New York: While a conclusive answer to why women have a longer lifespan as compared to men still eludes scientists, a host of factors - from differences in hormones to immune system variations - could be at work, suggests new research. In a perspective piece published in the journal Cell Metabolism, researchers from the University of Alabama at Birmingham explored what gives women the survival advantage. "Humans are the only species in which one sex is known to have a ubiquitous survival advantage," the authors wrote in their research review covering a multitude of species. "Indeed, the sex difference in longevity may be one of the most robust features of human biology," the researchers said. 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. The differences may be due to hormones, perhaps as early as the surge in testosterone during male sexual differentiation in the uterus. Longevity may also 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, the researchers said. Evidence of the longer lifespans 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. "Given this high data quality, it is impressive that for all 38 countries for every year in the database, female life expectancy at birth exceeds male life expectancy," authors Steven Austad and Kathleen Fischer wrote. Longer female survival expectancy is seen across the lifespan, at early life (birth to five 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 supercentenarians, those who live to 110 years of age or longer, the study pointed out.