It’s a fact that has been known for more than two centuries – wherever you are around the world, women on average live longer than men.
The primary reason for this intriguing trend has long eluded scientists, but an international team of researchers now claim to put the debate to bed.
They report that it is mainly, but not solely, due to something called the ‘heterogametic sex theory’.
Men are known as the ‘heterogametic sex’ because their sex chromosomes do not match – they have one X and one Y.
But this puts them at a disadvantage compared to females, who have two X chromosomes.
Because of their XY chromosomes, males are left more prone to harmful genetic mutations and disease – and ultimately an earlier death on average.
‘We believe that the heterogametic sex is partially the answer,’ Dr Fernando Colchero, study author at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, told the Daily Mail.
‘We found that there are factors which are tightly linked to our evolutionary history that contribute as well.’
Around the world, women on average live longer than men – and this striking pattern holds true across nearly all countries and historical time periods (file photo)
Since the 1740s it’s been known that women live longer than men on average – a pattern that’s consistent across nearly all countries and historical time periods.
Currently, the average global life expectancy is 73.8 years for women and 68.4 years for men, according to Our World in Data.
Although the figures vary over the years, women consistently have the higher average age.
And the trend can also been observed outside our own species, among some our closest relatives in the animal kingdom.
Female mammals such as baboons and gorillas also usually live longer than males, according to the international team of experts.
However, for other classes of animals, the reverse is true.
For example, in many birds, insects, and reptiles, males are the longer-lived sex, which clouds the picture to some degree.
In an attempt to get to the bottom of the lifespan gap, researchers studied records from 528 mammal species and 648 bird species in zoos worldwide.
XX and XY are sex chromosomes that determine a person’s biological sex. Females typically have two X chromosomes (XX), while males typically have one X and one Y chromosome (XY). Pictured, X-chromosomes (red) and Y-chromosomes (green) in embryonic stem cells of male (X/Y) and female (X/X) mice
Like humans, 72 per cent of mammals showed a female life expectancy advantage, while 68 per cent of birds showed a male advantage, they found.
As a primary explanation for this pattern in mammals, the researchers point to the heterogametic sex hypothesis.
Research suggests that having two X chromosomes protects females from harmful mutations, offering a survival advantage.
Unlike females, males have no second X chromosome to compensate for a single harmful ‘allele’ (a version of a genetic sequence at a particular region on a chromosome).
‘Basically, if you have two copies of the same genes, it’s better than one,’ study author Dr Johanna Stärk at the Max Planck Institute for Evolutionary Anthropology told the Daily Mail.
It is also possible that the the Y chromosome often contains long pieces of repetitive DNA which may be harmful, she added.
The team admit that there are other lesser factors that explain why female mammals live longer than their male counterparts.
Firstly, they point to ‘sexual selection’ – the ability to successfully obtain a mate at the expense of the competition.
Across mammals, females usually live longer. For instance, in baboons and gorillas females often outlive males. Pictured, male and female olive baboon (Papio anubis) in Bwindi Impenetrable Forest, Uganda
In the animal kingdom, males have evolved to develop conspicuous characteristics that catch the eyes of a female, such as large body size, colorful plumage or even ‘weapons’ such as antlers or large horns.
Although these evolved traits can increase reproductive success, researchers theorize that they come at the cost of shortening lifespan.
‘In theory, those are very costly to produce and maintain, while they are also linked to a higher tendency of males to fight to monopolize females,’ Dr Colchero told the Daily Mail.
Thirdly, the team think it might have to do with parental care.
The researchers found evidence that the sex that invests more in raising offspring tends to live longer.
Female mammals – usually the primary caregiver – have likely evolved to survive until their offspring are independent or sexually mature.
Another long-standing idea is that environmental pressures such as predation, pathogens and harsh climates drive the gaps between males and females – but the new study found little support for this.
The researchers used zoo populations to test this idea where such pressures are largely absent, but found that lifespan gaps persisted even under these protected conditions.
Comparing zoo and wild populations showed that the gaps were often smaller in zoos but rarely disappeared, they found.
Overall, the new study, published in the journal Science Advances, shows sex differences in lifespan are deeply rooted in both genetics and evolutionary processes.
Differences between the sexes are not only a product of the environment, but part of our evolutionary history – and will most likely continue to exist in the future.
However, Dr Colchero says there are likely social and behavioral factors not considered in this study that could also play a role.
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