Why Menopause?

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Last fall, a 96-year-old man named Ramajit Raghav became a father. No woman could become a mother at 96, or even 76. That’s because women typically lose the capacity to have children around the age 50–not because they become decrepit, not because civilization has poisoned them, but because they undergo a distinct biological transition, known as menopause.


Scientists have debated for years about why menopause exists. Some have argued that it’s a trait that evolved through natural selection in our ancestors. Women who stopped reproducing ended up with more descendants than women who didn’t. Some scientists proposed that older mothers were better off putting all their effort into caring for their children who were already born, rather than having new ones. As their limited supply of eggs deteriorated, they faced a higher risk of miscarriages and even death during childbirth. (In terms of reproduction, men have it easy by comparison: they can make new sperm through their whole life and don’t have to suffer any of the risks of pregnancy.)

But some studies raise questions about this hypothesis. The risks that childbirth poses to women later in life may not be big enough to make menopause much of an evolutionary benefit. Some scientists have come up with a different explanation: they argue that menopause provides the opportunity for women to help raise their grandchildren. Researchers who studied population records from Finland before the Industrial Revolutionfound that children were more likely to survive till adulthood if their grandmothers were still alive. Menopause might therefore be a winning evolutionary strategy because it leads to more grandchildren who can carry on Grandma’s genes.

But is it even necessary to think of menopause as a special adaptation in humans? Some scientists don’t think so. They argue that what happens to women as they get older is not terribly different from what happens to females of other species. In many species, females are born with a supply of eggs that then gradually deteriorate over their lifetime. They can invest energy into repairing the eggs, but if they invest too much, they have less energy for other tasks. This evolutionary balance leads females to eventually run out of viable eggs. Whether a female survives beyond that point or not simply has to do with how well her body is equipped to resist aging. There’s nothing special, then, about the fact that females in many species, including rats and elephants, can live past their reproductive years.

Written By: Carl Zimmer
continue to source article at phenomena.nationalgeographic.com

8 COMMENTS

  1. Researchers who studied population records from Finland before the Industrial Revolutionfound that children were more likely to survive till adulthood if their grandmothers were still alive. Menopause might therefore be a winning evolutionary strategy because it leads to more grandchildren who can carry on Grandma’s genes.

    This seems very probable, with human children taking a long time learning adult skills, to become independent and self supporting. The odds of an older mother dying of becoming too feeble to support children later in life, could well result in the loss of many late born offspring.
    The loss of a grandmother would simply leave them with a supporting mother, and wider family group.

  2. i strongly recommend “Homo Mysterious” by David P Barash. he goes into great depth with this and other questions that seem unique to homo sapiens providing all the questoins without giving bias to any of the postulated answers.

    only advice is remember the name and don’t ask the man at the counter to “get homo-curious” for you (thankfully he knew what i meant)

  3. I always thought that lifespans rarely exceeded 30 or 40 until the last two centuries, making menopause something rarely experienced. Women died young, often in childbirth. That said, having older women around to help young women take care of children does seem to make evolutionary sense in that it might help those children survive. Monthly ovulatory cycles, menstruation, pregnancy, and childbirth represent huge energy investments. It makes sense for women to have fewer children if more of those children survive; especially if she is eventually relieved of the physical cost of reproduction yet can continue to raise children and grandchildren.

  4. This tickles my group-selection tendency. A tribe with matriarchs will fare better. To me the best part about a group-selection argument is defeating it, thus illustrating design via the selfish gene. If anyone can swat this down, I would appreciate it.

  5. Keep in mind that over most of our evolution our life expectancy was 25. It is not much point in having children near the end of your life when you are too tired to care for them and when you will die before they are grown. Consider than in former times raising a child took much more effort than today, and even today is it is exhausting. You are better to put your energy into your grandchildren. Whether granny dies as a result of attempting kids or refrains from trying is not relevant to natural selection. She has already done her reproducing.

  6. In terms of reproduction, men have it easy by comparison: they can make new sperm through their whole life …

    Surely this feature of the male sex is also the result of natural selection?

    Any older male who’s reproductive opportunities are cut short by the early demise of available females, or ‘fitness’ (e.g. via loss of sexual selection features – being less ‘handsome’), would still have a chance to reproduce if young males were also, or later, to suffer from an increase in environmental factors that tended to ensure the early demise of young males.

    Social criteria such as fitness for hunting or gathering, for example, might favour young males leaving a home group to hunt or forage – thus potentially increasing exposure to predators, accidents and disease.

    Older males, perhaps given home guard duties, perhaps simply endured, would initially be cautious. But as the absence of young males continues beyond the norm, and females come ‘into season’, having sperm would be advantageous to those older males that have that trait – and would be passed on.

    ?

    My point is only that it makes sense, from an evolutionary standpoint, that males continue to produce sperm into old age. It is not some lucky chance, as the Original Piece appears to suggest.

    Peace.

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