In Westernized societies, women tend to outlive men. The established explanation for this inequality is that males undertake more risky behavior than females do and, as a result, perish prematurely. But new research published today in the journal Science suggests that parasites could be at least partially responsible.

Sarah L. Moore and Kenneth Wilson of the University of Stirling analyzed parasitic infections in 355 nonhuman mammal species and found that males were more likely than females to succumb to parasites. What causes this small but significant increase remains unclear. With their generally larger size, males may just make more attractive targets. Alternatively, because testosterone can suppress the immune system, it is possible that males are less able to protect themselves from a parasite's harmful effects. In an accompanying commentary, Ian P.F. Owens of Imperial College London suggests the findings may have implications for male mammals of the human variety, too. "In the United States, United Kingdom and Japan, men are approximately twice as vulnerable as women to parasite-induced death," he writes. In countries with a higher overall incidence of deaths due to parasites, such as Kazakhstan and Azerbaijan, men are four times as vulnerable as women are. "The next step," Owens concludes, "is to discover more about the precise physiological mechanisms that lead to the unusually high susceptibility of large mammals to parasitic diseases."