Women tend to live, on average, longer than men, a pattern observed in nearly every country and across time.
Although scientific advances, especially in medicine, and improvements in living conditions have, in some way, reduced this sex difference in human longevity, a group of scientists believes that it will never be truly erased. In a paper published recently in the journal ‘Science Advances’, they analyze the longevity of birds and non-human mammals to try to understand why males and females have different average longevities.
When it comes to mammals, females, as a rule and under normal conditions, tend to live longer than males, something that does not happen, for example, in birds. In mammals, males have sex chromosomes that differ—X and Y—while females have the same chromosomes, a pair XX. However, in birds the opposite holds: males have a pair of identical sex chromosomes—ZZ—and females have chromosomes ZW.
In a statement, the scientists note that there are studies suggesting that having two X chromosomes may protect females against harmful genetic mutations, giving them an advantage that males do not have.
Based on records of more than 1,176 birds and non-human mammals living in zoos in several countries, the team says they found a “striking contrast” between males and females regarding longevity. In most mammals, females lived, on average, 12% longer than males of the same species. In contrast, in most birds the males lived about 5% longer than the females.
However, not all species seemed to respect this chromosomal rule. For example, in birds of prey, females lived longer than males.
“Therefore, sex chromosomes are only part of the story,” Johanna Stärk of the Max Planck Institute for Evolutionary Anthropology and the paper’s first author said in a statement.
Sexual strategies and parental care can make a difference
If sex chromosomes are not the whole story, where is the rest of the explanation? The scientists point to reproductive strategies.
In polygynous species, male competition for the females has, over many years of sexual selection, led them to develop a range of traits that help them stand out from rivals and be more attractive to potential partners: larger bodies, more colorful plumage. But all this comes at a cost, and, while it may increase reproductive success, it can also reduce their longevity, the authors say. For example, brighter colors or larger bodies can make males more conspicuous and thus more vulnerable to predators.
On the other hand, in monogamous species, where males and females have only one partner and males do not have to compete as much to pass on their genes to the next generations, they, with lower competitive pressure, often live longer.
Moreover, there is also the issue of parental care. The researchers argue that the sex that spends more time caring for offspring is also the one that tends to live longer. In mammals, females are the ones who spend the most time caring for the offspring, and in species with higher average life expectancy, such as primates, this is an “advantage” that females have over males, because they need to survive until the offspring are independent or reach sexual maturity.
Thus, this investigation, the authors explain, shows that the differences in longevity between male and female, including our species, are the result of evolutionary history, and not only caused by environmental factors (such as diseases) nor by genetic aspects. That is why they say it is very unlikely that male mammals, humans included, will ever have an average longevity higher than females.
