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While menopause is commonly associated with middle-aged women, it is an uncommon occurrence in the animal kingdom. Aside from humans, only five mammal species are known to undergo this life stage.
As Forbes reports, a recent study published in Scientific Reports shows that female narwhals and beluga whales also go through menopause as they grow older. Previously, menopause was only documented in three other species—humans, killer whales, and short-finned pilot whales—who experience this phase and live for many years after.
Drawing from earlier studies, scientists from Exeter and York Universities in the UK, along with the Center for Whale Research in the U.S., aimed to determine whether other marine mammals undergo menopause. After studying the ovaries of 16 different whale and dolphin species, they identified narwhals and beluga whales as the only additional species known to experience this phenomenon.
Menopause is the phase when menstruation permanently stops, and females can no longer reproduce. Unlike humans and the four whale species previously mentioned, many females in the animal kingdom can continue reproducing well into old age. Elephants, for example, are among the animals with some of the longest lifespans on Earth and can still reproduce in their later years.
This phenomenon has long intrigued scientists. "For menopause to make sense in evolutionary terms, a species needs both a reason to stop reproducing and a reason to continue living afterward," explains Samuel Ellis of the University of Exeter, one of the authors of the new study, in an interview with Forbes.
Earlier studies on killer whales (which weren't part of this research) suggest that menopause may be connected to the relationship between grandmother whales and their younger descendants—a concept known as the “grandmother hypothesis.” Ellis points out that in killer whale families, both male and female offspring remain with their mothers for life, meaning that as the mother ages, her group expands to include more and more of her children and grandchildren.
As resources like food become increasingly shared among the whale's children and grandchildren, menopause may occur to halt her reproductive cycle. It is believed that grandmother whales live long after menopause—unlike other species that undergo a similar reproductive phase but die soon after—because they pass on valuable knowledge to their tightly-knit family groups. The authors of the study also suggest that human ancestors may have had similar social structures.
