Study: More susceptible to parasites, males live shorter lives

[The1calledTKE]

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


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

The reason, said Wilson, may be parasites.

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

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

Also, male defenses against disease may be more fragile than those of females, said Wilson. The male hormone testosterone, he said, is known to slightly weaken the immune system, allowing bacteria and other parasites to more easily thrive.

Animals in the study included rats, mice, monkeys, whales, deer, antelopes 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 years, but only 73 years 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 life expectancy among mammals has often been blamed on the riskier behavior of males, who often must fight other males 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.

"Taken together, these findings suggest that male-biased mortality occurs not only as a result of death through risky behavior, but also because males are more susceptible to parasitic diseases," said Owens.

He said there is some indirect evidence that the same principles could apply to humans.