Women, generally speaking, live longer than men. That’s true of humans across nearly all countries and historical time periods – and the male-female gap is even observed in other species. But why? A new study may shed some light on this long-standing mystery, hinting that it is deeply rooted in evolutionary history.
An international team, led by scientists at the Max Planck Institute for Evolutionary Anthropology, looked at 1,176 captive species – the most comprehensive analysis of sex differences in lifespan across mammals and birds to date. They found that female mammals live on average 12 percent longer than males, but the same didn’t hold true for birds: in avian species, males live about 5 percent longer than females.
One common hypothesis to explain this difference is the heterogametic sex hypothesis, which points to the sex chromosomes. In mammals, females typically have two X chromosomes, while males have one X and one Y (heterogametic). There is some evidence to suggest that having two X chromosomes may protect females from harmful mutations, offering a survival advantage. In birds, however, it is the females that are the heterogametic sex.
The team behind the new study found that 72 percent of mammals exhibited a female life expectancy advantage, while 68 percent of birds showed a male advantage – in keeping with the heterogametic sex hypothesis. However, sex differences varied widely.
“Some species showed the opposite of the expected pattern,” lead author Johanna Stärk said in a statement. “For example, in many birds of prey, females are both larger and longer-lived than males. So sex chromosomes can only be part of the story.”
So the researchers dug even deeper and looked at reproductive strategies. By sexual selection, males often develop characteristics that enhance their mating success but may be detrimental to their lifespan – things like flashy colors, large body size, and certain weapons. The team noticed that in polygamous mammals with strong competition, males generally die earlier than females, whereas birds are often monogamous, face less competition, and males can be seen to live longer.
Generally, polygamy and pronounced size differences were associated with a greater advantage for females.
The study also found evidence that the sex that is more heavily involved in raising offspring – in mammals, this is usually females – tends to live longer. In species with long lifespans like primates, this is likely to be an evolutionary advantage: females survive until their offspring are independent or sexually mature to ensure continuation of the species.
The species studied were all living in zoos, which offer protection from environmental stressors such as harsh climate, starvation, and predation. It has long been thought that such stressors drive the observed difference between male and female life expectancy. Although the gap was reduced in zoo animals, the female advantage found in wild populations still remained. This mirrors what we see in humans, where advances in medicine and living conditions have narrowed but not been able to eliminate the lifespan gap.
“Our findings may help explain why differences in ALE [adult life expectancy] between men and women are so consistent across time and cultures,” the researchers conclude. “Specifically, female-biased ALE appears to be common to chimpanzees and gorillas, suggesting that longer life expectancies for females are a characteristic long embedded in our evolutionary history.”
The study is published in Science Advances.
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