Study suggests more involved fathers have smaller testicles

In a study to figure out why some men are more involved fathers than others, a team of researchers have found that this trait seems to have a connection to testicle size, suggesting that evolution has keyed men to have a trade-off between mating and parenting ability.

Studying 70 men, who lived with their 1 to 2 year old biological child and the child's biological mother, the parents were interviewed (separately) about how much time and effort the men put into directly caring for the child — changing their child's diapers, feeding and bathing them, staying home to take care of them when they're sick or taking their child to the doctor. The men were then examined with functional magnetic resonance imaging (fMRI) techniques to determine their brain activity while looking at pictures of their children, and to measure the size of their testicles. Their testosterone levels were also measured.

The relation wasn't perfect, but those men that were found to take a more direct role in their child's care were more likely to have lower testosterone levels and smaller testicles. Those who did not take such a direct role were more likely to have higher testosterone levels and larger testicles. Although higher testosterone levels are often equated with greater virility, according to the study lead Jennifer Mascaro: "Testes volume is more highly correlated with sperm count and quality than with testosterone levels."

One of the more interesting ideas that this study supports is that men are either more 'geared' towards mating or they're more 'geared' towards parenting. Being better at one comes at the expense of the other.

"Our data suggest that the biology of human males reflects a trade-off between investments in mating versus parenting effort," says James Rilling, an anthropologist at Emory University, in Atlanta, GA, according to ScienceDaily.

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As the saying goes though, correlation does not imply causation. The researchers say that although this relation between testicle size and paternal care-giving exists, it could actually be the other way around — that the level of testosterone and size of testes may not dictate who will be a more involved father, but instead becoming a more involved father may lower testosterone levels and shrink the testes. Also, there was 'variance' in the results, so the correlation wasn't perfect.

"The fact that we found this variance suggests personal choice," said Rilling, according to ScienceDaily. "Even though some men may be built differently, perhaps they are willing themselves to be more hands-on fathers."

(Photo courtesy: Getty Images)

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