“'Cradle snatchers' cause menopause, says biologist,” is the bizarre headline in The Guardian today.
The menopause has always been a bit of an evolutionary puzzle. Evolution is about one thing only – reproducing genes. So why would genes that cause a woman to lose fertility halfway through her life survive?
Two main theories have been previously suggested:
- one theory suggests the menopause is a trade-off in increased fertility versus prolonged survival
- the second theory is known as the “grandmother effect”, whereby older, post-menopausal women are no longer fertile so they can instead help out with raising their grandchildren
A new study has provided a third theory. Humans started off with prolonged fertility, but if men theoretically preferred mating with younger women there would be no pressure to weed out the mutations that cause infertility in later life. According to this theory, over time mutations affecting fertility in older women accumulate – leading to most, and then all, women experiencing menopause.
Researchers used a complicated computer model to run a number of evolutionary cycles and found that the model was consistent with their theory. But it’s not possible to say conclusively that this model accurately represents what has happened in human evolution, and other factors may contribute.
The findings are interesting but they have no direct health implications. Menopause – for whatever reason it may occur – is a natural part of human female life, to which no blame should be attached.
Where did the story come from?
The study was carried out by researchers from McMaster University, Ontario, Canada, and was funded by the Origins Institute and Shared Hierarchical Academic Research Computing Network at McMaster University, and the Natural Sciences and Engineering Research Council of Canada.
The media reporting of the theory is broadly accurate but it is just that, a theory. This complex computer modelling research can only suggest certain scenarios that could potentially explain the observations seen. It is not possible to say conclusively that these models do represent what truly happened.
Headlines that suggested men are “to blame for the menopause” are a bit silly. Evolutionary forces are not something anyone has any control over. Blaming men for the menopause is like blaming someone other hereditary conditions that have evolved in humans over time, say, sickle cell anaemia.
A number of the news sources carry a quote from another expert critical of the theory. Dr Maxwell Burton-Chellew, an evolutionary biologist in the department of zoology at the University of Oxford, was quoted as saying: “This [theory] is probably the wrong way round - the human male preference for younger females is likely to be because older females are less fertile.”
What kind of research was this?
This was a computer modelling study trying to determine why women undergo menopause.
As the authors note, survival beyond the menopause is a characteristic that appears almost unique to humans with only odd exceptions (such as whales, and chimps in captivity). Evolutionary theories suggest that natural selection should operate against living beyond reproductive age and it is not clear (theoretically) why women live beyond the menopause.
At least two possibilities have been put forward to explain this phenomenon: a trade-off favouring longer lifespan over reproduction in women (because giving birth when older may increase mortality risk); and that menopause increases the reproductive success of the woman’s offspring (that is, the “grandmother effect” of older women being able to assist their children in rearing their own children).
Other researchers have suggested that living beyond menopause could be down to an influence from males. They suggest that the fact that men remain fertile throughout their lifespan would prevent accumulation of lifespan-shortening mutations, allowing men and women to live longer. However, this does not explain why women undergo menopause.
The current research aimed to test the effect of male mating preference on the evolution of the menopause. In particular, they were interested in the effect a male mating preference for younger females would have.
What did the research involve?
The research involved a complex computer model that looked at the effect on a population of male lifelong fertility and of male mating preference on female fertility.
The researchers modelled a population of a fixed size, which initially had pre-set fertility and survival probabilities. Each individual started the model in one of 18 increasing age classes. Survival probability for each age group (determined by the number and type of mortality-causing mutations introduced) was assessed at five-year intervals.
The different computer models then introduced mutations into the population that separately affected mortality and fertility – mutations that affected fertility did not affect survival, and mutations that affected survival did not affect fertility.
This determined whether any individual in each age group died or went into the next age bracket. All individuals that reached the oldest age class (class 18) died in the model.
Deaths in the model were replaced by new births assigned to the first age category. The births were simulated by randomly selecting a male from the surviving male population, and a female from the surviving female population. Male and female fertility probabilities in the model were influenced by the number and type of fertility-affecting mutations that had been introduced into the population.
The researchers used their model to look at two scenarios.
In their first scenario, men retained lifelong fertility, while women underwent menopause. In this scenario men did not have an age-specific mating preference. The researchers looked at the effect of introducing into the population theoretical genetic mutations that reduced lifespan but did not affect fertility.
In their second scenario, the researchers started out with men and women being fertile all their lives. They then introduced mutations that caused reducing fertility, and mutations that affected survival. They looked at the impact on this scenario of men having a preference for mating with younger females.
What were the basic results?
In their first scenario – where men did not have an age-specific mating preference and women underwent menopause – the researchers found that if men retained lifelong fertility this did prevent
mortality-causing mutations from accumulating in females. However, this does not explain why menopause came about.
In their second scenario - where both sexes started with lifelong fertility and men preferred to mate with younger females – over time the mutations reducing female fertility with age accumulated in the population, causing a decline in female fertility with age; effectively the menopause.
However, a similar effect was not seen if male mating preference was not influenced by women’s age.
How did the researchers interpret the results?
The researchers say that their model suggests that male mating preference for younger females could have driven the evolution of menopause. Their model did not need the presence of the other suggested explanations for the menopause to be present in order to work (the trade-off between fertility and lifespan and the “grandmother effect”). Instead they say that these explanations “may be insufficient factors in elucidating the origin of menopause”.
This computer modelling study has suggested that a male preference for mating with younger females could be the reason why menopause evolved in humans. However, whether this is truly the reason, or whether other factors some into play is not possible to say.
While this may be of interest to sociologists and others interested in the possibilities of human evolution, it has no direct health implications.
Unless someone comes up with a working time machine there is little we can do about the genetic cards that evolution has dealt us.
If you are experiencing the menopause, tempting as it may be to start blaming the men in your life, you are better off seeking support from your doctor, who may be able to offer treatment for particular symptoms of menopause.
Analysis by Bazian
Edited by NHS Website
Links to the headlines
The Guardian, 13 June 2013
BBC News, 14 June 2013
The Daily Telegraph, 13 June 2013
Mail Online, 14 June 2013
Links to the science
Published online June 13 2013