Menopause may explain why some female whale species such as belugas and narwhals live around 40 years longer than others, scientists have said.
Apart from humans, whales are the only mammals that go through menopause.
This natural biological process is known to exist in just five species of toothed whale: short-finned pilot whales, false killer whales, killer whales, narwhals and beluga whales.
The researchers said the findings, published in the journal Nature, suggest that menopause may have evolved to enable these whales to survive long enough to become grandmothers – so they can take care of their families by sharing food or “babysitting”.
Being post-menopausal also means these species can have long lives without competing with their daughters or granddaughters for mates, scientists say.
Professor Darren Croft, of the University of Exeter and executive director at the Centre for Whale Research, said: “The evolution of menopause and a long post-reproductive life could only happen in very specific circumstances.
“Firstly, a species must have a social structure in which females spend their lives in close contact with their offspring and grand-offspring.
“Secondly, the females must have an opportunity to help in ways that improve the survival chances of their family.
“For example, female toothed whales are known to share food and use their knowledge to guide the group to find food when it is in short supply.”
The researchers analysed data from scientific literature to construct the life history of 32 whale species.
The team said “male menopause” did not evolve in the same way because male whales are not in the same social group as their children or grandchildren.
Professor Croft said: “So they (male whales) don’t actually have the opportunity to be able to provide help that late in life to their close kin, so evolution just favoured them to continue to reproduce until the end of life.”
But this also means females often outlive the males of their own species.
For example, the researchers said, female killer whales can live into their 80s, while males are typically dead by 40.
They said that despite being separated by 90 million years of evolution, whales and humans have “remarkably similar life histories which have evolved independently”.
Prof Croft said previous work on killer whale populations has shown the life experience of post-reproductive females “is really crucial in dealing with environmental challenges in times of hardship”.
He said: “We see just the same patterns in human societies – in hunter-gatherer societies – in times of drought, or in during times of social conflict, where people would turn to the elders of their community who would have had the experience (and) the knowledge.
“I think it is important to draw that parallel between humans and cetaceans in terms of the role that these older matriarchs in the case of tooth whales, or grandmothers in the case of human societies, are playing.”
Lead author Dr Sam Ellis, from the University of Exeter, added: “There are more than 5,000 mammal species, and only six are known to go through menopause.
“So the question is: how and why did menopause evolve?
“Our study provides some of the answers to this fascinating puzzle.”
The study was funded by the Leverhulme Trust and the Natural Environment Research Council (NERC).
