Fighting with the in-laws: is menopause due to competition?

Published yesterday in the journal Ecology Letters, research by a team of scientists from the University of Turku in Finland, the Universities of Exeter and Sheffield in the United Kingdom, and Stanford University in the United States sheds new light on the mystery of the menopause.

Evolutionary biologists have long pondered why human women stop reproducing at such a relatively early stage in their lifespan, with various theories emerging as to why this might be. Data from Finland has afforded the rare opportunity to analyse the topic because it provides such a complete record of the offspring and living arrangements of successive generations.

The results of the study showed that when a mother-in-law and daughter-in-law had a baby at the same time, the likelihood of each child surviving to adulthood was reduced by as much as 50 per cent. They also demonstrated that women had more grandchildren when they stopped reproducing by the age of 50.

I spoke to one of the co-authors of the paper, Dr Andy Russell from University of Exeter’s Centre for Ecology and Conservation, who was able to explain why the evolution of the menopause is so intriguing and what has been gleaned from the Finnish records. He began by describing the motivation behind the study and why he and his colleagues were so keen to use this dataset, which had been available for some time.

"The data was collected by Dr Virpi Lummaa and Mirkka Lahdenperä, two of the authors on the paper, and consists of Finnish church records from 1700-1900," Dr Russell said. "This was a time before healthcare and before modern contraceptive methods. The church was under strict orders to make sure that all the births, deaths and migration events in a parish were noted down in the parish register. This gives us a very unusual chance to test some ideas on human evolution, because we have very complete data for at least three consecutive generations."

The data showed that when mothers-in-law and daughters-in-law breed at the same time, the offspring have very poor chances of survival. An infant in pre-industrial Finland was half as likely to survive to the age of 15 when a woman and her daughter-in-law had children simultaneously.

"We were quite pleasantly surprised when we saw that the correlation was that strong, but we didn’t expect it to be quite so strong," Dr Russell admitted.

A combination of the support that a woman who has stopped having children can offer to her daughter-in-law in terms of childcare – so-called ‘grandmothering’ benefits – and the slightly increased likelihood of dying in childbirth as one ages, resulted in women doing better by not reproducing after the age of 51. This coincides with the current average age for menopause. These are the factors which the team believe to be behind the results, and most likely behind the menopause, but it is difficult to pinpoint from this data exactly what causes the effect to be so pronounced.

"We can’t see from this study exactly what caused the effect," Dr Russell told me. "It might be to do with stress or competition over food or other resources somehow leading to children being malnourished. They would therefore be more susceptible to diseases, which were the main cause of child mortality in those days."

Interestingly, the competitive relationship was only found to exist between mothers and their daughters-in-law.

"We didn’t find any evidence that the same effect arises between mothers and daughters," said Dr Russell. "That was predicted because the mums and daughters are related to each other – they share 50 per cent of their genes. Of course you can still have competition between relatives, but it is likely to be much less than competition between unrelated females."

I asked Dr Russell why the menopause has proved such a mystery to evolutionary biologists over the years and why it is such an unusual phenomenon.

"No other animal, with the exception of two species of whale, is known to go through menopause. Not even human men go through it. Across the animal kingdom, when an animal reaches maturity they have reproductive potential for the rest of their lifespan. When they lose reproductive potential – which happens to many mammals in particular because they run out of eggs – they go through what might be called a menopause, but then they die.

"Humans and toothed whales go through the menopause and continue to live for another 20 or 30 years. Evolutionarily speaking, this doesn’t make sense when you consider that our behaviours and so on are governed by our genes and genes are passed on to the next generation by breeding. Why would you have genes that stop you from breeding? That’s why the menopause has posed such a big question for evolutionary biology."

The idea was first suggested in 2008 by Dr Michael Cant and Dr Rufus Johnstone, who studied the menopause in killer whales, that fighting or competition between in-laws – unrelated females within an otherwise family-based society – could have led to the development of the menopause. Grandmothering benefits alone could not explain the breeding habits of killer whales or pilot whales, just as they cannot explain the experience of the menopause in human women.

"The menopause is almost unbelievably invariable," Dr Russell explained. "We know from Aristotle’s writings that women more than 2000 years ago went through the menopause at around 50. Across the world, from hunter-gatherer communities in Africa to downtown Manhattan, women go through the menopause at approximately the same age. Of course there is a lot of variation between individuals and within populations, but the variation between populations is virtually nothing. That’s unusual."

Much has changed since the era in which the data for this study was recorded, and it is becoming more and more common for women to have children through their 40s and into their 50s. There are knock-on effects to grandmothering benefit if a woman begins to reproduce in her 20s and continues to do so for thirty years. I asked Dr Russell whether the menopause is still needed and how it may continue to evolve.

"Reversing the menopause will require either the ability to insert stem cells to allow us to make new eggs, or to be born with more eggs in the first place. Both of those would be difficult, but as we know, women are now freezing their eggs and having IVF when they are 55 or 60. Even if we can’t change the age at which women undergo the menopause per se, we can still increase the age at which women are able to give birth."

In light of advances in healthcare and medicine and alongside the changes that have been witnessed in family structures and hierarchies, it is perhaps questionable whether the menopause still serves the purpose it was meant to. Dr Russell pointed out that understanding the social and psychological implications of altering the age at which women can reproduce is vital; this is only possible if the origin of the menopause is properly elucidated.

"If we can understand why the menopause evolved in the first place, we can start to make predictions about the kinds of conflict which might start to resurface when women again have kids very late in life. The results of our study would suggest that, for a start, there would be conflict between mothers-in-law and daughters-in-law."