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Menopause trade-offs

Why do women, in contrast to our closest relatives, stop giving birth while they are still relatively young and healthy? This week's paper. "Testing Evolutionary Theories of Menopause", by Daryl Shanley and coauthors, published in Proceedings of the Royal Society, uses data from people living in The Gambia to test two different hypotheses.

Both hypotheses assume that human physiology has been shaped by natural selection to maximize the number of surviving descendants. (This seems to be the philosophy of fundamentalists of various religions as well, despite alternate interpretations of ancient religious texts.) As Darwin wrote:

I use the term Struggle for Existence in a large and metaphorical sense, including dependence of one being on another, and including (which is the more important) not only the life of the individual, but success in leaving progeny.

The two evolutionary hypotheses tested were:

1) if older women are more likely to die in childbirth, leaving their existing children at risk, then caring for existing children may be a better (DNA-programmed) strategy than bearing additional children.

2) grandmothers may enhance the survival of their grandchildren enough to more than make up for having more children of their own.

In this paper, they used data from The Gambia, collected from 1950 to 1975. (After that, improvements in medical care decreased infant mortality to levels no longer representative of most of our evolutionary history. )

Having a living mother increased survival of children under 2 more than tenfold. Having a living maternal grandmother increased survival twofold. Other relatives, including fathers, had no effect.

They used these data in a mathematical model simulating mothers of different ages, children, and grandmothers. If they assumed death in childbirth increased sharply with age, there was little net benefit (more children minus risk of existing children dying) in reproduction late in life, but no net cost. Combining the effect of mothers and grandmothers on survival to age 2 gave at most a slight benefit to menopause. However, if the effects of grandmothers on survival to age 15 were included, menopause at 55 was optimum (open circles in figure). Similar extensions of the maternal benefit had less effect, apparently because there are more children without grandmothers than without mothers.
There was some uncertainty in the estimates of the effects of having a mother or grandmother on survival, and even more uncertainty as to how dangerous childbirth at 60 would be (no data!). Data on chimps probably would not be helpful, as baby chimps have smaller heads. But maybe data for other human populations could be used to fit a curve that could then be extrapolated beyond the current range of human reproduction. I also wonder how relatively recent cultural advances, including agriculture and midwifery, affect lifespan, child-birth associated mortality, etc., even in hunter-gatherer societies.

This entry is dedicated to my mother in honor of her 80th birthday.


It's always great to see tests of the hypotheses that so often get dismissed as "adaptationist just-so stories". I'll add one more factor that I find interesting: the eggs are formed while the baby is still in the womb and held in suspendid meiosis for decades before they're used. Older eggs are more likely to have suffered things like non-disjunction, hence aneuploidies like Down's syndrome jump from fewer than 1:1000 where the mother is in her twenties, to more than 1:40 for mothers over 40. Sperm don't have a suspended meiosis phase, so paternal age is less of an issue. I'm not sure if there have been any studies indicating causation (which could, of course, go either way: perhaps the menopause means that there is little selection pressure for better meiosis checkpoints).

Another point to note is that other than briefly mentioning head size as a possible reason why death in childbirth is less of a pressure on chimps, your post doesn't really touch on this part of the question: "in contrast to our closest relatives..." One could speculate on a variety of just-so-stories – most obviously our extended period of vulnerability and reliance on others when young – to connect the dots and explain why this is a particularly human issue.

Thanks for the comments. Extended dependency is mentioned in the article. "Human infants are unable to provision themselves to any realistic degree before age 5, and even then the period of nutritional dependency may last until puberty (Kaplan 1996; Bogin 1997)." Both the similarities (even to bacteria) and differences (even to other apes) of humans and other species are interesting.

No doubt this is a very interesting piece of finding.

Few observations though!

Using accurate and updated data would bear more significance to your research- between 1950-1975 is such an awful period of time compared to now. Lots of things would have changed by now.

Secondly, why is your population sample only Gambia? Gambia only represents a minute amount of the entire population of the world and i also perceive that with different races brings different results in this hypothesis.

Concluding, these research is flawed by the reasons mentioned above.

Interesting that having a living grandmother increased survival rates by two. The nurturing provided by a more experienced, maternal figure (although not the primary maternal figure) is obviously important and that extra support probably helps the mother meet her child's dependency needs.

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