The Grandmother Hypothesis
February 28, 2019 3:27 PM   Subscribe

Killer whales, Japanese aphids and Homo sapiens — they're among the few organisms whose females live on long past the age of reproduction. Since the name of the evolutionary game is survival and reproduction, the phenomenon begs explanation — why live longer than you can reproduce? In the 1960s, researchers came up with the "grandmother hypothesis" to explain the human side of things. The hypothesis is that the help of grandmothers enables mothers to have more children. So women who had the genetic makeup for longer living would ultimately have more grandchildren carrying their longevity genes. (Sorry, grandfathers, you're not included in this picture.)

The first hard evidence for the grandmother hypothesis was gathered by Kristen Hawkes, an anthropologist at the University of Utah who was studying the Hadza people, a group of hunter-gatherers in northern Tanzania. Hawkes was struck by "how productive these old ladies were" at foraging for food, and she later documented how their help allowed mothers to have more children.

If our long post-reproductive lives evolved because of grandmothers, we should be able to find fingerprints of the benefits of grandmothering in many cultures. But the circumstances of modern life differ drastically from those we faced at the beginning of our evolutionary story.

The studies in Current Biology turned to the detailed records of two preindustrial populations, one in what is now Quebec and the other in Finland. The researchers mined these rich databases to quantify the reproductive boost that grandmothers provide and to help us better understand the limits of their help.

From NPR's Goats and Sodas newsletter. See also a Canadian take on the research, StatNew's take on the two new studies in the FPP, and Limits to Fitness Benefits of Prolonged Post-reproductive Lifespan in Women (February 2019).

(CW: female genital mutilation) In other grandma news, the potential power of grandmothers is the basis for The Grandmothers Project in Senegal. The project attempts to convince grandmothers of the dangers of female genital mutilation so they can stop supporting the practice and stop it instead. Thus far the results seem promising.
posted by Bella Donna (30 comments total) 20 users marked this as a favorite
 
I can kind of see a similar argument for the killer whales, but aphids?
posted by Joe in Australia at 3:30 PM on February 28, 2019 [2 favorites]


As I understand it, even non reproducing members of a community can "count" in some of these communally wholistic explanations of evolutionary advantage. Though I am not clear on the putative rigorousness of these kinds of analyses (not saying they're bad, just not personally familiar with their general rigorousness).
posted by kalessin at 3:33 PM on February 28, 2019


It's really interesting that the hypothesis is supported in agricultural communities, since Hawkes was mainly looking at hunter-gatherers. So it's not just that grandma is a good forager.

(Well, not my grandmother. She lived on coffee and coffeecake, and her idea of Kool-Aid when kids visited was water added to Jello mix.)
posted by zompist at 3:46 PM on February 28, 2019 [5 favorites]


Having a pool of free, otherwise unengaged child care that can tend to small children and allow their more vigourous mothers back into the field to tend crops would pretty much convey the same sort of benefit, I suspect, zompist. My mother couldn't farm for shit at her age but she'd probably murder a nun to have all day access to her grandkids.
posted by Jilder at 3:52 PM on February 28, 2019 [9 favorites]


Given the rightful (as I understand it; am not a scientist) skepticism regarding much research in evolutionary biology, I don't know what any of this means except that, on balance, nearby grandmothers are often handy for their families. Did anyone besides me find the StatNews piece kind of whiny?
posted by Bella Donna at 3:52 PM on February 28, 2019 [2 favorites]


A more extreme and sci-fi version of the idea:
One of the biggest moral bombshells handed to Billy by the Tralfamadorians, incidentally, had to do with sex on Earth. They said their flying-saucer crews had identified no fewer than seven sexes on Earth, each essential to reproduction. Again: Billy couldn't possibly imagine what five of those seven sexes had to do with the making of a baby, since they were sexually active only in the fourth dimension.

The Tralfamadorians tried to give Billy clues that would help him imagine sex in the invisible dimension. They told him that there could be no Earthling babies without male homosexuals. There could be babies without female homosexuals. There couldn't be babies without women over sixty-five years old. There could be babies without men over sixty-five. There couldn't be babies without other babies who had lived an hour or less after birth. And so on.

It was gibberish to Billy.

-Kurt Vonnegut, Jr. in Slaughterhouse Five (1969)
posted by Spathe Cadet at 4:06 PM on February 28, 2019 [18 favorites]


"Live long? 'Hell, we should have died in the big war! " --Grandpa Abehammerb

Did other people have different grandpa experiences? He was a hoot.
posted by Abehammerb Lincoln at 4:13 PM on February 28, 2019 [1 favorite]


I feel like I’ve seen this noted before on MF, but every time I’ve seen a “fertility goddess statue” it always looks like a woman who’s been through menopause. Like did the archeologists not know any post-menopausal women, or...?

