Menopause triggered by men's preferences

Evolutionary study from Canada: menopause triggered by men's preferences?

06/16/2013

During evolution, female menopause may have developed because of men's preference for younger women. This is roughly the interpretation of the conclusions that Canadian scientists came up with using computer simulations.

The preference of men for younger women
Has menopause developed in women because men have a preference for younger women? At least, the Canadian scientists at McMaster University in Hamilton claim in the journal PLOS Computational Biology. They came to this conclusion by analyzing different gender behaviors and the effects of mutations on human populations using computer simulations. This theory is in contradiction to previous attempts to explain the end of reproductive capacity in women long before the end of life, which says that this is a purely physiological consequence of aging. Against this theory speaks, however, that it comes only to humans in two other species to this phenomenon, the killer and the pilot whales. The females of all other species are capable of reproduction until old age. Also that of the chimpanzees, the closest relative of humans. Only in captivity could a kind of menopause be observed. Menopause is the time of the last spontaneous menstruation in a woman's life, which ends the fertility of a woman. In the years before and after menopause, which are also commonly referred to as menopause, there is a hormonal change in women. Little is known that it can also come in men to so-called menopause with menopausal symptoms, the andropause.

So-called grandmother effect controversial
Another explanation suggests that women's overall reproductive success could be greater if, instead of producing their own offspring, they raise grandchildren between the ages of 40 and 50. This approach, the so-called grandmother effect, refers to a possible evolutionary advantage that can be achieved through such behavior. Because in the end more young animals, including the grandchildren, would carry genetic material of the female, as if it exclusively relies on own children. "Genetic fitness" is what experts call it. Pregnancy and the care of infants are a major burden, and thus the likelihood for an older woman to successfully raise her children has been rather low in the past. Therefore, it is conceivable that mutations have prevailed that have led to the said grandmother effect within the human species. Rama Singh of McMaster University, however, does not believe in this idea. In his opinion, this would contradict the principle of natural selection. This would be preferred organisms that reproduce as effectively as possible. And those who stop doing so would be disadvantaged. So a reproduction strategy that is ineffective would ultimately lead to the extinction of its representatives. Moreover, the idea of ​​the grandmother effect would ignore the influence that masculine behavior can have on reproductive success.

Always a preference for younger women?
According to Singh's team, the development of menopause could be related simply to men's tendency to seek partners under the age of 40 or 50. As the evolutionary biologist told the BBC: "There is evidence in human history that there has always been a preference for younger women [among men]." In his reflections, he concludes that women and men are the whole Would be able to reproduce for a long time if women could have children until the end of their lives and men did not have the inclination to be younger. Ultimately, this would be the reason why older women then no longer have children because they simply lack a sexual partner. This eliminates the purpose of remaining fruitful and thus their reproductive capacity has disappeared in the course of human evolution.

Mutations suspected
Why did human evolution then lead to the development of the menopause? Related to this question, the Canadian scientists argue with suspected mutations. Thus, if young women are affected, changes in the genetic material that make them infertile, could not prevail. But if they do not have children, the mutation is in a dead end, so to speak. For seniors who are no longer having children because of the men's behavior, mutations that lead to the infertility of older women already look different. Because they could pass these gene changes already as young women to their daughters, who then become unfruitful as older women.

Positive effect on the reproductive success
The researchers used their computer models to study the development of a virtual starting population of men and women who could reproduce to the end of their lives. The scientists had occasional mutations affecting this population, which limited mortality and fertility in old age. In older women, mutations that limited reproduction when the male part had a preference for younger female partners accumulated. The conclusions of Singh and his team say that this ultimately led to the development of a menopause. Actually, mutations can prevail only under certain conditions to the extent that all representatives of a species ultimately show the corresponding characteristic and indeed that the consequences have a positive effect on the reproductive success. Or another factor works at the same time so that the reproduction is not restricted thereby and the feature simply gradually spreads in a population over time. The mutation would have no effect on reproduction in such a case, since no partner is available anyway.

Younger women can take care of children in the longer term
Singh told the British newspaper "The Guardian" that men in the 1950s would lose their fertility if the situation were the other way round, with women showing a tendency towards younger men. Life expectancy is a factor that could have played a role in men's preference for young women. In the past, women probably did not survive long beyond reproductive time. And so men had developed a preference for young women, as it was more likely that they could take care of the children in the longer term, as an older partner.

Clear criticism of the study
However, there is also clear criticism of the study. Maxwell Burton-Chellew from the University of Oxford in the UK continues despite the results that younger women are preferred by men because they offer a greater chance of recruiting, but this does not mean that the conscious desire to have children Drive leads. Rather, an unconscious inclination, which prevailed in evolution, was probably the decisive factor. Burton-Chellew told the BBC, "I think it makes more sense to look at male affinity first and foremost as an evolved response to menopause and to assume that our male ancestors were wise enough to unite with those women who have However, older women would then have faced an interesting evolutionary choice, namely, "to try to produce more children who are unlikely to be fully grown before the mother's death, or to stop procreation and instead focus on it helping the younger relatives to reproduce. "(sb)

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