Throughout history and in almost all countries, there has been a biological puzzle - women, on average, live longer than men.
Although medical advances have narrowed the gender gap in some countries, a new study suggests that the gap will not disappear anytime soon, as its causes are deeply rooted in evolutionary history, writes the American magazine Newsweek.
An international team of scientists led by researchers from the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, has conducted the most comprehensive analysis to date of sex differences in lifespan among mammals and birds.
Analyzing data on a total of 1.176 species, the researchers found that among mammals, 72 percent of females live an average of 13 percent longer than males, while among birds, 68 percent of males outlive females by about five percent.
One genetic explanation, known as the "heterogametic sex hypothesis", points to differences in sex chromosomes. In mammals, females have two X chromosomes, while males have only one X and one Y, making them a heterogametic sex.
Some research suggests that two X chromosomes may protect females from harmful mutations, giving them a survival advantage. Interestingly, this system is reversed in birds, so females are the heterogametic sex.
Lead author of the study, Johanna Stark, noted that there were exceptions. "Some species showed the opposite pattern to what was expected," she said in a statement.
"For example, in many birds of prey, females are both larger and longer-lived than males. So sex chromosomes may only be part of the story."
Reproductive strategies also play a role. According to the new study, in polygamous mammals, where competition is fierce, males tend to die earlier than females. On the other hand, in birds - which are usually monogamous - this competitive pressure is less, so males often live longer.
Parental care also plays a role. In nature, the sex that invests more in raising offspring - and in mammals, this is usually the female - tends to live longer. In long-lived species, such as primates, this probably represents a selective advantage: females survive until their offspring become independent or sexually mature.
To assess the impact of the environment, scientists also studied populations in zoos, where animals are protected from predators and harsh conditions, and found that the difference in lifespan decreases, but does not disappear completely.
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