Parasites prefer males

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Shorter lives are a price of machismo.

Male killer whales live for around 30 years, females about 50. Credit: © Getty Images

The bigger and badder a male mammal is, the more ridden with parasites he his, compared with the female members of his species. This could be one reason why males die sooner than females.

As the size gap between the sexes widens, so does the difference in parasite burden and lifespan, say Ken Wilson and Sarah Moore, two ecologists at the University of Stirling, UK.

They reviewed 355 studies on wild mammals' worms, bloodsuckers and bugs. But diseases caused by parasites, such as tuberculosis and malaria, also afflict men more severely than they do women.

Infestation seems to be a price males pay for sexual success. In species where males are much bigger than females, such as elephant seals, males fight fiercely for mates, and a few brutes father nearly all the offspring.

The finding shows that sex and death are linked in unexpected ways, says evolutionary biologist Marlene Zuk of the University of California, Riverside. "Sexual selection affects a lot more things in animals' and peoples' lives than was once thought," she says.

Heavy cost

It's not clear why rampant masculinity attracts parasites. The most important factor, says Wilson, is probably the cost of being big. "My gut feeling is that males have more parasites because they put resources into growth at the expense of immunity," he says.

Male killer whales, for example, weigh about twice as much as females. Female whales have an average lifespan of about 50 years; males do well to get beyond 30.

Testosterone also suppresses the immune system - castration increases longevity. Spayed dogs and cats live about three years longer than their intact counterparts.

But this can't be the whole story. Females who dwarf their mates, such as some rats and hamsters, suffer heavier parasitism. But they don't have more testosterone or aggression than species where females are the smaller sex.

Bigger might just be more risky, says Wilson. Large animals eat more food, and so are more likely to swallow a tapeworm egg. They are an easier target for mosquitoes and ticks. And they cover greater distances, making it more likely they'll step in something nasty.

Healthcare implications

Even where there is advanced medical care, such as in the United States and Japan, men are twice as likely as women to be killed by parasites. In places with more infectious disease, such as Kazakhstan, men are four times more likely to succumb.

Men's greater vulnerability to parasites could have implications for healthcare, suggests Zuk. "It suggests we ought to be looking at differences between men and women in responses to drug treatment, or recovery patterns," she says.

References

  1. 1

    Moore, S. L. & Wilson, K. Parasites as a viability cost of sexual selection in natural populations of mammals. Science, 297, 2015 - 2018, (2002).

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Whitfield, J. Parasites prefer males. Nature (2002) doi:10.1038/news020916-16

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