Maternal influence. When mom's around, lower-ranking bonobos get more of the action otherwise hoarded by the top ape.

Frans Lanting Studio/Alamy

Mama's Boys Get the Girls

Most people don't want their parents meddling in their sex lives. But for one species of ape, having mom nearby can actually increase the odds of hooking up with an eligible mate.

Bonobos—our closest living relatives, along with chimpanzees—aren't puritanical. Sex for these apes is a public, accepted form of social currency. They use it to acquire food from others, defuse conflicts, and ingratiate themselves with their superiors. But bonobos also live under a rigid social hierarchy. An ape retains its rank even when a community splits up into smaller groups to forage for food, which the primates do frequently.

Normally with bonobos, the highest-ranking male in the group also mates the most, typically with the nubile females. But male bonobos also stick close to their mothers, sometimes spending as much as 90% of their time in their company. Because the mother-son bond is so strong, biologist Martin Surbeck of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and colleagues wondered whether having a mother close by could upset the mating hierarchy.

For almost 2.5 years, the researchers observed a community of more than 30 wild bonobos in the Democratic Republic of the Congo's Salonga National Park, keeping a close eye on the adult and adolescent males—nine in all. When the apes split into small groups with fertile females but no moms, the highest-ranking male had about 40% of the intercourse with the females, the team will report online tomorrow in the Proceedings of the Royal Society B. But if every male's mom was also present, the top male managed only about 25% of the matings, leaving more for the subordinate males. Mom's presence didn't change the hierarchy, but it did level the playing field somewhat for the apes further down, says Surbeck. "The mother's like a social passport."

The researchers still don't know exactly why having a mom nearby makes such a difference. An older female might draw more fertile females, they speculate, giving her son more opportunities to interact with them. But the evolutionary advantage is unquestionable: To pass on her DNA, a mother bonobo needs grandchildren.

Given that male bonobos almost never leave the community of their birth, they tend to be "mama's boys," according to Frans de Waal, a primatologist at Emory University in Atlanta, who was not part of the study. If a mother is high ranking, that would probably boost her sons' standing with other males, giving them an edge in the mating game.

Primatologist Alan Dixson of the Victoria University of Wellington in New Zealand isn't surprised that animals as intelligent as bonobos might find other ways to get mates, such as trailing their mothers, if rank failed them. But he says it's not clear whether the lower-ranking males are actually trying to sire children when their moms are around; it could be just casual sex.