Their mothers are so keen for them to father children that they usher them in front of promising partners, shield them from violent competitors and dash the chances of other males by charging them while they are at it.

For a bonobo mother, it is all part of the parenting day, and analysis finds the hard work pays off. Males of the species that live with their mothers are three times more likely to father offspring than those whose mothers are absent.

Martin Surbeck, a primatologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, said: “We wanted to see if the mothers’ behaviour changes the odds of their sons’ success, and it does. The mothers have a strong influence on the number of grandchildren they get.”

Bonobo mothers seize every opportunity to give their sons a leg-up. In bonobo society, the lower ranks tend to be gender balanced, but females dominate the top ranks. Many mothers have social clout and chaperone their sons to huddles with fertile females, ensuring them better chances to mate. “The mothers tend to be a social passport for their sons,” said Surbeck.

But in the free-for-all that underpins bonobo sex, vigilance is the watchword. When their sons are finally copulating, bonobo mothers keep a wary eye on nearby males. Should any make a move to rush the busy couple – a tactic that is well-known – she can bound in and block the attack.

Such dirty tricks abound. When mothers spot other males on the job, they have been known to detach the hapless apes with a well-timed charge. On rare occasions, the mothers literally drag unrelated males off their sexual partners. “Once I saw a mother pulling a male away by the leg,” said Surbeck. “It doesn’t necessarily increase their son’s mating success, but it shows that they really get involved in the whole business.”

To assess the impact of mothers’ interventions, Surbeck and his colleagues observed several wild bonobo populations in the Democratic Republic of the Congo and wild chimpanzees in Tanzania, Uganda and Ivory Coast. Mothers from both species, which share the title of our closest living relative, helped their sons in fights, but only the bonobos boosted their sons’ mating success. In chimpanzee society, males are dominant, so the mothers have less influence.

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With a high-ranking mother on hand, the odds were particularly stacked in the male bonobos’ favour. For example, the young male of a high-ranking female might be allowed to lunch in the best feeding tree rather than being kicked out with the rest. “You see a lot of copulation while they go into these trees,” noted Surbeck. A report on the work appears in Current Biology.

In contrast to bonobo mothers, chimpanzee mothers had little impact on their sons’ reproductive success. If anything, based on the mating records the scientists analysed, chimp mothers had a slight negative impact on the chances of their sons having offspring.

While bonobo mothers looked out for their sons, the researchers found no evidence they helped their daughters in the mating game or in raising their offspring. But unlike the males, who hang around, the females usually leave the group to have their own families elsewhere.

Surbeck suspects bonobo mothers have hit on a winning strategy. In going the extra mile to get their sons mating, the mothers get to spread their genes without having to have more children themselves.