new groups. These apes are also famed for having lots of sex, for social reasons, as well as reproduction. It happens whether or not females are fertile — although when they are fertile they are more desirable.

Surbecks’ team has previously described the bonobo mothers’ over-involvement in their sons’ love lives while following a community of 35 animals in the Salonga National Park in the Democratic Republic of Congo. They found mating attempts are often disturbed by others, but mothers tend to hang around their sons and stop any such meddling. “If your son is copulating and another male tries to interfere, you chase this male away,” says Surbeck.

Mother knows best

When it comes to other males having sex, though, mothers are positively unhelpful. They may charge at the couple and threaten the male. Surbeck even saw one matriarch pull away a male by his feet in the middle of the act. “The males let go and run. They are careful not to be too aggressive because that can result in the females ganging up on the male,” he says.

Bonobos can also help their sons with the most fertile females – in this case simply by getting close and grooming them. “The only males that can be in that core part of the group are the ones who can use their mums as a passport to get in,” says Surbeck.

In the team’s latest work, they showed that such tactics work, as judged by the greater number of children fathered by males whose mothers are around. In contrast, this effect wasn’t there when the team followed groups of chimpanzees, where males are dominant, living in Uganda, Tanzania and Côte d’Ivoire.

However, the female bonobos may also contribute to their sons’ success in other ways than sex, such as by helping promote their status in the group hierarchy.

Read more: Our cousins chimps and bonobos use similar sign languages

“Mothers finding suitors for their sons is something we can relate to,” says Zanna Clay of Durham University, who was not involved in the work. “It reflects the differences in bonobo society where you have enhanced female dominance.”

Surbeck says that because bonobo mothers can boost their grandchildren in this way, they may also experience menopause, as humans do — because in old age they might get more return on helping their sons than having their own offspring. “They would be a very good candidate.”

The females of only a few species go through menopause, in other words, survive for long after they have lost the ability to reproduce, including orcas and short-finned pilot whales.

Journal reference: Current Biology, DOI: 10.1016/j.cub.2019.03.040