A Discourse on Why only Human and a Few Marine Mammal Females Are Menopausal
23 Pages Posted: 5 May 2021 Last revised: 25 Apr 2022
Date Written: April 28, 2021
A fundamental question, relating to menopause that has not been convincingly answered, is this: of the thousands of known mammals on earth, why are humans and a few marine mammals the only ones whose females are menopausal? Here we provide an answer to that key question by looking at three elemental criteria that all menopausal species must fulfill: first, it has to be long-lived (average female lifespan has to be over forty years); second, it has to be a social animal, that is, it must live in groups; and third, the average female-male lifespan differential has to be at least thirty percent or more. In addition, we also found a corollary criterion for menopause: for a species’ females to be menopausal, the Encephalization Quotient (EQ) for the species has to be 2.5 or more. Though humans do not fulfill the third menopausal criterion currently, we have shown that when the menopause phenotype first became common in human ancestors, in all likelihood, that principle was conformed to. Of the multitude of mammals around, only a few species satisfy all three menopausal criteria, and hence are the only ones whose females undergo the menopause phenomenon. Many hitherto unanswered questions with respect to menopause, such as, while long and short-finned pilot whales are close to each other both genetically and physiologically, why are short-finned females menopausal while long-finned females are not, why orca females are menopausal while elephant females are not, in spite of both being long-lived, etc., can be answered on the basis of those three criteria. It is also shown that grand-mothering and mate-choice are consequences of menopause and not its cause – those mechanisms had been the explanatory bases of some previously-advanced theories trying to explain why menopause occurs in the first place. It is also shown why there was no selection pressure for males to undergo advanced-age reproductive cessation in those few menopausal species. Lastly, why the majority of divorces occur in mid-life (40s-to-60s) is explained from a menopausal perspective.
Keywords: Menopause, why menopause occurs, menopause history, biology of menopause, factors that govern menopause, menopause criteria, why only females undergo menopause, menopause and orcas
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