Whether you’re a female human, orca, lion or elephant seal, you have a fairly good chance of outliving many of your male counterparts, according to a study published in the journal Proceedings of the National Academy of Sciences.
The fact that this phenomenon – well-known in humans – occurs in other mammals has long been suspected, but not previously quantified. What’s surprising is that males do not seem to age more quickly than females.
In all human populations, women are known to have outlived men since the mid-1700s, and nine out of 10 females are supercentenarians – 110 years old or more. This holds true even when each gender has similar social habits.
To explore sex differences in aging among other mammals, a European team led by Jean-François Lemaître, from Claude Bernard Lyon 1 University, France, collected reliable information on age-specific mortality for 134 populations of 101 wild mammalian species.
They found that females outlive males in 60% of species and have, on average, 18.6% longer lifespans – more than double that of humans (7.8%) – although there were considerable differences between groups.
While the researchers expected to see a female survival advantage, they didn’t expect this.
“It was surprising to observe that this gender gap in lifespan often exceeds the one observed in humans and is, at the same time, extremely variable across species,” says Lemaître.
“Moreover, it was surprising to see that, despite consistently living longer than males, females do not show a reduced rate of actuarial senescence. In other words, the risk of mortality does not increase more rapidly in males than in females across species.”
They believe the variation is a complex interaction between sex differences in life-history strategies – growth, survival and reproduction – and local environmental conditions.
In highly dimorphic species, for example, males allocate more resources to sexual competition and reproduction compared to less dimorphic species, which would logically lead to bigger sex differences in lifespans, Lemaître says.
But this doesn’t explain it all. The magnitude of the gap could also be modulated by local environmental conditions with a trade-off between reproduction and survival; roaming males might be vulnerable to more environmental pathogens, for instance.
This interaction was highlighted in their dataset by three populations of the polygynous bighorn sheep (Ovis canadensis) that had been monitored longitudinally.
In the National Bison Range population, where there was no shortage of regular resources, males and females enjoyed a similar lifespan. However, the harsh winter conditions of Ram Mountain produced notable sex differences in longevity.
The observation needs to be nutted out further though.
“Albeit challenging,” the team writes, “research programs that solve this complex network will undoubtedly provide innovative insights into the evolutionary roots and physiology underlying aging in both sexes.”