Women live longer than men by an average of six to eight years, according to the World Health Organization. This intriguing trend is seen in nearly every country around the world, suggesting that it may be driven by profound biological differences between the sexes. And longevity may not be limited to human females; according to a sweeping new study in the Proceedings of the National Academy of Sciences, a range of female mammals live longer than their male counterparts, too.

Scientists have long assumed this to be true, but according to the study authors, the assumption was based on a small number of case studies on wild mammals, or records of mammals housed in captivity, where lifespan and aging patterns are often not representative of conspecifics in the wild. For the new report, the researchers compiled and analyzed demographic data from different types of studies, including mortality estimates that had been obtained from long-term monitoring of wild populations, and mortality rates obtained from dead animals collected in the field. In total, the teams analysis covered 134 populations and 101 species, including lions, orcas, reindeer, and squirrels.

Among 60 percent of the populations studied, females lived longer than males. On average, their lifespans were 18.6 percent longer, which is considerably higher than the advantage for female humans, who live on average 7.8 percent longer than their male counterparts.

But why do such discrepancies exist between the sexes? Scientists have long sought to answer this question as it pertains to humans, and complex behavioral differences likely come into play. Men, for instance, are more likely to smoke, drink excessively and be overweight, Perminder Sachdev, a professor of neuropsychiatry at the University of New South Wales in Australia who has studied human longevity, told Times Markham Heid last year. They are also less likely to seek medical help and to adhere to medical treatments.

Biological factors may also drive the survival gap. Testosterone, for instance, increases levels of low-density lipoprotein cholesterol in men, putting them at greater risk of hypertension, stroke and heart disease.

Womens biology, on the other hand, may give them a boost. One theory posits that having two copies of the same sex chromosome confers protective benefits that govern longevity; women have two copies of the X chromosome, while men have X and Y chromosomes. Earlier this month, a study in Biology Letters gave credence to this hypothesis when it found a link between sex chromosomes and lifespan across more than 200 species. Female mammals, which have two of the same chromosomes, tended to live longer than males. The dual-chromosome trend applied to species that don't have X or Y chromosomes, too, and to species that in which males have two of the same chromosomes. For example, male birds, which have two Z chromosomes, have the survival advantage over females, which have one Z and one W chromosome.

The authors of the new study note that male mammals also devote substantial resources toward the growth and maintenance of secondary sexual traits, like larger body size or antlers. In certain environmental circumstances, these traits might come at a cost. When looking at bighorn sheep, for instance, the researchers found virtually no difference in lifespan between males and females in ranges where resources were consistently available. But in one location where winters are particularly harsh, there were significant sex differences in lifespan.

Male bighorn sheep use lots of resources towards sexual competition, towards the growth of a large body mass, Jean-Francois Lematre, first author of the new study, tells Matt McGrath of the BBC. [T]hey might be more sensitive to environmental conditions.

Both genetic variations and environmental conditions, in other words, likely play a role in sex differences in lifespan. Untangling these intertwined factors wont be easy, the study authors acknowledgebut further research, they write, will undoubtedly provide innovative insights into the evolutionary roots and physiology underlying aging in both sexes.

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Females Live Longer Than MalesAmong Humans and Other Mammals, Too - Smithsonian.com