Aside from humans, the five 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, involving researchers from the universities of York and Exeter and the Center for Whale Research, suggests menopause may have evolved independently in three whale species and in a common ancestor of belugas and narwhals.
For the menopause to have an evolutionary benefit, a species needs to have a reason for females to stop reproducing and a reason for them to live on afterwards.
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.
Dr Dan Franks, from the Department of Biology at the University of York, said: “Our study shows that menopause has evolved several times independently in toothed whales. We have provided the first evidence of menopause in beluga whales and narwhals which, when taken together with the evidence for killer whales and short-finned pilot whales, doubles the number of non-human mammals known to exhibit menopause in the wild.”
Based on the findings, the researchers predict that these species have social structures which – as with killer whales – mean females find themselves living among increasingly close relatives as they age.
First author, Dr Sam Ellis, from the University of Exeter, said: “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.”
Research on ancestral humans suggests our female ancestors also found themselves living with closer relatives as they aged. 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.
Although individuals of many species may fail to reproduce late in life, the researchers looked for evidence of an “evolved strategy” where females had a significant post-reproductive lifespan.
Analyses of ovarian activity reveal repeated evolution of post-reproductive lifespans in toothed whales is published in the journal Scientific Reports. The study was funded by the Natural Environment Research Council.
Source : University of York