March 4 (UPI) -- According to a new study, male mammals live shorter lives than their female peers as a result of sex chromosomes, not because of their risky behavior.
When an international team of researchers surveyed the scientific literature for links between sex chromosomes and lifespan across the animal kingdom, they found evidence supporting the "unguarded X hypothesis."
The unguarded X hypothesis supposes that males, because they have an X and Y chromosome, are less able to avoid harmful genes expressed on the X chromosome. Because the Y chromosome is often smaller than the X, it can's hide the flaws in the X chromosome.
Having two X chromosomes is beneficial because the homogametic chromosomes can be interchanged, with a healthy X chromosome taking the place of an X that has damaging genes.
Researchers found broad support for the hypothesis in available scientific literature. They published findings Wednesday in the journal Biology Letters.
"We looked at lifespan data in not just primates, other mammals and birds, but also reptiles, fish, amphibians, arachnids, cockroaches, grasshoppers, beetles, butterflies and moths among others," lead study author Zoe Xirocostas, doctoral student at the University of New South Wales, said in a news release. "And we found that across that broad range of species, the heterogametic sex does tend to die earlier than the homogametic sex, and it's 17.6 percent earlier on average."
Male animals don't always carry heterogametic sex chromosomes. Female birds, butterflies and moths carry ZW chromosomes, while the males carry homogametic ZZ sex chromosomes. Data analyzed by Xirocostas and her colleagues showed female birds, butterflies and moths tend to die sooner than their male counterparts.
The difference in lifespans between the carriers of ZW and ZZ chromosomes wasn't as great, however.
"We found a smaller difference in lifespan between the males and females in the female heterogametic species compared to males and females in the male heterogametic species," Xirocostas said. "In species where males are heterogametic (XY), females live almost 21 percent longer than males. But in the species of birds, butterflies and moths, where females are heterogametic (ZW), males only outlive females by 7 percent."
The findings suggest chromosomal differences help explain some of differences in lifespan between male and female mammals, but it's possible that differences in behavior further shrink the male mammal's already shorter lifespan.