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.