Does lust for sex kill males in their prime?

New Scientist, 24 May 1997

ALTHOUGH being female has never been easy, there has always been one clear advantage: longer life. But now a British geneticist is claiming that males, not females, are programmed to live longer. It is the relentless pursuit of sex that sends them to an early grave, he says.

In almost every species where one sex lives longer, from worms to cats and humans, females enjoy the longer life span. Scientists have often assumed that this is due to a superior "constitutional endowment"—females are simply designed to live longer. But work with worms has persuaded David Gems of University College London that they may have got it wrong: males have the greater underlying longevity.

He reached this conclusion while studying the nematode worm Caenorhabditis elegans. In this population there are very few males, most worms being hermaphrodites that reproduce by themselves. But the hermaphrodites are "essentially females capable of making a small number of sperm for self-fertilisation", says Gems, so for the purposes of his research he treated them as female.

Gems observed that when he put male worms together they died in about 10 days, sooner than when they were left with females. But when he isolated individual male worms, they lived for 20 days—longer even than the female average of 16 days. Isolating female worms had no effect on their life span. Gems put the premature male deaths down to too much activity: they were perpetually defending territory and seeking and competing for mates (New Scientist, Science, 12 October 1996, p 17).

To test the idea further, Gems measured the life span of worms with genetic mutations that made them less active. These laid-back males lived even longer, for 30 days. The same mutations did nothing to increase the life span of females.

"In males but not in [hermaphrodite females], life span is limited by the rate of movement," Gems told a meeting of the Zoological Society of London last week. Exceeding a threshold of activity, he suggests, shortens a worm's life. "Males are naturally above that threshold, and so their life spans are shorter," he says. He speculates that males have evolved this enhanced longevity to compensate for the dangers of risky mating behaviour. "The life span of a male relative to its constitutional longevity is cut to a third," he says.

Armand Leroi, an evolutionary geneticist at Imperial College, comments that Gems "dissects out in great detail for the first time in any animal the difference in longevities between males and females". Leroi suggests that the same tests be applied to a close relative of C. elegans that has true females to confirm that the effect is not specific to a hermaphrodite-dominated society. "It would be interesting to know whether the findings were the same," he says.

Gems argues that there is already plenty of evidence that males of other species would live longer than females if it weren't for their energetic sexual activity. If male marsupial mice are castrated they can live for years, says Gems, otherwise they die in just a few sex-crazed weeks. "They spend 5 to 11 hours a day copulating," he says.

Humans are no exception. A study of 319 eunuchs in 1969 showed that their median life span was 13.5 years longer than intact males. Gems also notes that although women tend to outlive men, there are more men above the age of 90 than women.