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Tuesday, October 13, 2009

The Adaptive Value of Menopause

One of the problems with adaptationist just-so stories is that they often sound so plausible that everyone just assumes they must be true and stops thinking critically. One of the classic examples is the adaptationist explanation of menopause.

The "Grandmother Hypothesis" says that menopause arose in primitive hominids because it prevented pregnancies in older women thereby freeing them to assist in the care of their grandchildren. This extra care had a significant effect on the survivability of the grandchildren thereby increasing the probability that the frequency of the menopause allele would increase in the population.

Over the course of thousands of generations, the allele for menopause became fixed in our ancestral populations because it conferred a significant adaptive advantage.

Razib Khan believes in this adaptationist explanation for menopause. He quotes from a recent study that seems. on the surface, to lend support to the idea. See the recent posting on Gene Expression: Menopause as an adaptation.

The study by Virpi Lummaa looked at the presumed benefit of grandmothers among Finnish families. She reports that children with the support of a grandmother are 12% more likely to survive than children without such support.

The prompts Razib to write.
12% is a very big effect and would lead to rapid evolutionary change (on the order of thousands of years in the most simple population genetic model of a single locus of dominant effect).
I posted a comment on his blog. I'm reproducing it here in order to get more feedback.

12% is, indeed, a very large effect but what does it have to do with evolution?

We're looking at a study where every single woman underwent menopause so one of the things we certainly aren't doing is testing to see whether menopause has an effect on the survivability of grandchildren.

Let's think about reasons why some families have grandmothers to help out and some don't. First, there's the relative proximity of living grandmothers. Then there's the question of the relationship between parents and grandparents. Let's not forget possible financial help that has nothing to do with direct caregiving. Finally, there's the issue of whether a family even has a surviving grandmother.

None of theses things are affected in any way by the fertility or non-fertility of the grandmother, right? The 12% difference has nothing to do with menopause.

Now let's think about a time in the past when menopause was presumably evolving. You had two kinds of females in the population, those who underwent menopause and those who didn't. There will still be all kinds of families who experience the help of a grandmother irrespective of whether she can still have kids or not. That's the background that a presumed adaptation has to deal with.

If there are non-menopausal women who live into their fifties, are close to their grandchildren, and whose husband is dead, then they will presumably help raise thier grandchildren. Same thing applies to non-menopausal women who simply don't risk getting pregnant any more even though their spouse is alive. (Just say "no." It's probably more common than you think. Women are not stupid.)

In fact, the adaptationist just-so story only really applies to that small subset of women who have the following characteristics.
  1. They live past 50 years old.
  2. They have grandchildren who are young and still need care.
  3. They don't have too many grandchildren in different families so their caregiving can be effective.
  4. They live near their grandchildren and can help out.
  5. They have a good relationship with their children and their spouses.
  6. Their husbands are still living.
  7. They choose to get pregnant.

That's the only group that menopause affects. It eliminates #7 but has no effect on any of the other factors. It certainly doesn't have any effect on whether the grandmother was dead or alive at 50 years old.

Given that there were many families that received no help from grandmothers, whether they had the menopause allele or not, and given that there were many families who received help even if the grandmothers did not have the menopause allele, the question is "what is the adaptive value of menopause under those circumstances."

What does the Lummaa study have to say about that?

Since you are a supporter of this adaptationist explanation can you describe for me the kind of society where you think this allele became fixed in the population? Was it a hunter-gather society of small bands or a large agricultural society of small towns? Or something else?

I'd like to hear more details about how this grandmother hypothesis actually worked in Australopithicus or Homo erectus societies. Please include your estimate of how many grandmothers survived past the age where menopause could make a difference as you estimate the fitness coefficient.

[Image Credit: "Rudyard Kipling’s illustration for The Elephant’s Child from Just So Stories (1902)." From Encyclopedia Britannica


Ian said...

Obviously I've misunderstood the whole debate. I thought the question was "how did life span evolve past menopause" (since women who survive their childbearing years tend to outlive men, despite the fact that men remain potentially reproductive into old age), not "why did menopause evolve"?

Adrian said...

I wonder what the populations were like when menopause was fixed, in particular I wonder about the life expectancy. In addition to death from disease, parasites and predators, women were also relatively likely to die during childbirth. How many women survived until menopause?

