That females live longer than males in numerous species is a topic of some interest to evolutionary theorists and other researchers in the life sciences. There are any number of possible explanations, but that this phenomenon exists in many different species tends to favor evolutionary arguments. Something fundamental to gender as it exists in most higher species is closely tied to aging, and the result is near always females that age more slowly than males. In the research noted here, scientists report on an experiment in fly populations that suggests this longevity difference will arise quite naturally from the differing mating strategies of male and female genders, each under selection pressure to maximize their success in reproduction.
Differences in aging and the length of life between males and females are common in the animal realm. Males often have shorter lifespans than females. Researchers used fruit flies, Drosophila melanogaster, to investigate whether sexual selection lies behind sex differences in aging. They wanted to determine whether the two sexes are affected differently when they are in poorer physical condition, in other words, when they have poorer access to nutrients and energy. In particular, they were interested in the ability of the flies to reproduce, and how this ability changes when the flies age, in a process known as "reproductive aging".
Researchers had manipulated the genetic material of some of the flies, such that they had many small harmful mutations in their genes. These mutations had a negative influence throughout life, meaning that an individual with such mutations converted food to useful energy slightly less efficiently. Thus, even though all of the flies had access to the same food and could eat equal amounts, the manipulated flies were in poorer physical condition.
In order to mate with available females, the aging males were compelled to compete with young males. It turned out, as expected, that males in good physical condition were better at this than those who were in poorer condition, independently of how old they were. The reproductive aging of males, however, decreased at the same rate, independently of whether they were in good or poor physical form. Things were different for females. Early in life, there was no difference between the number of offspring produced by females in good condition, who could use the available resources better, and the number produced by mutated females, who were in poorer condition. The two groups, however, aged at different rates. As the females became older, those who were in good physical form had more offspring than their less fortunate sisters.
"The results show that sexual selection contributes to the differences between the sexes in reproductive aging. This is probably because females in good condition, with good access to nutrients, invest the extra resources into maintaining their bodies, such that they can continue to reproduce to a more advanced age. Males, in contrast, seem to invest a great deal of their resources, independent of their condition, into trying to ensure that they achieve successful mating here and now."