Seriously, google search that image. Maybe older women were just running shit.
posted by schadenfrau at 4:37 PM on February 28, 2019 [19 favorites]


Aphids produce honeydew and some ants "farm" aphids to harvest the honeydew. It seem unlikely that aphids developed honeydew for the ant's benefit (though mutualism is a strange thing) so perhaps post-reproductive aphids somehow benefit their genetically identical sisters through their honeydew production? I have not found anything to back this up, but maybe someone here knows more about it.
posted by sjswitzer at 4:48 PM on February 28, 2019 [1 favorite]


I can kind of see a similar argument for the killer whales, but aphids?
Just be thankful it's not lobsters.
posted by L.P. Hatecraft at 4:55 PM on February 28, 2019 [4 favorites]


A recent article in Science News suggests that there are limiting factors on the GM effect: the age of the grandmother and distance. Interesting stuff.
posted by k8bot at 4:56 PM on February 28, 2019


(It occurred to me that aphids are basically tribbles. "... tribbles use over 50% of their metabolism for reproduction and ... they are born pregnant." Just like aphids.)
posted by sjswitzer at 5:14 PM on February 28, 2019 [3 favorites]


Best thing about all this rain? No aphids in the garden. Neat factoid about their lifestyle though.
posted by Homo neanderthalensis at 5:17 PM on February 28, 2019 [3 favorites]


her idea of Kool-Aid when kids visited was water added to Jello mix.

Somehow (well, I know how but it's a long story) my Grandma came into many, many cartons of cotton candy mix. She figured since it was basically sugar and flavoring, it was the same as Kool-Aid. Reader, it was not.

Without my Grandpa, who would have gotten my nose?
posted by The Underpants Monster at 5:30 PM on February 28, 2019 [9 favorites]


Isn't this "eusociality"? Only know about this theory from the science-fiction novel "Coalescent".
posted by jkaczor at 5:56 PM on February 28, 2019 [1 favorite]


Eusociality is one of the evolutionary arguments for gay people too. Gay uncles weren’t vying for mates or siring offspring (as much), but were still able to hunt and gather for the family, helping boost clan children’s survival.

Not sure where the theory stands, these days, though. It may have been debunked thoroughly by now. The grandmother thing seems pretty plausible, though.

posted by darkstar at 6:52 PM on February 28, 2019 [6 favorites]


..not my grandmother. She lived on coffee and coffeecake, and her idea of Kool-Aid when kids visited was water added to Jello mix.

au contraire, that is strong evidence for excellence at dumpstering. celebrate her strength, and find it within you.
posted by mwhybark at 7:24 PM on February 28, 2019 [1 favorite]


I feel like I’ve seen this noted before on MF, but every time I’ve seen a “fertility goddess statue” it always looks like a woman who’s been through menopause. Like did the archeologists not know any post-menopausal women, or...?

A recent episode of the No Such Thing as a Fish podcast made a case for pre-plough societies being matriarchal, and all of their important gods being female, but when societies adopted the plough, which was hard enough work that only men could do, suddenly all of a socity's important gods became male.
posted by Space Coyote at 8:20 PM on February 28, 2019 [4 favorites]


Given the rightful (as I understand it; am not a scientist) skepticism regarding much research in evolutionary biology

I don't think evolutionary biology in general has quite the same poor reputation that evolutionary psychology does.
posted by atoxyl at 9:05 PM on February 28, 2019 [10 favorites]


Though it seems a little weird to me that these articles keep framing it as "why do women survive past menopause" instead of of "why does menopause as a distinct transition in middle age exist?"
posted by atoxyl at 9:12 PM on February 28, 2019 [15 favorites]


So women who had the genetic makeup for longer living would ultimately have more grandchildren carrying their longevity genes. (Sorry, grandfathers, you're not included in this picture.)

I can see why natural selection favours grandmothers' longevity to facilitate their daughters' genetic output. Is that the same reason that grandfathers exist? To co-invest in their offspring's reproductive output?

If all children benefit from the grandmother effect, then male children also live longer when there aren't [apparently] a lot of reasons for them to do so. Do men live long past evolutionary usefulness as byproducts of The Grandmothers longevity natural selection?
posted by honey-barbara at 10:36 PM on February 28, 2019


I feel like I’ve seen this noted before on MF, but every time I’ve seen a “fertility goddess statue” it always looks like a woman who’s been through menopause. Like did the archeologists not know any post-menopausal women, or...?

Seriously, google search that image. Maybe older women were just running shit.
posted by schadenfrau


I think you're really onto something, schadefrau.

One study suggests that 15% of post-menopausal Orca continue to lactate, and there are reports that !Kung women continue lactating through menopause, as well.

