Why Menopause?

Last fall, a 96-year-old man named Ramajit Raghav became a father. No woman could become a mother at 96, or even 76. That’s because women typically lose the capacity to have children around the age 50–not because they become decrepit, not because civilization has poisoned them, but because they undergo a distinct biological transition, known as menopause.

Scientists have debated for years about why menopause exists. Some have argued that it’s a trait that evolved through natural selection in our ancestors. Women who stopped reproducing ended up with more descendants than women who didn’t. Some scientists proposed that older mothers were better off putting all their effort into caring for their children who were already born, rather than having new ones. As their limited supply of eggs deteriorated, they faced a higher risk of miscarriages and even death during childbirth. (In terms of reproduction, men have it easy by comparison: they can make new sperm through their whole life and don’t have to suffer any of the risks of pregnancy.)

But some studies raise questions about this hypothesis. The risks that childbirth poses to women later in life may not be big enough to make menopause much of an evolutionary benefit. Some scientists have come up with a different explanation: they argue that menopause provides the opportunity for women to help raise their grandchildren. Researchers who studied population records from Finland before the Industrial Revolution found that children were more likely to survive till adulthood if their grandmothers were still alive. Menopause might therefore be a winning evolutionary strategy because it leads to more grandchildren who can carry on Grandma’s genes.

But is it even necessary to think of menopause as a special adaptation in humans? Some scientists don’t think so. They argue that what happens to women as they get older is not terribly different from what happens to females of other species. In many species, females are born with a supply of eggs that then gradually deteriorate over their lifetime. They can invest energy into repairing the eggs, but if they invest too much, they have less energy for other tasks. This evolutionary balance leads females to eventually run out of viable eggs. Whether a female survives beyond that point or not simply has to do with how well her body is equipped to resist aging. There’s nothing special, then, about the fact that females in many species, including rats and elephants, can live past their reproductive years.

In the latest issue of Evolutionary Anthropology, three scientists take a closer look at the nature of human menopause. Daniel Levitis of the University of Southern Denmark, Oskar Burger of the Max Planck Institute for Demographic Research, and Laurie Bingaman Lackey of the International Species Information System started by comparing human biology to that of our primate relatives. Reviewing records from 66 species of primates, they found that in every case females could lived well beyond their last birth. Their post-reproductive life ranged between 25% and 95% of their breeding years.

Taken on its own, this result might suggest that human menopause isn’t anything special. But Levitis and his colleagues caution their readers to take it with a gorilla-sized grain of salt. Most of the records of longevity and births come from zoos, not surprisingly, where primates are well-fed, enjoy the attention of vets, and don’t face a daily threat from predators. Data on wild primates are a lot more sparse, understandably, but the picture that emerges from them is pretty brutal: only a tiny fraction of female primates survive to post-reproductive years.

Humans are different. A substantial portion of the women in any population are post-menopausal. This pattern is not limited to affluent societies. Take the Hadza, a group of people in Tanzania who survive by gathering fruit and killing game. A typical Hadza woman can expect to spend almost half her adult life in a post-fertile state. The slaves of Trinidad experienced some of the most brutal conditions ever recorded–so brutal, in fact, that their population was continually shrinking due to early deaths. And yet even among Trinidad’s slaves, a third of a woman’s adult life, on average, came after her last child.

It seems, then, that there really is something remarkable about the lives of human females compared to other primates. But is menopause what makes them remarkable, or is it just the side effect of something else that evolved in our ancestors? Humans have big brains, for example, and the bigger a primate’s brains, the longer its lifespan tends to be. This link may be due to the fact that big-brained babies demand a huge amount of energy and effort, both during pregnancy and afterwards. Those demands impose a slower pace of life on big-brained primates. So this pattern naturally raises the possibility that big brains in humans led to menopause.

Levitis and his colleagues don’t think so. They analyzed primate females, humans included, comparing their brain size to the age at which they stopped reproducing and the age at which they died. Even taking into account our gigantic brains, women are still odd. Compared to other primates, women stop reproducing sooner than you’d predict, and then go on living much longer than you’d predict.

