Does lust for sex kill males in their prime?
New Scientist, 24 May 1997
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.
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.
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 dayslonger 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.
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.
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.