I am inclined to like this hypothesis


I’m still going to criticize it, though.

For years, anthropologists and evolutionary biologists have struggled to explain the existence of menopause, a life stage that humans do not share with our primate relatives. Why would it be beneficial for females to stop being able to have children with decades still left to live?

According to a study published today in the journal Proceedings of the Royal Society B, the answer is grandmothers. “Grandmothering was the initial step toward making us who we are,” says senior author Kristen Hawkes, an anthropologist at the University of Utah. In 1997 Hawkes proposed the “grandmother hypothesis,” a theory that explains menopause by citing the under-appreciated evolutionary value of grandmothering. Hawkes says that grandmothering helped us to develop “a whole array of social capacities that are then the foundation for the evolution of other distinctly human traits, including pair bonding, bigger brains, learning new skills and our tendency for cooperation.”

I guess I’m personally inclined to appreciate the importance of grandmothers, having had a pair of good ones myself, and seeing how much time my wife invests in our granddaughter, but I’m less impressed with the study, which is based entirely on a computer simulation. I don’t trust simulations of complex phenomenon that necessarily have to simplify all the parameters. What about aunts and sisters? What about uncles?

What about the grandfathers?

None of those individuals are of interest, because this version of the hypothesis is structured around explaining menopause as the product of selection. Nope, I don’t buy it.

But why would females evolve to only ovulate for 40 or so years into these longer lives? Hawkes and other advocates of the hypothesis note that, without menopause, older women would simply continue to mother children, instead of acting as grandmothers. All children would still be entirely dependent on their mothers for survival, so once older mothers died, many young offspring would likely die too. From an evolutionary perspective, it makes more sense for older females to increase the group’s overall offspring survival rate instead of spending more energy on producing their own.

I’m willing to accept the benefit of an extended family and social cooperation, but the effort to justify menopause seems misplaced. There are many grandmothers who are not menopausal, and there would have been even more in ancient populations, where pregnancy shortly after the onset of menstruation would have been common. It also doesn’t explain the contributions of sisters and aunts to childrearing, or that brothers and sisters, who are also “distractions” from the business of raising a single delicate child. Why couldn’t it benefit a woman to raise her own child born late and also contribute to the well-being of grandchildren born to previous offspring? I suspect the simulation has assumptions built into the code about how much grandparental investment can be offered if they also have a child.

But, yeah, what about the grandfathers?

We help, too. So why isn’t there a male menopause where our testicles shrivel up and make us more willing to contribute to child-rearing? A man has a certain number of progeny, then boom, the reproductive urge goes away and he has to sit down and focus on taking care of the kids he’s got. Or his grandchildren. Or his nieces and nephews. That would be the logical endpoint of this arch-selectionist model, after all, and what’s good for the goose is good for the gander.

Yet somehow people feel compelled to come up with adaptationist explanations for accidents of evolutionary history.

Comments

  1. marcoli says

    It’s an emotionally appealing hypothesis, but natural selection would have done this … I dunno, 200,000 years ago, probably longer. How long were most of us living then? I don’t know if that’s been weighed, but right now it seems reasonable to say a 40 year old would have been rare.

  2. Allison says

    The hypothesis that I heard was that, normally, you’d expect humans to die not long after they ceased being able to reproduce.
    After all, they would be consuming resources, but not supplying new individuals. However, since human beings have culture — that is, much of our success is the practices that we pass on to our children — older individuals contribute to the reproductive success of the group by passing on the customs and knowledge. This would apply to both men and women.

    I.e., it is the survival of aged individuals that is explained, not the menopause itself.

    The existence of menopause (i.e., that at a certain age women stop being able to pass on their genes but men do not) is simply an accident of biology. The way human ovaries are set up, they contain all the (potential) ova that they are ever going to have, so if a woman lives long enough, she’ll run out of them — which is what menopause is. Testicles don’t seem to work that way.

    What I don’t know is whether there are other species which have a maximum number of ova that a female individual can produce over their entire lifetime. Or are humans (or primates? or mammals?) unique in this way.

  3. says

    @1 You beat me to it. Most primates eke out 30-40 years. So would we if we weren’t using our intelligence and abstract thinking but our bodies are still tuned to be dead by 40.

