Why do women have a menopause?

Abstract

In all multicellular animals the female germ cells are dependent on somatic organs for successful propagation, and it is the ageing of these organs, especially the uterus in viviparous animals, which limits reproductive output. Studies on laboratory rodents have shown that females cease producing litters well before the end of life, due to failure of the uterus to carry offspring to term. At this time and at death the ovary still has a store of oocytes. The major exception to this is women, in whom the ovary is depleted of oocytes during middle age; this gives rise to the menopause. It appears that the menopause is a non-adaptive consequence of two evolutionary developments. The first, which is common to all mammals and birds, is the entry of all female gametogonia into meiosis before birth, giving a finite store of oocytes in the ovary. The second, which is common to both male and female humans, is their very greatly increased longevity compared to other mammals of equivalent size. This results in the ovary using up its store of oocytes whilst ageing processes in somatic organs are at an early stage. In view of the dual origin of the human menopause and the fact that other mammals have an equivalent period of reproductive quiescence, it is unlikely that the menopause would have evolved in response to adaptive pressure.