Grandmother Knows Best
Recent Insights into Menopause in Toothed Whales

Menopause has become a “hot” and “flashy” topic. Too much? Perhaps. But it does seem that menopause is slowly edging into the zeitgeist as women start to talk about the realities of the phenomenon. From celebrities talking openly about “the change” to new companies that specialize in the health of middle-aged women, menopause is receiving more attention than it ever has. So why do humans go through menopause? Hints to the evolution of this unique developmental milestone may lie with an unexpected group of animals: toothed whales.

First, what is menopause? In 1981, the World Health Organization defined menopause as the permanent end of menstruation due to the loss of follicular activity in the ovaries (WHO Scientific Group on Research on the Menopause & World Health Organization, 1981). In reality, however, menopause is a complicated process, not a single event. It’s an important process, too! In the species where it has evolved, including humans, menopause seems to have considerable impacts on social dynamics and survival. Uncertainty abounds, however, as to why and how menopause evolves.

A recent study led by Darren Croft, executive director of the Center for Whale Research, has illustrated the importance of menopause in toothed whales and may provide clues to how it evolved (Ellis et al., 2024). Scientists have known for some time that killer whales (Orcinus orca) go through menopause (Brent et al., 2015), but, in fact, there are five species of toothed whales that undergo this process: killer whales, short-finned pilot whales (Globicephala macrorhynchus), false killer whales (Pseudorca crassidens), narwhals (Monodon monoceros), and beluga whales (Delphinapterus leucas). In the study led by Croft, researchers assembled a database of life history data for all species of Odontocete cetaceans, what we generally refer to as toothed whales and dolphins. One of the primary goals of the study was to test two competing evolutionary theories of menopause: the ‘live-long’ hypothesis and the ‘stop-early’ hypothesis (Ellis et al., 2024). Under the live-long hypothesis, menopause evolves by an increase in the total lifespan of a species, while the reproductive lifespan (the age when ovarian activity stops) remains the same. Conversely, the stop-early hypothesis proposes that menopause arises by shortening the reproductive lifespan of a species while the total lifespan is unchanged. Ellis and his team found that the whales who experience menopause live on average about 40 years longer than species without menopause. Notably, however, the reproductive lifespans of the species with menopause are no shorter than other species (Ellis et al., 2024), providing support for the live-long hypothesis. There also seems to be support for the live-long hypothesis in humans (Levitis et al., 2013), suggesting that menopause in humans and toothed whales may have evolved for similar reasons.

After determining that evolution in toothed whales follows the live-long hypothesis, Ellis and colleagues sought possible explanations for why menopause evolved in the first place. There are several competing theories about why menopause might evolve in a species. One long-standing theory is known as the ‘grandmother effect’, which posits that grandmothers receive indirect fitness benefits by helping to raise their grand-offspring (Monaghan & Ivimey-Cook, 2023). Some believe that grandmothering was the primary driving factor for extending female lifespan in humans (Hawkes, 2020). In their study of toothed whales, Ellis’ team found evidence to support the importance of grandmothering in driving the evolution of menopause (Ellis et al., 2024). Species who undergo menopause are alive at the same time as their grand-offspring for longer than species without menopause, meaning they have a greater opportunity to provide help to their younger family members (Ellis et al., 2024). The ‘grandmother effect’ requires certain qualities in a species, including limited dispersal of family members, substantial parental care, and overlapping generations (Monaghan & Ivimey-Cook, 2023). All of these are characteristic of human and toothed whale societies. 

Grandmothers in killer whale societies, in particular, seem to have a lot to offer to their grand-offspring. These matriarchs store extensive ecological knowledge that is crucial to the success of their kin. In a previous study of killer whales led by researchers at the Center for Whale Research, it was found that post-reproductive females improve the probability of survival in their grand-offspring (Nattrass et al., 2019). In addition, post-reproductive females are the most likely to lead salmon foraging endeavors, especially during times when the abundance of salmon is low (Brent et al., 2015). The importance of post-reproductive females is especially relevant for highly endangered populations of whales, such as the J Pod, who frequent Puget Sound in the Pacific Northwest.

Beyond simply an interesting discovery, this recent study of toothed whales has the potential to guide our perspective on menopause in humans. Menopause only seems to evolve in highly intelligent species with complicated social dynamics. Instead of viewing this process as a nuisance to ignore or endure, we should remember that it likely evolved to facilitate meaningful connection with younger generations. Menopause, in our own species and others, is truly a reminder of the power that females possess to shape sociocultural dynamics.