Gender, aging and life expectancy
Female mammals were not found to have a tendency to age slower than males
Polina Loseva, N+1
Scientists have combined population data on the aging of hundreds of mammalian species and have not found common sex differences for all. Despite the fact that females in most cases live longer than males, not all species actually age more slowly. The researchers propose to explain the difference in mortality by sexual dimorphism: differences in the work of male and female organisms make them susceptible to different environmental factors. The work was published in the journal Proceedings of the National Academy of Sciences (Lemaître et al., Sex differences in adult lifespan and aging rates of mortality across wild mammals).
It has long been known that women live on average longer than men. This is true not only for the species Homo sapiens as a whole, but also for each population individually. However, it is still unclear what caused this difference. According to one theory, it's about sex chromosomes: recently scientists have calculated that animals of a heterogamous sex (with different chromosomes) live on average less than a homogamous one (with the same). If this is the case, then there is nothing special about women and men — they simply obey a common pattern for animals.
At the same time, the difference in life expectancy can be caused by different reasons. Perhaps the fact is that one sex, due to its genetic characteristics, ages faster than the other. Aging is now often defined as an increase in mortality over time (although we have already talked about why this is not the best definition). Therefore, if one sex ages faster than the other, the mortality curve should bend up more sharply. But another development is also possible: one of the sexes may simply have a higher background mortality rate — that is, its representatives die more often on average, regardless of age — and then the curve for one sex should lie higher than for the other.
To find out if there is a pattern common to mammals, Jean-François Lemaître from the University of Lyon, together with colleagues, collected data on 134 populations belonging to 101 species of mammals. The authors of the work used open mortality tables for these conclusions, or modeled the dynamics of mortality based on individual data — for example, determining the age of dead individuals found in nature.
It turned out that female mammals live longer than males in about 60 percent of populations. Moreover, in most cases, they have a higher advantage than women over men: if for humans the difference in life expectancy is on average 7.8 percent, then for other mammals it is 18.6 percent.
At the same time, scientists could not find a general pattern in the mortality curves. They identified species in which males age faster than females (the curve grows more sharply), for example, deer, and those in which female elephants age faster. And in primates, the rate of aging turned out to be almost the same, and the difference in life expectancy was caused by a difference in mortality, that is, the curve for males lay above the curve for females.
Relative aging rate (left) and mortality curves (right) for males (blue) and females (red) of different mammalian species. A drawing from an article in PNAS.
This means that the presence of a particular set of sex chromosomes in itself does not cause the animal to age faster or slower. The difference in mortality appears for other reasons specific to each species. The authors of the work believe that these reasons are mainly related to external factors: for example, the body of males may be more vulnerable to temperature changes or parasite attack. This is confirmed by the fact that in populations that people hunt, the difference in favor of females is higher than in those that do not suffer from hunting (34.5 percent versus 16.7, respectively). Sexual dimorphism can play a role in this: secondary sexual characteristics in males develop under the influence of testosterone, which suppresses the work of the immune system, which reduces the protection of the male's body from pathogens.
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