  4. says

    Older mothers are at higher risk for maternal death and problem pregnancies, so I would expect that without menopause the additional reproductive potential would not be very great — and few people lived that long in the first place. Men don’t go through menopause per se but they do tend to become less, ahh, frisky in old age. Even so the burden they assume for child rearing is less, so grandfathering isn’t much compromised by the need to also father. So the idea I think is more logically sound than PZ seems to think, though it is certainly speculative.

  5. says

    Oh, and I should add that overpopulation was commonly a problem for paleolithic people. They likely practiced various means of avoiding pregnancy and even infanticide. More babies wasn’t usually something they would have needed. All of the people in grandma’s band would have been related to her so serving as a source of wisdom for them all might well have been more beneficial to her genetic heritage than pumping out one more baby.

  6. Rowan vet-tech says

    In dogs and cats, the older the dog or cat the more likely they are to experience complications and dystocia. The same is true for humans. Pregnancy is hard and dangerous. Dogs can rapidly become emaciated trying to feed a large litter without additional support, and even though related queens will co-raise litters they still become skinny and lots of the kittens die. Without modern medicine it would be one of the leading causes of death for childbearing humans. Making sperm isn’t likely to kill you. Also, an older male mammal can impregnate many females still so genes to continue producing sperm would be beneficial.

  7. says

    How long were most of us living then?

    Yeah, this. We have expanded our lifetimes greatly. Just like we’ve done with our captive our domestic animals. Very few reach their maximum age in nature.

  8. Rowan vet-tech says

    If I remember correctly, once you take things like childhood mortality out of the equation, reaching late 50s early 60s, even mid 70s was not unusual. Once you made it to adulthood, and provided you didn’t die of childbirth or war or random plague… You still actually could live a long time. It’s just that most people died as children or infants and when you take an average that’s going to wildly skew it younger.

  9. lochaber says

    I was always partial to the idea that menopause made it more likely for a woman to live to an advanced age, where her accumulation of knowledge and history of past events would be of greater use to her descendants than having a couple more kids directly. I think when I initially read this proposal, it cited environmental threats, like tsunamis, major storms, droughts, etc. – events that aren’t everyday occurrences, but could easily wipe out a small village of people if know one knows the warning signs or how to deal with it.

    Also, echoing what Rowan vet-tech said, if we look at recent pre-agricultural human groups, there was a high infant/child mortality rate, but once an individual hit their teens or so, they had a decent chance of making it up until their seventies or so. This is a bad misreading of “life expectancy” that just won’t die…

  10. kome says

    This doesn’t even pass the sniff test of an explanation. One can become a grandmother at 30. Independent of cultural values, taboos, and/or laws, 15-year-olds can (and, recalling my high school days, do) get pregnant and give birth and become parents.

    So why would menopause begin in the 40s if “grandmothers” is the alleged explanation? They’d be better off argueing that evolution selected for great-grandmothers. At least that (still very stupid) idea fits better with the timeline of average human reproductive capabilities.

  11. garnetstar says

    @9 you’re so right about the usefulness of old people: I remember Jared Diamond discussing how incredibly valuable even one old person is to a tribe. The example he cited was a famine, and an old person may have experienced that as a child, and so be able to tell the tribe what wild fruits, etc. were poisonous and not to eat.

    I recall that, in the first outbreak of Ebola in then-Zaire in the 1970s, it was the elders in villages who stopped community spread because they knew from past experience what to do: they barricaded every road in and out of the villages with thorn bushes, allowed no one in or out.

    But that applies to both men and women, so that particular value wouldn’t really contribute to menopause. I kind of like the ova explanation of Allison @2: the ova may either be all used up or just too degraded at some age, so why bother with the energy expense of ovulation, menstruation, etc.

  12. cartomancer says

    There are plenty of other mammal species that live in social groups and benefit from elders passing on their experience. Elephants are a good one. There’s even a famous Attenborough segment on it. I’ve seen grandmother cats helping to look after their grandkittens. And grandfather cats. And unrelated cats who took a shine to the youngsters. If this explanation were true then might we expect to see numerous other menopause-suffering animals also?

    To be fair my own grandparents contributed quite a few things to my upbringing. Second-hand smoke, homophobia and the constant smell of cooking lard mostly. But they meant well.

