The scientific community has spent years wondering if females live longer than males.
After studies carried out by Xirocostas et. al (2020) and others it is clear that the answer is “it depends”.
If we analyse our own species, there does seem to be an apparent correlation between being a woman and greater longevity. This occurs generically in more and less developed countries, sometimes with a margin of up to 10 years of difference in life expectancy. Men do have higher overall mortality in all age ranges, but there are certain ranges in which the difference is especially significant. For example, in the United States men aged between 15 and 24 years are 3 times more likely to die than women, largely due to the more reckless behaviour that leads to more accidents and fatal incidents. These data could lead to the conclusion that men are socio-culturally conditioned to take more risks.
However, if we ignore the environmental factors that can influence behavioural development and look at the data of the individuals before or immediately after birth, we can observe that there is still a higher risk of premature death in boys than in girls. Therefore, we could conclude that there are biological factors that determine a greater longevity in women.
Among the studies related to this topic, different hypotheses, including a more active female immune function, the protective effect of oestrogen, the compensatory effects of the second X chromosome or the influence of oxidative stress on aging, have been considered. Nevertheless, none of them have been conclusive.
Besides, research investigating the influence of telomere length and the difference in telomere shortening between genders has also been conducted. However, the telomeric shortening ratio shows similar results.
These conclusions are consistent with other previous studies that show telomere shortening with age and a gender difference in this trend. Although the key factors for gender differences are not fully understood and further studies are clearly needed to establish whether there are gender differences in telomere-length dynamics.
In the animal kingdom, differences in longevity seem to be much more clearly linked to having the homogametic sexual chromosome pair (same type of chromosome). Not all species express the same gender in homogamic individuals. In mammals, it is the female that has the same pair of sex chromosomes (for example, in humans XX), while, in the case of birds (ZZ) or other organisms such as butterflies, it is the males that have the pair of sex chromosomes of the same type.
The study by Xirocostas et. al (2020) concludes that homogametic gender lives 17.6% longer than heterogametic individuals (different sex chromosomes) and that there are substantial differences on average in life expectancy; if the male is the heterogamic individual, the homogamic gender lives 20.9% more, and if the female is the heterogamic individual, the homogamic individual lives 7.1% more.
In general, everything points to the fact that the homogametic gender confers a certain resistance and longevity to the individual, although there is still a long way to go to glimpse if there are more determining biological factors, also without forgetting the importance and effect that sociocultural and environmental factors have.