Parasites' bias for big animals gives female mammals longevity. (Gender Gap).

Scientists often attribute the tendency of male mammals to die earlier than females to hunting, fighting for mates, and other risky behaviors that the males engage in. In the Sept. 20 Science, however, Sarah L. Moore and Kenneth Wilson of the University of Stirling in Scotland point to another reason: Parasites infect males more often than females, apparently because the males are bigger in most mammalian species, including people.

"Our analysis suggests that parasites may be contributing to the sex difference in mortality," says Wilson.

Two lines of data originally prompted Moore and Wilson to undertake an extensive look at the scientific literature on mammalian sex differences in parasite susceptibility. First, they were aware of research showing that the hormone testosterone suppresses the immune system and thus may leave males, which make more of it than females do, particularly susceptible to parasites. "There were some studies suggesting that males tended to have more parasites than females, but there didn't seem to be any consistent pattern emerging," Wilson notes.

The second impetus derived from observations of Soay sheep on St. Kilda, an island group off the northwest coast of Scotland. Among Soay sheep, males tend to die significantly earlier than females and are more susceptible to parasitic worms that infect their gastrointestinal tracts. In experiments several years ago, Wilson's colleagues gave some sheep drugs to eliminate the infections. In his own analysis of the results, Wilson found that the mortality rates of treated males and females evened out.

So, he, along with Moore, began reviewing the scientific literature for any data that document the incidence of parasitism in male versus female mammals. The researchers ended up with more than 350 reports detailing infections with worms, arthropods, and single-cell parasites. From that bounty of data, Moore and Wilson discerned a clear pattern: In mammals where the males are larger than the females, the males suffer more parasitic infections.

In a commentary accompanying the researchers' Science paper, Ian P.F. Owen of Imperial College London suggests that a parasite bias for men generally also holds true. "Human demographic data support the idea that parasites are an important determinant of male-biased mortality. Although sex differences in suicide and homicide grab the headlines, males are also more prone to a range of parasitic and infectious diseases," he writes.

Challenging the notion that testosterone's immune-suppressing actions explain their findings, Moore and Wilson note that there are some mammalian species in which the females are more susceptible to parasites. In most of those species, which include some bats and rats, the females are larger than the males. That indicates that in the end, it's size, not sex, that matters, Moore and Wilson conclude.

"Sometimes [the parasite incidence] is male-biased, sometimes it's female-biased," says Wilson. "The extent of the bias is correlated with the difference in body size between the two sexes."

It's not clear yet why being bigger poses a greater risk of infection, but there are several theories. First, species in which males are much bigger than females are usually ones in which males intensely compete for or defend mates. That behavior may leave the males weaker and more vulnerable to infection. Second, growing big may simply involve the tradeoff of having a weaker immune system. Third, being large may increase exposure to parasites because bigger animals forage farther, eat more, and present larger targets.