The reason why women live longer than men

Women have always lived longer than men, regardless of country, culture, race or historical moment. Although life expectancy increases when living conditions improve, as has happened in many countries over the last decades, the difference between genders remains.

Good living conditions make the most of our potential, but there is something structural that persists that is different between genders.

Why? To understand what living beings are like, we have two types of reasons: proximate causes and ultimate causes. Proximate causes refer to the mechanisms that give rise to the effects we see. For example, aging can be caused by oxidative stress in cells, shortening of telomeres, loss of the Y chromosome in many cells in the case of males, etc.

But behind all this there is an ultimate cause, we could say “by design”, and that is that the organism is prepared by natural selection for a duration that is not infinite. And that duration is not just any, but the best to achieve what really counts in natural selection: that the genetic instructions on how to build an organism endure through the generations.

These genetic instructions remain through time thanks to reproduction. Organisms with different reproductive strategies have different optimal longevities. For example, a field mouse whose strategy is to produce many offspring in a few years will make more copies of its genes than one that produces one offspring per year and is designed to live for many. And it is so, simply, because the probability of living those years is very low if we take into account the large number of predators that can end their lives at any time.

In deer, females live almost twice as long as males.

In many species, males and females have quite different reproductive strategies. Let’s forget about humans for a moment and talk about deer, which is an animal that my research team has worked on for many years. The males base their reproductive success on becoming dominant for at least a few years, since this way they will be able to cover many females in the typical harems of this polygamous species. Females, on the other hand, only produce one offspring per year in the best of cases.

No matter how good a male is, he will not be able to remain dominant over his rivals for more than 3-4 years. He is like an elite athlete at the top: a lot of glory but necessarily short. On the contrary, to leave many offspring, females must live for many years. Iberian red deer males can live a maximum of 11-12 years while females reach more than 20.

The interesting thing is that in our studies we have found that the design in males was adjusted to that longevity compared to that of females. Specifically, the teeth in males are smaller than their size, so they wear down sooner than in females. Studying the evolution of deer and other species, we found that natural selection had increased the overall size of males, necessary to win competition against other males, but had not proportionally increased the size of their teeth. For the males of these species, if they wanted to have more children, it was preferable to give everything to impregnate many females in a few years of social dominance than to live longer.

Polygynous species: larger males that age and die earlier

Polygamous species, and specifically those called polygynous (in which each male could fertilize many females), usually present sexual dimorphism, that is, different morphologies in males and females. Many species, such as humans, in which males are larger on average than females, in addition to different sexual traits, such as deer antlers, peacock tails or beards in men, tend to also show differences between the sexes in longevity, in accordance with the degree of polygyny.

Very polygynous species such as red deer, where a male can cover dozens of females in a season, have a life expectancy difference of 8 years (40% reduction in males compared to females). However, in humans, with an evolutionary history of moderate polygyny, the difference between men and women in longevity is approximately 7%.

The case of deer teeth is just one example of proximate cause. In us humans, it is likely that many processes are occurring simultaneously that give rise to this difference in longevity between men and women.

It does not matter whether or not we currently maintain the polygynous behaviors of our past. It doesn’t matter whether we retain some degree of natural selection or not in our modern world. What we are today is the result of that evolution and its transfer throughout recent generations through phylogenetic inertia, which means that most of our genes are copied as is to the next generation.

Organisms are designed through natural selection and only by knowing how this selection works can we understand the ultimate reason for how we are.

Juan Carranza Almansa, Professor of Zoology, University of Cordoba

This article was originally published on The Conversation. Read the original.