Menopause evolves in toothed whales, such as orcas and belugas, by extending their lives without changing the duration of their reproductive capacity. This allows older women to care for their grandchildren without competing with their own daughters and sisters, a new study reveals.
The study, published today (March 13) in the journal Nature, is the first to examine menopause in multiple species.
“This is work that absolutely needed to be done,” Megan Arnot, an evolutionary anthropologist at University College London, who was not involved in the study, told CNET.
Living long after menopause is very rare in nature, with humans and five species of toothed whales being the only mammals known to possess this trait. Chimpanzees in a wild population also go through menopause, although it is not clear whether this is genetically coded or simply an accidental byproduct of an optimal environment.
At first glance, menopause seems to violate the rules of evolution. In order to have the best chance of passing on genes, it would make sense for an individual to reproduce for the rest of its life.
So why have some species evolved toward menopause? According to the “grandmother hypothesis,” females stop reproducing to help their children and grandchildren. This would indirectly help women pass on their genes by increasing the survival of their grandchildren.
As for the evolution of menopause, there are many ideas. One, called the “lifespan hypothesis,” posits that animals simply live longer over time, while their reproductive lives remain the same. Toothed whales provide an interesting test case because menopause has evolved independently on several occasions.
“What we wanted to do with this study was exploit this repeated evolution to ask very general questions about how and why menopause evolves,” said the lead author. Sam Ellis, a psychologist at the University of Exeter in the United Kingdom, told CNET.
To test different theories about how and why menopause evolves, researchers examined data collected by generations of scientists to reconstruct the life history, total lifespan, and reproductive lifespan of as many species of toothed whales as possible. Then, they compared the data from menopausal species and those from non-menopausal species.
Females of all five species of postmenopausal toothed whales have lifespans about 40 years longer than expected for species of their size, while their reproductive lifespans are the same as species of comparable size. These results support the “lifespan” hypothesis.
Toothed whales like orcas are menopausal, and females live decades after they stop reproducing.
The researchers also showed that females of menopausal species, thanks to their longer lives, are able to spend significantly more time with their offspring, showing that grandmothers have an important role to play in helping subsequent generations. . Additionally, by no longer reproducing, older females do not compete with members of their own family.
Together, the findings suggest that “extending post-reproductive lifespan has been the target of evolution – it’s a strategy,” Ellis said.
How this translates to humans is unclear. “We don’t know if this is exactly the same way it evolved in humans,” Arnot said, “but it’s certainly a better hypothesis at the moment.”
However, it provides supporting evidence that there is a “common pathway by which menopause evolves” in both whales and humans, Ellis said.
But caution should be taken in interpreting the results, Rebecca Sear, professor of population and health at the London School of Hygiene and Tropical Diseases, who was not involved in the study, wrote in an accompanying commentary for News and opinions on nature because “studying the demography of whales is not easy […] The data therefore contains biases and may concern small sample sizes”. This is because the data often relates to populations of stranded whales, which may not be representative of the age distribution of healthy populations.
“Research on menopause in men has tended to focus primarily, but not exclusively, on finding evidence of a helpful grandmother, and has found this in abundance,” she wrote.
But that doesn’t mean menopause has actually evolved to provide this benefit.
“Contemporary grandmothers might help their grandchildren, either because menopause has evolved to create helpful grandmothers or because menopause means older women have no choice but to invest in their grandchildren rather than in their children.”
This article is originally published on crumpe.com
