Women tend to have longer lifespans relative to men and it might be linked to having double X chromosomes. Photo / 123rf
Women tend to live longer than men. There are traditional explanations: Men smoke more. They drink more. They tend to engage in riskier behaviour.
The fact that this lifespan gap holds true regardless of country or century indicates something deeper is also at play.
A growing body of evidence suggests that women’s relative longevity may derive, in part, from having double X chromosomes, a redundancy that protects them against harmful mutations.
That theory was further bolstered today with the publication of the most sweeping analysis to date of the lifespan differences between males and females in more than 1000 mammal and bird species.
“From a human standpoint, it’s really remarkable that women live longer across almost every country in the world,” said Johanna Staerk, an evolutionary demographer at the Max Planck Institute for Evolutionary Anthropology in Germany.
“So we were interested at looking at this from a broader taxonomic perspective.”
A quick biology lesson on the birds and the bees … well, birds and other animals:
In mammals, including humans, sex is generally determined by the X and Y chromosomes. If a baby has a pair of X chromosomes, she’s a girl. If the baby inherits an X chromosome and a Y chromosome, he’s a boy.
In birds, however, the situation is reversed. Female birds have a pair of unlike sex chromosomes while males have the like pair. (In this sort of sex-determination system, scientists use the letters W and Z and talk about a ZW pair for female birds and a ZZ pair for males.)
The “heterogametic sex hypothesis” holds that if something goes haywire with a gene on one of a woman’s X chromosomes, her cells have a spare to rely on.
But men, with only a single X chromosome, have no such reinforcements. The same sort of problem may happen with a male’s unpaired Y chromosome.
In men, “if there’s any deleterious mutations or any mutations that will reduce the lifespan, you don’t have a back-up,” said Fernando Colchero, who is also with the Max Planck Institute.
For their study, Colchero, Staerk and their colleagues collected data on the lifespans of 528 mammal species and 648 bird species kept in zoos.
The team found that most other mammals are like humans, with the females of nearly three-fourths of mammal species outliving their male counterparts.
But in birds, 68% of species studied showed a bias toward male longevity, as expected from their chromosomal make-up.
The peer-reviewed results were published in the journal Science Advances.
Pau Carazo, an associate professor at the University of Valencia in Spain, said the study is one of several in recent years lending evidence to the hypothesis, but, by including more than 1000 species, it is the most comprehensive.
“This is a very welcome contribution to the field,” said Carazo, who was not involved in the study.
Colchero and Staerk cautioned that chromosomes don’t tell the whole story. Scientists don’t know exactly which genes on the X and Y chromosomes are important for longevity, they said.
“Looking at the physiological mechanisms is much more complicated,” Colchero said.
“Those mechanisms may change from species to species.”
And an animal’s body and behaviour are important, too.
Males that have to compete harder to secure a mate - either by growing bigger bodies or by acting more boldly because their kind doesn’t pair off monogamously - tend not to live as long as females of the same species, according to their study.
Masculine characteristics such as long antlers in moose or meaty muscles in gorillas may “allow them to produce babies”, Colchero said. “But many times, those come at a cost” to their own longevity.
And then there are some animals that seem to defy all the rules.
For example, despite lemurs being sexually promiscuous, there is little difference between male and female lifespans in those primates. And female hawks, eagles, and vultures all tend to live longer, despite being bigger than males.
“In birds of prey, everything is reversed,” Colchero said. “There’s still a lot to look into.”
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