Abstract
There is an ongoing debate why a trait like human menopause should have evolved. Adaptive explanations explain menopause with fitness benefits of ceasing reproduction, whereas non-adaptive explanations view it as an epiphenomenon. Here we present data in support of non-adaptive explanations of menopause suggesting a maximum shelf-life of oocytes. By analyzing the association between lifespan and age at reproductive senescence across 49 mammal species, we find that the positive association levels off in long lived species, indicating that the age at reproductive senescence has an upper limit. Only in baleen whales there seems to be no evidence for reproductive senescence. We suggest that apart from the baleen whales, the confinement of reproductive senescence in long-lived species may be the result of physiological constraints imposed by the long period of time oocytes remain inactive in an arrested phase of meiosis from their production in utero until ovulation. We therefore conclude that menopause may be an implication of the long duration of meiotic arrest caused by semelgametogenesis together with long lifespan.
Highlights
Human menopause is the irreversible cessation of menstrual cycling that typically occurs at the age of 45 to 55 years[1], leaving about 30 years of post-reproductive life-span
Exhausting viable egg supply resulting in a depletion of viable egg stores is considered the crucial physiological constraint causing reproductive senescence in female mammals[27,28] according to vom Saal et al.[29] the view that oocyte exhaustion is the only cause of reproductive senescence in mammals is too simplistic
In order to expand the number of long-lived species, we further performed a literature search in Google scholar on all species listed in the AnAge database with a maximum lifespan of 50 years and older and included each species in the analysis, for which we found data on age of reproductive senescence (11 species)
Summary
Human menopause is the irreversible cessation of menstrual cycling that typically occurs at the age of 45 to 55 years[1], leaving about 30 years of post-reproductive life-span. It has been suggested that the length of time oocytes can remain viable may be the limiting factor of reproductive lifespan, assuming that this limitation of reproductive lifespan would constrain reproduction in female mammals whose lifespan exceeded such a putative maximum shelf-life of oocytes[34,35]. The present study was aimed at testing the assumption of a maximum shelf-life of oocytes by analyzing the association between maximum lifespan and the age of reproductive senescence (proxied by the age at last reproduction or, in humans and two non-human primates, the age at menopause) across 49 mammal species.
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