It is well known that women live longer than men. I had not been aware that this was not true for all species and that when comes to birds, the opposite is true. Now some researchers are finding new evidence to support an idea that what causes the difference is whether one has an unpaired sex chromosome or not.
From humans to black-tailed prairie dogs, female mammals often outlive males – but for birds, the reverse is true.
Now researchers say they have cracked the mystery, revealing that having two copies of the same sex chromosome is associated with having a longer lifespan, suggesting the second copy offers a protective effect.
The idea that a second copy of the same sex chromosome is protective has been around for a while, supported by the observation that in mammals – where females have two of the same sex chromosomes – males tend to have shorter lifespans. In birds, males live longer on average and have two Z chromosomes, while females have one Z and one W chromosome.
Scientists say they have found the trend is widespread. Writing in the journal Biology Letters, the team report that they gathered data on sex chromosomes and lifespan across 229 animal species, from insects to fish and mammals. Hermaphroditic species and those whose sex is influenced by environmental conditions – such as green turtles – were not included.
The results reveal that individuals with two of the same sex chromosomes live 17.6% longer, on average, than those with either two different sex chromosomes or just one sex chromosome.
The team say the findings back a theory known as the “unguarded X hypothesis”. In human cells, sex chromosome combinations are generally either XY (male) or XX (female). In females only one X chromosome is activated at random in each cell.
As a result, a harmful mutation in one of the female’s X chromosomes will not affect all cells, and hence its impact can be masked. By contrast, as males only have one X chromosome, any harmful mutations it contains are far more likely to be exposed.
I had also not known that bird chromosomes are identified by different letters than those used for human chromosomes.
You can read the paper here.