Men with smaller testes tend to be more involved with hand-on childcare of their own offspring.
Dads with smaller testes also showed a higher level of nurturing-related brain activity when looking at pictures of their children, an Emory Health Sciences press release reported.
"Our data suggest that the biology of human males reflects a trade-off between investments in mating versus parenting effort," Emory anthropologist James Rilling, said.
Researchers already knew lower levels of testosterone often meant more involved fathers, and those with higher levels were more prone to having multiple partners. Testes produce both testosterone and sperm.
"Testes volume is more highly correlated with sperm count and quality than with testosterone levels," study leader and post-doctoral fellow in the Rilling lab, Jennifer Mascaro, said.
Past studies have looked at the effects of environment and social situations on parenting, but this study looked at biological correlations for the first time.
"Our study is the first to investigate whether human anatomy and brain function explain this variance in parenting effort," Mascaro said.
The researchers analyzed 70 men with children between the ages of one and two-years old, the subjects were also still married to the mothers of their children.
Both spouses were interviewed on the father's involvement with the child's upbringing. The men also had their hormone levels measured, and their brain waves read through functional magnetic resonance imaging as they looked at pictures of their kids.
"The findings showed that both testosterone levels and testes size were inversely correlated with the amount of direct paternal caregiving reported by the parents in the study," the press release stated.
The study also found the volume of the men's testes was also correlated to the amount of brain activity exhibited as they flipped through their children's photo albums.
"The fact that we found this variance suggests personal choice," Rilling said. "Even though some men may be built differently, perhaps they are willing themselves to be more hands-on fathers. It might be more challenging for some men to do these kinds of caregiving activities, but that by no means excuses them."
A question the researchers would like to answer next is if childhood stress affects testes size.
"Some research has shown that boys who experience childhood stress shift their life strategies," Rilling said. "Or perhaps fatherless boys react to the absence of their father by adopting a strategy emphasizing mating effort at the expense of parenting effort."
