Gene Expression

Menopause as an adaptation

I’ve talked about menopause before. One question in evolutionary anthropology is whether it is an adaptation, a derived trait in our species which emerged due to the force of natural selection, or simply a physiological byproduct of some other phenomenon. The key point is the peculiar asymmetry in male and female reproductive potentials; males decline gradually over time, while the general suite of female reproductive function simply shuts down at during middle age. Eric Michael Johnson reviews a new paper by the redoubtable Virpi Lummaa, Fitness benefits of prolonged post-reproductive lifespan in women. Eric concludes:

Dr. Lummaa has done just that in her study published in the journal Nature, demonstrating that children are 12% more likely to survive to adulthood when they have a grandmother’s support than when they don’t. That may not seem like a lot, but consider all of the descendants from that surviving 12%, each carrying the trait for reproductive senescence, and you can see how it wouldn’t take long for the trait to become fixed in a population. Furthermore, one of the key innovations of her study was her choice of sample set. By using Finnish records dating from the 18th and 19th centuries she could ensure that any modern health benefits wouldn’t influence the results and would therefore accurately pinpoint the grandmother’s role.

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). Seeing as to the fact that there are several models of Pygmy stature arguing that it is a product of life history changes, exploring this group’s physiological defaults might be interesting. A friend once told me that anthropologists report that Bantu populations regard Pygmy women as particularly fertile. Finally, one major consideration about menopause for me is that I was once told by a biological anthropologist who collaborated with some doctors in this area is that the process has many “moving parts” and seems to involve proactive shutting down a host of physiological processes. Whatever the fitness implications, the physiological tightness of menopause and the rarity of the phenomenon among animals suggests to me that the spandrel model should be viewed with skepticism.

Comments

  1. #1 hot flash
    October 13, 2009

    “males decline gradually over time, while the general suite of female reproductive function simply shuts down at during middle age”

    The female reproductive funtion also declines gradually over time starting around 35. The difference between men and women is that there is a common and finite end to a female’s reproductive life.

    Why we like to compare woman to animals in this sense I don’t know. The effort to birth and raise a human child is enormous compared to an animal. Why would anyone wonder why the ability to do so actually ceases when in fact the will to do so also ceases.

    Except for the media grabbing few, women don’t want to give birth to babies at 60 so why waste time and money working out why we don’t?

  2. #2 Larry Moran
    October 13, 2009

    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.

  3. #3 JL
    October 13, 2009

    It’s an old paper, published in 2004.

  4. #4 EMJ
    October 14, 2009

    @Larry Moran: I have responded to your critique of my and Razib’s support for the grandmother hypothesis at The Primate Diaries: http://scienceblogs.com/primatediaries/2009/10/reply_to_moran_the_adaptive_va.php

    @Razib: You don’t allow hotlinks? Dude, what gives?

  5. #5 razib
    October 14, 2009

    @Razib: You don’t allow hotlinks? Dude, what gives?

    if you’re talking about the SB site, i don’t control access to that. i think tim blocked hotlinking a long time ago. if you’re talking gnxp.com, yeah, i blocked it cuz of some malicious types who kept switching websites a few years ago (no need to go into details).

  6. #6 Michael Watts
    October 14, 2009

    I wonder what the effect is if you control for age of the parents? It strikes me as reasonable to believe that children without “a grandmother’s support” are likely to have older parents (possibly because this is a neat account of my immediate family history). As a related measure, what is the effect of parent age at birth on child survival?

  7. #7 Tony Jeremiah
    October 15, 2009

    The key point is the peculiar asymmetry in male and female reproductive potentials; males decline gradually over time, while the general suite of female reproductive function simply shuts down at during middle age.

    This does not seem all that peculiar given the asymmetry in having children.

    Though men and women contribute genetic material during sex, only women experience the birthing process. Assuming the birthing process involves additional risks (e.g., birthing complications) relative to sex alone, and these risks increase with maternal age, it would seem evolutionary advantageous for physiological mechanisms (i.e., menopause) to evolve that would stop reproductive capacity at some point (e.g., around middle age). Such a mechanism would seem beneficial to the long-term survival of mother and infant. The best evidence for this that comes to mind at the moment include: the tendency for women who have not had children to reach menopause sooner (suggesting the presence of a biological clock that is sped up when a woman does not have a child after a critical age), and women with the largest time gap between age of pubertal onset and age of having their first child increases risk of birthing complications. The current explanation for the latter finding, is that a larger time gap corresponds with increased exposure to the hormones estrogen and progesterone, which somehow impair uterine contractions during labor. This also seems to explain the finding of more cesarean sections with increased maternal age, and suggests there’s also an optimum start time for the biological clock (e.g., the age of pubertal onset) as well.

    …children are 12% more likely to survive to adulthood when they have a grandmother’s support than when they don’t…

    Is this finding supposed to explain how the physiological mechanisms associated with menopause evolved? If so, I’m having some difficulty grasping the connection. If it is not meant to explain how menopause itself evolved, then there are relatively more mundane (sociocultural) explanations for post-reproductive lifespan as suggested by Michael Watts’ questions.

    I wonder what the effect is if you control for age of the parents? It strikes me as reasonable to believe that children without “a grandmother’s support” are likely to have older parents (possibly because this is a neat account of my immediate family history). As a related measure, what is the effect of parent age at birth on child survival?

    Approximately 5% of the U.S. and Canadian population are from skipped generation families (i.e., children being raised by grandparents). These families occur among all ethnic groups, but is more frequent among ethnic minorities (e.g., African American, Hispanic, Aboriginals). I suspect part of the reason for the difference stems from higher teenage pregnancy rates, but also, from the more collectivistic mindset of minority groups relative to mainstream culture.

    So I suspect the grandmother effect would also be observed when we compare survival into adulthood rates of children in skipped generation families vs. children living with parents, especially if the parents are teenagers and/or had children during their early teens. (On a related side note: research indicates a tendency for puberty to start earlier in children who grow up in high conflict homes, and later for women who grow up in low conflict homes).

  8. #8 Mill Sbuchanan
    October 17, 2009

    Hormone replacement therapy (HRT) is the main treatment for menopausal symptoms. It relieves hot flushes and vaginal dryness, and reduces the chance of urinary infections. It can also help if you feel depressed. Try B-6 it helped with the hot flashes and the night sweats, it will make your urine dark almost orange, but it does help. Also good cardio workouts, they help with the mood swings.

  9. #9 Joe Mondo
    October 18, 2009

    @hotflash “Except for the media grabbing few, women don’t want to give birth to babies at 60 so why waste time and money working out why we don’t?”

    Because it’s the job of science to look at how things are and try to figure out how/why.

  10. #10 Aaron
    October 19, 2009

    @hotflash

    “Why we like to compare woman to animals in this sense I don’t know”

    It really isn’t a comparison since we are, in fact, animals. Our emotions and other aspects of us are controlled by things we don’t yet fully understand. We really don’t understand alot about ourselves, including basic instincts, emotions, sexuality and reproductive functions, personality, decision making processes, morality, ect.

    The will to have children may be an effect of menopause rather than an independant function, or it may be the result of cultural adaptation rahter than physical adaptaion.

    And clearly you have no passion for science whatsoever, asking why we spend money on learning new things that aren’t of immediate use is like asking why we went to the moon. And to be fair, we got teflon out of that. The potential for a vast growth in knowledge from any discover far outweighs the money spent on such research.