“I’ve had enough of you children.” – Mother Nature
To many women, menopause is a dreaded accompaniment of age. Yet so natural to our state of being is it that we scarcely realise that it need not necessarily be so. We may go our entire lives without even realising that there is question lurking here: “Why do women undergo menopause?”
Some authorities have argued that the phenomenon does not, in fact, require a special explanation. A women is only born with a certain number of follicles (egg cell plus surrounding support cells) in her ovary, from which several (6-12) are chosen each month to produce oestrogen. Halfway through the monthly cycle, the egg from the dominant follicle is expelled and launched towards where a hopeful sperm cell might arrive, while the other follicles degenerate. The following month, the same process starts again, with the same result. Thus there is a continual loss of egg cells, and since the ovaries, unlike the testes, don’t produce new egg cells, eventually the number of eggs in the basket must eventually run out. This usually happens somewhere between 45 and 55 years of age, and the symptoms are readily explicable – infertility (since no eggs are expelled) and symptoms of oestrogen deficiency (hot flushes, mood instability, dryness and thinning of the lower urinary and genital tracts, osteoporosis, etc.). So women simply suffer the menopause because they run out of ovarian follicles as they age and use them up.
But to argue like this misses the point. Every aspect of the design is up for grabs – why should we simply accept that in a woman, no new egg cells can be created, whereas a man suffers no such threat to his posterity? Or even if there is some abstruse reason for this, why could women not be equipped with, say, 93 billion egg cells – enough that they’d never run short? What exactly is evolution doing here? On first glance, it would seem an utterly ruinous evolutionary strategy to be programmed to abandon all hope of producing descendents at a certain age. Men seem to have ‘realised’ this, after all – a man’s fertility simply fades gradually as the years march on, in line with the general deterioration in all bodily functions that accompanies senescence. Apparently, the oldest female to give birth was 63, yet men can regularly impregnate women at this age, and there are numerous reports of men in their 90s fathering children. Another puzzling aspect of the whole process is that menopause is confined to the tiniest minority of species – there is controversy, but according to the strict definition of menopause I favour, only one other species suffers it: the short-finned pilot whale! Even if the definition is expanded, less than 0.001% of species that could experience the phenomenon do so.
There are a few contenders to explain menopause, but in my view there is one theory that comfortably stands head and shoulders above the rest, and it is brilliant. It’s sometimes called the “grandmother theory”. The first step is to understand that while there are obvious evolutionary benefits, there is also a cost (or perhaps more accurately a risk) to bearing and giving birth to a child. In this distinctly unnatural modern world of antibiotics, ultrasound, science-based health carers, pharmacological cures, and caesarian sections, there is still a greater than a 1% chance of the average woman dying from any single childbirth in many countries[1]. This rate must have been enormously higher in our (evolutionary) past. Clearly if the woman dies, the chances of her creating more offspring are zero. Furthermore, we are a species with an incredibly long dependency on our parents, a point I was reminded of by Jared Diamond’s superb essay on this topic[2]. In his own words,
[H]uman hunter-gatherers acquire most food with tools (digging sticks, nets, spears), prepare it with other tools (knives, pounders, huskers), and then cook it in a fire made by still other tools. Furthermore, they use tools to protect themselves against dangerous predators, unlike other prey animals, which use teeth and strong muscles. Making and wielding all those tools are completely beyond the manual dexterity and mental ability of young children. Tool use and toolmaking are transmitted not just by imitation but also by language, which takes over a decade for a child to master. As a result, human children in most societies do not become capable of economic independence until their teens or twenties.
The awful consequence of this is that if a mother should die as a result of pregnancy, she may well inadvertently kill off some of her other children too, should they not have reached the age where they can look after themselves. Worse still, the mother doesn’t have to die to effect these changes; any pregnancy-induced pathology that severely limits her activities of daily living will do just as well. Strokes, necrotising infections and kidney diseases are among pregnancy’s legacies that, while not killing the mother, will kill her children just the same.
The next step is to understand other ways to be evolutionarily successful without giving birth and raising children. Here it gets interesting – recall that evolution cares about genes, not individuals. Genes make bodies only in order to propagate themselves, recalling the saying that “a chicken is the egg’s way of making another chicken”. From an evolutionary point of view, creating offspring is a very productive way of benefiting the genes, since exactly 50% of your genes are present in each child. (This is the highest possible degree of relatedness that two individuals can share in a sexually reproducing species like our own, apart from identical twins.) But looking after a child is only a special case of helping one’s own genes replicate. Your genes will be present in your children’s children too: your grandchildren each carry an average of a quarter of your genes. Thus, another way of helping my genes would be to help my grandchildren survive. Similarly, a percentage of my genes will also be present in other relatives: brothers, cousins, and the like. Theoretically, I could expend whatever ‘parental investment’ (food, skills-training, shelter, etc.) I would have given to my child on my grandchild instead. Usually however, this would still be a bad strategy, since they only carry half as many of my genes as a child would, and I only have a fixed quantity of ‘parental investment’ to spend (there are only so many hours in a day, or so much food I can gather, etc.). Thus, all things being equal, a child is ‘worth’ more to me than a grandchild, but we have at least been shown another, less potent way of being evolutionarily successful. This will come in handy later.
