Parasites Favor Big and Juicy Males

Male mammals are bigger and juicier targets for parasites than females, and two Scottish scientists suggest that may be one reason the males often have shorter life spans.

Kenneth Wilson and Sarah L. Moore of the University of Stirling say in a study that appeared Friday in the journal Science that males have a shorter life expectancy in animal species where they are larger than females, which is the general rule among mammals.

The reason, Wilson said, may be parasites. Parasites of all types -- from bacteria that invade and cause disease to insects that bite, chew and suck blood -- tend to favor the larger males. This may cause the males to age and die more swiftly than females, he said, although he acknowledged that this conclusion is just educated speculation.

“Simply by being big, you expose yourself to more parasites,” Wilson said. Bigger animals must eat more, increasing the risk of swallowing bacteria. Mosquitoes, fleas and ticks can find bigger animals more easily, he noted.

Also, male defenses against disease may be more fragile, Wilson said, noting that the male hormone testosterone is known to slightly weaken the immune system.

Animals in the study included rats, mice, monkeys, whales, deer and monkeys, but not humans.

Wilson said he could only speculate about whether parasites explain the difference in the life spans of human men and women.

“Probably the same principles that apply in other mammals set the scene for what happens in humans,” he said.

In the United States, life expectancy for females is 79, but only 73 for males, based on 1996 figures.

In an editorial in Science, Ian P.F. Owens of the Imperial College in London said the gender difference in mammals’ life expectancy has often been blamed on the riskier behavior of males, who often must fight for mating rights.

He said the study by Wilson and Moore shows that in species where the males die younger than females, “the males suffer a disproportionate high rate of parasitism,” which is most extreme in those species where the male-to-male competition is intense.