AGE-OLD QUESTION: Scientists explore why women live long after they lose their ability to reproduce whereas many other female organisms die once they can no longer reproduce.
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The origin of menopause has puzzled evolutionary biologists for the last half-century. Three new studies attempt illumination. The real question, though, is probably not: Why menopause? Rather, it is: Why do women long outlive their fertility?
Human ovaries tend to shut down by age 50 or even younger, yet women commonly live on healthily for decades. This flies in the face of evolutionary theory that losing fertility should be the end of the line, because once breeding stops, evolution can no longer select for genes that promote survival.
The most popular explanation, the "grandmother hypothesis," argues that a generous post-reproductive life span makes sense if a grandmother improves the survival and reproduction of her grandchildren, thus ensuring continuation of her own genes—including genes that contribute to longevity. But skeptics say the math is askew. From an evolutionary perspective, it is hardly ever better for a woman to give up a chance to bear additional children of her own, and so pass on half her genes, for the sake of improving the survival of her grandchildren, who carry only a quarter of her genes.
"The problem is that these grandmother benefits aren't big enough to ever favor stopping breeding between the ages of 40 and 50," says Michael Cant, an evolutionary biologist at the University of Exeter in England and co-author of a new study on the genesis of menopause published this week in Proceedings of the National Academy of Sciences USA. "When you look at data from hunter-gatherers and other natural fertility populations, the sums just don't add up." Grandmothers do benefit their descendants, he says, but the genetic payoff is small compared with those of producing another child.
Cant and co-author Rufus Johnstone, an evolutionary biologist at the University of Cambridge in England, used game theory to argue that menopause is early cessation of reproduction that originated through reproductive conflict between generations. In most cooperatively breeding species, reproduction is suppressed in younger females, who act as helpers to older reproducing females. By contrast, they say, younger women in human social groups win the reproductive sweepstakes, because the older ones stop having babies.
"We showed that, compared to other primates that exhibit a post-reproductive life span, humans really stand out, because there is absolutely no overlap in reproduction between generations," Cant says. "Women stop breeding on average when the next generation starts to breed."
This makes evolutionary sense, Cant and Johnstone say, because, contrary to most mammals, young women tend to move to their mates' communities, where they become immigrants whose only genetic kin are their own children. There is no genetic profit in helping their mothers-in-law bear more children, because they will not share any genes with those children. But an older woman who helps her son's wife reproduce will benefit by bequeathing 25 percent of her genes to her grandchildren.
"We show that the mother-in-law's best strategy is to stop breeding, avoid competition, and allow the daughter-in-law to breed and help her," Cant says. "It's the first time anyone has taken the idea that humans evolved with this sex bias in dispersal and looked at the implications for how these conflicts will be resolved within the family."
The mother of the grandmother hypothesis, anthropologist Kirsten Hawkes of the University of Utah, says Cant and Johnstone are right to focus on intergenerational conflict. Elephants have babies in their 60s, and some whales give birth in their 80s. "It's clearly something selection can adjust," she says. "So explaining why it hasn't in us has to be part of the story." But she disputes their claim that female-bias dispersal is, in fact, the universal human/ape residence pattern, pointing out that half of the young female chimps at anthropologist Jane Goodall's Gombe Stream Research Center remain with their mothers, and that recent studies show that hunter-gatherers often live with the wife's family as well.
Another explanation for menopause is the "mother hypothesis," which holds that it occurs because older mothers might profit more, genetically speaking, by investing resources in their existing children than in giving birth to new ones. Researchers at the Max Planck Institute for Demographic Research in Rostock, Germany, make the case for this in the American Journal of Physical Anthropology (AJPA), concluding that menopause is advantageous when a woman has aged enough to face an increased risk of stillbirth, birth defects and her own death in childbirth.
Researchers of a different AJPA study, based on 400 years' worth of data on births in Costa Rica, believe that postmenopausal longevity is associated with an increased number of children but a decreased number of grandchildren—a finding that supports mothers over grandmothers.
"We're not saying grandmothers do not provide benefits in some societies," says study co-author Lorena Madrigal, an anthropologist at the University of South Florida in Tampa. But, "we should not assume that one pattern fits all."
