If human beings enjoy a much longer life than that of a primate of their size, it is thanks to menopause. This is demonstrated by the first research that has systematically analyzed the evolution of menopause in different species.
According to the results presented today in the journal Nature, menopause lengthened the life of females so that they could help raise their offspring. But he did so by depriving them of the ability to reproduce in the extension years so that they would not compete with their own daughters, as it would have reduced their daughters' chances of having offspring. Men also seem to have benefited from these additional years gained thanks to menopause, whose lives have also lengthened.
Menopause is “a really unusual trait,” said Samuel Ellis, a biologist at the University of Exeter (United Kingdom) and first author of the research, at a press conference yesterday. The human species is the only land mammal in which it appears to have evolved, which “has baffled biologists and anthropologists for decades,” the researchers point out in Nature.
But Ellis and Darren Croft, also from the University of Exeter, discovered in 2018 that menopause has also evolved in some marine mammals. They showed that it has appeared at least four times independently in the last twenty million years (and a fifth including the human species). And that currently there are five species of toothed cetaceans where the females have menopause: killer whales, black killer whales, belugas, narwhals and tropical pilot whales.
This has allowed them to compare the life cycle and social organization of these five species with those of other cetaceans and thus contrast the different hypotheses that have been proposed to explain the origin of menopause. The results “provide important knowledge to understand the evolution of menopause, including human menopause,” they write in Nature.
Researchers have observed that female cetaceans in which menopause has evolved live, on average, about forty years longer than would correspond to their size. On the other hand, the age up to which they can reproduce is the same as that of other cetaceans of the same size. Therefore, the evolution of menopause has caused an increase in life expectancy (which confirms the so-called long life hypothesis) and not a reduction in reproductive life (which refutes the hypothesis of early interruption of fertility) .
But why, if life is lengthened, is reproductive life not also lengthened? At first glance it might seem that if a female has offspring for more years, she will have more offspring. “Competition between mothers and their daughters is costly,” replied Darren Croft at the press conference. In orcas, it has been observed, for example, that when a mother and her daughters are fertile at the same time, the offspring live shorter. So extending reproductive life would lead, paradoxically, to having fewer offspring. The data from cetaceans thus support the so-called reproductive conflict hypothesis (or intergenerational damage hypothesis).
Extending life, but not reproductive life, allows older females, in matriarchal societies like those of these five cetaceans, to contribute to the survival of their descendants. They can help by sharing food, taking care of grandchildren, or guiding the group to find food or avoid danger. “The experience acquired about their environment throughout their lives is very valuable; We see the same thing in the human species,” declares Darren Croft, for whom the development of intelligence has contributed to the evolution of menopause.
These conclusions partly support the popular grandmother hypothesis, which explains human menopause based on the contribution of women of post-reproductive age to the well-being of the collective. But, unlike what the grandmother hypothesis proposes, in cetaceans it is not observed that the effort to raise the offspring is greater in species that have developed menopause - something that is observed in the human species, where childhood is longer than in other primates.
The most surprising thing, according to Samuel Ellis, is that “every time this [menopause] strategy has evolved, it has done so in the same way.”
“The same evolutionary process that led to menopause in humans can be found in the ocean,” corroborates Darren Croft.
If menopause is advantageous, why hasn't it evolved in more matriarchal species like elephants or matrilineal species like dolphins, which have also developed intelligence? “The five species [of cetaceans] that have developed menopause have very unusual social structures; “Mothers maintain a relationship with their offspring” even when they are adults, as occurs in the human species, explains Croft. “In elephant societies, children disperse. What is really important [for the evolution of menopause] is the family structure.”
