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.