It’s fairly obvious that women live longer than men on average. This pattern holds true across most countries and historical time periods. While that longevity gap between the sexes has narrowed a bit in some countries (but widened in others) due to improved living conditions and medical advances, it is not likely to disappear anytime soon, according to new research.
The causes of this gap are deeply rooted in evolutionary history and can be observed in several other animal species, not just humans. Female mammals live on average 12 percent longer than males, with mating strategies playing an important role. The findings are detailed in a study published today in the journal Science Advances.
XX vs. XY
Across mammal species, from baboons and gorillas up to humans, females usually live longer than the males do. However, this pattern is not universal throughout the animal kingdom. Males are the longer-lived sex in several bird, insect, and reptile species.
One genetic explanation for this gap called the heterogametic sex hypothesis points to differences in sex chromosomes. Female mammals generally have two X chromosomes, while males only have one X and one Y chromosome, making them the heterogametic sex. Some research suggests that females might be protected from certain harmful genetic mutations because of the two X chromosomes. Although, females are the heterogametic sex in birds, with one Z and one W chromosome.
To compare longevity in mammals and birds, the team on the study used records of 1,176 bird and mammal species in zoos around the world. They found a decrease in lifespan, which supports the heterogametic sex hypothesis and the idea that two X chromosomes could give a genetic advantage.
In mammals, the majority (72 percent) of females lived longer by 12 percent on an average. In most bird species (68 percent), males lived longer by an average of five percent overall. The study did find some interesting variations.
“Some species showed the opposite of the expected pattern,” Johanna Stärk, a study co-author and evolutionary anthropologist at the Max Planck Institute for Evolutionary Anthropology in Germany, 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.”
It’s a male-eat-male world out there
Reproductive strategies also played a role, not just genetics. Through sexual selection, males in particular develop ways to stick out among the crowd, including colorful plumage, mating dances, or unique vocalizations, which can increase reproductive success. However, these can shorten their lifespan.
The data from the study supports this assumption. In polygamous mammals with strong competition for mates, the males generally die earlier than females. By comparison, many birds are monogamous, so the competitive pressure is lower. This, in turn, appears to help them live longer. Overall, the longevity differences were smallest in monogamous species, while polygamy and pronounced size differences were associated with a greater advantage for females.
Parenting also plays a role. The team found evidence that the sex that invests more time in raising offspring tends to live longer. For mammals, that is typically the females. In long-lived species such as primates, this is likely a selective advantage. If females survive until their offspring are independent or sexually mature, the species will ultimately be better off.
The deep evolutionary roots
One other long-standing theory is environmental pressures. Natural occurrences such as predation, pathogens, or harsh climates are thought to drive the longevity gaps between males and females.
To test this environmental theory, the team looked at zoo populations, where these kinds of pressures are largely absent. Even without predators or natural disasters, the lifespan gaps still persisted. Comparing the zoo and wild populations revealed that the gaps were often smaller in zoos, but rarely disappeared. This zoo comparison mirrors the human longevity situation, since advances in medicine and living conditions have narrowed, but not completely eliminated the lifespan gap.
According to the team, these findings suggest that sex differences in lifespan are a deeply evolutionary process. It’s shaped by sexual selection and parental investment, while genetic differences in sex determination (like chromosomes) may also play a role. While environmental factors influence the extent of these differences, it does not completely eliminate them.
Ultimately, life expectancy between the sexes is not only a product of the environment we are raised in, but part of our evolutionary history–and will probably continue to exist.
