A study has shown a trade-off between mating prowess and parenting involvement – in other words, men with smaller testicles appear to be better fathers.

Credits: SAM EDWARDS/ALAMY

Not only are fathers with smaller testicles more involved when it comes to taking care of their offspring, but their brains are also more responsive when they see picutres of their children – which seems to indicate they care more about them.

Evolutionary biologists have long observed a trade off in primates between mating efforts to produce more offspring and the time males which spend more time taking care of their offspring. For example, male chimps, which are notoriously promiscuous have testicles twice as big as humans, they produce a lot of sperm, and they usually don’t provide parental care; they also don’t pay child support. By contrast, male gorillas have relatively small testicles, and they almost always take care of their young ones.

James Rilling, an anthropologist at Emory University in Atlanta, Georgia set out to figure out what makes a father more interested in parenting, and if there is any connection to testicle size. The researchers recruited 70 fathers of children aged between one and two years, which to me doesn’t seem like quite a relevant sample size, and scanned their brains and testes at a magnetic resonance imaging (MRI) machine. They (as well as the mothers of the children) were asked to rate “parenting involvement”. The results seem to be relevant – bar the small sample size.

“It’s a very provocative and important step,” says Sarah Hrdy, an emeritus anthropologist at the University of California, Davis. She adds that more research is needed to establish whether certain men are predisposed by biology to be more nurturing. The study’s authors say that even if men are predisposed to a certain style of parenting, nurturing dads can be made as well as born. That levels of testosterone changed as a father spent more time with his child suggest flexibility in a man’s inclination toward fatherhood.

Still, it’s not easy to draw a causality between testicle size and parenting skills. Charles Snowdon, a psychologist at the University of Wisconsin-Madison believes this study explains only a small variation in paternal care.

“There are lots of other variables that affect fatherhood,” he says, citing as examples social environment and prior experience looking after younger siblings when the men were children themselves.

Via Nature