  13. Frederic Bourgault-Christie says

    What bothers me about this selectionist approach (though from what I can see this seems to be a much more reasonable and tentative hypothesis than a lot of bad EP) is that it just assumes ahead of time that all traits have to have a specific reason rather than being, say, the result of competing costs. Menstruation has costs so it makes sense that the body only sets aside enough space and energy to make a finite number of eggs. Combine that with the fact that the average primate would indeed have died from 30 to 40 and it could just be the result of other energy expenditures being prioritized.

  14. hillaryrettig says

    there are other species where grandmothers make a huge contribution – orcas and elephants and sperm whales.

    are there any species where grandfathers do? the adult males of the above species are mostly loners, I think.

    maybe it’s harder to transition from a fighter / dominator / competitor to a nurturer and font of wisdom? or maybe the males’ “lifestyles” means they don’t reliably live as long? although I imagine they did live pretty long prior to human predation.

  15. KG says

    Wanting to check on what Rowan-tech@8 and lochabar@9 say (I think I’ve read the same), I used my favourite search engine* to look up “paleodemography”. The top item was an advert: “Meet Interracial Singles – Cute Interracial Singles”. WTF?

    *Which isn’t Google – I got tired of being constantly pestered either to reaffirm that I don’t want them to track and collect information about me, or be permanently logged in to my Google account so they remember this.

  16. KG says

    Further to #16, the most relevant article I’ve found so far is here. It indicates that there was some increase in survival past young adulthood from Austropithecines to early Homo to Neandertals, but then a large change from the latter to Early Upper Paleolithic AMH. A commentary here (firewallled) points out that EUP AMH and Neandertals are from different lineages. My overall impression is that direct information about age distributions in ancient (specifically, pre-agricultural) populations is sparse – we have better information from modern foraging populations, but we don’t know how far they resemble ancient ones.

  17. kestrel says

    @Allison, #2: Quote: “What I don’t know is whether there are other species which have a maximum number of ova that a female individual can produce over their entire lifetime. Or are humans (or primates? or mammals?) unique in this way.” End of quote…

    Yes there are. Chickens for example. They are born with the number of ova they will be able to produce over their lifetime. Humans have deliberately bred chickens to produce their ova every single day as much as possible… so that they lay all their eggs, then “burn out” and can no longer lay eggs. For production I suppose this makes sense, the hen is “no longer any good” once she’s about 3 years old and in some cases less. Heritage breeds produce eggs far more slowly, and tend to be able to lay as 5 and 6 year olds. Lifespan of chickens is considered 5 to 10 years. I have personally known of some to live a lot longer than that, up to 16 years.

    Why are chickens like that? Because grandmother chickens are super important? No. Although chickens are social and live in groups, there is no evidence that I am aware of that chickens are somehow greatly benefited by grandmother chickens. I don’t think humans are either. I think that sometimes stuff just happens. Being humans we keep trying to find reasons for it, however that does not mean there is actually a reason for that trait to exist. As far as I am aware, evolution is not directed and does not have a goal.

  18. says

    15 – The “grandmothering” argument does sound a bit like evolutionary psychology. But unlike other claims, it’s not rationalizing male sexual desires. I think.

    1, 3, 7 – Agreed. Evolutionary change takes long periods of time. We’re essentially the same animals we were when mammoths existed and few lived in organized settlements with agriculture. It’s doubtful people lived past fifty in those times.

  19. anat says

    A dated online book about human (and some non-human) paleodemography: Between Zeus and the Salmon. Back then, based on estimated ages of skeletons plus actuarial modeling, some authors concluded that the average of the paleolithic populations in their sample, while life expectancy at birth was 23 years, chance of surviving to 45 was 17% and to 65 was 5%. And that of the total female population alive at any given point 10% were 45 and over, and 1% were 65 and over.

    There is also analysis of the economic contribution of individuals at various ages in modern foraging societies.

    Oh, and a quick quote regarding male ‘menopause’:

    A brief survey of the evidence may be useful. To begin with, we must operationalize “post reproductive.” In developed countries, the mean age at menopause ranges from 47 to 50 years, but menopause comes some years after the effective cessation of the ability to bear children. In noncontracepting agricultural populations, the mean age at last birth is usually in the range of 39 to 41 years (Bongaarts, 1983:124-127). The mean age at last birth for the forest dwelling Aché hunters and gatherers is slightly higher (42.1 years; Hill and Hurtado, 1996:254) and for the !Kung is substantially lower (35.4 years; Howell, 1979:130). Nonetheless, many females in noncontracepting populations continue to bear children after age 40, so it is preferable to use age 45 for present purposes. By this age, about 70 percent of couples are sterile (Bongaarts, 1983:126), although often the sterility of the couple is due to sterility of the male rather than the female.

