How evolutionary biology can explain why human and a few marine mammal females are the only ones that are menopausal

Abstract

Two fundamental questions related to menopause that have not been answered are: (1) why does menopause even occur? And, (2) of the more than six thousand known mammals extant today, why human and four whale females are the only ones that are menopausal? Answers to both of these basic questions are provided here on the basis of evolutionary biology. From observational data, it was found that there are three elemental criteria that all menopausal species must fulfill: first, it has to be long-lived (average female lifespan of the species has to be forty years or more); second, it must live in groups; and third, the average female-male lifespan differential has to be at least thirty percent or more. In addition, a corollary criterion for menopause was also established: 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, it has been shown that when the menopausal mechanism 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 menopausal process. 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 short-finned females are 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. Why there was no selection pressure for males to undergo advanced-age reproductive cessation in those few menopausal species and why the majority of divorces occur in mid-life (40-to-60s) are also explained from a menopausal perspective.