The real reason women outlive men: it's all a matter of breeding
Theory concludes that men are genetically more 'disposable', reports Steve Connor
Monday 25 October 2010
The reason women live longer than men – and why the final act of sex discrimination favours females over males – may at long last have a scientifically valid explanation.
Scientists believe we are close to understanding why men on average die younger than women. Life expectancy in Britain has risen steadily for both sexes over the past few decades and even though the gender gap has narrowed, women are still significantly more likely to live longer than men.
A newborn baby boy in Britain today can expect to live 77.7 years; a newborn baby girl can expect to live to 81.9 years. That is a difference in life expectancy of more than four years. A British man who has already reached 65 can expect to live a further 17.6 years, and a woman of 65 another 20.2 years.
The gender differences become more apparent with age. There are roughly six women to every four men by the age of 85, and the ratio is more than two to one at the age of 100. The oldest documented person to have ever lived was Jeanne Calment, a French woman who died in 1997 at the age of 122 years and 164 days.
Past explanations for why women live longer than men, such as men having physically demanding jobs or engaging in riskier activities such as smoking and drinking, cannot fully explain the gender gap. It has been a major scientific puzzle why members of the "fairer sex", who have in the past been able to retire earlier than men, live longer.
Now the answer to one of the biggest conundrums of human biology may come down to the fact that the female body seems to be better at carrying out the "routine maintenance" that keeps cells alive and ageing at bay – despite the widespread belief in cosmetic circles, based on skin changes alone, that men age more slowly than women.
Professor Tom Kirkwood, a leading gerontologist at the University of Newcastle, believes there is now growing evidence to suggest that men are literally more disposable than women, because the cells of their bodies are not genetically programmed to last as long as they are in females.
The theory builds on a "eureka moment" that first came to him while having a bath one winter's night in 1977. Called the "disposable soma" theory, it has become the leading scientific explanation for why we age, why we cannot live forever – and now the reason why women live longer than men.
The disposable soma theory states that, although the genes are immortal and can indeed "live forever" by being continually passed on to subsequent generations, the body or "soma" is disposable because it is designed to live only long enough to act as a vehicle for carrying genes to the next generation.
The body, just like a car, needs to be maintained continuously to keep it on the road, but as time progress, the faults and errors build up within the cells and tissues. These faults are energetically expensive to fix and with time, they become so common that the body eventually succumbs and dies. When this occurs depends on how much effort the body spends on fixing its mistakes.
"Could it be that women live longer because they are less disposable than men? This notion, in fact, makes excellent biological sense," Professor Kirkwood suggests in an article to be published in the November issue of the magazine Scientific American.
"In humans, as in most animal species, the state of the female body is very important for the success of reproduction. The foetus needs to grow inside the mother's womb, an the infant needs to suckle at her breast.
"So if the female animal's body is too much weakened by damage, there is a real threat to her chances of making healthy offspring. The man's reproductive role, on the other hand, is less directly dependent on his continued good health," Professor Kirkwood writes.
There is growing evidence to back up the idea. Females of most species tend to live longer than males, and experiments in Professor Kirkwood's own laboratory have shown that animals that are naturally long-lived have better maintenance and repair systems than shorter-lived species.
Studies have also found that cells taken from a female body are better at repairing damage compared with cells from a male body. Interestingly, this difference between the sexes is eliminated if the female cells come from a body where the ovaries have been surgically removed.
Equally, being a fully functional male seems to be bad for longevity compared with a male that has had his testes removed. Neutered male cats and dogs, for instance, often live longer than normal males, and even in humans there is evidence that castrated men live longer than other men, Professor Kirkwood said.
A further strand of supporting evidence comes from recent studies in Japan where scientists have created female mice with no fathers. The "super female" mice, which were created from genetic material taken from two females with no male genes from a father, lived on average 186 days longer than ordinary female mice – a third longer than expected.
It all points to the idea that women have bodies that are less disposable, and so more likely to live longer.
"It's always going to be difficult to say things with absolute assurance, but I think we've got a very convincing explanation for why women live longer than men. I'm reasonably confident that this is the explanation," Professor Kirkwood said.
"In females, reproductive success is so inextricably bound up with the integrity of the body and the evidence is really clear right across the life course – men have a statistically significant, higher likelihood of dying at all ages compared to women.
"It seems to be deeply engrained in the biology. Obviously differences in lifestyle may add or subtract to it, but I'm absolutely convinced that there's an underlying biological explanation for the gender differences we see between the life expectancy of the sexes."
The speed of ageing
Why do animals die, and why do some species live short lives while others can live for many decades or even centuries?
Biologists have puzzled over such questions for many years. The most likely explanation to ageing and death is called the 'disposable soma theory'. It states that while the genes are immortal and are passed on to subsequent generations, the body, or 'soma', is disposable. The genes dictate how much effort goes into maintaining the soma and fixing the errors and mistakes that build up over time to cause ageing.
Some species like the mouse only live for a couple of years because more effort goes into reproduction than error fixing. Other speices such as the bat – which is a similarly sized animal – can live for 30 years or more because its soma is treated as being less disposable.
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