Men with larger testicles tend to be less involved fathers than
those with smaller testes, a new study suggests.
The findings, detailed today (Sept. 9) in the journal Proceedings
of the National Academy of Sciences, are correlational, so they
can't say exactly why the trend exists but only that there is a
link.
But men who produce more sperm have bigger testes,
and sperm
production is extremely energy intensive for the body, so
it may be that fathers "face a trade-off between investing energy
in parenting and investing energy in mating effort," said study
co-author James Rilling, an anthropologist at Emory University in
Atlanta. [Sexy
Swimmers: 7 Surprising Facts About Sperm]
Involved
dads
Scores of studies have shown that children with involved
and caring fathers tend to do better emotionally, socially
and educationally.
So Rilling and his colleagues were interested in understanding what
makes some men stellar
dads and others AWOL.
A 2011 study in the Philippines suggested that men who have high
testosterone levels are more likely to marry. Even so, those men
who are eventually more involved in day-to-day child care duties —
such as changing diapers, running the bath or kissing scraped
boo-boos — see their testosterone levels drop more than men who
remain aloof after having children.
But testosterone has
many roles in the male body, so it wasn't clear whether the drop in
the male hormone occurred because men were investing more in
parenting than in mating.
Rilling and his colleagues surveyed 70 married men ages 21 to 55
who had between one and four children about their involvement in
caregiving. Only four of the men routinely did more caregiving than
the mothers.
The researchers then used magnetic resonance imaging to scan the
brains of the fathers while they looked at images of their
children. The team also scanned the participants' testes to assess
volume and measured testosterone levels from blood samples.
Men's testes' volume varied considerably — from a little more than
a tablespoon in volume to about a quarter cup.
Men with bigger
testes had a more hands-off parenting style, and the
reward centers of their brains activated less when the men viewed
their children's pictures. These fathers also tended to have higher
testosterone levels.
The findings are fascinating and provocative, said Sarah Hrdy, an
anthropologist at Citrona Farms who was not involved in the
study.
"For me, the take-home message is, there is this tremendous
potential for nurture in human males, albeit a potential that in
different cultural and social situations is not always being
tapped," Hrdy told LiveScience.
But the study also raises a number of questions, added Hrdy, who is
professor emerita at the University of California, Davis; associate
in the Peabody Museum of Archaeology and Ethnology at Harvard; and
A.D. White professor-at-large at Cornell University.
For instance, no one knows how testicular volume changes over time
(although testes do tend to shrink with age).
In addition, scientists aren't sure whether men who make more sperm
are genetically wired to be detached dads, or whether early life
experience or the act of caring for children leads men's bodies to
invest less in sperm-making, thereby causing their testicles to
shrink, she said.
Another possibility is that the trend reveals a trade-off that
varies with men's mating strategy, said Robert Martin, a biological
anthropologist and author of "How We Do It: The Evolution and
Future of Human Reproduction," (Basic Books, 2013).
"A man can either invest in looking after the child of one wife, or
he can invest more in sperm
production if he has several wives," Martin, who was not
involved in the study, told LiveScience.
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So ladies.. instead of the 5C's, dont forget the measure your man's
testicles. LOL