The reason why men die younger than women

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According to popular theory, men live shorter lives than women because they take bigger risks, have more dangerous jobs, drink and smoke more, and are poor at seeking advice from doctors.

But research by scientists at the University of NSW in Sydney suggests the real reason may be less related to human behaviour and more to do with the type of sex chromosomes we share with most animal species.

In a study, UNSW researchers analysed all available academic literature on sex chromosomes and lifespan – and tried to establish whether there was a pattern of one sex outliving the other throughout the animal kingdom.

Specifically, they wanted to test the ‘unguarded X hypothesis’, which suggests that the Y chromosome in heterogametic sexes – those with XY (male) sex chromosomes rather than XX (female) sex chromosomes – is less able to protect an individual from harmful genes expressed on the X chromosome. The hypothesis suggests that, as the Y chromosome is smaller than the X chromosome, and in some cases absent, it is unable to ‘hide’ an X chromosome that carries harmful mutations, which may later expose the individual to health threats.

Conversely, there is no such problem in a pair of homogametic chromosomes (XX), where a healthy X chromosome can stand in for another X that has deleterious genes to ensure those harmful genes aren’t expressed, thus maximising the length of life for the organism.

First author on the paper and PhD student Zoe Xirocostas says that after examining the lifespan data available on a wide range of animal species, it appears that the unguarded X hypothesis stacks up. This is the first time scientists have tested the hypothesis across the board in animal taxonomy. Previously, it was tested only within a few groups of animals.

“We looked at lifespan data in not just primates, other mammals and birds, but also reptiles, fish, amphibians, arachnids, cockroaches, grasshoppers, beetles, butterflies and moths among others,” she says.

“And we found that across that broad range of species, the heterogametic sex does tend to die earlier than the homogametic sex, and it’s 17.6 per cent earlier on average.”

Interestingly, the researchers observed this same pattern in the classes of animals possessing their own unique pair of sex chromosomes that are the reverse of all other animals.

In birds, butterflies and moths, it is the male of the species that has the homogametic sex chromosomes (denoted by ZZ) while the female has the heterogametic chromosomes (ZW).

Female birds, butterflies and moths were usually found to die earlier than their male counterparts, giving credence to the unguarded X hypothesis – although strictly speaking, it’s an unguarded Z in this case.

While this study confirms the unguarded X hypothesis is a reasonable explanation for why one sex outlives the other on average, one statistic emerged that took Ms Xirocostas by surprise.

“We found a smaller difference in lifespan between the males and females in the female heterogametic species compared to males and females in the male heterogametic species,” she says.

“In species where males are heterogametic (XY), females live almost 21 per cent longer than males. But in the species of birds, butterflies and moths, where females are heterogametic (ZW), males outlive females only by seven per cent.”

Simply put, heterogametic males (XY) die sooner than heterogametic females (ZW) when compared to the opposite sex in their species. Does this mean there is something still fundamentally life-shortening about being a male member of any species?

Ms Xirocostas says this could be the case, and lists side effects of sexual selection, the degree of Y chromosome degradation and telomere dynamics as possible explanations for this surprising finding.

“I was only expecting to see a pattern of the homogametic sex (XX or ZZ) living longer, so it came as an interesting surprise to see that the type of sex determination system (XX/XY or ZZ/ZW) could also play a role in an organism’s longevity.”

Ms Xirocostas says future studies of this phenomenon should test a hypothesis raised in the paper that the difference in lifespan between sexes is proportional to the difference in chromosome length between sexes, which could help us further understand the factors affecting ageing. But for now, she believes, the unguarded X hypothesis stands.

Why do you think men die younger than women?

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Written by Ben

24 Comments

Total Comments: 24
  1. 0
    0

    I think that it is the luck of the draw. There was a gentleman on Sunrise who just turned 110 years of age.

  2. 0
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    I think that it is the luck of the draw. There was a gentleman on Sunrise who just turned 110 years of age.

  3. 0
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    Because they want to ????

  4. 0
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    I believe that in most cases men die earlier than women because of work, food choices and those who smoked, consumed significant amounts of alcohol and disregarded the threat of Diabetes.

  5. 0
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    I believe that in most cases men die earlier than women because of work, food choices and those who smoked, consumed significant amounts of alcohol and disregarded the threat of Diabetes.

  6. 0
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    My fathers brothers wife is alive and well here where I am living and she is 105. My other and all her sisters died in there early nineties or late eighties. My father died when I was 27. One of my brothers has passed at 64 and 2/3 of the rest have serious medical situations. Every situation is different but a good barometer is look at the Funeral notices in any paper and work out the percentage of men/women and their age.

  7. 0
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    My fathers brothers wife is alive and well here where I am living and she is 105. My other and all her sisters died in there early nineties or late eighties. My father died when I was 27. One of my brothers has passed at 64 and 2/3 of the rest have serious medical situations. Every situation is different but a good barometer is look at the Funeral notices in any paper and work out the percentage of men/women and their age.

  8. 0
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    According to Wiki out of 100 verified living super oldies the first 54 are women.

    Men know when to quit.
    https://en.wikipedia.org/wiki/List_of_the_oldest_living_people

  9. 0
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    Putting oneself in harm’s way for the overall good of the ‘tribe’ has always been a part of men’s makeup… without the effort and expenditure and the adventurism many of the things that we take for granted these days would not exist. The fundamental biologic al differences and requirements on men and women make a lot of difference – hence we can never be ‘equal’ by enforcing numbers in any way.

    Besides – it’s better to work as a team than as two armed enemies holding an uneasy truce.

    • 0
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      Could not agree more!
      Look at “the average” family or couple of dying age, now.
      He would have got his job in the fities or sixties (as in, 1950′ or 1960’s) and worked at a physical job until retirement. Contrast that to the gentle sex who may have had a job in their early adult life, then left to have the kids, and look after the “home duties”. I am not saying that is any easier, by the way.
      Now before you take offence, look at your parents’ lives… Or maybe your own.
      So, not only do the (traditional) men do the risky stuff but they also take on the stressful parts: Jobs, money management, health of the family decisions. Maybe even have gone to war…
      If that’s all correct, we should see an equalisation of death ages in the next 20 to 30 years.

  10. 0
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    Women live longer because they have it easy. They usually don’t do the hard physical yakka that wears the body out.

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