Pause for thought

Most wild animals remain fertile until they die. So do most human males. But human females undergo a steep decline in fertility from about the age of 40, leading to complete sterility within a decade or so.

To lay people, menopause is an inevitable fact of life, albeit often a painful one, anticipated with foreboding. But to evolutionary biologists, human female menopause is an aberration and an intellectual paradox. The essence of natural selection is that it promotes genes for traits that increase the number of your descendants bearing those genes. How could natural selection result in every female member of a species carrying genes which throttle her ability to leave more descendants? All biological traits are subject to genetic variation. Once female menopause became fixed in humans, why was its moment of onset not gradually pushed back until it disappeared again? After all, those women who experienced the menopause later could have left behind more descendants.

Many biologists dismiss this as an issue because they regard female menopause as an artefact of the recent increase in human life span. Of course, the female reprod-uctive tract was programmed to shut down by the age of 40, they argue, because it would not have had the opportunity to operate thereafter anyway. The increase in human life span has developed much too recently in our evolutionary history for the female reproductive tract to have had time to adapt.

However, this view ignores the fact that the male reproductive tract, and every other biological function of both women and men, continue to function in most people for many decades after 40. We would therefore have to assume that every other biological function was able to adapt quickly to our new life span, leaving female reproduction uniquely incapable of doing so.

An evolutionary account of human female menopause must explain how a woman's apparently counter-productive strategy of making fewer babies could actually result in her making more babies. Evidently, as a woman ages, she can do more to increase the number of descendants bearing her genes by devoting herself to her existing children and her potential grandchildren than by producing another child.

This evolutionary chain of reasoning rests on several facts. One is the human child's long period of parental dependence. Hence a hunter-gatherer mother who already had several children risked losing some of her genetic investment in them if she did not survive until the youngest was at least a teenager. Another, crueler, fact underlying menopause is that the birth of each child jeopardises a mother's previous children because of the mother's risk of death in childbirth. In most other animal species, that risk is insignificant. For example, in one study encompassing 401 pregnant female rhesus macaques, only one died in childbirth. For humans in traditional societies, the risk was much higher and increased with age. Even in affluent, 20th century western societies, the risk of dying in childbirth is seven times higher for a mother over the age of 40 than for a 20-year-old mother. Each new child puts the mother's life at risk not only because of the immediate risk of death in childbirth but also because of the delayed risk of death related to exhaustion from lactation, carrying a young child around, and working harder to feed more mouths.

Furthermore, infants of older mothers are less likely to survive or to be healthy because of age-related increases in the risks of stillbirth and genetic defects. For example, the risk of a foetus carrying the gentic condition Down's syndrome increases with the mother's age-from one in 2,000 births for a mother under 30, to one in 300 for a mother between the ages of 35 and 39, one in 50 for a 43-year-old mother, and the grim odds of one in 10 for a mother in her late 40s.

In effect, the older mother is taking on more risk for less potential gain. That is why menopause is logical, resulting in women giving birth to fewer children but ending up with more survivors. Natural selection has not programmed menopause into men because men never die in childbirth, rarely die while copulating, and are less likely than mothers to exhaust themselves caring for infants.

A hypothetically non-menopausal old woman who died in childbirth would be throwing away even more than her investment in her previous children. That is because a woman's children eventually begin producing children of their own; and those grandchildren count as part of the woman's genetic investment. So less is more-and menopause turns out to be one of the traits which contributes to making us distinctively human.