Menopause is an important topic in considerations of the evolution of aging, alongside the unusual longevity of humans in comparison to other primates. Any evolutionary theory worthy of the name has to explain why both of these features exist. The Grandmother hypothesis has been deployed to try to explain human longevity, that our intelligence and culture allows for the selection of increased lifespan through the influence of older individuals on the evolutionary fitness of their descendants. Lacking that intelligence and culture, other primates are not as long-lived as we are. What of menopause, however, and how to explain the observation that we share it with some toothed whales, but with none of our closest primate relatives?
Scientists have discovered that beluga whales and narwhals go through the menopause, taking the total number of species known to experience this to five. Aside from humans, the species now known to experience menopause are all toothed whales - belugas, narwhals, killer whales and short-finned pilot whales. Almost all animals continue reproducing throughout their lives, and scientists have long been puzzled about why some have evolved to stop.
The new study suggests menopause has evolved independently in three whale species (it may have evolved in a common ancestor of belugas and narwhals). "For menopause to make sense in evolutionary terms, a species needs both a reason to stop reproducing and a reason to live on afterwards. In killer whales, the reason to stop comes because both male and female offspring stay with their mothers for life - so as a female ages, her group contains more and more of her children and grandchildren. This increasing relatedness means that, if she keeps having young, they compete with her own direct descendants for resources such as food. The reason to continue living is that older females are of great benefit to their offspring and grand-offspring. For example, their knowledge of where to find food helps groups survive."
The existence of menopause in killer whales is well documented due to more than four decades of detailed study. Such information on the lives of belugas and narwhals is not available, but the study used data on dead whales from 16 species and found dormant ovaries in older beluga and narwhal females. Based on the findings, the researchers predict that these species have social structures which - as with killer whales - mean females find themselves living among more and more close relatives as they age. Research on ancestral humans suggests this was also the case for our ancestors. This, combined with the benefits of "late-life helping" - where older females benefit the social group but do not reproduce - may explain why menopause has evolved.