Going through the menopause helps whales become long-lived grandparents

Giving up on reproduction early may help cetaceans – the group that includes whales and dolphins – to live for longer.

Orcas and beluga whales are some of just a handful of animals, including humans, where females stop being able to have children midway through their life. Why this process, known as menopause, evolved and what benefit it offers has puzzled scientists for a long time.

A new paper, published in Nature, suggests that menopause actually allows success over the longer term. While it might limit the number of offspring a female has, a longer life means she can raise her children and grandchildren, helping to ensure their survival.

Dr Sam Ellis, the lead author of this research, says, “Female whales that go through the menopause live around 40 years longer than similarly-sized whales which don’t, while their reproductive lifespan is about the same.”

“As they’re no longer reproducing after the menopause, these females aren’t competing for resources against their own descendants. Instead, they can spend more time with their grandchildren and provide intergenerational help to them.”

Though the study focused on whales, it ties into research that shows why humans, unlike all other primates, also go through the menopause.

Professor Darren Croft, a co-author of the research from the University of Exeter, adds, “it’s fascinating that we share this life history with a taxonomic group we’re so different from.”

“Despite these differences, our results show that humans and toothed whales show convergent evolution in their life history – just like in humans, menopause in toothed whales evolved by selection to increase the total lifespan without also extending their reproductive lifespan.”

What’s the purpose of the menopause?

From an evolutionary perspective, the menopause is extremely unusual. Anything that limits the ability of an animal to have children would normally be lost in the process of natural selection.

“As the menopause is found in humans, we forget what an uncommon evolutionary strategy it is,” Sam says. “The process of evolution favours traits and behaviours by which an animal passes its genes to future generations, so getting as many offspring as possible into the next generation seems to be the goal.”

“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.”

“So, the question is: how and why did menopause evolve?”

While humans might be the only primate with the menopause, it’s a more common phenomenon in the toothed whales. Most research has focused on orcas, but it’s also evolved in beluga whales, false killer whales and narwhals.

Anything that evolves multiple times is likely to have some kind of advantage, rather than being a fluke of evolution. There are two main hypotheses for the benefits menopause offers.

The grandmother hypothesis explains that while having children is important for passing on genes to the next generation, ensuring their survival is important too. In this case, putting effort into raising existing children and grandchildren might be a better option than continuing to have more offspring.

The reproductive conflict hypothesis, on the other hand, suggests that when mothers and daughters have children at the same time, their offspring compete for the same resources. If an older female instead gives up reproducing, then there’s less competition and more chance of her descendants surviving.

Long-lived guardians

By comparing the different lifestyles and characteristics of toothed whales, the researchers found that both the grandmother and reproductive conflict hypotheses seem to apply to these animals.

In orcas, for example, experienced older females take over during lean times to find new sources of food. If they were instead focused only on their most recent offspring, this could lead to grandchildren from older births become weaker and perhaps even dying.

The menopause seems to have evolved in these animals and other toothed whales as a result of a longer life for females. Menopausal and non-menopausal whale species reproduce for around the same amount of time. But menopausal species live decades longer.

This means that whales with the menopause are likely to live to see at least one of their grandchildren reach maturity, having helped to raise them. Non-menopausal whales, meanwhile, will on average die when their grandchildren are only a third of the way to adulthood.

The researchers also investigated whether the longer lives of females with the menopause might reflect wider selection for longer lives across the whole species. But there was no evidence to suggest that males live any longer in species which have the menopause.

“There are different pressures on males and females in mammals,’ Sam explains. “While females know who their offspring are, males may breed with multiple females throughout their life and never know their children.”

“The best-known examples are in killer whales, where males and females stay with their mothers for life. Mating only occurs when two groups come together before they go their separate ways. This means that the offspring of males end up in another group where a grandmother can’t help them.”

“In this case, it might be better for males to have as many children as possible, whereas females get more of an evolutionary advantage by helping to raise their grandchildren.”

With the new research providing an insight into why menopause might have evolved, the challenge is now to look into how it occurred. Examining populations where this process might be in action, such as the chimpanzees of Ngogo, Uganda, could help to piece together the evolution of humans and toothed whales alike.