Washington, Sept. 10 (ANI): A new study has revealed that the relative size of a man's testicles can influence his involvement in parenting, asserting that men with smaller testes are more likely to be involved in hands-on care of their toddlers.
The study, conducted by anthropologists at Emory University, finds smaller testicular volumes also correlate with more nurturing-related brain activity in fathers as they are looking at photos of their own children.
"Our data suggest that the biology of human males reflects a trade-off between investments in mating versus parenting effort," Emory anthropologist James Rilling, whose lab conducted the research, said.
The goal of the research is to determine why some fathers invest more energy in parenting than others.
"Our study is the first to investigate whether human anatomy and brain function explain this variance in parenting effort," Jennifer Mascaro, who led the study as a post-doctoral fellow in the Rilling lab, said.
They knew that lower levels of testosterone in men have been correlated with greater paternal involvement, and that higher levels of the hormone predict divorce as well as polygamy.
The testes, in addition to producing testosterone in males, also produce sperm.
"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 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.
While testosterone levels may be more related to pre-copulatory, intrasexual competition, testicular volume may reflect post-copulatory mating investment, the researchers theorize.
The findings are published in Proceedings of the National Academy of Sciences (PNAS). (ANI)