Men who have smaller testes are more likely be better dads to their toddlers, according to a study published in The Proceedings of the National Academy of Sciences.
Researchers from Emory University in Georgia set out to determine why some fathers invest more energy in parenting than others, after prior research has shown that children who have a better relationship with their fathers have better social, psychological and educational outcomes.
Previous studies have shown that lower levels of testosterone in men have been linked to increased parental involvement. But the researchers were interested to see whether testicle volume, more closely linked to sperm count and quality rather than testosterone levels, also played a part.
For the study, the researchers analyzed 70 biological fathers who had a child between the ages of 1 and 2, and who were living with the child and its biological mother.
Researchers say that fathers who have smaller testes are more likely to be better dads
Both the mothers and fathers took part in interviews to determine the father's involvement in the care of their child.
Information was gathered on whether the father changes diapers, feeds and bathes the child, whether they stay home to care for their child when it is sick, and whether they take their child for doctor visits.
After the father's testosterone levels were measured, they underwent functional magnetic resonance imaging (fMRI), which measured their brain activity as they viewed photos of their own child with happy, sad and neutral expressions.
They were also shown photos of an unknown child and an unknown adult showing similar expressions.
Paternal caregiving dependent on testes size
The findings showed that testosterone levels and the size of testes correlated with the amount of "direct paternal caregiving" that was reported by the parents in the study.
Fathers who had smaller testes showed increased nurturing-related brain activity in the ventral tegmental area (VTA) - an area of the brain linked to reward and parental motivation - when viewing photos of their own children.
The study authors explain:
"In response to viewing pictures of one's own child, activity in the ventral tegmental area - a key component of the mesolimbic dopamine reward and motivation system - predicted paternal caregiving and was negatively related to testes volume.
Our results suggest that the biology of human males reflects a trade-off between mating effort and parenting effort, as indexed by testicular size and nurturing-related brain function, respectively."
Involvement still determined by personal choice
However, the researchers add that although the correlation between the size of testes and the nurturing activity of the father is significant, it is still down to personal choice as to how involved the father wants to be in their child's care.
"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," says James Rilling, anthropologist at Emory University and study author.
The researchers also point out that testes size may not be fully accurate in predicting a father's involvement in their child's lives.
"We're assuming that testes size drives how involved the fathers are," adds Prof. Rilling. "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."
Prof. Rilling adds that another important question is whether the environment a father was brought up in as a child could effect his testes size:
"Some research has shown that boys who experience childhood stress shift their life strategies. Or perhaps fatherless boys react to the absence of their father by adopting a strategy emphasizing mating effort at the expense of parenting effort."