Reviewing the Evidence for the Unguarded X Hypothesis of Shorter Male Life Span
Why do males of all species have a shorter life expectancy than that of females? There are numerous perspectives on this question, from viewing it as a natural evolutionary outgrowth of mating strategies, to the more mechanistic concerns of differences in metabolism, appetite for risk, and so forth. One popular hypothesis is that the Y chromosome is less capable of covering for mutational damage to the X chromosome than is a duplicate X chromosome. This will gender-bias the effects of inherited mutations on the evolution of longevity, and perhaps also magnify the effects of stochastic mutational damage occurring across a lifetime. Experiments in using male mice engineered to have two X chromosomes provide supporting evidence for the proposition, and, as noted here, the balance of the rest of the evidence in the literature tends to follow along in that support.
Researchers analysed all available academic literature on sex chromosomes and lifespan - and they tried to establish whether there was a pattern of one sex outliving the other that was repeated across the animal kingdom. Specifically, they wanted to test the 'unguarded X hypothesis' which suggests that the Y chromosome in heterogametic sexes - those with XY (male) sex chromosomes rather than XX (female) sex chromosomes - is less able to protect an individual from harmful genes expressed on the X chromosome. The hypothesis suggests that, as the Y chromosome is smaller than the X chromosome, and in some cases absent, it is unable to 'hide' an X chromosome that carries harmful mutations, which may later expose the individual to health threats. Conversely, there is no such problem in a pair of homogametic chromosomes (XX), where a healthy X chromosome can stand in for another X that has deleterious genes to ensure those harmful genes aren't expressed, thus maximising the length of life for the organism.
After examining the lifespan data available on a wide range of animal species, it appears that the unguarded X hypothesis stacks up. This is the first time that scientists have tested the hypothesis across the board in animal taxonomy; previously it was tested only within a few groups of animals. "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. 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."
Interestingly, the researchers observed this same pattern in the classes of animals possessing their own unique pair of sex chromosomes that are the reverse of all other animals. In birds, butterflies and moths, it is the male of the species that has the homogametic sex chromosomes (denoted by ZZ) while the female has the heterogametic chromosomes (ZW). Female birds, butterflies, and moths were usually found to die earlier than their male counterparts, giving credence to the unguarded X hypothesis - although strictly speaking, it's an unguarded Z in this case.