  20. Allison says

    @Allison, #2: Quote:

    “What I don’t know is whether there are other species which have a maximum number of ova that a female individual can produce over their entire lifetime. Or are humans (or primates? or mammals?) unique in this way.”
    End of quote…

    Yes there are. Chickens for example….

    Now I wonder how widespread this is (among species.) Is this a general phenomenon? (With exceptions, I’m sure.)

    Anyway, assuming it is a widespread phenomenon, that would suggest that menopause simply is, and whatever evolutionary function it may have (or be a consequence of), if any, has nothing to do with any “function” in humans.

    That still leaves open the possibility that there might be some advantage to human populations for some members to live well past their prime reproductive years, maybe even enough to drive some sort of selection. But it probably doesn’t have anything to do with menopause, other than the fact that post-menopausal humans are older than pre-menopausal ones.

  21. chrislawson says

    Allison — I had a very superficial look at the literature. There are now five known species that experience menopause: humans, orcas, short-finned pilot whales, belugas, and narwhals. Of course, menopause is by definition limited to placentals, but more broadly speaking there are a wide variety of species that experience what is called “post-reproductive lifespans” including guppies, aphids, nematodes, and yeast. Interestingly, several non-human primates have postreproductive lifespans but are not known to go through menopause, which immediately suggests that there will be no simple evolutionary explanation.

    Also of interest: the whale species that show evidence of menopause are matrilineal. If one were to invoke the usual simplistic interpretations of the evopsych crowd, the obvious implication would be that humans must also be matrilineal by nature and therefore the patriarchal societies we observe are political monstrosities that violently oppress our preferred social structures and must be resisted! I won’t hold my breath waiting for them to apply their own logic consistently…

  22. says

    What about grandfather’s? They are too busy chasing fertile women to sire more offspring. After all there are plenty of spare grandmother’s to go around.

  23. says

    Yeah, that’s another thing that goes against this hypothesis : without contraception, a person who reaches menopause is very likely to still have very young children. Our childhoods are long. It takes about a decade and a half to get kids to the point where they can effectively sustain themselves.
    If menopause has an evolutionary advantage, getting your youngest children to adulthood because you have 15 years left after your last pregnancy is a much more direct advantage than increasing your grandchildren’s chances.
    It seems to me that much of the hypothesis is based on our current experience with grandmothers: women whose own children are grown, who were SAHM or are already retired because we are now spacing generations and who therefore have time to look after the children.
    Hell, in my mother’s generation of non existing public childcare and disdain for working mothers, being a working mum was only possible if a grandma was ready to do the child rearing.
    For which I am grateful. My grandma was good at it. My mum, not so much.
    Now, this has of course another set of modern assumptions, namely that the birth parent is the one responsible for the children, while it does not make sense to have 10 women individually look after their own kids when 2 or three people in a group can do that just as well, and it makes sense that the people who have troubles with hunting or foraging due to their age do this job. But that applies to all sexes. In that setting it is also irrelevant how many of the kids are yours or your descendants. You get fed because you contribute to the community.

  24. KG says

    chrislawson@23,

    No, no – the implication can’t possibly be correct: lobsters are well-known to be the only valid comparator species for human social arrangements.

  25. logicalcat says

    @10

    Did human females from way back then even start mentruating at the same age as modern females today?

  26. MadHatter says

    @29 logicalcat

    I was fairly sure that the average age of menstruation was much later for hunter-gatherer females as it often depends on nutrition (cant find the study right now). I had thought that there were historical records suggesting that there was a significant class difference well into the 18th century, aristocracy and girls from well-off families tended to reach it earlier (11-14) vs peasants or poor girls in the towns (closer to 16).

    Whether this relates at all to menopause I have no idea. Since childbirth was the number one cause of death for women until very recently in most places it would seem like menopause is more of a biological accident than an evolutionary contribution. I can see how having a larger kin-group would help ensure survival of your children, but as hunter-gatherer groups tend to be related and share child-rearing I’m not sure how grandmothers contribution would be so much greater than aunts or even older cousins.

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