Bonobo Mothers Interfere in Their Sons’ Monkey Business

They find suitable mates for their offspring and chase away intruders once the mating begins, boosting fertility rates

Martin Surbeck

You probably won't be getting lucky if your mom sticks around on your date. But in the primate world, things work a little differently. New research shows that male bonobos whose moms set them up with ovulating females and shoo away competitors while the pair consummate their chimp-lust sire three times as many offspring as bonobos navigating the primate dating world solo.

Bonobo sexuality is a lot more complicated than copulation in most other animal species. Besides humans, bonobos are one of the most promiscuous species in the world and one of the few that engage in sexual acts for bonding or pleasure. They engage in pretty much every sex act and combination imaginable, except for mother-son copulation, which is taboo. According to evolutionary biolgist Ben Garrod, writing at The Conversation, about 75 percent of bonobo sexual encounters are solely for pleasure. With all that monkey business going on, it’s probably easy for bonobos to forget the baby-making aspect of sex.

That’s where the mothers come in. In the bonobo hierarchy, high-status females are at the top. When a would-be grandma notice females of the species ovulating, they will bring their son nearby hoping to begin a sexual encounter. Then, the new study in the journal Current Biology found, they watch over the couple to make sure things go according to plan. And in the sexual free-for-all that is a bonobo community, that’s necessary since other individuals are not shy about joining in.

Once the hanky-panky begins, the moms chase away interloping males, sometimes physically preventing them from interfering. “Once I saw a mother pulling a male away by the leg,” lead author Martin Surbeck, primatologist at the Max Planck Institute for Evolutionary Anthropology, tells Ian Sample at The Guardian. “It doesn’t necessarily increase their son’s mating success, but it shows that they really get involved in the whole business.”

Overall, however, mothers and sons working in tandem do improve mating success threefold. “This is the first time that we can show the impact of the mother’s presence on a very important male fitness trait, which is their fertility,” Surbeck says in a press release. “We were surprised to see that the mothers have such a strong, direct influence on the number of grandchildren they get.”

Surbeck tells Ed Yong at The Atlantic that he’d noticed the weird behavior of high-status females running interference for mating males several times over 16 years of observing bonobos. But it wasn’t until his team sequenced DNA from bonobo scat that they confirmed that mothers were keeping guard over their sons' trysts. That’s when he decided to look into the phenomenon more closely.

Surbeck and his colleagues watched bonobos in the Democratic Republic of the Congo, the only country in which the endangered primates live, and chimpanzee groups in Tanzania, Uganda and Ivory Coast. In both species, they found, mothers were willing to step in and throw a punch or two when their sons got embroiled in a fight. But other than that, chimpanzee mothers weren’t as involved. The difference is likely because chimp society is male-dominated, meaning the mothers don’t have the same social clout to repel interlopers, reports Sample. The bond between mother and son didn’t seem to have any positive effect on chimpanzee male fertility and may have had a slightly negative effect.

Yong reports that with bonobos, the benefits of hanging out with mom mainly stem from her social status. Their sons are given access to the inner sanctums of the 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.”

He tells Jason Bittel at The Washington Post that males without moms to introduce them to high society tend to hang out on the edges othe bonobo group and sire fewer offspring.

Bonobos don’t practice the same type of helicopter parenting with their daughters, likely because female offspring eventually leave their birth group to find mates while sons stick close to mama their whole lives.

Bittel reports that the finding raises some comparisons to the controversial Grandmother Hypothesis. In most animal species, females usually die after passing their reproductive prime. But in humans and a few other species including killer whales and some aphids, women live long stretches of time after menopause. The hypothesis says that they stick around so long to help support their grandchildren and ensure they pass on their genetic legacy.

In the press release, Surbeck says this might be what the helicopter bonobo moms are doing. “These females have found a way to increase their reproductive success without having more offspring themselves,” he says.

Currently, Yong reports, there’s no evidence female bonobos go through menopause, but it might be something researchers have overlooked.

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