In many species, females have a tendency to live longer than males. Our own species is no exception: the average human life expectancy for females is 74.2 years compared to just 69.8 years for males. This chasm in lifespan is often explained by environmental or social factors, such as males undertaking more dangerous jobs, indulging in riskier behavior, or taking less care of their health.
However, it's starting to look like it might have something to do with doubling up on sex chromosomes. A new study has found that having two copies of the same sex chromosome is associated with having a longer lifespan. In humans, sex chromosomes are generally either XX (female) or XY (male).
Some species of bird, fish, reptile, and insect have a different system of sex determination based on Z and W chromosomes, where the males have ZZ sex chromosomes and females have ZW chromosomes. Interestingly, even under this "reversed" system, the theory remains true: the males, which have two copies of the same sex chromosome, generally outlive the females.
Reporting in the journal Biology Letters, researchers from the University of New South Wales (UNSW) wanted to see whether this trend could be seen among a wider variety of animals. They looked at sex differences in lifespan in 229 species – spanning 99 families, 38 orders, and eight classes – and noted whether the longer-living sex had homogametic chromosomes (such as XX or ZZ) or heterogametic chromosomes (XY).
As expected, the sex with homogametic chromosomes tended to have a longer life across most species.
“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,” Zoe Xirocostas, first author on the paper and PhD student at UNSW, said in a statement. “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.”
“In species where males are heterogametic (XY), females live almost 21 percent longer than males," she added. "But in the species of birds, butterflies and moths, where females are heterogametic (ZW), males only outlive females by 7 percent.”
This idea is known as the “unguarded X hypothesis." While this theory has been floating around for some time, this is the first time a scientific study has tested the hypothesis across such a wide range in animal taxonomy.
The theory goes that individuals with heterogametic sex chromosomes, such as XY, are less able to protect against harmful genes expressed on the X chromosome. On the other hand, those with homogametic sex chromosomes have a second copy that could serve as a “back up.” Alternatively, it might have something to do with how the Y chromosome degrades and telomere dynamics as possible explanations for this trend.
However, there are examples of heterogametic species that buck this trend, so though it appears to certainly be a factor in the longevity of some species, other factors may also play a role.