Killer Whales Undergo Menopause, and Scientists Now Know Why:  Data suggest menopause evolved to enable older female whales to help younger generations survive, and how researchers made a cellular map of the developing human heart

Why humans, five (5) whale species and some chimpanzees are the only known mammals to go through menopause has long been a mystery…Now, researchers at the University of Exeter in the United Kingdom think they have found the answer:  It is all about survival of the species.

It turns out that females of five toothed-whale species that experience menopause — short-finned pilot whales, false killer whales, killer whales, narwhals and beluga whales — live about 40 years longer than other female whales of a similar size.  By living longer without extending the years in which they breed, they have more time to help their children and grandchildren.  And it does not extend the time when mothers and daughters are both breeding and raising babies at the same time.

Researchers said their work shows that whales and humans have remarkably similar life histories despite being separated by 90 million years of evolution.  “The process of evolution favors traits and behaviors by which an animal passes its genes to future generations,” said Lead Study Author Dr. Same Ellis, a Lecturer in Psychology.  “The most obvious way for a female to do this is to breed for the entire lifespan — and this is what happens in almost all animal species.”

There are exceptions; however, the scientists pointed out.  Of more than 5,000 mammal species, only the five whale species and humans had been thought to go through menopause.  That is when changes in hormone levels cause a female’s menstrual cycle to end, ending her reproductive life.  Researchers also reported recently in the journal Science that some wild chimpanzees in Uganda also experience menopause, living more than 50 years.

As well as outliving females of other similar-sized whale species, females in the five whale species studied typically outlive males of their own species, Ellis’ team said.  Female killer whales can reach their 80s, while male whales typically die by age 40.  The whale findings were published on 13 MAR 2024 in the journal Nature.

“The evolution of menopause and a long post-reproductive life could only happen in very specific circumstances,” said Darren Croft, a Professor of Behavioral Ecology at the University of Exeter and executive director of the Center for Whale Research in Friday Harbor, Wash.  “Firstly, a species must have a social structure in which females spend their lives in close contact with their offspring and grand-offspring,” he said in a university news release.  “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.”

His University of York colleague Dan Franks noted that previous research on the evolution of menopause has focused on a single species.  This effort is the first to cross several species, enabled by the recent discovery of menopause in multiple whale species, he said.  “Our study provides evidence that menopause evolved by expanding female lifespan beyond their reproductive years, rather than from reduced reproductive lifespan,” he said.

Franks said it is fascinating that humans share this life history with a species it is so different from in so many ways.  “Despite these differences, our results show that humans and toothed whales show convergent life history,” he said.  “Just like in humans, menopause in toothed whales evolved by selection to increase the total lifespan without also extending their reproductive lifespan.”

REFERENCE:  HealthDay; 13 MAR 2024; Carole Tanzer Miller