Among mammalian species, male fitness is typically more variable than female fitness. Explain why this is often the case. What implications does this have for evolution acting on males and females?
This is mainly the result of sexual selection, a special form of natural selection. It can take place either when members of one sex choose to mate with members of the other sex with certain characteristics (intersexual selection), or when members of the same sex compete with each other to get access to members of the other sex (intrasexual selection).
For most mammals, females’ reproductive success mostly relies on their access to resources and food, while males’ reproductive success is more related to their access to females. As we can see, due to their high parental investment, females can only have limited amount of offspring in their whole life span. After giving birth to the child, they still need to take care of these altricial offspring to an extent that they can become relatively independent. However, as males’ reproductive success does not depend greatly on their investment in caring for their offspring, they would prefer to mate with more females to reproduce more offspring, as greater quantity of offspring suggests higher possibility that his genes would be passed on.
As a result, males and females have developed very different mating strategies throughout the evolutionary history. Males often need to compete with each other for access to female mates and therefore reproduce. This kind of intrasexual selection acting on males is extremely strong, as males’ reproductive success varies a lot based on the results of the competitions. Individuals who succeed in the competition may sire many offspring, while ones who lose may fail to reproduce at all. And after several generations, these characteristics that enhance males’ competitiveness, such as large body size, strong horns, and large canine teeth, would be “favored” by natural selection, resulting in greater sexual dimorphism and even stronger competition between males.
Therefore, male fitness is typically more variable than female fitness among mammalian species.