I also wonder about the structure of these societies. Today, we can accumulate wealth which we can spend in our old age to provide care for ourselves, our children and grandchildren. Would primitive societies be the same or would the elderly become a net drain on the assets of the family, requiring more energy than they could bring in?

aluchko said...

"2. They have grandchildren who are young and still need care."

Well if they're going to pass on their genes than grandchildren, or other young relatives, are very helpful. I don't know what age they'd typically reproduce but I think it's almost certain there'd be children of an age where they could offer assistance.

"3. They don't have too many grandchildren in different families so their caregiving can be effective."

True but I think it's a safe assumption that for most of human evolution the population was relatively stable so this number shouldn't be too large.

"4. They live near their grandchildren and can help out."

Again a bit of an assumption but for the tribal environment this was probably a fairly common scenario.

"5. They have a good relationship with their children and their spouses."

Probably very typical.

"6. Their husbands are still living."

That's assuming a monogamous culture.

"7. They choose to get pregnant."

Assuming that it's their choice and that there aren't other benefits to vaginal intercourse for menopausal women.

There's also other health effects of menopause but I don't really know enough to say if they're a net cost or benefit.

Still those assumptions you list all seem to be fairly safe assumptions to make. And they only serve to reduce the amount of selective pressure, not eliminate it.

Anonymous said...

The funny thing about 'just-so stories' is that they are nonadaptive. They are about serendipitous events that produce outcomes unrelated to function. While they are historical (ultimate causation) explanations, they're not adaptationist. If any set of explanations should be called just-so stories it's nonadaptive evolutionary models - that is, ones that invoke genetic drift, developmental constraint, contigency and so on.

Frank said...

I recall reading in "Darwin's Ghost" that the daughters of British peers who were born to fathers in their 50s had a life expectancy about a year less than that of those born to fathers in their 30s. This is most likely an effect of mutational load.

Now consider women, who have a greater investment than men in offspring's survivability (our species may be R-limited for men and K-limited for women, in ecological terms). A mechanism that front-loads reproduction would have correspondingly greater advantages for women's reproductive success.

Now, why don't women simply die off earlier than men? In fact, until the recent past, they did. I was at an early 19th century graveyard a few weeks ago. There were a large number of 50ish women lying next to their septagenarian husbands. In other words, their lifespan didn't go very much longer than their reproductive capacity, notwithstanding deaths in childbirth.

I don't doubt the existence of spandrels but think you've chosen a fairly poor example here.

Anonymous said...

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Unknown said...

Not to support this particular just-so story but there are more options. If pre-human society was like chimp society we do see support for grandchildren when grandma is childless and competition from grandma when she is not. Suppressing the fertility of your own offspring ought to have rather heavy consequences for females (not for alpha males)
Don't know of evidence for "just say no" in other primates so that whimsy was as bad as most just-so stories.

DiscoveredJoys said...

I wonder if the menopause is evolutionarily determined as a 'primary' fact to be explained, or is it just a byproduct of limiting the number of babies a woman can have and rear successfully?

I remember reading about reproductive investment. The point was there there evolutionary forces optimising the number of offspring. Too many and a greater proportion of offspring would die, due to lack of caring resources. Too few and others would outbreed the couple (or whatever) - contributing 'more offspring' genes to the gene pool.

One of the ways in which the optimum number of offspring could be 'selected' by evolution could be for genes to limit the woman's period of useful fertility.

Using guesstimates as an example, it could be that the optimum number of babies (on average, of course) in a hunter/gatherer type of life is around 8 live births, with around 4 surviving to adulthood. From these notional figures, if you assume that the average time between conception, birth, and end of weaning is around 3 years then a woman's fertility needs to extend to around 22 years. If she starts conceiving around age 14, then she needs to be reasonably fertile until around age 36. After this age it would be evolutionarily beneficial for her to stop having children. I understand that fertility does start to decline in modern women around this age.

One of the ways to reduce fertility to the optimum age range would be for genetic variation and natural selection to ensure just enough 'good' eggs to cover this period. The menopause could be just a later consequence of this selected reproductive strategy; it would probably be beneficial for the mother to continue to live and raise the later children to adulthood.