I said something similar in a thread about nursing back in 2012:
Humans are one of only three species known to experience menopause rather than remaining fertile up until near the end of their lifespan, the others being killer whales and pilot whales, and orca have been observed to continue nursing calves for most of a decade into their menopause, so I've been wondering whether menopause in humans might not have served to produce a corps of ready and willing wet nurses-- which could have been all the more critical as our rapidly expanded brains raised the level of maternal mortality at birth.

Perhaps the venuses of Willendorf and Dolní Věstonice could be seen as celebrations of a cadre of nurses as well as simple primary fertility.
but back then, I couldn't find any reports of women continuing to lactate after menopause. And since then Narwhals have been added to the roster of mammals that experience menopause.
posted by jamjam at 10:43 PM on February 28, 2019 [8 favorites]


If all children benefit from the grandmother effect, then male children also live longer when there aren't [apparently] a lot of reasons for them to do so. Do men live long past evolutionary usefulness as byproducts of The Grandmothers longevity natural selection?

Probably. Either that or since men can keep reproducing more or less until death that there is a single age limit for humans, it's just that men just didn't get the gene to shutdown reproduction during their middle age.
posted by jmauro at 1:55 AM on March 1, 2019


If all children benefit from the grandmother effect, then male children also live longer when there aren't [apparently] a lot of reasons for them to do so. Do men live long past evolutionary usefulness as byproducts of The Grandmothers longevity natural selection?

Probably. If it's biologically advantageous for women, it's carried on the X chromosome, which means it's hard not to drag us freeloading XY's into old age, short of something on the Y chromosome specifically selecting for shorter lifespans.

Also, anecdotally, my experience with grandmas versus grandpas vis-a-vis their helpfulness in child-rearing suggests that this phenomenon is approximately 10000% coded female.
posted by Mayor West at 4:21 AM on March 1, 2019 [1 favorite]


If all children benefit from the grandmother effect, then male children also live longer when there aren't [apparently] a lot of reasons for them to do so. Do men live long past evolutionary usefulness as byproducts of The Grandmothers longevity natural selection?

This question loses sight of the original "problem." Organisms are "evolutionarily useful" as long as they can have offspring.

The initial assertion is basically that it's surprising women live so long instead of dropping dead at menopause. There's not this puzzle with men: Someone who can live to 70 instead of dying at 50 could easily have one or two more kids; living to 70 in males has some selection pressure in favor of it.

The hypothesis is interesting and the follow up studies are neat, but to my (non-biologist eye) they may be in the "assume every trait is an adaptation" realm.

The extreme non-adaptive explanation could be as simple as our increased life span is a side effect of the changes that led to our prolonged juvenile state, and in between this being a relatively recent change and life expectancy seldom approaching theoretical life span there has been no strong evolutionary pressure against it, let alone sexually differentiated pressure.

I really like atoxyl's take on the question, not "why do women live so long" but "why is there menopause?" which kind of flips it around. Though if it's an adaptation in humans, as opposed to non-adaptive trait, the grandmother hypothesis would absolutely be relevant. FWIW having googled chimpanzee and bonobo menopause it seems this may be a fuzzy area.
posted by mark k at 7:16 AM on March 1, 2019 [2 favorites]


Eusociality is one of the evolutionary arguments for gay people too. Gay uncles weren’t vying for mates or siring offspring (as much), but were still able to hunt and gather for the family, helping boost clan children’s survival.

I am actually currently working on a manuscript on evolutionary discussion of why is same-sex sexual behavior in animals so common, but the short answer in humans is that this does not particularly seem to be the case in our species; cis gay men are generally not any more likely to look after their nieces and nephews than cis straight men are. Here's a downloadable copy of VanderLaan et al. 2015 which has a pretty good review of that work in its introduction; basically, one highly-studied Samoan cultural category of transfeminine MAAB people does display more avuncularity, but androphilic cis men from a variety of other tested cultures including a number of Western countries and also Japan don't.

Generally speaking, humans are not eusocial although we are cooperative breeders with a monogamous history (which is the background eusociality always, as far as we can tell, seems to evolve from); humans who look after and help raise offspring of relatives or siblings are not necessarily much less likely to go on to produce offspring of their own after dispersal, which is the definition of a cooperatively breeding species rather than a eusocial one. Whether or not gay people are necessarily much less likely to reproduce than completely heterosexual people is kind of debatable depending on cultural context and how you define "gay" on a Kinsey scale kind of basis, and also based on whether you lump trans* people into the same category as gay or not, which is a pretty common feature of this kind of theorizing.

posted by sciatrix at 8:59 AM on March 1, 2019 [11 favorites]


Though if it's an adaptation in humans, as opposed to non-adaptive trait, the grandmother hypothesis would absolutely be relevant.

Yes - in fact I've seen it framed as an answer to the version of the question I asked.

My point was basically just that framing it in terms of longevity makes it seem like we have a clear answer to why menopause happens, which I'm not sure is true beyond the observation that menstruation and pregnancy are pretty resource-intensive if you don't need them.

