The Evolution of Menopause

This one seems a bit dubious, but we’re here to give you the information. At the website of the University of Exeter in South West England we find this news: Menopause evolved to prevent competition between in-laws. Here are some excerpts, with bold font added by us:

The menopause evolved, in part, to prevent competition between a mother and her new daughter-in-law, according to research published in the journal Ecology Letters.

Here’s a link to that paper: Severe intergenerational reproductive conflict and the evolution of menopause. You can read it without a subscription, but let’s stay with the story at the university’s website:

The study – by researchers from the University of Turku (Finland), University of Exeter, University of Sheffield and Stanford University (US) – explains for the first time why the relationship women had with their daughter-in-laws could have played a key role.

Okay, we’re curious. What’s it all about? The article continues:

The data showed that a grandmother having a baby later in life, and at the same time as her daughter-in-law, resulted in the newborns of each being 50 per cent less likely to survive to adulthood. The analysis helps to solve one of nature’s great mysteries: why female humans, unlike most other animals, stop reproducing so early in life.

There you are, ladies. Those hot flashes are for the benefit of your daughter-in-law and her children. Here’s more:

It also adds weight to the theory that the menopause evolved to allow women to focus on their grandchildren. Traditionally, this role included providing food for the family and protecting young children from accidents and disease.

Nice story, but how did they figure this out? The article explains:

The topic has rarely been analysed, because it requires detailed data on the reproductive success of several generations of women, with knowledge on who lived with whom and when. Scientists analysed 200-years’ worth of data collected by Dr Virpi Lummaa of the University of Sheffield and her student Mirkka Lahdenperä of Turku University, Finland, from church registers of pre-industrial Finland. They looked at information on birth and death rates from 1700 to 1900, before the advent of modern contraception or healthcare.

The study reveals that women had more grandchildren if they stopped reproducing around the age of 50. The research team believes this was partly because of reduced competition between the older woman and her daughter-in-law and partly because of the support she could offer her grandchildren.

Why do they keep emphasizing daughters-in-law? What about daughters? Moving along:

A child born to families with a mother-in-law and daughter-in-law reproducing simultaneously was twice as likely to die before reaching the age of 15. However, this was not the case in the instances when a mother and daughter had babies at the same time. This suggests that related women breed cooperatively and unrelated women breed conflictually.

There is a clear biological benefit to a woman cooperating with her daughter: the women share 50 per cent of the same genes so being in competition for food and other resources makes little sense. This is not the case for a mother-in-law and daughter-in-law: they are not related, so it is logical they should compete to maximise on their chances of spreading their genes.

Consequently, the Finnish data shows that the average woman would benefit from stopping reproducing at the age of 51 if she risked breeding with her daughter-in-law, but not her daughter.

Our Curmudgeon sense tells us that a tidier solution would be for the matriarch to toss her son out of the house and tell him and his bride to raise their kids under another roof. But what do we know? Anyway, that’s the news. Make of it what you will.

Copyright © 2012. The Sensuous Curmudgeon. All rights reserved.

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14 responses to “The Evolution of Menopause

  1. Several observations; first the daughter-in-law’s children would share some genes with the mother-in-law via her son’s contribution, it just would be less than her daughter’s. Secondly, for early hominids the chances of a female making it to her fifties was most likely a rare event so this would suggest menopause is a relatively recent addition to the human genome.

  2. In many cultures, apparently including Finland, it is traditional for daughters to live with their husband’s families. Hence the data on in-laws, instead of natural descendants. The daughters have all moved away.

  3. Jim Thomerson

    In cattle, there is a “freemartin” phenomenon when there are different sex fraternal twins. The heifer is sterile. I wondered if there was some similar effect in humans. I found a study using the same set of data mentioned here, which found that female twins with a fraternal twin brother had lower rates of marriage and reproduction than other females. I can’t find the reference, but thought it quite ingenious to use the church records as they did.

  4. Ceteris Paribus

    I’m just waiting for a scientific explanation for why male humans evolved to eventually need to wake up 4 times every night just to go to the bathroom, and simultaneously lose the ability to remember to put the seat back down without fail.

    Forget just one time, and the resulting “intergenerational conflict” with the Trophy Wife ™ is sufficient for a PhD thesis .

  5. Conversely, children with a surviving grandmother are more likely to survive to adulthood as well. It would seem that ensuring the grandmother is in a situation to help raise the grandkids is great for their survival.

  6. Ceteris Paribus says: “I’m just waiting for a scientific explanation for why male humans evolved to eventually need to wake up 4 times every night just to go to the bathroom”

    When the whole clan is living in one cave, they could be vulnerable to attack at night if everyone were sleeping at the same time. We evolved so the old guys would always be on sentry duty. Frequently getting up to go is for the benefit of everyone else.

  7. The Curmudgeon conjectures: “We evolved so the old guys would always be on sentry duty.”

    My advanced years permit me to verify this hypothesis. Many’s the time and oft, during nocturnal treks to the outhouse, that I have shooed away prowling sabre-tooth tigers seeking to dine on my spouse and offspring.

    But they are protected, not so much by a Thin Blue Line as by a Thin Yellow Stream…

  8. megalonyx says: “But they are protected, not so much by a Thin Blue Line as by a Thin Yellow Stream…”

    Verily, Rudyard Kipling lives among us!

  9. …explains for the first time why the relationship women had with their daughter-in-laws could have played a key role.

    That should be “daughters-in-law,” not “daughter-in-laws.” I hate to be nit-picky, but we are talking about in-laws.

  10. NeonNoodle says:

    That should be “daughters-in-law,” not “daughter-in-laws.”

    The story comes from Wales. Complain to them.

  11. Troglodyte “I know nothing”

    I am wondering how/why the grandmothers and the daughters-in-law would conceive at the same time?
    Family incest is keeping it in the family?

  12. Troglodyte “I know nothing” asks: “I am wondering how/why the grandmothers and the daughters-in-law would conceive at the same time?”

    Anyone want to offer Trog a non-incestuous explanation of this, ah … inconceivable phenomenon?

  13. Retired Prof

    No incest is necessary.

    Assume early Homo lived in troops of ten to twenty individuals, with at least some offspring from the group staying with it into adulthood. Humans tend to choose mates outside their own village or troop, thus minimizing incest. It is fairly common (but by no means universal) for sons to stay with their home group and and for daughters to move to their husbands’ group. Whichever gender normally moves out, presumably the young people would be getting it on with their own mates and the older generations would do the same.

    If members of the parents’ generation are still fertile, the troop will consequently include some aunts and uncles of the same age as some of their nieces and nephews, all clamoring for attention and resources at the same time.