Women live longer on average than men. Scientists have observed this trend for some time in humans, with 90 percent of supercentenarians being women, but now scientists have found the same holds true for female mammals in the wild too.
Why is this so? To delve into the longevity mystery, the team compiled mortality data for 134 populations of 101 species, from orcas and kangaroos to squirrels and lions. The results from this mammalian panel came to a final verdict in what the team says is the most complete compilation to date.
In 60 percent of cases, females lived 18.6 percent longer on average than males. For humans, this difference was less than half that of wild mammals at around 7.8 percent longer. Examples of animals where females considerably outlasted the males include sheep, Rocky Mountain goats, lions, brown long-eared bats, Mongolian gazelles, and white-footed mice. To swing the pendulum in the other direction, males outlived females in species such as European rabbits, brown mouse lemurs, white sifaka, African buffalo, Dall's porpoise, and wild boar.
"It was surprising to observe that this [sex] gap in lifespan often exceeds the one observed in humans and is at the same time extremely variable across species," team lead Jean-François Lemaître, a CNRS researcher at the Biometry and Evolutionary Biology laboratory, told IFLScience.
The pickings for both groups are an eclectic bunch from the animal kingdom. Yet the team did not detect any difference in aging rates – essentially males do not appear to be aging any faster than females. They also dismissed the possibility that differences in sex chromosomes are largely influential as the pattern stayed true even for species that share the same sex-determination system.
"We did not find any clear evidence that sex differences in life history strategies (e.g. differences in the strength of sexual competition) explain the sex differences in lifespan (or aging rate)," said Lemaître.
Instead, their findings in the Proceedings of the National Academy of Sciences suggest lifespan is predominantly shaped by an interaction between environmental conditions and sex-specific reproductive costs. Environmental conditions, such as climate harshness, may impact males more heavily than females since males allocate a substantial amount of resources toward growth and maintenance of secondary sexual traits, possibly making them more vulnerable than females.
"If you take the example of a dimorphic species, males allocate more resources to sexual competition/reproduction compared to less dimorphic species which should lead to bigger sex differences in lifespan," said Lemaître. "Yet, even for such a dimorphic species, the magnitude of the [sex] gap might be modulated by the local environmental conditions (e.g. male survival costs of reproduction might be amplified when there are lots of pathogens in the environment). These hypotheses remain to be tested."
The team also proposes the possibility that male hormones at high levels impair some aspects of their immune defense. Trophy hunting also impacts males more than females, likely because they have physically larger traits than the females, and so are deemed a more desirable prize.