THURSDAY, Sept. 19, 2002 (HealthDayNews) -- Everyone knows that women live longer than men -- in the United States the difference is about three years -- but no one knows why.
Now British researchers suggest the reason may be a sex-based excess of life-weakening parasitic infections in males.
"Males" is the word used in a paper in tomorrow's issue of Science reporting the theory because, says Kenneth Wilson, co-author of the report, "our work was done in wild populations, not in humans." Nevertheless, says Wilson, a lecturer in evolutionary ecology at the University of Stirling, "this is the first time that reliable data have been gathered across species to show a difference in parasitic infections."
Wilson and his colleague, Sarah L. Moore, combed the world literature looking for reports on sex differences in parasitic infection rates in a wide range of species, including mammals. Their carefully phrased conclusion is that their analysis "is consistent with the hypothesis that increased parasitism is a cost of sexual selection in mammals."
Humans, of course, are mammals, and Wilson says there is good evidence that the finding applies to them. "There is reasonable evidence from World Health Organization statistics that often men have higher rates of parasite-induced deaths," he says. "It looks like, even in the United States, deaths due to parasites and infectious diseases are higher in males than in females."
There are a couple of possible explanations for the difference, Wilson says. One is that the male sex hormone testosterone is no help. "In the last 10 or 15 years, there have been studies suggesting that higher levels act as a suppressor of the immune system, so that men have higher levels of parasitic infections," he says.
A more compelling explanation, Wilson says, is that their larger body size makes men more vulnerable to parasites. Studies of a variety of species indicate that when females are larger than males, they tend to have more parasitic infections, he says: "My feeling is that if you want to grow your body larger, energy levels are the limiting factor. The energy goes either into growth or immunity."
Men (and males of other mammalian species) tend to be larger than women because of the rules of the game in nature, Wilson says. "Female success is measured by their ability to have offspring," he says. "For men, success is measured by how many matings you have. So for males, there is a premium in growing large. It is less important for males to live for a long time than to grow large enough to win the mating game against their competitors."
An accompanying editorial by Ian P. F. Owens of Imperial College London says the parasite theory is a reasonable alternative to the traditional explanation, which blames the short life span of men on their tendency toward more risky behaviors. Statistics do show men are more likely to die in auto accidents or by murder, suicide, and accidents caused by firearms, Owens says, but "human demographic data support the idea that parasites are an important determinant of male-based mortality."
The testosterone theory should not be abandoned, Owens says, because of evidence such as studies showing that castrated men outlive intact men by up to 15 years, with the difference greatest in those who are castrated earlier in life. More research is needed to determine the precise mechanisms that make males more susceptible to parasites, he says.
Does this scientific debate have a practical application? "One could argue that extra attention should be paid to men's health," Wilson says.
What To Do
Learn more about parasites from the Ohio State University. For information on death statistics, try the Centers for Disease Control and Prevention.
