On average, men die about seven years earlier than women, and male lions, rodents, and spiders
often go belly up before their female counterparts too. One big reason males live shorter lives than
females might be hidden away in the chromosomes that help determine sex in the first place.
Welcome to MinuteEarth. In lots of species, males are more aggressive and
take more risks, so you might expect they’d die first - and in many species, like humans, they do.
But males aren’t always more aggressive, and the more aggressive sex doesn’t always die first.
Instead, when you look across the animal kingdom, the animals that die earlier,
regardless of their sex, have something else in common: they have a small sex chromosome.
Sex chromosomes generally come in two sizes, big and small; part of what determines the
sex of an individual is the combination of sizes they inherit. In mammals like humans,
females inherit two big sex chromosomes, while males inherit one big and one small. But in birds,
butterflies, and some reptiles and amphibians, the opposite is true: males get two bigs,
while females get a big and a small. In most cases, the sex with the small
chromosome tends to die first, regardless of whether they are the males or females.
There are two possible explanations for why this trend exists: one is that having more
big sex chromosomes is better, and the other is that having a small sex chromosome is worse.
Having more big sex chromosomes might be better because big sex chromosomes have
lots of genes on them; a human’s big chromosome has 800 or so genes that code for many things
unrelated to sex - from color vision to proper wound healing to muscular function.
Someone with two big chromosomes has TWO copies of each of those genes.
So if one big chromosome carries a harmful mutation, the other is likely to have a working
version of the gene. But someone with only one big chromosome only has ONE copy of all those
genes - they have no backup copies. That’s why colorblindness, hemophilia, and muscular
dystrophy are much more common in human males. While there may be an advantage to having two big
sex chromosomes, there's also evidence there's a disadvantage to having a small chromosome.
Small chromosomes happen to be particularly vulnerable to malicious viral genes called
transposons, which can trigger lots of harmful mutations in genes on other chromosomes.
So having a small sex chromosome at all may put a critter at risk for a shorter life.
These hypotheses - that big chromosomes are beneficial, and that small chromosomes are
detrimental - are not mutually exclusive…it’s likely that both are true. Scientists are still
debating how these processes interact - and how big a role they play in animals’ lifespans
versus other factors. That may take a while, so if you want to be around when we figure out
X-actly Y all this happens , you should probably hope you don’t have a small chromosome.
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