Why does menopause occur in humans

They evolved big brains and acquired new capacities—for making more versatile tools, for example, and for communicating with language. These factors may have allowed hominins to live longer.

As a result, more females lived beyond their reproductive years. Now the benefits of life after menopause could emerge. Any genes that enabled women to live longer would be favored by natural selection, because older women could raise the odds of their descendants surviving. Over many generations, women evolved a life in which they spent a dramatically larger part of it not having children.

In this new hypothesis, human menopause becomes at once special and yet not unique. In many species, females have the capacity to live beyond reproduction, but they rarely do, depriving evolution of the opportunity to expand that stage of life. But if other animals get that chance—for whatever reason—they may evolve to be menopausal too. Even insects can benefit from menopause. In species known as the Japanese gall aphid, females stop reproducing midway through their lives. Now that their abdomens are no longer dedicated to growing eggs, they can use that space to manufacture a sticky chemical.

When a predator attacks the aphid colony, the menopausal females rush forward and glue themselves to its body. The predator is swamped by the heroic females, which die in the process. The evolutionary forces behind menopause may differ between humans and aphids, but the outcome is the same. All rights reserved.

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Environment As the EU targets emissions cuts, this country has a coal problem. Rather, it is: Why do women long outlive their fertility? Human ovaries tend to shut down by age 50 or even younger, yet women commonly live on healthily for decades. This flies in the face of evolutionary theory that losing fertility should be the end of the line, because once breeding stops, evolution can no longer select for genes that promote survival.

The most popular explanation, the "grandmother hypothesis," argues that a generous post-reproductive life span makes sense if a grandmother improves the survival and reproduction of her grandchildren, thus ensuring continuation of her own genes—including genes that contribute to longevity.

But skeptics say the math is askew. From an evolutionary perspective, it is hardly ever better for a woman to give up a chance to bear additional children of her own, and so pass on half her genes, for the sake of improving the survival of her grandchildren, who carry only a quarter of her genes.

Cant and co-author Rufus Johnstone, an evolutionary biologist at the University of Cambridge in England, used game theory to argue that menopause is early cessation of reproduction that originated through reproductive conflict between generations. In most cooperatively breeding species, reproduction is suppressed in younger females, who act as helpers to older reproducing females. By contrast, they say, younger women in human social groups win the reproductive sweepstakes, because the older ones stop having babies.

There is no genetic profit in helping their mothers-in-law bear more children, because they will not share any genes with those children. But an older woman who helps her son's wife reproduce will benefit by bequeathing 25 percent of her genes to her grandchildren. Elephants have babies in their 60s, and some whales give birth in their 80s.

Another explanation for menopause is the "mother hypothesis," which holds that it occurs because older mothers might profit more, genetically speaking, by investing resources in their existing children than in giving birth to new ones. Researchers at the Max Planck Institute for Demographic Research in Rostock, Germany, make the case for this in the American Journal of Physical Anthropology AJPA , concluding that menopause is advantageous when a woman has aged enough to face an increased risk of stillbirth, birth defects and her own death in childbirth.

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