Of course I have speculated about a single cause for a biological fact. You know and I know that there will be many different genetic, environmental and social pressures all interacting. There may well be a 'granny effect', but I expect it is not the root cause for the menopause.

Rich Lawler said...

In terms of its evolution, I believe the argument is that menopause can spread in a population since it allows the interbirth interval (IBI) of daughters to be shortened (since grandma can take care of their daughter's kids), AND--and this is the "iffy" part--that children born to older women (i.e., grandmothers at age 50) are less viable than those born to women at age 25. Hence, the trade-off is: an "average" IBI in mothers + less viable offspring in grandmothers VERSUS a reduced IBI in mothers (i.e., more kids per time) but no offspring in grandmothers. At least that's what I recall.

Alan Rogers modeled the evolution of menopause in an article in evolutionary ecology back in the early 90's. I think his model showed that it could work in principle, but it was highly dependent on how it was parameterized. Something like that...

Iktomi said...

i agree with discoveredjoys. i am no scientist but i always thought that nature just "cuts off" reproduction due to the stress pregnancy/birth/motherhood causes to an older person. physical stress inhibits viability of offspring anyway. (animals who are stressed do not conceive, or miscarry.) perhaps menopause is just nature's way of avoiding the catastrophe of reproducing after the body is stressed due to age.
also, simply the shorter life span would make more sense than the "grandmother theory." if a person was likely to die at any time past age 50, it wouldn't make sense for women to continue having babies, because that might lead to orphans. (since women are traditionally the caretakers of the young.) whereas men can still father children, because physically the children are not dependent on the father after conception. if a woman has a child at 25, she needs at least 10, probably more, years until the child can survive on its own. and while other relatives can care for the child if the mother dies before then, it's advantageous to have the mother care for her children as long as possible. if women were dying at age 60 or so, then it would be advantageous for them to stop reproducing at 45-50, so that they could care for their last child until at least age 10.

Eric Michael Johnson said...

Larry - I have responded to your critique of my and Razib's support for the grandmother hypothesis at The Primate Diaries: Reply to Moran: The Grandmother Hypothesis of Menopause

gsw said...

Menopause has certainly helped women to stay alive but, like grey hair, arrives to late to have any effect on our progeny.

Please note: The phenomenon of women living long enough to actually enjoy the menopause is still relatively new.

We need some statistics - is it happening to women earlier or later the say 100 years ago?

What effect does HRT have on all this?

Still too little data for a theory.

Larry Moran said...

el cid says,

Don't know of evidence for "just say no" in other primates so that whimsy was as bad as most just-so stories.

Good point, although I wasn't really making up a just-so story. I was trying to raise questions about the prevailing explanation.

There's a lot we don't know about how our ancestors really behaved but that doesn't seem to stop people from making assumptions and basing their science on those assumptions.

We don't know whether our ancestors were monogamous and we don't know whether elderly females even cared about their grandchildren. We don't know whether grandfathers played a role. For all we know, small tribes could have been dominated by elderly males who were fiercely protective of their grandchildren and did everything they could to help them.

Perhaps these males preferred to mate with younger females while, at the same time, preventing any other male from mating with the elderly grandmother.

It's hard to see how menopause could be a significant benefit it that kind of society.

The point is not that my scenario is more likely than any other. The point is that adaptationists make a lot of assumptions when creating their just-so stories.

Corneel said...

Ian raised an excellent point at the beginning of the thread, that everybody ignores but I think is worth repeating.
Menopause could easily evolve if women didn't live beyond 50 years in prehistoric societies (very likely), because of genetic drift.
The real question is why female fertility is abolished at this age, when longevity is not.

You can make up lots of explanations, even non-adaptive ones. For example: No mutations existed that decreased late-age survival without deleterious effects early in life. Therefore, these did not acumulate in the human genome. The point is: You will need to make assumptions for non-adaptive explanations too. There does not exist a most parsimonious explanation here.

Ms. Crazy Pants said...

1) Some other animals have menopause
2) Some of those animals are social animals and some are not
3) In some animals that have menopause, the menopausal animal does not care for the young, either their own or grandchildren
4) In some menopausal animals, the animal dies soon after menopause

Just because humans grandparents sometimes (not always) care for their grandchildren does not necessarily show a reason for menopause to have evolved.

Bryan said...

