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Women go through the menopause to stop them competing with their daughters-in-law for resources, claim scientists

  • Study looked at  birth and death rates from 1700 to 1900
  • If grandmother and daughter-in-law had babies at the same time newborns were 50 per  cent less likely to survive, historical  records reveal

By Claire Bates

PUBLISHED:05:17 EST, 23  August 2012| UPDATED:13:18 EST, 23 August 2012

The menopause partly evolved to prevent  competition between mothers and their daughter-in-laws, according to  researchers.

A study found that when a  grandmother had a baby later in life, and at the same time as her  daughter-in-law, both newborns were 50 per cent less likely to survive to  adulthood.

The researchers, from the University of  Sheffield and Turku University in Finland among others, said it could explain  why women stop reproducing so early in life unlike most other  animals.

Researchers believe women stop reproducing around 50 because they can then provide support for their grandchildren Researchers believe women stop reproducing around 50  because they can then provide support for their grandchildren

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.

Dr Virpi Lummaa, from the University of  Sheffield said: ‘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 menopause.’

Read more: http://www.dailymail.co.uk/health/article-2192440/Why-women-menopause–stop-competing-daughters-law.html#ixzz24QKtAdrW

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