When 109-year-old Jessie Gallan was asked about the secret to her long life, she replied “staying away from men.” Other people older than 100 have extolled the virtues of everything from crossword puzzles to tap dancing. One thing they don’t usually mention: chromosomes. Yet, across the animal kingdom, individuals with identical sex chromosomes—including women with double Xs—live nearly 18% longer than their counterparts with mismatched chromosomes, a new study reveals.
In most animals, sex chromosomes help determine whether an individual develops as a male or female. In mammals, females typically have two identical X chromosomes, whereas males have one X and one much smaller, or “reduced,” Y chromosome. Sexes of some animals, such as most male arachnids, lack a second sex chromosome entirely. These chromosomes contribute to the physical differences between males and females. Birds with ZZ sex chromosomes, for example, are male and tend to be more colorful, whereas ZWs are females with typically blander plumage.
Physical traits aren’t the only differences between the sexes. Researchers hypothesize that animals with mismatched sex chromosomes, such as XY male mammals, could be more vulnerable to genetic mutations, which could result in a shorter life span. But until now, scientists haven’t studied this effect across the animal kingdom.
So researchers at the University of New South Wales, Sydney, scoured scientific papers, books, and online databases for sex chromosome and longevity data. They compared the life spans of males and females of 229 animal species across 99 families, 38 orders, and eight classes. On average, the sex with identical chromosomes lives 17.6% longer, they report today in Biology Letters. The longevity pattern holds for humans, wild animals, and captive animals across the evolutionary family tree.
“I thought it was really cool how, across insects and fish, we’re all showing the same sort of response,” says the study’s lead author, ecologist Zoe Xirocostas.
Still, the researchers found that the life span disparity varies markedly between species. At one extreme, female German cockroaches (Blattella germanica), with XX sex chromosomes, live 77% longer than single-X males. The disparity also varies depending on whether the animal with matching sex chromosomes is female or male. Females with identical sex chromosomes—such as mammals and some reptiles, insects, and fish—live an average of 20.9% longer than males, but in males with matching sex chromosomes, such as birds and butterflies, the life span gain over females is just 7.1%.
This unevenness hints that factors other than the presence of certain sex chromosomes might also strongly influence longevity, the team says. One of these factors could be sexual selection. Exaggerated physical traits and elaborate behaviors make males of some species more attractive to females but require large amounts of energy and take a toll on overall health.
“We know that sexual selection is stronger in males,” says evolutionary biologist Gabriel Marais from Claude Bernard University Lyon, who was not involved in the research. Males “pay the cost of this sexual selection by faster aging, and they will die younger,” Marais says.
If those males also have reduced or absent sex chromosomes that leave them vulnerable to mutations, the deleterious effects on life span add up, Marais says. In comparison, female birds and butterflies with mismatched sex chromosomes might be more vulnerable to mutations, but they don’t face the life span reduction of intense sexual selection.
Further work could help researchers understand how sex chromosomes impact life span. For example, researchers don’t yet know whether the size of the reduced sex chromosome corresponds to the difference in life span between males and females. “There are so few papers about this question,” Marais says. He praises the new study as an important step in the right direction.