"I'd like to hear more details about how this grandmother hypothesis actually worked in Australopithicus or Homo erectus societies"

Or, for that matter, paleolithic homo sapiens (which, if I remember, lived on average to their mid-30s).

In reality, women who lived to menopause (or at least the age where we see menopause today) were an abnormality until the last few thousand years.

It seems more likely to me that menopause is simply a product of there being nearly no selection in women over 40 until very recently (due to an absence of over-40 year old women).

From that point of view menopause isn't a selected trait, but rather is simply what happens when womens biology runs for longer than it did in our evolutionary past. Basically "old" parts wearing out.


Unknown said...

Ms. Crazy Pants said...
1) Some other animals have menopause
2) Some of those animals are social animals and some are not

I found non human primate menopause very useful.

It would be nice to moderate the adaptationist stories to refinements on the timing of menopause. And rather than an advantage of having grandma, I do wonder about the advantage of not having mommy competing with daughter #1 when both have babes, more on the infantacide theme. I agree social memes are major variables with simplistic notions of "motherly" behavior requiring support.

daedalus2u said...

We know the average number of children that each female had who survived and reproduced over evolutionary time was two. The same for each male. No more, and no less. If the number was even slightly greater than two, the population would have expanded exponentially and reached levels we know did not happen. If it was slightly less than two then humans would have gone extinct.

Is there any evidence for a “menopause allele”?

What is the “default” phenotype without a “menopause allele”?

Which is the more difficult evolutionary task, evolving for a long life (i.e. delaying the senescence of every organ system), or accelerating the senescence of a single organ system (i.e. menopause)?

I think it is more likely that the evolved “feature” is the delayed senescence of all organ systems with the senescence of the female reproductive system not keeping up with the rest because there is little benefit to female reproduction after age 45, not because there is a benefit for female infertility after age 45.

Larry Moran said...

Corneel says,

There does not exist a most parsimonious explanation here.

The only scientific answer to the question of menopause is that we don't have an answer.

We don't know when it evolved in our lineage and we don't know how it evolved. We don't even know which genes are involved.

You are correct to point out that we shouldn't mistake parsimony for truth. Sometimes the "best" answer is simply "we don't know" even though it's not nearly as satisfying as pretending we do know.

Ms. Crazy Pants said...

"And rather than an advantage of having grandma, I do wonder about the advantage of not having mommy competing with daughter #1 when both have babes"

I was thinking about guppies and menopause. I think guppies have menopause as well and there isn't any caring about babies going on there. I'm not sure if there is cross-over though between mother and daughter laying eggs.

The Other Jim said...

Slight tangent, but relevant. Search and replace field names at will...

Volume 63 Issue 10, Pages 2487 - 2490
Rasmus Nielsen


Gould and Lewontin's 30-year-old critique of adaptionism fundamentally changed the discourse of evolutionary biology. However, with the influx of new ideas and scientific traditions from genomics into evolutionary biology, the old adaptionist controversies are being recycled in a new context. The insight gained by evolutionary biologists, that functional differences cannot be equated to adaptive changes, has at times not been appreciated by the genomics community. In this comment, I argue that even in the presence of both functional data and evidence for selection from DNA sequence data, it is still difficult to construct strong arguments in favor of adaptation. However, despite the difficulties in establishing scientific arguments in favor of specific historic evolutionary events, there is still much to learn about evolution from genomic data.

daedalus2u said...

I suspect the main advantage of elderly members of a tribe was for the knowledge they had. Before writing, the only way to store information was in the memories of living individuals. There is a Discovery article that discusses this in the context of knowing which foods were ok to eat in the aftermath of a cyclone that destroyed other food sources.

This might even provide a reason for old people to become frail; a frail person is not a threat to a younger person and won’t compete for mates.

Anonymous said...

I want to mention two "models" that tried to see whether postreproductive females can provide sufficient help to their younger kin to offset the reproduction they forfeited when menopause occurred.

A model by Hill and Hurtado (1991) found that postreproductive women did not provide enough assistance.

Another approach, used by Rogers in 1993, showed the postmenopausal women "would have had to double the number of children that ALL of her children had, as well as completely eliminate infant deaths" for menopause to have been adaptive.

This was from Austad's 1997 book: "Between Zeus and Salmon"