Editor, Earth News
What are platonic friends for?
Male and female baboons form platonic friendships, where sex is off the menu.
Having a caring friend around seems to greatly benefit the females and their infants, as both are harassed less by other baboons when in the company of their male pal.
But why the males choose to be platonic friends remains a mystery.
The finding published in Behavioral Sociobiology and Ecology also suggests that male baboons may be able to innately recognise their offspring.
Primatologist Nga Nguyen decided to investigate the occurrence of 'platonic' friendships in four groups of yellow baboons living in Amboseli, Kenya.
To do so, Nguyen, now at California State University Fullerton, teamed up with colleagues Russell Van Horn of the Zoological Society of San Diego, California, and Susan Alberts of Duke University in Durham, North Carolina and Jeanne Altmann of Princeton University, New Jersey.
Male and females of a few species of monkey, including baboons, macaques and others are known to form so-called 'friendships', where particular males and females will spend a lot of time in each other's company.
These friendships are often strictly platonic, and don't seem to involve sex. But no-one knows why they occur.
"We don't really know what males or females get from these friendships," says Nguyen. "Males should be off trying to get other females to mate with them, not squandering their time on a female with a young infant."
So Nguyen's team investigated whether these chaperone males were actually fathering infants with their female friends.
They studied the behaviour of more than 500 male and females in the four groups, and used genetic tests to determine the paternity of 183 of the baboons, including 23 young infants being cared for by a mother and her chaperone.
Half of all the male chaperones did turn out to be the father of the infant whose mother they befriended.
That is highly surprising in one respect, because each of the females mated with multiple males around the time they conceived. "But of these potential dads, only the genetic dads became friends," says Nguyen.
"To my knowledge, human males cannot tell their own offspring from unrelated offspring, but somehow baboon dads can tell."
But the study revealed an even bigger surprise.
"Half of the friends were not genetic fathers. But these guys weren't even potential fathers, that is, they didn't even mate with the female when she conceived the infant, and these guys didn't receive mating benefits."
Friends don't always become sexual partners
"So we really don't know what these guys got out of the friendship, other than maybe spending time with a mum and a new baby and having other females seeing this."
The suggestion here is that by chaperoning a female in a platonic relationship, a male might advertise his parental skills to other females, who then might consider him a worthy partner. But as yet, there's no evidence for this or any other reason why males become chaperones.
However, for the females, the benefits of having a chaperone are clear.
"We found direct evidence that friendships provided a social benefit to mothers and infants," says Nguyen.
"We found that mother-infant pairs who spent a lot of time with their male friends received a lot less harassment from other females in the group, and the infants cried a lot less too, than pairs who spent less time hanging out with their male friends. This could translate into big gains for infants who may be more likely to survive infancy, as harassment can lead to injury," she says.
"It was especially exciting when I looked and saw what a huge difference having a friend around means for the mother and infant. We've long suspected that mother-infant pairs got some social benefit from the male friends, but this benefit had never before been documented."