Why do women live longer than men (on average)? The answer comes down to evolution.
An international team of researchers led by scientists from the Max Planck Institute for Evolutionary Anthropology in Leipzig have conducted a comprehensive study of lifespan differences between the sexes in mammals and birds. The findings shed new light on one of biology’s enduring mysteries: why males and females age differently.
Across nearly every country and historical era, women tend to live longer than men. While medical advances and improved living standards have reduced this gap in some places, new findings suggest the difference is deeply rooted in evolution and unlikely to vanish.
Similar patterns appear across many animal species, hinting that the roots of longevity go far beyond modern life. For example:
- Mammals vs. birds: Of the 1,176 species analyzed, female mammals lived an average of 13 percent longer than males. In contrast, among birds, males lived about five percent longer than females.
- Mating strategies matter: In species where competition for mates is intense — as is true for most mammals — males tend to die younger. In species that form monogamous pairs, such as many birds, males often outlive females.
- Zoo comparisons: The gap between male and female lifespans is greater in wild populations than in zoo environments. This pattern indicates that both genetics and external conditions influence how long each sex lives.
The theory is the heterogametic sex hypothesis. This links differences to sex chromosomes:
Mammalian females possess two X chromosomes, while males have one X and one Y (making them the heterogametic sex). Having a pair of X chromosomes may shield females from harmful mutations and extend their lifespan. In birds, the system is reversed: females are the heterogametic sex.
The researchers used data from more than 1,176 mammal and bird species in zoos around the world. Here they observed a striking contrast that supported this hypothesis. In most mammals (72 percent), females lived longer, by an average of twelve percent. In most bird species (68 percent), males were the longer-lived sex, averaging five percent longer lifespans.
Yet the pattern was far from universal. Some species showed the opposite of the expected pattern. For example, in many birds of prey, females are both larger and longer-lived than males. Therefore, sex chromosomes are only part of the story.
Parental care and reproduction
The researchers found reproductive strategies also play a role. In polygamous mammals with strong competition, males generally die earlier than females. Many birds, on the other hand, are monogamous, which means that competitive pressure is lower and males often live longer. Overall, the differences were smallest in monogamous species, while polygamy and pronounced size differences were associated with a more pronounced advantage for females.
Parental care also plays a role. The researchers found evidence that the sex that invests more in raising offspring — in mammals, this is often the females — tends to live longer. In long-lived species such as primates, this is likely to be a selective advantage: females survive until their offspring are independent or sexually mature.
The findings indicate that lifespan differences between males and females are deeply embedded in evolution. They are shaped by sexual selection, parental care, and genetic factors linked to sex determination.
The researchers conclude: “These contrasts between the sexes are not simply a product of circumstance — they are woven into our evolutionary past and are likely to persist far into the future.”
The research appears in the journal Science Advances, titled “Sexual selection drives sex difference in adult life expectancy across mammals and birds”.