Whereas - are there any examples of a 30 percent difference in lifespan by sex in mammals? I have no idea, to be honest. It just feels a little weird.

Though - not to get too morbid but if women were giving birth in their 60s and 70s, before the advent of modern medicine, it would probably cut a number of years off average lifespan in practice, if not maximum lifespan. So it all kind of converges back to the idea that the advantage to your descendants of you being around declines more slowly than the advantage of actually being fertile, when the costs of being fertile are high.
posted by atoxyl at 10:57 AM on March 1, 2019 [1 favorite]


(Should make it clear that this is idle speculation from a guy who read a few articles about something at some point - it's not my field, unless biology expertise is, uh, genetic.)
posted by atoxyl at 11:04 AM on March 1, 2019


The phenomenon (of women living past their reproductive age) only requires explanation if you think that natural selection is extremely strong and purifying.

Here's a thing that people don't necessarily realize about natural selection -- the *efficacy* of natural selection is equal to the effective population size times the strength of selection. For most of our existence on the planet, humans have had an incredibly small effective population size relative to many other kinds of organisms. Furthermore, there's nothing about living past reproductive age that prevents you from reproducing, you know? Living past reproductive age is definitely not detrimental to fitness. So the strength of selection can't be particularly strong. (by contrast, bacteria have incredibly large effective population sizes, and so even very mild selection pressures can be extraordinarily efficacious.)

atoxyl, I think you're barking up the right tree. Here's another thing that's different about humans relative to most other mammals -- we menstruate. This Forbes article captures a particularly good Quora answer about the evolution of periods. The long and short of it is that human females take a really hard hit to bring infants to term. Maybe menopause evolved just to help us avoid being killed by our own fetuses once we're a little too old to safely give birth? Maybe menopause didn't evolve in other mammals because there's no harm in being pregnant up until the point they die of old age? Though actually I can't really think of how there would be selection pressures for that, either; if you've already reproduced, you've passed on your genes, and your fitness is only going to be a function of how many of your offspring survive to reproduce. Anyhow, wouldn't grandpas be good, too? How come men don't survive past reproductive age? (their reproductive age lasting so very much longer, of course.)

One of my major beefs with the Grandmother Hypothesis for menopause has been that the ways researchers test for it are evo-psychological. For example, a study that we read in my grad evolution and ecology course hypothesized that grandmothers should be more beneficial for the female children of their daughters than for the male children of their daughters, because they're more likely to share more genetic relatedness with female children of their daughters (b/c of passing on x-chromosomes). Or something like that. At any rate, they had a bunch of different datasets from very different time periods, and while the US and Gambian datasets supported that hypothesis, the Japanese dataset contradicted it. The authors explained that contradictory result by appealing to cultural mores, saying that sons were more highly valued in Japan than daughters, and thus the cultural values over-rode the evolutionary imperatives. If evolutionary imperatives are so fragile that they can be overridden by culture, why are they strong enough to cause lasting reproductive history changes in the species?

Here's what I wanna be clear about -- Evo psych is super dumb even when it says things that we think sound nice. Furthermore, it trades on biological essentialism about sex and gender and sexuality, which I think is the point that sciatrix was making in an extremely polite and professional way.

------
Since I am an entomologist who specializes in hemipterans, I had to look up the Japanese aphids thing. Here's a description of their post reproductive females' grandmotherly roles in society:

Moving beyond vertebrates, Quadrartus yoshinomiyai, a Japanese gall-forming aphid has a high representation of post-reproductive females with an important role in the success of their kin group. In Q. yoshinomiyai, a clonal colony grows inside a sealed gall. When the gall is opened to allow dispersal, the risk that a predator will enter the gall increases. The colony defends itself against this by having a significant proportion of its adult females break down their reproductive organs and instead fill their abdominal cavities with enlarged wax glands. These post-reproductive females position themselves near the gall entrance with their abdomens filled with a sticky wax. When a predator attempts to enter the gall, post-reproductive aphids attack to cover the predator with wax, often killing themselves in the process. Experimental removal of these defenders leads to predators more successfully entering and hunting inside. Gall defense (most often by pre-reproductive nymphs or non-reproductive soldiers) has been shown to improve selective outcomes for the clonal group in several aphid species. Source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5088610/

By contrast, humans build cities. What can we possibly hope to say about human society by reference to aphids? Those are some bad ass grandmas, though.
posted by Made of Star Stuff at 2:56 PM on March 1, 2019 [9 favorites]


And it must be stressed that through most of their reproductive cycle, aphids reproduce parthenogenetically, so the "granddaughters" the aphid "grandmothers" are are protecting are genetically themselves (clones).
posted by sjswitzer at 3:25 PM on March 1, 2019 [3 favorites]


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