A lioness photographed on November 18, 2012 in a national park in Zimbabwe. – MARTIN BUREAU / AFP
- Bats, lions, orcas. A team led by Jean-François Lemaître, CNRS researcher in evolutionary biology, has compiled the demographic data of 134 populations of wild mammals, from 110 species.
- As in humans, the observation is obvious: females live longer than men. Their longevity is on average 18.6% longer than that of males, when it is “only” 7.8% in humans.
- To biological and behavioral causes, Jean-François Lemaître also adds the environmental factor. The researcher answers 20 minutes.
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Life expectancy at birth reached 79.8 years for men and 85.7 years for women in 2019 in mainland France, according to INSEE. What applies to France applies to the vast majority of countries: the life expectancy of women is greater than that of men.
Why ? Science gropes, reminded Slate in a February 2019 article. The causes would be partly biological – testosterone could alter the life of the man from a medical point of view – and partly behavioral (on average, men drink more, smoke more, have more risky behaviors …).
And what about other mammals? This is the question posed by fifteen researchers, including Jean-François Lemaître, CNRS researcher at Biometrics and evolutionary biology laboratory, in a study that appears this Monday in the scientific journal Pnas (Proceedings of the National Academy of Sciences). Jean-François Lemaître, researcher at the biometrics and evolutionary biology laboratory, answers our questions.
What is the genesis of this study already?
One of the themes we are working on at the Laboratory of Biometrics and Evolutionary Biology is the ecology of aging. In other words, we are looking to
understand the evolutionary origins of the aging process as well as explain the differences in aging between species, populations of the same species and between individuals. Through a partnership with the French Office for Biodiversity, we are studying in particular the deer, in which we follow individuals from two populations, from birth to death.
Among the factors that can play on aging, there is one that is particularly highlighted, taking the human being as an example: it is sex. We went to see if these differences between males and females are also found in nature. Similar studies have been conducted in the past. But either on datasets smaller than ours, or on captive populations. The interest of our study is to have collected demographic data on 134 wild populations of 110 mammal species, and thus to be able to draw a more general picture.
You come to the observation that, for 60% of the 134 populations studied, females live longer than males …
In this study, we calculated four measures of longevity, comparing each time males and females. Namely the maximum longevity (the maximum age attained by males and females), the average adult longevity (i.e. the life expectancy for an adult having survived until the age of reproduction) ), the median adult longevity (the age at which half of the individuals in the same population died), and finally the age at which 80% of the individuals in the same population died. Of these four longevity measures, for 60% of the populations studied, the females presented superior results to the males. If we take these measures one by one, we often very much exceed this 60%.
If we take the example of median longevity, that of females is on average 18.6% higher than that of males, while the difference is only 7.8% in humans. For some species, the differences are even more significant. This is the case of the lion, a species for which we have studied two populations. Three years after reaching the age of first reproduction, 50% of the males have already died. This was not the case until seven years later for the females. The differences are also very marked in orcas or bovines such as the great kudu or the sheep of Soay.
Is it the reverse for some species: males live longer than females?
When we say that for 60% of the populations studied, females live longer than males, for the remaining 40%, in reality, the differences in longevity between the two sexes are very often almost nonexistent. In rare cases, males live significantly longer than females in the species we studied. This is the case for example of two species of bat: the murine of Daubenton or the small brown bat.
How to explain these differences in longevity in animals? Are the causes, there too, biological and behavioral?
By noting that overall, in most mammals, females live longer than males, we tend to show that this difference is biologically very entrenched. the differences in longevity between women and men in human populations are biologically very entrenched even if they can be modulated by certain behaviors.
However, aren’t there also differences in behavior between males and females in many mammals?
Yes, there are differences in life strategies. In many species, males will have to allocate a lot more resources to sexual competition, to the control of a harem or to the defense of a territory …
It is indeed a factor that tends to modulate differences in longevity
between males and females. In our view, the local environment plays a lot
in the differences in longevity between males and females, more than one could
imagine it. We can see that in our study on species for
which we were able to study several populations living in environments
different. Between these populations, differences in longevity between the two sexes
can be important. The local environment can thus come to modulate these differences
longevity between the sexes, we can hypothesize that in the
environments very rich in pathogens, males already allocating a lot of
resources for sexual competition will suffer from increased mortality, exacerbating
thus differences in longevity between the sexes.
