Men with smaller testicles are more nurturing and tend to be hands-on fathers than those with larger testes, a new study suggests.
Smaller testicular volumes also correlate with more nurturing-related brain activity in fathers as they are looking at photos of their own children, the study shows.
"Our data suggest that the biology of human males reflects a trade-off between investments in mating versus parenting effort," said Emory University anthropologist James Rilling, whose lab conducted the research.
The goal of the research was to determine why some fathers invest more energy in parenting than others.
"It's an important question because previous studies have shown that children with more involved fathers have better social, psychological and educational outcomes," Rilling said.
Evolutionary Life History Theory posits that evolution optimises the allocation of resources toward either mating or parenting so as to maximise fitness, researchers said.
"Our study is the first to investigate whether human anatomy and brain function explain this variance in parenting effort," said Jennifer Mascaro, who led the study.
While many economic, social and cultural factors likely influence a father's level of care-giving, the researchers wanted to investigate possible biological links.
"Testes volume is more highly correlated with sperm count and quality than with testosterone levels," Mascaro said.
The study included 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.
The mothers and fathers were interviewed separately about the father's involvement in hands-on childcare, including tasks such as changing diapers, feeding and bathing a child, staying home to care for a sick child or taking the child to doctor visits.
The men's testosterone levels were measured, and they underwent functional magnetic resonance imaging (fMRI) to measure brain activity as they viewed photos of their own child with happy, sad and neutral expressions, and similar photos of an unknown child and an unknown adult. Then, structural MRI was used to measure testicular volume.
The findings showed that both testosterone levels and testes size were inversely correlated with the amount of direct paternal care-giving reported by the parents in the study.
And the father's testes volume also correlated with activity in the ventral tegmental area (VTA), 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 said.
The study was published in the journal Proceedings of the National Academy of Sciences (PNAS).