Data on great ape fertility is spotty, but what there is shows that our closest relatives—chimps, bonobos, gorillas, even orangutans—stop having babies about the same age that we do: the late 30s. The difference is, they generally die a short time later. "The thing that makes us different from apes is not the age of fertility decline, it's the lack of aging in other systems," says Hawkes. "I have been saying this for a long time and I don't think it's what anybody is hearing. Probably what a lot of people are prepared to listen to is the way Cant and Johnstone have framed this, that the real question is: 'Why do we stop [reproducing] so early?' I think the bottom line is that, compared with our closest living relatives, we don't."
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37 Comments
Add CommentI don't see any mention of what seems to me to be the self-evident answer - mothers continue to live because their children continue to need them for many years after birth. Humans have a remarkably long period of dependency on their parents, compared to other species. When fish die shortly after spawning, it doesn't matter to their offspring one way or another. I suppose it even helps the offspring - by reducing competition for food, for instance. If human mothers died shortly after their fertility ended, they would leave behind their still-dependent last-born children, who would be much less likely to survive to reproductive age themselves. A woman who lived long enough after the end of her fertility to raise her last-born baby to adulthood would be more likely to pass on her genes to a greater number of descendents - including her "live-for-years-after-menopause" genes! Right?
Reply | Report Abuse | Link to thisOutlive?? In ancient Rome, the average life expectancy was 25 years (41 years for a 10-year old child) -- see http://www.utexas.edu/depts/classics/documents/Life.html.
Reply | Report Abuse | Link to thisWhat was it before that, in lesser civilizations? Clearly, the "outlive" phenomenon is a consequence of the longer lives we now live. Duh! I am amazed at how unscientific and anti-scientific has Scientific American become, and how quickly. But how politically correct at that! Certainly, these two developments are just the two sides of one coin.
I agree with "teller"; we often project our own environments to the past, assuming that to be the status quo. I've read any number of accounts of men losing several wives due to complications of childbirth in the not so distant past. Typical life spans were only 40 years as recently as the turn of the last century. "So why do women outlive fertility?" Because they live in a different world!
Reply | Report Abuse | Link to thisgrandmothers are the touchstone in the family. the glue that binds them. they share the family traditions and keep the generation ebb and flow and mend the family when it gets its tears here and there. They are the quilt.. and the members are the patches. If you dont think that.. who continues to have the dinner table.. the center of bringing all together and networking. working out the differences and teaching. Every holiday...who is the one that is the most in charge in the family unit? the man? no.. the grandmothers! the great aunts are indirect.. but in the line up.. and second after that, the mothers and sisters in laws... the men.. just eat and follow the lead. who ends it? the women. who ends it finally..? the grandmother.
Reply | Report Abuse | Link to thisI think it's clear that women now live more than usual for homosapiens, and I excpect, over the following millennia women will have longer periods of reproductivity. i.e the change in longevity didn't affect our genes yet.
Reply | Report Abuse | Link to thisteller,
Reply | Report Abuse | Link to thisYour statements weren't exactly accurate either... Yes, the average life expectancy may have been the the 40s but that INCLUDES the huge influence of infant mortality and the harsh lives of the poor. IF you lived past your teens and your were wealthy, you had a good chance of living to a ripe old age. Many people lived into their 80s. Check your history...
I do agree about grandmothers holding families together. Men don't have that emotional attachment that women do.
Low life expectancies in more primitive times was due to high infant and childhood mortality. If you made it to adulthood, your chances of living to 60 or 70 years old was actually pretty good.
Reply | Report Abuse | Link to thisI was perhaps in error on the 40s thing, but we know there are always exceptions. My Father once lamented after his last child left that he was no longer needed. My Wife & I are now "Empty Nesters", and that statement has more meaning. True, patriarchal & matriarchal structures still exist. But with grandma & grandpa off to the retirement community (or home) and the children to pursue their careers in various parts of the country, it seems the exception rather than the rule here. You have to work at being a good grandparent, I'm keenly aware of that. But I see no implicit "natural mechanism" that infers that either gender has special status in "holding families together". Even Norman Rockwell was reputed to have said; he didn't paint reality!
