New Delhi, Sept. 9: Men with small testes are more likely to be responsible fathers, providing care to their toddlers, according to new research.
Earlier studies had established that testosterone levels can influence paternal care-giving.
The latest study, by anthropologists at Emory University in the US, was published today in the US journal Proceedings of the National Academy of Sciences. It provides the strongest evidence yet in humans for a theory in evolutionary biology called Life History Theory, which proposes that mating and parenting involve trade-offs.
The theory predicts that men who invest time and energy in providing care to their children have less resources to pursue mating efforts.
“We believe this is the first study to link human anatomy with behaviour to explain differences in parenting patterns,” said Jennifer Mascaro, a post-doctoral researcher at Emory University and the first author of the study.
Mascaro and her colleagues examined 70 men, each of whom was living with his own biological child and the child’s biological mother. The scientists questioned the parents about the father’s involvement in tasks such as changing diapers, feeding, and bathing the child.
The researchers used magnetic resonance imaging (MRI) to measure testicular volumes, then scanned the brains of the men while they looked at photos of their own child and similar photos of an unknown child and an unknown adult.
They found that men who had relatively higher testosterone levels and larger testicular volumes were likely to be poorer caregivers to their toddlers. And men with larger testes, when shown pictures of their child, also displayed lower levels of activity in a key region of the brain called ventral tegmental area (VTA) which has been shown to be connected with the motivation to nurture offspring.
The findings, Mascaro said, suggest that activity in the VTA in response to the child’s picture “augments or supports a father’s motivation to nurture his child --- and fathers with smaller testes have more of this response”.
“We find this biological influence, but we don’t know the direction of causality,” James Rilling, associate professor of anthropology and a team member, told The Telegraph.
“It is possible that pre-existing testis size influences care-giving, but it could also be that as men become more involved as caregivers, their testes shrink. We don’t know whether the size of the testes is genetically determined or can be shaped by development environments.”
Many studies over the past two decades had suggested that high testosterone levels drive men towards increased mating efforts and reduced parenting efforts.
“While testes produce testosterone, most of their volume is involved in sperm production than with testosterone,” Rilling said.
A measure of testicular volume is a measure of sperm-production capacity. The new findings indicate that testicular volume may be a better measure of the capacity for parenting than testosterone.
Scientists caution that the study does not in any way suggest that a man’s biology alone determines his parenting quality.
“Humans have culture, social norms, and ethics,” said Anindya Sinha, a primatologist at the National Institute of Advanced Studies, Bangalore, who was not connected with the study.
“And humans can reflect on their own actions and modify behaviour, imposing certain norms on themselves.”
The Emory study is significant because while Life History Theory is “a big concept in evolutionary biology”, it has not been easy to test it on humans, said Anindita Bhadra, a biologist at the Indian Institute of Science Education and Research, Calcutta.
“They’ve done a good job and come up with clean results,” Bhadra said. “It is a nice study from the perspective of a behavioural biologist.”
While the results suggest that some men are naturally more inclined towards childcare, Mascaro said, many biological, societal and cultural factors also influence a father’s potential for parenting.
Testicular volume is only one among multiple factors. “I suspect that all men are capable of learning to become good fathers,” Rilling said, “especially if they make the effort to get involved early on.”