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A new analysis shows that beluga whales also experience menopause, bringing the number to five known species with post-reproductive lifespans.

Lisa Barry, NOAA/NMFS/AFSC/MML

Females that live long, post-reproductive lives are rare in the animal kingdom. Until recently, reproductive senescence, or menopause, had only been observed in humans and in two species of toothed whales: killer whales, Orcinus orca, and short-finned pilot whales, Globicephala macrorhynchus [see “The Menopause Monologues,” 6/15]. Research into the prevalence of menopause in toothed whales has been hindered by difficulties in accessing and observing wild populations. A new study circumvented these challenges by applying novel analyses to published physiological data and found two additional toothed whale species that have significant post-reproductive lifespans.

A research team led by behavioral ecologist Samuel Ellis of the University of Exeter in the United Kingdom searched through the published literature for age-specific counts of the number of ovulations over a lifetime, as determined from examinations of ovaries from deceased whales. The relationship between age and reproductive activity in several individuals of the same species then allowed the researchers to infer the length of that species’ post-reproductive life. Calculations indicated that beluga whales, Delphinapterus leucas, and narwhals, Monodon monoceros, also experience menopause, doubling the number of non-human animals known to exhibit menopause. Furthermore, analysis of the evolutionary tree of toothed whales shows that species with evidence for menopause fall on three separate branches, suggesting that menopause evolved independently at least three times in toothed whales.

While it is not known exactly why menopause is so concentrated in whales, Ellis believes “that the development of menopause is driven by social structures.” Many whale species are long-lived. As females age, they tend to stay with their pods, becoming important sources of knowledge—especially during hard times—about such matters as the location of food. They also become related to more members of the pod. Once they stop reproducing, they require less food and fewer resources, which are then available to younger kin.

While toothed whale species are still living in situations close to the ones in which they evolved, humans are living further away from those conditions, making us poor test cases for our own evolution, says Ellis. He notes, however, “If we can understand how this strategy has evolved in whales, we can understand how it’s evolved in us, which gives us insight into the structure of our own lives.” (Nature Scientific Reports)