Reply | Report Abuse | Link to thisLet's also consider that it takes 20 years to rear a human child to adulthood, and most of that work will be done by the child's mother. A female human who reproduces at the age of 80 (or likely 60) will not be alive and healthy to care for her child. A motherless child is at a great disadvantage. A fatherless child will be at some disadvantage, much of which can be ameliorated so long as the mother and child have a source of economic support.
Reply | Report Abuse | Link to thisAlso, the human birth process is difficult and women having children at older ages would be less likely to survive the birth.
"bluebird"; you're reinforcing the obvious. I would never consider at 65 taking on a job I once did as a 25 year old, (the job market is more than aware of this) nor should a woman my age (if she could) consider childbirth. Too physically demanding in either case. For that matter, a man my age would be wise not to consider starting a family either ( if he could ) for the very same reasons you've mentioned. Unless, of course, he has a huge ego.
Reply | Report Abuse | Link to thisThe risk of death in childbirth applies to the young mother, then what happens to grandchildren without grandmother? Don't underestimate ferocity of mothers protective instinct, which is even greater when it comes to grandchildren.
Reply | Report Abuse | Link to this> teller,
Reply | Report Abuse | Link to this>
> Your statements weren't exactly accurate either...
> Yes, the average life expectancy may have been the
> e the 40s but that INCLUDES the huge influence of
> infant mortality and the harsh lives of the poor.
Were you reading my post really? For I did say there: "In ancient Rome, the average life expectancy was [...] 41 years for a 10-year old child", which excludes infant mortality.
Next, there were very few rich enough people In Rome to live a long life. But even this means little. For clearly, I gave Rome just as a reference point.
Biologically, mankind today is about the same the Crow-Magnon people, and now think: how long could the richest of the Crow-Magnon guys have lived?
"kay moran"; While I think your passions are honorable and I'm certainly supportive of them, I think you're being a bit subjective. The intensity of these feelings are not universal, certainly not to make the case that nature provides a special provision so that woman can live past menopause. We seem to forget that not too long ago most people simply didn't live long enough to become grandparents. In any case, the "grandchildren" simply adapted to their particular circumstances.
Reply | Report Abuse | Link to thisteller,
Reply | Report Abuse | Link to thisThere are a LOT of rich, important, famous, Romans living well into their 80s.
The "age" for a human body seems to be right around 100 years. Given everything working out perfectly, you can expect 100 years as a rough estimate of human body longevity. An extreme few go farther, but for most extents and purposes, 100 is the limit.
A Roman citizen could easily reach their 80s and some their 90s. Wealth, and standard of living has a huge effect on longevity.
In fact, I'll tell you this too...the US has the BEST health care in the world. Yet, we rank below other countries with inferior systems. Want to know why? We TRY, I mean REALLY try, to save even the most "ill prepared" to make it in this world. We try to save preemies MONTHS early. Those countries don't. They'll allow it to die at birth and is therefore not counted. Miscarriages and dead births are not counted in any statistics. The more you try to save, the more you're going to lose. It's all a numbers game.
A grandmother remembers much further into the past than a mother. If a clan is facing a famine, a single grandmother who remembers that you can eat the roots of that plant can save the entire clan. (Which means the entire genetic pool of her relatives.) The tribe which focuses on mothers at the expense of grandmothers may thrive, but only till the next drought, or exceptionally cold winter. Then they all die.
Reply | Report Abuse | Link to thisHuman evolution is probably defined better by catastrophe survival than by "averages."
Come on now...we ALL know the truth here. Women are "taken care of" in modern societies. They don't suffer the competitive stresses that men face. Once they make it past the last childbirth, they're pretty much home free. Kick back, relax, a stress free, taken care of existence.
Reply | Report Abuse | Link to thisMen ALWAYS have to deal with the effects of their male hormones. Even OLD men compete...
It's as simple as "old women have less stressors and therefore live longer".
You would have to consider prehistoric man more than historic man. I would assume that with hunter gatherers wealth did not translate into longevity. Likewise, adulthood was probably considered to be reached with fertility if such a distinction was even made. I can't remember any fossil records of excessively aged people being found. It would make sense that humans in their natural state would out live the reproductive cycle by about as long as the last child was conceived by about 13yrs or so when that generation was raised and fertile. Probably between 35-to 45 yrs old. I doubt there were many 50-70 yr old hunter gathers. Life was hard and disease did what it would unregulated. People were subjected to clan territoriality, climatic events...predators of the wild. The whole premise seems flawed to me in this article. I think it may be more correct to assume grandparentage was more a factor once agriculture was introduced and humanity became domesticated. Make sense?
