Menopause remains one of the oldest mysteries in evolutionary biology. Why do women stop reproducing when they still have decades to live? Doesn’t that limit their reproductive success?
But no, shows a study looking at the only five other animals known to go through menopause: killer whales and four marine cetacean relatives (false whale, beluga, narwhal and bottlenose dolphin). The findings support the so-called “grandmother’s case,” according to which menopause evolved in some social species because it allows females to care for their children and grandchildren, carriers of the family genes.
The British study published in the journal Nature collected data on 32 different odontocetes, marine mammals that carry teeth (as opposed to whales that have baleen, horned scallops that filter food). The five menopausal species live in matriarchal groups consisting of females, daughters and their young grandchildren. Thanks to their valuable experience, dominant females take it upon themselves to guide the group through difficult times. Sometimes they even leave half-chewed food for the younger members, behavior unheard of in males.
In this social environment, menopause allows females to care for the family without reproducing at the same time as and competing with their daughters, the researchers report. This tactic benefits the children and grandchildren, who have been found to have a higher chance of survival than their non-menopausal counterparts.
Still, the findings appear to disprove the hypothesis that menopause evolved as a switch that prematurely stops reproduction. As the study points out, of the five odontocetes that go through menopause, females live about four decades longer than males or females of similar-sized species that do not go through menopause. Female orcas, for example, often live past 80, while males rarely live past 40.
These differences indicate that menopause is not a premature cessation of reproduction, but instead evolved as an extension of life beyond the reproductive period. An adaptation that helps the family and at the same time gives extra years to the females.
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