Human males living with their moms may not expect to have much luck hooking up this Valentine’s Day. But among the northern muriqui monkeys, males that spend the most time around their mothers seem to get an added boost when mating time rolls around.

The findings, published in the Proceedings of the National Academy of Sciences USA, suggest that females in some species may have evolved to play a critical role in their sons’ reproductive success. Karen Strier, the paper’s lead author and a professor of anthropology at the University of Wisconsin–Madison, says the paper “extends” the so-called grandmother hypothesis, a concept in which human females evolved to live past their prime reproductive years to spend more time helping offspring.

The research team observed and collected genetic data from a group of 67 wild monkeys living in a protected reserve in Brazil’s Atlantic Forest: infants, mothers and possible sires. They found that six out of the 13 adult males they studied spent more time in close proximity to their mothers than would be expected by chance. These same six monkeys, on average, sired the greatest number of offspring.

The investigators are still trying to figure out why. “It’s not like we see moms intervening and helping their sons out,” Strier says. “Maybe by sitting near their moms, they get to see when females are sexually active, or maybe they just get more familiar with other females.” Strier also found that there was no inbreeding among sons and their close female relatives, a process that might also be mediated by mothers. “Mating may be less random than we think, perhaps because of the influence of the mothers,” she says.

The findings can help with future conservation efforts for the critically endangered species. “The last thing we would want to do is take a male out of its natal group,” Strier observes.