Reply | Report Abuse | Link to thisThink of pre-birth control pill days, where women did still bear children right up to just prior to menopause. She would still need another 15 odd years after to bring up her last child, at a time when her physical strength wasn't like when she was in her 20s. So it makes sense to menopause to allow her to focus her energy to bring up that last child of hers!!!!!
Reply | Report Abuse | Link to thisOvulation stops when the body is too old to bear children safely. The woman lives on because she is still mentally and physically active. She wants to live!
Reply | Report Abuse | Link to thisGenetic evolution is slow and now pretty much defunct because of culture which is what make humans human. Duh
Obviously woman have alot more to offer than to just bear children. Like they say...a woman's work is never done!
Reply | Report Abuse | Link to thishey man
Reply | Report Abuse | Link to thisI have some issues with the logic of this article. If female great apes other than human ones, die soon after menopause, what about the males? Do they outlive females by many years? I can't find any information that says that males live longer. Also, before the last couple of centuries, the average lifespans of males and females was around 40. Perhaps humans didn't evolve to live beyond that anyway, whether male or female. Perhaps females or males living past 40 was an anomaly, not really having anything to do with evolutionary reproductive capability. We just happen to have older females (and males too) around now because of improved conditions. Evolutionary forces couldn't really play out historically with regards to fertility because basic survival took precedence. What we see now is a result of the survival capability, not fertility. Along this vein would explain why fertility did not last beyond 40-50 - because there it was unlikely that they would survive that long
Reply | Report Abuse | Link to thisIf anyone out there has an answer to this question, I'd like to read about it. I've always wondered why childbirth is so difficult. Is there any data out there that shows that babies born to difficult births are less likely to survive than those that pop out easier? Why hasn't it been the case through evolution that births get easier over time (history) since one would think that only strong and resilient mothers would survive childbirth and pass on those strong genes to their daughters - and only strong and resilient babies would survive. So shouldn't births be getting easier (and I don't mean by intervention as in recent history).
Reply | Report Abuse | Link to thisDoes anyone know of any studies that predict how the gene pool is getting weaker with the advent of improved health care? It's probably politically incorrect to think this way, but there are many people who are reproducing today who never would have survived to adulthood in the past and passed on their genes - e.g. maybe we have so many asthmatics today because their parents would not have survived in the past. And there are many other serious conditions/diseases that can be treated today. However, with medical intervention, the strength of the species is probably diminishing.
Reply | Report Abuse | Link to thisThe grandmother who helps look after the grandchildren will then be looked after herself by her own children as a reward. So being nice to the grandkids helps increase her OWN life span when she becomes too fragile to hunt and gather for herself. Babysitting the grandkids frees up her own kids to do the work required to gather food for HER.
Reply | Report Abuse | Link to thisOh, I don't believe that in primitive times it was likely for young adults to live to be 69-70 years old. There were too many ways to get killed: infetious diseases, wars, preditors, accidents (something like a fractured pelvis or hip was a death sentence).
Reply | Report Abuse | Link to thisHowever, so many people forget that in ancient times, it was well-known that a certain small percentage of people liked to be about 70 years old. There is even a classic phrase for this classic lifespan: Three score and ten.
These were the ancient patriarchs and matriarchs. They were uncommon, but they existed.
Let me repeat myself: three score and ten has been known since the times of the Ancient Egyptians, Babylonians, Phoenicians, Greeks, and Romans. Read up on it.
There are plenty of comments here that just assume that in ancient times, NOBODY lived beyond 40 or so.
I am not religious, but I do think that there are some things in the Bible that are historical records. The existence of some people like David, Solomon, etc., is confirmed by non-Biblical records and archaelogy. An notable fact about these folks was that they lived to be really old - and that was one thing that made them worthwhile to write anything about. The number three score and ten is even mentioned in the Bible as the lifespan of a man, assuming that nothing radical happened to him: disease, war, famine, accident, murder, etc.
The obvious reason why childbirth is generally difficult for human beings, as compared to other mammals, is that human babies have much larger skulls and MUCH larger brains at birth than other mammalian species. How is this even possible?
