All over the world, women generally outlive men. In New Zealand, a boy born today can expect to live to 70 whereas a girl can expect to live to 75. While it used to be thought this was because men live more dangerous lives than women, new research out this week finds that it is probably less to do with our behaviour and more to do with our sex chromosomes.
There are many reasons why women seem to live longer than men and, for a long time, it was thought to be behavioural. Data shows there are some behavioural themes that do seem to affect more men than women. For example, in general, men tend to seek medical care less, which makes them much more likely to die early from treatable illnesses.
Statistically, men tend to smoke more cigarettes, drive more recklessly and are more likely to be overweight than women, putting them at a higher risk of preventable diseases. With almost every person being bitten by a shark being a man, they are also either much more tasty to predators than women or are just more likely to put themselves in places of higher risk. While these increased risk behaviours do seem important, the biological factors that differentiate the two genders appear to be more important, according to research published in the journal "Biology Letters".
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The study published last week shows that these lifespan disparities aren't just a human problem but occur across the whole animal kingdom. They also seem to be much more related to our sex chromosomes than our behaviour.
Humans have 23 pairs of chromosomes with the 23rd pair determining our gender. Women have two X sex chromosomes whereas men have one X and a short, degraded Y chromosome. The X chromosome contains many important genes whereas the Y chromosome has limited contribution to anything beyond fertility and reproduction.
Birds have Z and W chromosomes and it's the males that have two Z chromosomes while the females have one W and one Z. Like humans, the W is shorter and more degraded than the Z. In birds, it is the males that live longer than the females – the opposite of humans, but the same when it comes to longevity being tied to sex chromosome matching.
In the study, the researchers compared the lifespans and chromosomes of males and females of 229 different animal species. These included mammals, reptiles, fish, amphibians and insects.
They found that on average the gender that had the identical sex chromosomes, such as an XX or a ZZ, lived 17.6 per cent longer. The biggest winner of the study was the female German cockroach Blattella germanica. With their XX sex chromosomes, they live on average 77 per cent longer than their male counterparts, who only carry a single X chromosome.
Scientists have previously thought that a shorter, degraded chromosome could be a weak spot and this new research supports that theory.
The theory is that having two identical chromosomes automatically means there is a back-up available if needed. Called the unguarded X hypothesis, the study suggests that the smaller Y or W sex chromosome is a disadvantage. This is because they are less protected from any harmful genes that can cause health issues, which are expressed on the X chromosome as they don't have another one to rely on.
While this new research does show that men are potentially at a slight disadvantage biologically, it also shows that male longevity could also be extended with simple behaviour changes, such as going to the doctor more frequently and choosing to live a more healthy lifestyle.