Menopause evolved to prevent competition between mother and daughter-in-law, researchers say

Aug 22, 2012

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

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

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.

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.

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.

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.

Different theories have been put forward for the evolution of the menopause in humans, including the idea that it evolved to protect older women against the danger of dying during pregnancy or childbirth. However, under two per cent of the pre-industrial Finns in this study died in childbirth in their mid-40s, and such risks of dying in childbirth are similarly low in hunter-gatherers today.

Co-author Dr Andy Russell from the University of Exeter's Centre for Ecology and Conservation said: "We are so used to the fact that all women will experience menopause, that we forget it is seriously bizarre. Evolutionary theory expects animals to reproduce throughout their lifespan, and this is exactly what happens in almost every animal known, including human men. So why are women so different? Our study shows for the first time that the answer could lie in the relationship between a mother-in-law and daughter-in-law."

Dr Virpi Lummaa, from the University of Sheffield's Department of Animal and Plant Sciences, said: "The research adds weight to the argument that menopause evolved because of the vital role that grandmothers played in rearing grandchildren in traditional societies. Although family roles have changed, many grandmothers still play a vital role in caring for their grandchildren and in western society a large number provide daycare. It is interesting that even today, mothers rarely choose to have children at the same time as their offspring: even if they have not yet been through the ."

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User comments : 12

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KalinForScience
4.3 / 5 (7) Aug 22, 2012
Right... Have they ever considered the null hypothesis that menopause has not evolved at all, under netheir selection nor drift, but is a "side effect", a consequence of the deterioration of body functions with age... Or what about the possibility that menopause is the result of indirect selection on something else...

"Different theories have been put forward for the evolution of the menopause in humans, including the idea that it evolved to protect older women against the danger of dying during pregnancy or childbirth."

Well, I find this unlikely because childbirth-related mortality at old age would be less screened by negative selection, because those women would have already given birth at an earlier age; thus, whatever genetic factor helps to prevent "childbirth-at-old-age", it would be passed to the next generation during reproduction at young age... This is similar to one of the hypothesis about aging...
KalinForScience
5 / 5 (5) Aug 22, 2012
IMPORTANT NOTE: I AM NOT A CREATIONIST :) I AM A DEVELOPMENTAL BIOLOGIST with immense interest in evolution of developmental mechanisms. I thought that my prevous comment might mislead the audience about my affiliations.
JonA
4.5 / 5 (2) Aug 22, 2012
"50 per cent less likely to survive to adulthood" is not the same as "twice as likely to die before reaching the age of 15".
Shakescene21
3.7 / 5 (3) Aug 22, 2012
Is this statement really in a peer-reviewed article?

"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."

This is false -- the Daughter-in-Law is carrying the Mother's grandchild, who should possess 25% of the Grandma's genes. The Daughter's children would also possess 25% of Grandma's DNA. A grandmother's grandchildren will possess 25% of her DNA (on average) regardless of whether they are children of her sons or her daughters.
There is no difference in the evolutionary benefit, unless the daughter-in-law has been cheating on the Mother's Son.
PoppaJ
2.8 / 5 (8) Aug 22, 2012
There is a flaw in this. 200 years of data cannot provide the needed data to determine the development of a process that is tens of thousands of years in the making. Also going through menopause at 50 is not an "early in life" process since most people did not live past there late 30's until we were well into the industrial revolution. bad science.
NotAsleep
4 / 5 (4) Aug 22, 2012
Did they find a population of women that don't experience menopause to compare this theory against? As far as I know, such a population doesn't exist and claiming "evolution proves one better than the other" ignores the fact that there are a variety of species that prosper without it.

And this made me smile:
"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"

I don't think breeding works the way they think it works...
krundoloss
2.3 / 5 (3) Aug 22, 2012
Come one guys, you know researchers and scientists get bored and do studies like this to pass the time. If that was my job I would ask myself "Isn't there something useful I can do with my time?"
Modernmystic
not rated yet Aug 22, 2012
Well...even if you accept the article's conclusion....it didn't really "work"...

:)
evercurious
1.5 / 5 (6) Aug 22, 2012
Not to mention that poor people breed more than rich people precisely because the mortality rate of their children is higher - so older couples might be having children because their other children have died - does the study count children or the chronology of the child birth. But, yes menopause wasn't "invented" in the last 200 years - and I think with the high competitiveness of women, a daughter might want to keep her mother's paws off of her husband.

On the surface, sickle cell anemia doesn't make any sense. But, it both weakens african populations and helps them survive malaria.

For menopause to be in all populations, it must have evolved very early on in our history, within the first few tribes - but why that weakness? It's hard to believe it evolved independently in many tribes.

Our early ancestors were nomadic. We have one or two children at a time so they can be carried.. the advantage could have been fewer mouths to feed. Just enough children, but not too many.
julianpenrod
2.3 / 5 (6) Aug 22, 2012
This "study" covers only the last two hundred years. No such major claimed evolutionary variation occurred in the human physiognomy in just two hundred years! Evolution would require that something like this have formed long, long ago. Likely when humans supposedly were similar to other apes. And living in conditions far different from the average modern industrial city in a world of constant warfare! In other words, it would have had to develop during a time when it shouldn't have developed! And, anmong other things, note the speciousness of the argument that mothers breeding at thre same time as their daughters preserved the mother's genes, and so supposedly would be favored by evolution, while mothers breeding with the daughters-in-law would promote different genes and so would presumably not be favored. Yet developments even like sex is claimed to have been favored by evolution because it promoted varieties of genomes rather than a single one.
TheGhostofOtto1923
3.6 / 5 (17) Aug 23, 2012
Right... Have they ever considered the null hypothesis that menopause has not evolved at all, under netheir selection nor drift, but is a "side effect"
"Menopause is an evolutionary puzzle since an early end to reproduction seems contrary to maximising Darwinian fitness...There may be little advantage for an older mother in running the increased risk of a further pregnancy when existing offspring depend critically on her survival. An alternative theory is that within kin groups menopause enhances fitness by producing post-reproductive grandmothers who can assist their adult daughters."
http://www.ncbi.n...11223885

-Because the human brain is so LARGE, babies are naturally born prematurely, allowing the head to pass through the pelvis. As such it is born weak and vulnerable. It comes out face-down and so needs another set of hands to safely deliver it.

These are 2 more possible reasons for the longevity of post-menopausal women.
TheGhostofOtto1923
3.8 / 5 (15) Aug 23, 2012
a "side effect", a consequence of the deterioration of body functions with age...
Another consequence of aging is death. The question is why has evolution selected for post-menopausal women, who offer no clear advantage to their genes? As the study implies, competition for resources within family units increases infant mortality. Families supporting grandmothers who do not assist in their survival, would have been selected against.
Evolution would require that something like this have formed long, long ago. Likely when humans supposedly were similar to other apes.
Correct. I suppose one would have to believe in evolution for any of this to make 'sense', right 'julian'?

Intraspecies competition driven by weapons use forced our brains to grow far faster than the rest of our physiology could compensate. Human change became culture-driven. Menopause is also a cultural, and not an evolutionary, adaptation.

Other domesticated animals exhibit similar such unanticipated defects.
jonnyboy
Aug 25, 2012
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