Reply | Report Abuse | Link to thisWell, if you look at the fossil records, you will see that the pelivs of the primitive cavewoman was much smaller than of modern woman. That implies that cavewoman gave birth to babies with much smaller heads. The fossil record shows that over millions of years, there has been a dramatic increase in the size of the pelvis of our remote ancestors and of primitive cavewomen.
I don't know why it isn't in the brains of all men, but I am personally a lot more attracted to a woman with measurements of 38-28-38 than I am to one with measurements of 32-22-32. My brain is just subconsciously wired to be attracted to females who can carry and safely give birth to large, healthy babies, and take care of them (not having died in childbirth). That is the way I am, and I cannot understand the minds of men who are attracted to skinny women. Sincerely, DAW
I wonder if evolutionary biologists have noticed that even fertile humans don't usually live just to reproduce. Probably not, but normal people have known for millennia that there's more to life than what can be explained by evolution.
Reply | Report Abuse | Link to this--
Edited by booyalab at 04/12/2008 11:25 AM
accidental double-post
Reply | Report Abuse | Link to this--
Edited by booyalab at 04/12/2008 11:24 AM
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Edited by booyalab at 04/12/2008 11:24 AM
Evolutionary biologists are looking for the answer to this question. Just because they don't have the answer, yet, does not mean it doesn't exist. That's the nature of science.
Reply | Report Abuse | Link to thisIn many primative societies it is the long-lived grandmothers that are the resevoir of information that gets the tribe through periods of famine and droughts. They know what alternative foods can be eaten in times of hardship and where to dig for water...
Reply | Report Abuse | Link to this"In many primative societies it is the long-lived grandmothers that are the resevoir of information that gets the tribe through periods of famine and droughts. They know what alternative foods can be eaten in times of hardship and where to dig for water..."
Reply | Report Abuse | Link to thisYou believe that?
My take is this: Men USUALLY do the dangerous jobs...more likely to die of accident than old age. Women USUALLY do safe, "stay at home" type jobs...LESS likely to die of an accident, more likely to die of old age.
Even an old man wants to go hunting. When he's too old to go, he feels worthless and dies from "shame". An old woman can still do what she's always done...care for others.
It's the same in our society. Many men die soon after retirement because they feel worthless. They die of depression. Women don't have that problem...yet.
A note to all you women: Be careful what you wish for, you may get it. You want equality in EVERYTHING, you just may get it. Studies show that working, career, women suffer the same fate as the men. More stress, health problems from working, early deaths.
You've been warned!
You had it so easy...placed on a pedestal, worshiped, courted, taken care of...now you're just competition...you blew it...
Are there any data about having sex after menopause?
Reply | Report Abuse | Link to thisIs it possible that women keeping up their sexual appetite are prolonging their lifetime? I wonder.
Robertsor
Yes...women have sex after menopause.
Reply | Report Abuse | Link to thisMenopause ain't big issue nowadays... Just use something like http://www.menoaid.com/ and you fine
Reply | Report Abuse | Link to thisSeems to me the author is inserting a little teleology into natural selection. Asked why women live so long after menopause, as good an answer as any is, "Why not?"
Reply | Report Abuse | Link to thisThe way my college biology prof explained it, reproduction is the "target" for living things. Success or failure as biological individual depends solely on that. In humans the prime reproductive age runs from around fifteen to twenty-five years, when we are most robust. Reproduction is still possible later on, but odds become increasingly remote.
Whether the individual reproduces or not, as prime reproductive years recede it becomes less relevant. Nature (so to speak) holds no grudge against the useless, merely indifference.
For whose who wonder why natural selection does not shut them down, perhaps the answer is, "It doesn't care."
The authors claim the numbers don't add up,but I wonder if they are looking at both sides of the equation. Not only the value of grandmonthering but the lower payoff of older mothering. They don't mention that the chance of death from childbirth goes up exponentially with age and number of children after the first few. Meanwhile the chance of having a healthy child plummets. As does your chance of surviving for long enough to raise a viable child.
Reply | Report Abuse | Link to thisIt would seems to me that the breeding payoff gets really damn small for an older woman.
It is also or noting that the grand-mothering hypothesis contains in it the mothering hypothesis.