Age-Specific Fertility Decline
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In natural fertility human populations (those not practicing family limitation) both means and medians for age at last birth are close to forty years and age-specific fertility approaches zero before the age of fifty. The other living great apes also have last births in their forties, although their adult mortality rates are notably higher than human rates, and, unlike humans, few outlive their fertility. Other primates also reach sterility if they can be kept alive long enough. Macaques are especially well studied. In several species latest ages at last birth are in the mid twenties. Captive females may live into their thirties but are decrepit. In contrast, women while reaching terminal fertility at ages similar to the other great apes, usually remain strong and productive for decades longer. In primates, as in mammals generally, female fertility depends on ovulation from a stock of primordial follicles that is established around birth and subsequently depleted exponentially with increasing age. Recent comparisons between humans and chimpanzees show the rate of decline with age in the number of primordial follicles from birth to 47 years to be statistically indistinguishable in the two species. This similarity suggests that ovarian aging in humans and chimpanzees has been conserved from a common ancestor while slowed aging in other physiological systems is derived in the human lineage.