Men's aptitude for childcare may be reflected in
the size of their testes, according to a study by
US scientists.
Researchers found that men with smaller testes
were more likely to take charge of children's
bath-time, visits to the doctor, night-time
comforting, and other parenting jobs than
others who have larger testicles.
The same story was borne out by brain scans
which showed that men with smaller gonads
reacted more strongly to photographs of their
own children than did men with larger ones.
The findings are the strongest evidence yet that
variations in male anatomy reflect competing
evolutionary strategies that can be distilled
down to mating as much as possible versus
investing more in parenting. Both are effective
ways to maximise an animal's chances of having
offspring that continue their lineage.
Jennifer Mascaro, an anthropologist at Emory
University in Atlanta, said that while her work
revealed a correlation between testes size and
parenting, it said nothing about the causes. She
suspects the size of a man's testes influences
how involved he gets in childcare. But the
reverse could be equally true: indulging in
childcare may make a man's testes shrink.
"We are not saying you can determine a man's
parenting aptitude based on their individual
biology. But it does suggest that some men may
be wired to participate in childcare more than
others. They may take to it more readily,"
Mascaro told the Guardian.
Mascaro based her experiments on what
evolutionary biologists call life history theory.
The theory can explain how evolution shapes
organisms to optimise their chances of having
fertile young. The different strategies are clear
in nature. Sea turtles have scores of offspring
but do little to raise them, while chimpanzees
have far fewer young, but invest heavily in their
upbringing.
The Emory group asked 70 co-habiting couples
with children aged 1 to 2 to complete a
questionnaire on how they divided up parenting
jobs. Their answers were ranked from one to
five, with one meaning the mother was almost
always responsible, and five meaning the father
was the main carer. The lowest male score was
in the low 40s; the highest scores, from stay-at-
home-dads, were in the 80s.
With the questionnaires filled out, the scientists
went on to measure the size of the men's testes
with a scanning technique called magnetic
resonance imaging. Testes size is a proxy for the
quantity and quality of sperm a man produces,
so men with larger testes may be biologically
more tuned towards mating than parenting.
Taken as a pair, the men's testes varied in size
from 18cm3 to 60cm3. The typical pair was
around 38cm3.
Mascaro found a clear trend, with parenting
scores rising as testes size fell. The effect
remained when she controlled for the men's
height and testosterone levels.
Since parenting behaviour must manifest itself
in some way in the brain, the scientists looked at
how men with different sized testes responded to
pictures of their children. In a brain scanner,
they showed each man photos of adults,
unrelated children and their own children.
When shown snaps of their own children, the
men with smaller testes showed more activity
than others in part of the brain linked to
nurturing, called the ventral tegmental area.
Testes size cannot be the only factor linked to a
man's aptitude for childcare. Differences in size
accounted for only a fifth of the variation in
men's parenting scores, Mascaro said. Details of
the study are reported in Proceedings of the
National Academy of Sciences.
"It is important to recognise that these data are
only correlatory, and no cause and effect are
demonstrated," said Carl Soulsbury, a senior
lecturer in zoology at Lincoln University,
"However, they provide an important basis for
future study and to examine whether this links
more directly in sexual behaviour and
reproductive success."
James Swain, a child psychiatrist at the
University of Michigan and Yale, whose work
covers the psychology of social bonding and
parenting, called the report a "thought
provoking study, suggesting biology that may
underlie parenting v mating trade-offs".
He added: "We are still a long way from this
work leading to better biological understandings
of fathering that could lead to improving such.
[Those] advances will require connecting with
other aspects of the psychobiology of parenting
behaviour."

Source - www.theguardian.com/lifeandstyle/2013/sep/09/testicle-size-men-childcare-aptitude-parenting-us

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