Playing for Longevity

Women generally outlive men, a pattern present throughout history and across the world.

Still, the gap is narrowing because of advances in medicine and improved living conditions.

Now, a new study says this difference is unlikely to disappear in the near future. That’s because the differences in lifespans are deeply rooted in evolutionary history and are even found in other species.

“From a human standpoint, it’s really remarkable that women live longer across almost every country in the world,” lead author Johanna Stärk told the Washington Post. “So we were interested in looking at this from a broader taxonomic perspective.”

Researchers have carried out the most comprehensive analysis of sex differences in lifespan across mammals and birds to date. They found, while examining mammals, such as baboons and gorillas, that females usually outlived males. However, among birds, insects, and reptiles, the opposite is usually true.

One possible genetic explanation, known as the heterogametic sex hypothesis, suggests that differences in sex chromosomes could explain why one sex outlives the other, according to researchers. In mammals, females have two X chromosomes, while males have one X and one Y – meaning males are the heterogametic sex. In birds, however, the system is reversed, and females are the heterogametic sex. Some research suggests that having two X chromosomes could protect individuals from harmful mutations, giving them a survival advantage.

Analyzing data on the lifespans of 528 mammal species and 648 bird species in zoos worldwide, the team uncovered a striking difference in lifespan that supports the heterogametic sex hypothesis.

For example, in 72 percent of mammals, females lived longer – 12 percent on average. But in 68 percent of bird species, males lived longer – 5 percent on average. Still, there were noteworthy variations with many exceptions.

“Some species showed the opposite of the expected pattern,” 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.”

Alongside genetics, reproductive strategies also play a role. Males often develop conspicuous characteristics such as colorful plumage, weapons, or large body size through sexual selection. While improving reproductive success, these traits can reduce longevity.

Meanwhile, in polygamous mammals with strong competition for mates, males usually live less than females. Many birds, instead, are monogamous, meaning there is less competitive pressure, and males often live longer.

In general, lifespan differences were minimal in monogamous species, while polygamy and greater size differences tended to give females the advantage.

Parental care is also a factor, as researchers found that the sex most involved in caring for offspring generally lives longer – in mammals, this is usually the females.

Also, the team looked at zoo populations, where certain pressures such as harsh climates or predators were absent. They discovered that the gap in lifespan, while smaller, still existed.