Parasites May Help Explain
Gender Longevity Gap
Excerpt By Merritt McKinney, Reuter's Health

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."

SOURCE: Science 2002;297:2015-2018.

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