The post-reproductive stage in women (and men)

From the ages 35 to 50 years there is a gradual decline of performance and fertility. The fertility decline of women is most easily noted, but it may also be detected in terms of the quantity and quality of spermatozoa and the reproductive performance of men. These signs of physical degeneration are measurable but may be compensated by a variety of adaptations in behavior, technology, clothing, cosmetics, and medical treatments including drugs. By the age of 50 years most women become infertile, meaning that they no longer produce egg cells that can be fertilized. The end of women’s fertility is sometimes called menopause, but often the process of reproductive system decline ending in infertility is likewise called menopause. Women in some parts of the world report that the menopause process is characterized by other physical and emotional events, such as changes to the frequency and intensity of menstrual bleeding, «hot flashes», headaches, and reduced sex drive (libido). Other women report no menopausal symptoms or report increased libido and social happiness as fertility declines. Menopause at about age 50 years is known for a few other mammalian species —Asian elephants, but not African elephants, and four whale species, beluga, narwhals, killer whales, and short-finned pilot whales. All primates studied, including our closest ape cousins, show no fertility decline before death. One reason why menopause is a rare phenomenon for other species is that very few mammals live to the age of 50 years. But this does not explain why those few mammals that do live past 50 remain healthy and vigorous, but the females lose fertility.

Searching for the answer to this is an active area of research. One of the explanations with research support is that all the species that live past age 50 years have social systems with intensive care and protection of pregnant females and their infants and youngsters. The young of these species are slow to develop and mature to adulthood and with the special care they receive early in life they become high quality adults with a reserve of physical and mental capacities that allow them to live and be healthy past age 50 years. All mammal females have a limited supply of egg cells and by age 50 years this supply is depleted. Infertility, then, is a consequence of slow decline and longer life. With menopause, the infertile females, who are both mothers and grandmothers, can invest even more strongly in their descendants. In the whale species and in human societies, menopausal women often adopt a strategy of investment in younger generations to enhance reproductive success and human capital. This is called the «grandmother effect». Men also live past age 50 and even if fertile they have biocultural limitation on fertility. Biologically, they have a higher risk of producing sperm with genetic defects. Culturally, grandfathers may adopt an investment strategy like that of the grandmothers, especially in societies where the men are married to menopausal women and social rules make it unlikely that they will father offspring with other women. [Barry Bogin]