Men with smaller testicles tend to be more involved as fathers, a new study suggests.
James Rilling and his colleagues at Emory University in Atlanta, Georgia, used MRI scans to measure the volume of the testicles of 55 fathers. The team asked these men – and the mothers of their children – a series of questions to determine how involved they are as fathers. The team also used fMRI brain scanners to look at the brain activity of the men as they viewed photos of their kids.
Men with smaller testicles got the best parenting scores in the questionnaires. When looking at pictures of their children, these men also showed more activity in regions of the brain associated with empathy and motivation to care for offspring than men with bigger testes, which suggests they are more nurturing fathers.
Previous studies have suggested that men with more testosterone are more sexually active, but that levels of the hormone drop once men become fathers. But the new link between testicle size and involvement in parenting could not be explained by the participants' testosterone levels.
"Circulating testosterone isn't the whole story," says Susanne Schultz, an evolutionary biologist at the University of Manchester, UK, who describes the finding as "a very surprising result".
Energy equation
Men with bigger testicles tend to produce more and higher-quality sperm. There may be a trade-off between the amount of energy a man invests in making sperm, which can be thought of as a mating strategy, and parenting, Rilling says. "We all do really have a finite amount of energy that we can spend on things," he says.
Rilling does not know how smaller testicles might make men better fathers. "We're assuming that testis size drives how involved the fathers are, but it could be that when men become more involved as caregivers, their testes shrink."
The findings do not necessarily mean that men with bigger testicles will be worse fathers. Most fathers choose how involved they are in their child's upbringing, says Rilling. "It might be more challenging for some men to do these kinds of caregiving activities, but that by no means excuses them," he says.
Journal reference: PNAS, DOI: 10.1073/pnas.1305579110
If you would like to reuse any content from New Scientist, either in print or online, please contact the syndication department first for permission. New Scientist does not own rights to photos, but there are a variety of licensing options available for use of articles and graphics we own the copyright to.
Have your say
Only subscribers may leave comments on this article. Please log in.
Only personal subscribers may leave comments on this article
All comments should respect the New Scientist House Rules. If you think a particular comment breaks these rules then please use the "Report" link in that comment to report it to us.
If you are having a technical problem posting a comment, please contact technical support.
