On average, females of a species tend to outlive males, which was previously attributed to differences in behaviours and physical traits. But the reason for this lifespan disparity may be less down to risky behaviour and more to do with sex chromosomes, according to a new study published on 4 March in Biology Letters (1).
The lifespan of males and females is known to differ in many species. One hypothesis suggests that mismatched sex chromosomes, such as XY male mammals, could make an individual more vulnerable to genetic mutations. This so-called ‘unguarded X hypothesis’ might provide a reasonable explanation for why one sex outlives the other on average.
The unguarded X hypothesis
The unguarded X hypothesis says that the reason one sex dies younger than the other is that the mismatched XY sex chromosomes — referred to as heterogametic — cannot ‘hide’ harmful mutations on the X chromosome, and could, therefore, potentially expose the individuals to more health problems.
On the other hand, when there are two X chromosomes, if one of the X chromosomes contains deleterious genes with recessive mutations, then genes on the healthy X chromosome can be expressed instead, thereby ensuring the harmful mutations are not expressed.
Until now, the well-known — yet unproven — effect of mismatched chromosomes has not been studied across the animal kingdom. So, the team of researchers from the University of New South Wales in Sydney Australia decided to analyze all of the available literature on sex chromosomes and lifespan.
They compared the lifespans of males and females of 229 animal species across 99 families, 38 orders, and eight classes. “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 lead author and PhD candidate Zoe Xirocostas in a statement.
The analysis revealed that the sex with two matching chromosomes (e.g. XX) lives 17.6 per cent longer. However, interestingly, the scientists also discovered that this life span disparity also varies quite dramatically between species. For instance, the female Blattella germanica (German cockroach), which has the XX sex chromosomes, will live 77 per cent longer than their single-X male counterparts.
Furthermore, they showed that the difference in longevity also varies depending on whether the animal with heterogametic sex chromosomes is female or male: heterogametic males (XY) die sooner than heterogametic females (ZW), relative to their female and male species counterparts.
“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”, explained Xirocostas.
“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”
The authors also hypothesize that difference in lifespan between sexes is proportional to the difference in chromosome length between sexes, which they hope to investigate further. Furthermore, sexual selection and telomere dynamics might also play important roles in longevity, but more research is still needed.
(1) Xirocostas, Z.A., Everingham, S.E., and Moles, A.T. The sex with the reduced sex chromosome dies earlier: a comparison across the tree of life. Biology Letters (2020). DOI: 10.1098/rsbl.2019.0867
Image: Male and female cardinal birds, STidwell via Getty Images.