Reproductive Senescence in Primates Occurs Because of Ovarian Inadequacy, Not Inadequacy of Gonadotropin Secretion
The end to ovarian activity that occurs in primates is called menopause. In humans, for example, it usually occurs between 45 and 50 years of age. Menopause results from the depletion of oocytes, which has occurred throughout the reproductive life of the individual; in essence, it represents ovarian failure.
It is not clear whether follicles fail to develop from their primordial state because of an absolute, or relative, reduction in follicle numbers, or whether the absence of gonadotropin receptors prohibits follicles from entering the gonadotropindependent stage of growth. The initiation of menopause often involves cyclical irregularity caused by failure of follicle development and ovulation. Gonadotropin secretion can be increased, or can be normal, because of the lack of estrogen and therefore lack of negative feedback on gonadotropin secretion. In the end, ovarian follicle activity ceases, estrogen concentrations decline, and in the absence of negative-feedback inhibition, gonadotropin concentrations increase dramatically.Reproductive senescence is not recognized in domestic animals. This is partly because some domestic species have lives that are shortened for economic or humane reasons. Nevertheless, a phenomenon such as menopause clearly does not occur in domestic animals. One effect of age can be noted in the dog: estrous-cycle intervals gradually increase from the norm of 7.5 months to 12-15 months toward the end of the life span. Also, litter size diminishes, and increased neonatal mortality occurs with increasing age of the dam.