So Levitis and his colleagues are left with the fact that human females typically enjoy a vastly longer post-reproductive life than their fellow primates, a life that can’t be explained away by other factors such as the size of their brains. Through human history, it appears, at least a third of women were post-fertile at any moment–a percentage that’s ten times higher than among the most pampered zoo-dwelling primates. There seems to be something special, something worth explaining, about menopause.

To explain this remarkable turn of evolutionary events, the scientists offer up a hypothesis. Six million years ago, our early hominin female ancestors were like other female primates. They could potentially live beyond their last childbirth, but they almost never did because most were dead by then.

And then hominins took an unusual evolutionary course. They evolved big brains and acquired new capacities–for making more versatile tools, for example, and for communicating with language. These factors may have allowed hominins to live longer. As a result, more females lived beyond their reproductive years.

Now the benefits of life after menopause could emerge. Any genes that enabled women to live longer would be favored by natural selection, because older women could raise the odds of their descendants surviving. Over many generations, women evolved a life in which they spent a dramatically larger part of it not having children.

In this new hypothesis, human menopause becomes at once special and yet not unique. In many species, females have the capacity to live beyond reproduction, but they rarely do, depriving evolution of the opportunity to expand that stage of life. But if other animals get that chance–for whatever reason–they may evolve to be menopausal too. Even insects can benefit from menopause. In species known as the Japanese gall aphid, females stop reproducing midway through their lives. Now that their abdomens are no longer dedicated to growing eggs, they can use that space to manufacture a sticky chemical. When a predator attacks the aphid colony, the menopausal females rush forward and glue themselves to its body. The predator is swamped by the heroic females, which die in the process. The evolutionary forces behind menopause may differ between humans and aphids, but the outcome is the same.

33 thoughts on “Why Menopause?

  1. This seems a little adaptationist. Hasn’t life expectancy been historically very low? Wikipedia indicates that humans have never had a life expectancy over 40 until the last century (http://en.wikipedia.org/wiki/Life_expectancy#Life_expectancy_variation_over_time). If menopause doesn’t happen until the late 40s, then how could natural selection be acting on any benefits post-menopausal women can have on offspring?

    Perhaps in enough generations it will, but for the moment, this is surely more like a spandrel than an adaptation.

  2. As always: where are grandpa and papa the other tool-makers? Did they have (then and now) anything to do with ma and grandma surviving longer?

  3. Why on earth would anyone propose an adaptive explanation for menopause, or take one seriously? Haven’t they read the spandrels paper?

    [CZ: Konrad, Gould and Eldredge were not dismissing ALL adaptations. They suggested that some traits that have a function now might have “come along for the ride” as another trait was selected. And I discussed a couple of these possibilities in this post. The thing is, those explanation do not explain the evidence well.]

  4. You seem to make an implicit assumption that human females have lived long lives in the last 6 million years. Do we know this? It seems more plausible to me that most of the last 6 millions years, human females did not live as long as is required for menopause to be selected for.

  5. Michelle, the wikipedia article aggregates peer reviewed sources for the ages it gives. You can see those sources by clicking through the links.

    It seems others were struck by the same question. If humans haven’t, until very recently, lived long enough to regularly reach menopause, then it seems impossible for natural selection to be the cause. Any benefit then would have to be a happy accident of some other evolved trait. It doesn’t appear that scientists have hit on the right associated trait. CZ does a good job of explaining why a couple hypotheses probably aren’t correct. Still, I think the null hypothesis has to be that it’s an accidental benefit.

    [CZ: Ben–At some point in hominin history, our ancestors changed from the mortality pattern seen in other primates to one that’s found in, say, the Hadza. When that occurred, the authors of the new study argue, selection could act on women’s post-reproductive lifespan. And while I’m a big fan of Wikipedia, the study I write about has a much more detailed analysis of demographics than you can find there.]

  6. With regards to Ben’s comment on 40yr life span: life expectancy is an average of all lifespans in a society and includes infant mortality. Infant mortality was much higher in past centuries, which brought down average life expectancy. An individual’s life expectancy improves as they get older. So while your average life expectancy at birth is only 40, your average life expectancy at 5yrs may be 70.

  7. And in case it’s not obvious, what HHD’s explanation means is that even when average life expectancy is only 40, plenty of females will live long beyond that age and there is therefore the possibility of natural selection acting to develop the menopause.

