Parasites May Help Explain Gender Longevity Gap
Thu Sep 19, 5:47 PM ET
By Merritt McKinney
NEW YORK (Reuters Health) - It's no secret that women tend to live longer
than men, but it may not be just that men drive too fast and take other
risks, new research suggests. Parasites could be to blame, at least in part,
according to a study from the UK.
In an article in the September 20th issue of the journal Science, Drs. Sarah
L. Moore and Kenneth Wilson of the University of Stirling report evidence
that male mammals are more susceptible than females to parasites, which are
a common cause of death in wild animals.
And there is some evidence, according to an editorial that accompanies the
study, that parasites could affect sex differences in death rates among
people, too, since men are more likely than women to harbor parasites.
Exactly why parasites seem to have it in for men is uncertain, but size
might have something to do with it, study co-author Wilson told Reuters
Health.
Wilson explained that in most mammal species, males compete with each other
for the right to mate with females. He noted that in many species, including
gorillas, baboons, antelopes and wild sheep, males have evolved to be
bigger, on average, than females "since large males tend to do better in
fights."
"What our study shows is that one of the downsides to evolving to be large
is that the risk of becoming parasitized increases," Wilson said.
In an analysis of parasite infection rates in a variety of mammal species,
Wilson and Moore found that male mammals were more likely to be infected
with parasites than female mammals. The difference was small, but it was
large enough that it most likely did not occur by chance.
Being infected with parasites "appears to be costly," Wilson said. He noted
that in mammals, the sex that is larger and thus most likely to have
parasites, usually the male, tends not to live as long as the smaller sex.
But Wilson said the link between parasites and a shorter life holds true
even for the minority of species in which females are larger than males and
have higher rates of parasites--some kinds of gerbils, rats, bats and
rabbits, for example.
"Our study suggests that parasites are contributing to sex differences in
mortality rates in wild mammal populations," Wilson said.
The gender gap in parasite infection rates was most pronounced, the study
found, in species where there were greater size differences between the
sexes or where there was more intense mating competition among males.
Why size puts larger animals at greater risk of parasites is still a
mystery, according to Wilson. Testosterone, which is more plentiful in
males, is known to suppress the immune system somewhat, Wilson said, but
that would not explain why large females are also more likely to have
parasites. Another possibility, according to the UK researcher, is that
larger animals simply provide a bigger target for mosquitoes that often
carry parasites.
Or something in the lifestyle of larger animals may put them at greater risk
of parasites, according to Wilson. "Perhaps they forage over wider areas
looking for food and this exposes them to more parasites," he said.
More research is needed, Wilson said, to understand what causes these
differences and to confirm that parasites play a role in the sex differences
in longevity.
In an accompanying editorial, Dr. Ian P.F. Owens of Imperial College London
points out that human males also seem to be more vulnerable to parasites
than women.
"In the United States, United Kingdom and Japan, men are approximately twice
as vulnerable as women to parasite-induced death," Owens writes. And he adds
that in Kazakhstan and Azerbaijan, this rate is quadrupled in men.
In his comments to Reuters Health, Wilson said, "In fact, the magnitude of
the sex difference in mortality due to parasitic and infectious diseases is
similar to the sex difference in mortality due to homicide, so parasites and
diseases do appear to be important in generating sex differences in life
span."
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