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Around the world, women on average live longer than men. This striking pattern holds true across nearly all countries and historical time periods. Although the gap between the sexes has narrowed in some countries due to medical advances and improved living conditions, new research now provides clues as to why this difference is unlikely to disappear anytime soon. The causes are deeply rooted in evolutionary history and can be observed in many animal species.

An international team led by scientists at the Max Planck Institute for Evolutionary Anthropology in Leipzig, together with 15 co-authors worldwide, conducted the most comprehensive analysis of sex differences in lifespan across mammals and birds to date. Their findings, in Science Advances, provide novel insight into one of biology's long-standing puzzles: why males and females age differently.

Longevity: A question of chromosomes?

Among mammals, females usually live longer—for instance, in baboons and gorillas, females often outlive males. Yet this pattern is not universal: In many birds, insects, and reptiles, males are the longer-lived sex. One genetic explanation, the heterogametic sex hypothesis, points to differences in sex chromosomes.

In mammals, females have two X chromosomes, while males have only one X and one Y (making them the heterogametic sex). Some research suggests that having two X chromosomes may protect females from harmful mutations, offering a survival advantage. In birds, however, the system is reversed: females are the heterogametic sex.

Using records from over 1,176 bird and mammal species in zoos worldwide, the researchers found a striking contrast in lifespan, supporting the heterogametic sex hypothesis: in most mammals (72 percent), females lived longer, by on average twelve percent, while in most bird species (68 percent), males lived longer, overall by an average of five percent.

Still, there was remarkable variation with many exceptions. "Some species showed the opposite of the expected pattern," says lead author Johanna Stärk. "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."

Sexual selection and parental care shape lifespan differences

In addition to genetics, reproductive strategies also play a role. Through sexual selection, males in particular develop conspicuous characteristics such as colorful plumage, weapons, or large body size, which increase reproductive success but can shorten lifespan. The new study supports this assumption: 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.

Zoo life reduces—but does not erase—lifespan gaps

A long-standing idea is that environmental pressures—such as predation, pathogens, or harsh climates—drive the observed gaps between males and females. To test this, the researchers turned to zoo populations, where such pressures are largely absent.

They found that lifespan gaps persisted even under these protected conditions. Comparing zoo and wild populations showed that the gaps were often smaller in zoos but rarely disappeared—mirroring the human case, where advances in medicine and living conditions have narrowed but not eliminated the lifespan gap.

The findings suggest that sex differences in lifespan are deeply rooted in evolutionary processes—shaped by sexual selection and parental investment and that genetic differences in the sex determination system may also play a role. Environmental factors influence the extent of the differences, but cannot eliminate them. The differences between the sexes are therefore not only a product of the environment, but part of our evolutionary history, and will most likely continue to exist in the future.