Data has shown conclusively that even though women tend to have poorer health than men during their adult lives, they live longer than men. Around 90 percent of septuagenarians are female. There have been a variety of theories about why that is so; some have suggested it is the effect of behavior, others think it is related to hormones, and some have theorized that it's genetic.
The unguarded X hypothesis states that mutations on the X chromosome that have built up over generations must be expressed, and therefore, impact physiology, because there is not a second copy of the X chromosome to step in if the first becomes non-functional or dysfunctional. Since females carry that second X chromosome, they are protected from those mutations. New research has backed up that last theory and suggested that the XY sex chromosomes that males carry are not as equipped to protect an individual from a failure of a gene on the X chromosome.
Reporting in Biology Letters, researchers at the University of New South Wales Science's School of Biological, Earth and Environmental Sciences have assessed studies on sex chromosomes and lifespan across a range of species to check this hypothesis.
"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," said the first author of the study, graduate student Zoe Xirocostas. "And we found that across that broad range of species, the heterogametic sex (XY) does tend to die earlier than the homogametic sex (XX), and it's 17.6 percent earlier on average."
This pattern held true for other animals that carry unique sex chromosomes. In birds, males are the ones carrying two of the same sex chromosome; they are homogametic (ZZ) and females are the heterogametic chromosome carriers (ZW). In their case, it is females that tend to die before males - the unguarded Z, so to speak.
One observation surprised the researchers. "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," said Xirocostas. "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."
It may then be possible that genetics are a major influence on sex differences in lifespan, but other factors are also at work.
Xirocostas suggested that this might be the case, and that sexual selection, Y chromosome degradation, and telomere dynamics might be involved in this trend.
"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," she added.