  8. And for those making snarky comments about Wikipedia – you can use Wikipedia to find realiable information.

    Some articles (about 10%, I’d say) are well written and sourced.

    The problem is that you need to evaluate the quality of what has been written and the sources used, which invariably means you need to develop some knowledge of the subject yourself.

    This can be an interesting and useful educational experience – but does mean that drawing quick conclusions is not necessarily possible!

  9. More on Ben’s comment. Jim Oeppen and James W.Vaupel published a paper in 2002 (http://user.demogr.mpg.de/jwv/pdf/scienceMay2002.pdf) that showed that “Before 1950, most of the gain
    in life expectancy was due to large reductions in death rates at younger ages.” In addition to the Hadza, life tables from Sweden from the 18th century have shown that about 1/3 of the female population was beyond child bearing years. Long life span in humans is NOT due to recent advances in health care. It’s a trait of modern humans and perhaps of the genus Homo as well.

  10. Is this hypothesis actually new? Take a look at some of Kristen Hawkes’ publications on menopause (http://www1.anthro.utah.edu/faculty/kristen-hawkes.html). I need to look over the Levitis et al. paper more closely but it sounds essentially like what she’s suggested long ago with her grandmother hypothesis. Not surprisingly, her hypothesis was inspired by her ethnographic work with the Hadza in which she documented the advantage to a young child’s survival if a grandmother was there to give mom a hand when a new baby sister/brother came along.

    [CZ: Hawkes proposed the grandmother hypothesis several years ago, and critics have raised doubts about it for some time. This new paper seeks to evaluate the evidence marshaled in the debate in a more rigorous way than in the past.]

  11. I think we are missing one important issue. Age of the eggs. Since women are born with a finite number of eggs, as the woman ages, the risk of having unhealthy children increases. Remove modern medicine and the risk could be very high. If women generated new eggs as men generate new sperm, this may not be an issue. I would suggest this is the real reason. Viable offspring, born strong and healthy early in a woman’s lifespan. And, as people age, our bodies are less capable of child rearing. Would a 60 year old woman actually want to give birth with all its attendant risks?

  12. A couple of notes:
    1) this paper (upon which this blog is based) does not say that post-fertile lifespan is adaptive. It demonstrates that some degree of post-fertile life is *extremely* common and people seeking explanations for this extended survival have been off-base. But part of the problem is that they haven’t measured it properly either. It then shows that the special thing for humans, at least among mammals, is that this post-fertile survival is really like a ‘stage’ in the life course, because such a large fraction of the women born will outlive their fertility, even in extremely high mortality environments. However, demonstrates no adaptive function (it doesn’t try to). The aphids are an example of a clearly adaptive post-fertile lifespan (and how!). Previously no one had even shown that the degree of post-fertile survival was unique in humans. …
    2) The statement in this blog about brain size is wrong. The authors show that brain size in captive primates predicts lifespan and the age at last birth quite well.
    3) did someone seriously post a link to the spandrels paper? that’s funny. Definitely not relevant to the blog post, or the paper.

  13. Surviving for years after the last child is born may merely reflect that human babies take a long time to mature. Children may do better when they have grandmothers alive, but for sure they do very badly when their mothers are not around. Living 10-15 years after your last child is born makes evolutionary sense for human females.

  14. I suggest, decreasing proportion of females still alive as age rises. Law of diminishing returns, evolutionarily speaking, making enough of the finite supply of eggs to last, as the chance of being able to use them decreases.

  15. At the beginning of mankind old and long living women were necessary as a sort of living archive for knowledge.They enabled mankind just with the force of their memory to accumulate important knowledge about dangerous plants, enemies, friends, creating myths, history, a context to ancestors and identity and the basis of the treasury of knowledge of mankind. Those very few very old needed for their job as human memory nothing but support by their squod, excluded from the dangers of energy consuming child birth and -raising. They created human society instead and thus our evolutionary advantage …

  16. Actually the maximum biological life expectancy is probably unchanged somewhere around 120-130 years. What has changed is much reduced infant mortality. You have to remember that previously average life expectancy included all this vast infant mortality. So when you read that life expectancy was 40 years old, remember that most adults would live much longer than that. In reality, if you made it past the first years of life, you had a good chance to live long.

