Dads With Smaller Testes More Likely To Be Involved In Parenting
September 10, 2013

Dads With Smaller Testes More Likely To Be Involved In Parenting

April Flowers for - Your Universe Online

A new study from Emory University, aimed at determining why some fathers invest more energy in parenting than others, has discovered that the size of a man's testicles make a difference. The findings, published in Proceedings of the National Academy of Sciences (PNAS), reveal that men with smaller testes are more likely to be involved in the hands-on care of their toddlers.

As they look at photos of their own children, the study finds that smaller testicular volume correlates with more nurturing-related brain activity in fathers. "Our data suggest that the biology of human males reflects a trade-off between investments in mating versus parenting effort," says Emory anthropologist James Rilling, whose lab conducted the research.

Understanding why some fathers invest more energy in parenting than others is important "because previous studies have shown that children with more involved fathers have better social, psychological and educational outcomes," said Rilling.

Evolution optimizes the allocation of resources toward either mating or parenting to maximize fitness, according to the Evolutionary Life History Theory. "Our study is the first to investigate whether human anatomy and brain function explain this variance in parenting effort," says Jennifer Mascaro, who led the study as a post-doctoral fellow in the Rilling lab.

The research team acknowledges that many economic, social and cultural factors likely influence a father's level of caregiving; they wanted to investigate possible biological links. Prior studies had shown that lower levels of testosterone in men have been correlated with greater paternal involvement. Those same studies revealed that higher levels of the hormone predict divorce as well as polygamy.

In addition to their role in producing testosterone in males, the testicles also produce sperm. Testes volume is more highly correlated with sperm count and quality than with testosterone levels," Mascaro says.

The current study had 70 participants -- all biological fathers who had a child between the ages of 1 and 2. These fathers were all living with the child and the biological mother.

Mothers and fathers were interviewed separately regarding the father's involvement in hands-on childcare. Such tasks could include changing diapers, feeding and bathing a child, staying home to care for a sick child or taking the child to doctor visits.

The male participants' hormone levels were measured. Afterwards, they underwent functional magnetic resonance (fMRI), measuring brain activity as they viewed photographs of their own child with happy, sad and neutral expressions. They were also shown similar photos of an unknown child and an unknown adult. Structural MRI was then used to measure testicular volume.

The researchers found that both testosterone levels and testes size were inversely correlated with the amount of direct paternal caregiving reported by the parents in the study. They also found that the testes volume correlated with activity in the ventral tegmental area (VTA). The VTA is a part of the brain system associated with reward and parental motivation. "The men with smaller testes were activating this brain region to a greater extent when looking at photos of their own child," Mascaro says.

The researchers theorize that testosterone may be more related to pre-copulatory, intrasexual competition, while testicular volume may reflect post-copulatory mating investment. The research team cautions that the results are statistically significant; however the correlation between testes size and caregiving was not perfect.

"The fact that we found this variance suggests personal choice," Rilling says. "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."

The study did raise the question of casualty. "We're assuming that testes size drives how involved the fathers are," Rilling says, "but it could also be that when men become more involved as caregivers, their testes shrink. Environmental influences can change biology. We know, for instance, that testosterone levels go down when men become involved fathers."

Whether childhood environment can affect testes size is another important question of the study. "Some research has shown that boys who experience childhood stress shift their life strategies," Rilling says. "Or perhaps fatherless boys react to the absence of their father by adopting a strategy emphasizing mating effort at the expense of parenting effort."

The research team focused on direct forms of paternal care only, not indirect forms such as protecting children and earning a living to provide for them.

In the decades since the 1960s, the number of women raising children on their own in the United States has risen dramatically. "Although there are more households with no fathers, when the fathers are around, they tend to be much more involved than in previous decades," Mascaro says.

Rilling notes that most of the existing scientific literature concerning nurturing is focused on mothers. "Mothers definitely have more of an impact on child development, but fathers are also important and their role is understudied."