FEMALE GAMETE COMPETITION: A NEW EVOLUTIONARY PERSPECTIVE ON MENOPAUSE

Menopause has long perplexed evolutionists because it is hard to see how a trait that precludes reproduction for a large portion of a woman’s adult life could have spread evolutionarily. Neither of the two common explanations is scientifically satisfying. The Extended Lifespan Hypothesis-- that menopause is simply a byproduct of the modern expansion of human lifespans-- is plagued by its inability to account for gender differences and the fact that many non-human animals also cease reproducing well before the end of life, among other problems. The Grandmother Hypothesis argues that women net more genes in future generations by assisting their descendants than they would by continuing to reproduce directly, but gains large enough to offset a woman’s direct reproductive potential have never been demonstrated. Approaching the phenomenon from the perspective of the gamete rather than the individual suggests a new hypothesis: Female gametes are competing to achieve ovulation, and the high rate of gamete loss that results leads directly to the follicular depletion that triggers menopause. Thus, menopause is a cost imposed upon individuals by gametes that are behaving in their own evolutionary best interest.