Study Explains Why Toothed Whales Evolved Menopause

Menopause is exceptionally uncommon across the animal kingdom, having been recorded in only a small number of species, humans among them.

Yet it has appeared in several species within one taxonomic group. Female toothed whales – including orcas, beluga whales and narwhals – can also survive for many years after their reproductive lives have ended.

This scarcity is understandable. A species' success can be substantially improved by maximising reproduction, so evolving a characteristic that stops an animal producing offspring might appear counterproductive.

That prompts an obvious question: why has menopause developed in these particular species? A new study of toothed whales, led by animal behaviourist Samuel Ellis at the University of Exeter, offers an explanation.

Menopause gives female toothed whales longer lives

Female whales in species that experience menopause do not have briefer reproductive lifespans than similarly sized species. Instead, their total lifespans are longer: they live roughly 40 years beyond those of comparable whale species that do not undergo menopause. This benefits the whole clan.

"Previous research on menopause evolution has tended to focus on single species, typically humans or killer whales," says behavioral ecologist Dan Franks of the University of York in the UK.

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

Much about toothed whales remains poorly understood, partly because observing them in their natural environment is so challenging.

However, research has revealed some unexpected advantages associated with whale menopause. In orca pods, the presence of grandmothers increases the survival rate of young animals in the wild. They lead their pods and can direct them towards feeding areas, particularly during difficult periods.

Why menopause may have evolved in whales

The evolutionary route behind this remains unknown. Menopause may have arisen because the species' overall lifespan increased, or because its reproductive lifespan became shorter.

Only a few years ago, scientists established that, alongside orcas (Orcinus orca), short-finned pilot whales (Globicephala macrorhynchus), false killer whales (Pseudorca crassidens), narwhals (Monodon monoceros) and beluga whales (Delphinapterus leucas) also experience menopause.

Because several related animals appear to have independently developed this trait, Ellis and colleagues had an excellent opportunity to compare them with other cetacean species and search for clues.

Their analysis showed that female whales undergoing menopause live markedly longer than females in comparable species.

Notably, males in menopausal species do not always receive the same advantage. Female orcas, for example, cease reproducing at around 40 but may survive into their 90s. Male orcas generally have a natural maximum lifespan of about 60 years.

This approach is consistent with the grandmother hypothesis. Knowledgeable, robust leaders remain within the group while no longer competing reproductively with their daughters and granddaughters. It also reduces pressure on food supplies for calves, benefiting every member of the social group.

"The evolution of menopause and a long post-reproductive life could only happen in very specific circumstances," says marine biologist Darren Croft of the University of Exeter.

"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."

Human and animal evidence of menopause

To date, little evidence has been found for menopause among other land-based species.

Female Asian elephants, for instance, stop reproducing near the ends of their lives, but studies indicate that very few elephants live long enough for this to occur. Scientists have also recently identified postmenopausal females in a small population of wild chimpanzees in remote Uganda, although none have yet been observed elsewhere.

Although the sample is limited, the parallels with human menopause make whale menopause a useful comparison with a ‘wild’ species.

"It's fascinating that we share this life history with a taxonomic group we're so different from," Croft says.

"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."

The team's research has been published in Nature.