  17. Interesting the twist on mothers needing to survive long enough for their last kids to grow up. I could see some natural selection under that angle. Probably more likely than the grandmother hypothesis. If a mother has kids until an age close to her death, her last kids will most likely die when she does (or if she becomes impotent after a stroke, fracture, etc.). So in a world of competing females, females that stop reproducing at 50 could produce more living adults compare to those that produce kids until the end, say 65 year old with little hope for their last kids to survive. Over millions of years, a balance is found between life expectancy and number of kids a women can bear, i.e. menopause. For males, as long as a male can find female mates that are fertile, he can still hope to pass on his genes. Even if he does die or becomes impotent soon after, it does not really matter since survival of the kids would be linked to the survival of their mother, males being not so much involved in the survival.

    I suspect that most of that evolution of menopause must have happened millions of years ago when we were not much different from the ancestor of the chimpanzees.

    Very interesting topic.

  18. An anthropologist around here remarked to me once regarding modern hunter-gatherers that once age 30 has been reached survival to old age was the most likely.

  19. That’s nature way of saying “know your place woman, and raise your kids”

    [CZ: Hard for me to imagine a comment on this post being more wrong.]

  20. hummm…. how is Johne Deere’s comment “wrong”? Anti-social by our current era’s standards, but a reality of biology. How is it “wrong”, author?

  21. Rather than harping back to demographic studies done prior to the industrial age , combining recent developments in our own culture with what we saw and now what we see,i think it would be interesting to do some demographic studies between the states of Florida , whose snow bird culture has created entire gated communities where youngsters are turned away at the gate no doubt due to their disruptive patterns of evolution that create noise and activity where no noise and activity, [ at the levels sustained by youth] are required. And compare the mortality rate with the elder members of the tribes on the other end of the country where financial stability has disallowed the choice to migrate to climates more suitable for aging body parts and in most cases results in both male and females staying in the work forces much longer than originally planned. . Working Grandmas spend much less time with their offspring and are no doubt much more cranky. It was an interesting blog though . Thanks for writing it .

  22. Re: chapster — probably because only the “raise your kids” clause stands. The rest of it is irrelevant, highly-simplified and sits rather badly with us, societal-values-wise. So, in general, the comment earns great disapproval.

  23. I’ve heard that mortality in childbirth for older mothers was so high in the preindustrial era that to become pregnant over age 40 was considered a virtual death sentence. Does anybody have any info on this?

  24. May be this is the way nature controls primate populations. Look at world population now. Natural resources cannot cope with the demand of humans as their population exploded. We compete with wild animals and cultivable land. We need to manipulate our food sources to support humans, like we use fertilizers, hormones to make plants, animals grow faster; we use preservatives to keep them longer. These are all chemicals. They are harmful to humans or animals, in some way. That’s why we get new types of diseases, we develop all sorts of allergies, etc. I think it is same sex unions are in the same category that nature tries to find a way to curb population human growth.

  25. This post doesn’t include another argument for menopause. This is that humans evolved with arranged marriages, and since males and females didn’t select each other, females have a good reason to look elsewhere. Males often have to leave the community periodically for hunting etc., and sisters can be exogamic, so the only continuously present kin member to watch over a male’s wives is his mother. If she can prevent one act of cuckoldry, this is the same as preventing the death of a grandchild, in genetic terms. This argument reflects the antagonism between women and their mother-in-laws. The arguments concerning older females caring for their daughter’s children makes no sense since females are often exogamic, thus an older woman may have no daughters present. See The Structure of Humanity.

  26. ok … calm calm …. maybe , just maybe, the reason why the whole human race has thrived , is because the gentle and focused attention necessary to raise a very un protected and long maturing little creature, which is a person , actually meant that the whole species benefitted from the wisdom that developed inside of that process – that all live beyond their usefullness, based on the love ( like, nurturing and personal appreciation and affection ) that was necessary to raise a child. Maybe , its not why we women live after menopause, but what happens when a woman extends herself into the well being of another , for years – Breastfeeding actually is a 4 to 7 year process . Elephants and Whales have the same extension of life , past reproduction . Its almost like, the amount of extended concern into the first years, is echoed into the later years . – any takers ? The answer is usually inside the question .

  27. Menopause is not the only time women are not fertile, lactation and pregnancy is the other time . The answer is in the pause , not in the product.

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