Menopause explains the longevity of whales

Very few female mammals go through menopause.

For centuries it was assumed to be something unique to humans.

But so far in the 21st century, it has been discovered that up to five different species of whales also live well beyond the end of their reproductive life.

They are all cetaceans that have teeth instead of beards (odontocetes) and, like the human species, live in social groups made up of several generations.

Now, the comparison between dozens of these marine animals brings them even closer to humans: they live longer because it helps the group, because they take care of their grandsons and granddaughters.

“The five species of odontocetes that evolved towards menopause live about 40 years longer than expected for the same species that do not have menopause,” Samuel Ellis, a researcher at the University of Exeter (United Kingdom) and first author of this research, published in the scientific journal

Nature

.

The data is reminiscent of the case of the human species, in which women live more than 40% of their lives after the reproductive phase.

Ellis highlighted that the end of the menstrual period emerged in different species independently and not from a common ancestor.

Among them are killer whales, the black killer whale (also called false killer whale), pilot whales, narwhals and belugas.

In addition to outliving females of other species of similar size, those of these five species outlive males of their own species.

For example, female killer whales can live to be 70 or 80 years old, while males usually die at 40. Although the reasons are not entirely clear, something similar happens among humans.

More information

Orcas live longer when they have a grandmother

Why live more?

The mystery of menopause is that, from an evolutionary point of view, the longer the ability to reproduce is maintained, the better it should be for the species.

It seems like a contradiction and a waste of resources to extend life without being able to have offspring, which would mean a selective disadvantage compared to others.

In fact, of the more than 5,000 species of mammals there are, only six (or seven, according to some studies) have unmatched their longevity from ovarian production.

But the beginning of the climacteric could serve the same objective, but in a different way: the care not of children, but of grandchildren.

The comparative analysis between the different odontocetes revealed the key: in the species that developed menopause, the females that already have it overlap this part of their lives with that of their grandchildren.

Specifically, killer whales, pilot whales or greater belugas live up to 36% longer coinciding with the offspring of their daughters, compared to similar species, such as, respectively, the white-beaked dolphin, the melon-headed dolphin or the smooth porpoise.

“This way they have more time for intergenerational care,” said Ellis.

It is the animal version of grandmother's hypothesis.

Grandmother's hypothesis

The idea that post-reproductive women were essential in human evolution shaped that grandmother hypothesis.

With offspring that need years of care, groups made up of several generations and the need for cultural transfer, so many years without fertility were not a disadvantage, quite the opposite.

In orcas, it had already been observed that calves live longer when they have a grandmother.

A greater number of deaths than those without it then suggested a familial function for menopause.

But what they have observed in odontocetes is something more complex.

These same researchers already discovered a few years ago that the mortality rate of offspring increases dramatically when mothers are older.

Specifically, when a mother and a daughter coincided in having offspring, the mother's daughter was 1.67 times more likely to die.

That is, in the competition for resources, the daughters of the oldest were the losers.

These results pointed to a possible connection with the arrival of menopause and the end of reproductive age based on the costs of reproduction.

“The females of these species have minimized competition for reproduction, lengthening their life span, but keeping their reproductive life shorter.”

Darren Croft, University of Exeter

It is defended by Darren Croft, also from the University of Exeter and senior author of this research.

“The second part of the story has to do with reproductive competition between generations,” he says.

“What we see in the populations we have studied is that the females of these species have minimized competition for reproduction, lengthening their life span, but keeping their reproductive life shorter.

It is the same vital pattern that we observe in humans.

It is very striking that we can make this comparison with such different animals, but with similar social structures and dynamics.

It is very intriguing that we find this vital trait typical of human societies in the ocean, but not in other mammals,” he concludes.

Many connections can be found between human and animal menopause, according to researchers.

As with humans, among orcas (the species they have studied the most), there are different forms of social organization.

Among residents of the northern Pacific coast, Croft provides information that fits with the grandmother hypothesis in humans: “One of the key benefits that we have seen (in previous work) is that non-reproductive females help the group family is storing ecological knowledge of where and when to find food.

This experience that they acquire throughout their lives is crucial when they have to face times of scarcity.

And we see the same patterns in human hunter-gatherer societies in times of drought or in times of social conflict, when they turn to their elders.”

There is another thing that humans and odontocetes (but not other whales) share.

In most mammal species, the young leave when they grow up.

Sometimes both males and females do it.

And in others, only the first or only the second do it.

“But for both to remain in the group (philopatry), for both daughters and sons to remain in the family group, is really rare among mammals,” says Croft.

And he compares it to elephants, also with complex social structures, accumulation of knowledge from grandmothers and care of calves, but who do not have menopause until the end of their days: “A very striking difference between the societies of elephants and orcas It has to do with what happens with [male] children.

Among the orcas, they stay with their mothers, among the pachyderms, they leave,” he says.

In fact, among these odontocetes, mothers continue to care for their older sons, something they do not do with daughters when they reach reproductive age.

It would, therefore, be a kind of evolutionary convergence in which similar selective pressures gave rise to similar adaptive solutions.

However, researcher at the London School of Hygiene and Tropical Medicine and reproduction expert, Rebecca Sear, recalls a case that does not fit with this: the recent discovery that a population of chimpanzees are going through menopause.

“It is surprising, given that chimpanzees do not seem to provide much help to their grandchildren,” she writes in a commentary also published in

Nature

.

Sear points out another possible bias: much of the research on menopause in humans has focused on finding evidence that grandmothers are useful, and, of course, they have found it.

“Contemporary grandmothers might help grandchildren because menopause evolved to create helpful grandmothers or because menopause means older women have no choice but to invest in grandchildren rather than children,” she recalls.

And she leaves for last a decisive question that was argued in the case of chimpanzees: “There are many other hypotheses to explain menopause.

One is that this is simply an artifact of declining mortality, which has extended life expectancy overall, while reproductive life expectancy has remained the same.”

You can follow

MATERIA

on

Facebook

,

X

and

Instagram

, or sign up here to receive

our weekly newsletter

.