It’s queer but true – women have a longer lifespan compared to men.

Researchers Steven Austad and Kathleen Fischer of the University of Alabama explored this riddle in their latest perspective piece.

“Humans are the only species in which one sex is known to have a ubiquitous survival advantage,” the researchers write in their review covering a multitude of species.

“Indeed, the sex difference in longevity may be one of the most robust features of human biology,” they added.

Though other species, from roundworms and fruit flies to a spectrum of mammals, show lifespan differences that may favour one sex in certain studies, contradictory studies with different diets, mating patterns or environmental conditions often flip that advantage to the other sex.

With humans, however, it appears to be all females all the time.

“We don’t know why women live longer. It’s amazing that it hasn’t become a stronger focus of research in human biology,” said Austad.

One of the evidences of the longer lifespan for women includes ‘The Human Mortality Database,’ which has complete lifespan tables for men and women from 38 countries that go back as far as 1751 for Sweden and 1816 for France.

Again, longer female survival expectancy is seen across the lifespan, at early life (birth to 5 years old) and at age 50. It is also seen at the end of life, where Gerontology Research Group data for the oldest of the old show that women make up 90 percent of the super centenarians, those who live to 110 years of age or longer.

Longevity may relate to immune system differences, responses to oxidative stress, mitochondrial fitness or even the fact that men have one X chromosome (and one Y), while women have two X chromosomes.

But the female advantage has a thorn.

“One of the most puzzling aspects of human sex difference biology,” write Austad and Fischer, “something that has no known equivalent in other species, is that for all their robustness relative to men in terms of survival, women on average appear to be in poorer health than men through adult life.”

This higher prevalence of physical limitations in later life is seen not only in Western societies, they say, but also for women in Bangladesh, China, Egypt, Guatemala, India, Indonesia, Jamaica, Malaysia, Mexico, the Philippines, Thailand and Tunisia. But this is just one of several plausible hypotheses for the mystery of why women live longer, on aver age, than men.