http://www.newscientist.com/article.ns?id=dn2825 Males suffer more parasitic infections than females, which could help explain why they die earlier, say Scottish researchers. The traditional explanation for the longer lives of females is that males undertake more risky behaviours. Men living in the West are consistently more likely to die as a result of motor vehicle accidents, murder and suicide. A 1998 study in the US found that men are twice as likely as women to die by the age of 50. Sarah Moore and Kenneth Wilson at the University of Stirling have now re-analysed existing data on body size, lifespan and parasitic infections for 355 non-human mammal species. Overall, they found a small but significant increase in the rate of parasitic infection in males. This sex difference in infections could also be important in understanding human mortality rates, says Ian Owens of Imperial College London. Owens re-analysed World Health Organization data. He found that in Japan, the US and the UK, men are twice as vulnerable to death by parasites as females. That figure increases to four times in countries like Kazakhstan and Azerbaijan, where parasitic infection is more common. "Together with Moore and Wilson's study of nonhuman mammals, these data suggest that differences between males and females in 'immunocompetence' may underlie the increase in male-biased mortality," he writes in a commentary on the research in the journal Science. Easy target Testosterone is known to suppress the immune system. Experiments with rats have shown that castrated males live longer and healthier than intact animals. Castrated men live up to 15 years longer. But higher levels of the hormone in males do not explain the new findings. In fact, Moore and Wilson found that males had higher rates of parasitic infection only in species where males are significantly larger than females. In species where females are larger, they found the reverse. "Males are not special, they just tend to be big," Owens says. Moore and Wilson speculate that larger animals present an easier target for parasites, because bigger animals eat more and have to roam more widely when foraging for food.