Menopause is rare in the animal kingdom. While many species may be less likely to reproduce as they near the end of their life, until now only three animals were known to have an ‘evolved strategy’ where females have a significant post-reproductive lifespan: humans, killer whales and short-finned pilot whales. But now researchers at the University of Exeter and the University of York have added two more toothed whale species to that list: belugas and narwhals.
The team studied dead whales from 16 species and found dormant ovaries in older beluga and narwhal females, indicating that they had gone through the menopause. The finding suggests that these species are likely to have social structures that involve female beluga whales and narwhals living among a greater number of close relatives as they age.
“For menopause to make sense in evolutionary terms, a species needs both a reason to stop reproducing and a reason to live on afterwards,” said Dr Sam Ellis, of the University of Exeter. “In killer whales, the reason to stop [reproducing] comes because both male and female offspring stay with their mothers for life, so as a female ages her group contains more of her children and grandchildren. This increasing relatedness means that, if she keeps having young, they’re competing with her own direct descendants for resources such as food. The reason to continue living is that older females can be of great benefit to their offspring and grand-offspring. For example, their knowledge of where to find food helps the group as a whole survive.”
Studies of ancestral human remains suggests they had similar social structures, which may explain why menopause has evolved in our own species, the researchers say. “Looking at other species like these toothed whales can help us establish how this unusual reproductive strategy has evolved,” said Prof Darren Croft, also of the University of Exeter.
This is an extract from issue 327 of BBC Focus magazine.
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