Female Menopause

Menopause is permanent cessation of menstruation, often assessed retrospectively in women after 12 months have passed with no cycling. Each ovarian cycle in female mammals includes the development of a vascularized uterine lining to begin a pregnancy. In most mammals this endometrium is resorbed when no fertilized egg implants. In Old World Primates including humans the endometrium is relatively large and much of it is shed as menses. Menstrual cycles begin at menarche with the ovulation of oocytes from primordial stocks established years before in fetal ovaries. Once established these initial stocks are depleted by atresia at an exponential rate, with most oocytes lost before menarche. Menopause occurs when too few oocytes remain to produce hormone levels necessary for cycling. All mammals studied show steep rates of depletion in oocyte stocks beginning near birth, so all would run out of eggs if they live long enough. Only the ones who menstruate, humans and other Old World Primates, can experience literal menopause. In Old World Primates except humans, as in mammals generally, few individuals outlive their fertility. Since fertility falls to zero before menopause, even fewer individuals have been observed to do so in several species of captive macaques and in captive chimpanzees. Average age at menopause is around fifty in women, with substantial individual variation within populations and some variation between them. Hominids generally can have last births in their forties, but humans differ in usually remaining strong and healthy as their fertility ends. Even in high mortality populations, like hunter-gatherers, girls who live to adult hood usually outlive their fertility. More than a quarter of the women are past their child-bearing years. Termination of fertility at mid-life in women as long been viewed as an evolutionary riddle on the assumption that menopause is "stopping early." But similar ages of female fertility termination in hominids generally indicate that it is lower adult mortality and longer adult lifespans that are the derived feature in the human lineage. Economic subsidies that older women are observed to supply to their daughters and grandchildren point to the distinctive selection pressures likely to have favored post-menopausal survival in ancestral populations.