Background
In female placentals, the number of ovarian oocytes is fixed during embryonic development; possibly as an adaptation to reduce the accumulation of mutations. At birth there are, typically, one million ova; when menopause begins, only 400 eggs would have actually matured. The intriguing question is why somatic cells decline at a slower rate and why humans invest more in somatic longevity than other primates.
More important than the question of why longevity has been extended, however, is why selection has not adjusted female life-history to match. The most frequently cited adaptive causes for the menopause are variations on the ‘mother’, or ‘grandmother’ hypothesis. These theories advocate that the high costs attributed to female reproduction could prevail over the benefits of continuous propagation. It is true that with advancing age and decreasing fertility, there is also a corresponding increase of miscarriages and birth defects, such as Down’s syndrome. Age is less significant in the increased foetal abnormalities than is the number of the ova left in the ovarian follicular reserves.
A possible explanation is the rate of oocyte depletion. With ova numbers fixed before birth, it is logical to think extension of fertility would require increased oocyte stocks, which may be a limiting factor, or a slower rate of follicular attrition. In humans, the rate of follicular atresia increases at older ages (around 38-40), for reasons that are not known. In other mammal chimpanzees and Japanese macaques there is no similar acceleration in the rate of follicular atresia, so this pattern remains an evolutionary puzzle. A model of "biphasic" or abruptly accelerating follicle loss has been contested on statistical grounds Oocyte stocks and the rate of follicular attrition vary between mammal species, but it remains puzzling why humans start out with so many oocytes and show a late-life acceleration in the rate of oocyte atresia. See also non-adaptive hypotheses for menopause
Read more about this topic: Grandmother Hypothesis
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