In most species, including our own, females live longer than males. Why this is the case is likely one of those simple questions that lacks a simple answer. At the root of it all are evolutionary pressures relating to sex-specific differences in mating strategy, but that says little about how and why an emergent property such as sex-specific life span differences actually emerges. Researchers here find a great deal of variation from species to species in the degree of the female longevity advantage, complicating the picture.
The researchers compiled demographic data for more than 130 wild mammal populations and were able to estimate the average longevity and the rate of increase in the risk of dying as a function of age for both sexes. The analyzes led to unexpected results. Not only do females generally live longer than males in wild mammals, but the difference in longevity between the sexes, although very variable depending on the population, in the vast majority of cases exceeds the difference observed in human populations. The average female wild mammal lives 18.6% longer than her male counterpart. In humans the difference is “only” 7.8%. The greatest differences are found in animals like Common brushtail possum, lion, killer whale, moose, greater kudu, and sheep.
For about half of the mammal populations studied, the increased risk of mortality with age is actually more pronounced in females than in males. These results show that the larger longevity of females than males is most likely due to other factors that affect individuals during their entire adult life. To reach this conclusion, researchers calculated the average age at death, as well as the rate at which mortality increases with age.
There is a common belief that males engage in potentially dangerous sexual competitions and live riskier lives than females, and that this could account for their shorter lifespan. Contrary to this idea, this study reveals that the intensity of sexual selection does not directly modulate the amplitude of the differences in longevity observed between the sexes. The results rather suggest that complex interactions between the physiological characteristics specific to each sex and local environmental conditions are at play.
Why do the females live longer? One explanation is that males often are larger and put more energy in sexual characters such as growing larger horns than females. This requires energy, and if the animals live in a harsh climate, the males may be more vulnerable to these extreme environmental conditions. Another explanation is that males produce more androgens than females. Androgens modulate immune performance and when present at high levels, they can impair some aspects of the immune defense, making males more susceptible to infections and diseases.