Women live longer than men on average in each country
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We now have a better idea of the reasons why women live longer than men, on average, thanks to the most complete analysis to date differences in lifespan between males and male and female birds.
Average life expectancy is around 74 years for women and 68 for men. There are various ideas to explain why women tend to live longer than men, including the suggestion that young men are more likely to die in accidents or conflicts, and that women are better protected against potentially harmful mutations in determining chromosomes of sex than men, but the image is far from complete.
To search for clues of other animals, Johanna Stärk at the Max Planck Institute for evolutionary anthropology in Leipzig, Germany, and his colleagues have analyzed data on life expectancy in 1176 species – 528 mammals and 648 birds – in zoos as well as in wild populations.
They found that in 72% of mammal species, females live longer than men, from 12 or 13% on average. But in birds, males tend to survive women in 68% of the species, surviving about 5% more on average.
Researchers say that this trend supports the idea that sex chromosomes represent some of the differences in lifespan.
In mammals, having two copies of the X chromosome makes a woman genetically individual, while males have two different sexual chromosomes, one X and A Y. In theory, women are better protected against harmful mutations in sex chromosomes, because the second copy of the chromosome X acts as a backup.
In birds, the sex determination system is the opposite: women have two different sexual chromosomes, called Z and W, while males have two Z chromosomes. Thus, the different trends in life expectancy in mammals and birds support the idea that sex with different chromosomes – heterogamic sex – causes a cost of longevity.
“But what was very interesting is that we found exceptions,” explains Fernando Colchero, member of the team, also at the Max Planck Institute for Evolutionary Anthropology. “And these exceptions, our idea was to test other evolutionary hypotheses to explain why these sexual differences occur.”
By deepening the data, the team found that mating systems seem to play a role. In polygamous mammals where there is strong competition for friends – such as baboons, gorillas and chimpanzees – males generally die earlier than women.
“Due to competition for mating opportunities, individuals – generally men – will invest in the features favored by sexual selection, such as large body size, ornamental feathers or woods,” explains Nicole Riddle at the University of Alabama in Birmingham. “These features are expensive to produce, and there are generally other costs associated with competition for mating opportunities, for example by fights with other men.”
These factors will mean that the individual has less resources to invest in his own long-term survival, she said.
Men who invest in expensive features to win companions may have a shorter lifespan accordingly
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This is also true for birds with polygamous mating systems. “Overall, this can also explain why the male advantage in birds is considerably lower than the female advantage among mammals,” explains Pau Carazo at the University of Valencia in Spain.
He says that in mammals, the genetic factor and sexual selection lines work in the same direction to shorten the male life, while in birds, pressures can balance each other, because men are often involved in a strong sexual selection, but women bear the costs of heterogamy.
Stärk and his colleagues also found that sex that invests more in the elevation of offspring tends to live longer. In mammals, they are often women. In species with long lifes like humans or other primates, this is probably advantageous, because it helps women to survive until their offspring is themselves sexually ripe.
However, there have been exceptions. “Birds of prey are the opposite of everything we find in other species,” says Stärk. “Women are larger, and it is often women who engage much more in the protection of the territory, but women always live longer.” Why is a mystery, she said.
The differences in life in the sexes are smaller in zoo populations than in wild populations, explains Carazo, probably because life in captivity minimizes environmental pressures such as fights, predation and disease. This control over the environment could also be the reason why the differences in life in the sexes in humans have decreased, he says, although they never disappear entirely.
“There are still very strongly coded differences – physiological differences and genetic differences – between men and women,” explains Colchero. “Who knows where the medical sciences will take us, but in general, we do not expect these differences to disappear completely.”
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