The final step in our chain of reasoning is to understand how these variables change with age. Perhaps the most vital statistic is that the risk of childbirth increases tremendously as the years go by. For instance, a study in Saudi Arabia[3] showed that the maternal mortality rate increased by a factor of 10 between pregnant women less than twenty years old and pregnant women older than forty. Older pregnant women are at increased risk of the lethal pregnancy-induced hypertension, tears to the birth canal, and postpartum haemorrhage, to name but a few. All this is exacerbated by the fact that, even when controlled for age, the evolutionary risk of pregnancy would often still increase. This is because even with the risk to her kept constant, she is putting more at stake – all her young children – each time she falls pregnant again. And then to rub salt in the wound, a child of an older woman is also far less likely to survive – there is a higher incidence of virtually every obstetric complication, including miscarriage and congenital abnormalities. Thus, the average gain in falling pregnant is also diminished in an older woman.
So, with all the pieces at hand, how does nature put the puzzle together? Basically, the idea is this. When the woman is young, nature ‘permits’ her to fall pregnant. At this age, the costs of pregnancy are relatively low and the benefits are simply too juicily seductive (a high yield propagation: a child) to ignore[4]. But with age, the risks of pregnancy increase for the genes, and the benefits decrease. For many years, the net equation will still favour having children, since on average the genes will be most benefited this way. However, there comes a point when the risks of pregnancy to our female ancestors’ genes plus the lure of alternative methods of evolutionary success outweigh the benefits of further progeny. Now, genes for shutting down the fertilisation mechanism at this point would tend to prosper, since they would out-compete genes that were making bodies indefinitely fertile. Via this brilliant switch, menopause actually becomes evolutionarily desirable. To paraphrase Diamond’s words, menopause comes about because a woman’s apparently counterproductive evolutionary strategy of making fewer surviving gene copies via children actually results in her making more copies overall. “Evidently, as a woman ages, she can do more to increase the number of people bearing her genes by devoting herself to her existing children, her potential grandchildren, and her other relatives than by producing yet another child.”
What an ingenious argument! We can even now tie up loose ends. Why don’t men get menopause? The answer is fairly obvious given the preceding litany of mortal potholes in the pregnancy road: men don’t give birth. More accurately, men bear very little intrinsic cost in making a child: they don’t grow a baby for nine months from food that went into their own mouths, they don’t risk dying during childbirth, and they usually provide less for the baby after its birth too. Seeing that the woman inevitably carries most of the obligatory costs of pregnancy, a man’s genes never face the scenario where the risks of pregnancy outweigh its benefits. It virtually always pays a man’s genes to keep fathering sons (with younger women if necessary), rather than to forgo this bountiful method of genetic replication and concentrate exclusively on other watered down methods of evolutionary success.
And why don’t most other female animals have menopause? There are at least two cogent reasons for this. Firstly, most other animals don’t have nearly as high a risk of mortality in childbirth. One reason for this is that our big brains are so evolutionarily favoured that our bodies delay labour until the last possible moment to allow our brains more time to grow. The obvious downside to this is mechanical: in around 1% of cases, nature gets it a little wrong, and the baby’s head doesn’t fit out the mother’s pelvis. Often, the result is the worst of possible consequences for the mother – death – and this must have been especially true in the days before caesarian sections. The second reason why animals don’t usually get menopause is that there are often no viable alternatives to childbirth available to them. Clearly, if I plan to assist my genes by helping out my grandchildren and cousins, et al., I will be stymied if they’re not around. Many animals do not live in family groups as large and interconnected as we do. Solitary animals like the sloth have to continue to produce children or else exit the game altogether, whereas we have other options.
That all these factors are taken into consideration may boggle the mind unaccustomed to evolutionary methods. Yet in many respects, the Blind Watchmaker that Darwin discovered has it easy – a variety of options are randomly thrown up in the form of mutations, and the best of these tends to prevail, no matter how complicated the reason for its success. It is left to us to wipe away the dust hiding our evolutionary past and marvel at life’s grandeur.
[1] According to estimates from WHO, UNICEF and UNFPA in 2000, Sierra Leone had the highest maternal mortality rate (2000 per 100 000), followed closely by Afghanistan (1900 per 100 000).
[2] http://discovermagazine.com/1996/jul/whywomenchange817
[3] http://www.kfshrc.edu.sa/annals/154/94181/94181.html
[4] Actually, this must obviously be true of any successful species at some stage – at least some individuals must reproduce, for if there were no reproduction, not only would there be no offspring, but there would also be no extended family to help out as an alternative strategy!
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