The Y Chromosome: Why Do Men Tend To Die Younger?

The unguarded X hypothesis suggests that as the Y chromosome is smaller than the X and in some cases absent it is unable to hide an X chromosome that carries harmful mutations that may expose the individual to health threats. But there is no such issue in a pair of homogametic chromosomes where a healthy X chromosome can stand in for another X that has deleterious genes to ensure harmful genes are not expressed maximizing length of life. 

After examining available lifespan data on a wide range of species the unguarded X hypothesis appears to hold up, according to the researchers who tested the theory across the board in animal taxonomy according to first author Zoe Xirocostas. 

"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," says Xirocostas. "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 per cent earlier on average."

The same pattern was also observed in classes of animals possessing their own unique pair of gender chromosomes that are the reverse of all other animals such as some birds, butterflies and moths wherein the male of the species has the homogametic gender chromosomes. In these reversed cases the females of the species were found to die younger than the male counterparts giving more credence to the unguarded X hypothesis theory, although in these species it is an unguarded Z chromosome. 

Even though this study confirms the unguarded X hypothesis as a possible reason as to why one gender may be outliving the other, another statistic emerged from the data that came as a surprise:

"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," she says. "In species where males are heterogametic (XY), females live almost 21 per cent longer than males. But in the species of birds, butterflies and moths, where females are heterogametic (ZW), males only outlive females by 7 per cent."

In other words, heterogametic males simply die sooner than heterogametic females when compared to the opposite gender of their species; this may mean that something still remains that is fundamentally life shortening about being a male member of any species. Xirocostas lists side effects of sexual selection, degree of Y chromosome degradation, and telomere dynamics all as being possible explanations for the surprising find. 

"I was only expecting to see a pattern of the homogametic sex (XX or ZZ) living longer, so it came as an interesting surprise to see that the type of sex determination system (XX/XY or ZZ/ZW) could also play a role in an organism's longevity."

Additional research of the phenomenon should explore the hypothesis raised in this study of the difference in lifespan between the genders being proportional to the difference in chromosome length between the genders, which may help further understandings of the factors affecting aging.