Menopause extends the lives of certain whales

(CN) — Menopause is not exclusive to humans, and according to a new study on Wednesday, it may be what keeps certain whale species alive for so long.

A team of five international researchers set out to understand how and why menopause evolved independently in humans and five species of toothed whales.  

“Menopause is a rare phenomenon where humans are the only terrestrial mammal that evolved an extended post-reproductive lifespan under natural conditions,” the researchers write in the study published in Nature Wednesday.

The average woman experiences more than 40% of their adult life in a post-reproductive stage, according to the study's authors.

The researchers say this has puzzled biologists and anthropologists for decades, as evolutionary traits tend to favor animals that can reproduce throughout their adult lives.

“The process of evolution favors traits and behaviors by which an animal passes its genes to future generations,” lead author Sam Ellis, a lecturer of psychology at the University of Exeter, said in a statement. “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 more than 5,000 mammal species, and only six are known to go through menopause.”

The other species that evolved to experience menopause are five species of toothed whales from the suborder Odontoceti: short-finned whales, false killer whales, killer whales, narwhals and beluga whales. Those whales, as it turns out, have remarkably similar life histories to humans — and that’s despite having evolved 90 million years apart.

Through a comparative analysis of the whales’ life histories, the researchers found that menopause appears to be an evolutionary adaptation under the “life-long” hypothesis, which predicts that menopause extends a species’ lifespan without lengthening the time they spend reproducing.

In another similarity to humans, menopausal whales can live long enough to improve the survival rates of offspring and grand-offspring without competing with their daughters.

The researchers say it has been argued that this intergenerational help is the selection pressure that pushed females to extend their lifespan.

“It’s fascinating that we share this life history with a taxonomic group we’re so different from,” said study co-author Dan Franks, a professor of biology at the University of York, in a statement. “Despite these differences, our results show that humans and toothed whales show convergent life history — just like in humans, menopause in toothed whales evolved by selection to increase the total lifespan without also extending their reproductive lifespan.”

Additional study findings indicate how menopausal whales outlive their male counterparts and even females of similar-sized species by 40-plus years. As to how the evolutionary adaptation of menopause occurs, study co-author Darren Croft explained it requires very specific circumstances.

According to Croft, a professor of behavioral ecology at Exeter and the executive director at the Center for Whale Research, the first requirement is a social structure where females live in close proximity with their offspring and grandoffspring.

“Secondly, the females must have an opportunity to help in ways that improve the survival chances of their family,” Croft said. “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.”

Altogether, the researchers say their results provide an informative comparison of how menopause evolved in humans and, potentially, how it evolved in toothed whales.

“This study is the first to cross several species, enabled by the recent discovery of menopause in multiple species of toothed whales,” Franks said. “Our study provides evidence that menopause evolved by expanding female lifespan beyond their reproductive years, rather than from reduced reproductive lifespan.”

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