There was an interesting article in the Guardian a couple of days ago about the biological reasons for menopause. I have not read the studies that this story is based on, so commenters with more knowledge of this stuff please feel free to correct anything that I have misconstrued.
Human females are one of three species currently known to undergo menopause at a relatively young age compared to average lifespan. The other two are pilot and killer whales. Female chimpanzees usually lose their fertility late in life as do other more closely related species to humans than killer whales. This leaves the reasons behind menopause wide open to speculation.
My preferred theory is the Grandmother theory: a woman in a subsistence environment has more chance of having her children survive into adulthood if she has older women to help her i.e. it takes a village to raise a child. Thus women surviving past their childbearing years has benefits beyond those inferred by fertility.
The other theory is that men have influenced human female biology by consistently choosing younger mates so that older women lost their fertility as it wasn’t being used. This theory neatly centres men in the evolutionary cycle. But it doesn’t explain why women still live so long after menopause when other species simply die. If older women are that useless surely we would just die off? I also like how this places human men as special snowflakes in evolutionary terms. In other species the male is a sperm donor and either buggers off soon after, is eaten, or dies. But not the human male, he decides the destiny of the human race.*
The theory of men influencing menopause makes a couple of assumptions that I find interesting. Firstly it assumes that menopausal women aren’t sexually attractive to anyone. Human sexuality is one of those fascinating things that I don’t think can be tied down to theories like this. I’m thinking that if you are a beta guy who wants a bed mate then an older woman is probably going to be a good option for you. Or you’re a bloke who is just looking to have a bit of fun with someone. It isn’t like all menopausal women immediately lose interest in sex or the ability to have sex. Effectively for the male selection theory to work all older women would have had to stop having any sex ever for menopause to become a evolutionary reality. I’m not sure that there would have been enough comely young women to ensure that every bloke who wanted a woman as a partner/shag/friend with benefits to ensure that outcome.
It also ignores the agency of women in choosing their partners. The common assumption is that blokes did all the choosing but do we know this to be absolute across all humans through time? Would young women necessarily choose older men? This theory seems to presuppose girls being married off to men to secure family alliances etc. Again, I’m not sure that this was going on for long enough to cause evolutionary change.
SotBO: yes this is tongue in cheek.
Categories: Life, medicine, skepticism
Working Mum it’s all your fault too. 
Hoyden About Town 
Maybe it’s just that the more children you give birth to, the greater the toll on the female body and the higher the risk of dying in childbirth. Stopping women from having infinite numbers of babies ensures they don’t die and are unable to care for their other children.
The time that it takes offspring to reach maturity may be a factor. Humans do take a long time to grow up and would be disadvantaged if their mothers died of old age before the process was complete.
According to a doco on kangaroos I saw years ago, older mothers were more likely to have male offspring (which require maternal care for a shorter timeframe). Female joeys stay with their mothers in the mob and learn mothering skills over several years. Apparently male joeys are also more common in times of drought and scarcity.
The original study is Morton RA, Stone JR, Singh RS (2013) Mate Choice and the Origin of Menopause. PLoS Comput Biol 9(6): e1003092. doi:10.1371/journal.pcbi.1003092
It’s a modeling paper, and assuming that the model and its parameters are reasonable (I’m not competent to check this with any reasonable time investment), they’re showing that male preference for younger mates is sufficient to account for menopause rather than older women simply dying at menopausal age. It seems though, that they leave open the question that sprang to mind when I read the post, which is where in turn the assumed prior male preference for younger mates arises from. (I recall Belle Waring, many years ago, pointing out that you can come up with a very plausible just-so story about why people with testes should prefer mates who’ve birthed one child already, namely that they have demonstrated their ability to survive our species’ dangerous childbirth where young slim nulliparas have not. It’s not a perfectly obvious immutable fact that people with testes prefer younger mates.)
This isn’t necessarily a criticism of the paper — it’s not fair to criticise it for not fufilling what my aims would have been if I’d been studying this — but I’d be much less inclined to treat male preference for younger mates as an assumption. If nothing else, there’s still the question about why either (a) humans would have this preference when other species do not thus (possibly) leading to human menopause or (b) why this effect (might) exist in many species but leads to menopause in humans specifically.
The main problem I see with this idea is that the process of natural selection is a great driver for evolving new traits/features, but doesn’t provide as much pressure for removing them. Things don’t go away just because you aren’t using them anymore. Many species have features that are of no particular use to them, but are a relic of their ancestry (whales have finger and toe bones for example). A feature only tends to go away if it is detrimental (or becomes detrimental). And here’s the thing – it needs to be detrimental not to the individual but to the prospects of the individual passing on their genes. If a woman who has already reproduced stops using her fertility later in life (for whatever reason) it will not go away unless continuing to possess it is detrimental to the survival of her children.
Humans tend to live longer than other apes. Its conceivable (pun intended) that we retain the programming for a chimp-like number of eggs, but just live longer after we’ve run out.
As with so much in human biology, it could just be an accident. These days in the West, a typical cis woman will lose her fertility twenty or thirty years before death, but then a typical woman is an incredibly strong and healthy specimen with access to modern medicine, good nutrition and no natural predators. Historically, although it has been genetically possible for women to survive for long periods after menopause (and for men to survive long after the quality of sperm has begun to deteriorate), it is not at all clear that many did.
For example, it wouldn’t be right to imagine that modern Amazonian tribes people exactly represent our ancestors, but their life expectancies tend to sit in the early 40s. Similarly, in Victorian London, although a lucky person could live to 90, it averaged out around 42, even with the many advantages of civilisation. Add a few sabre-tooth tigers, render folk more vulnerable to unseasonable weather changes and very few people are going to make old bones.
Point is, perhaps humans are just more successful at living for a long time in the modern age. The fluke of surviving well into middle age has become a nearly universal expectation. And that gives society a huge advantage having all these experienced folk about whose energies aren’t consumed with caring for small children.
What happens to chimp-like young whose mothers die when they are still younger than they would normally be when their mothers stop caring for them? Are they cared for by other chimps?
I wonder if mothers who go through menopause have offspring with better outcomes because stopping giving birth allows the mother to concentrate on the offspring she already has.
The Goldfish – those very low life expectancies are largely due to high infant mortality. Most people who managed to survive the various perils of life in the Middle Ages or Victorian times would have had a reasonable chance of making it to 60 or thereabouts. For most of human history a woman who didn’t die of childbirth or mischance would likely have enjoyed about ten years of menopause (assuming it kicked in around the same time in the past as it does now).
What angharad said. Life span has been c. 70, barring accidents, disease, etc. E.g. the biblical “three score years and ten.” That wasn’t just a figure of speech, based on available evidence.
Then, biology: the large heads of human infants plus our upright posture places huge (huge!) mechanical stresses on the body during pregnancy and delivery. Connective tissue needs to be both strong and flexible, and it just isn’t good enough after about fifty. Without modern medicine, pregnancy around that age is an almost-certain death sentence.
As for the question of why have old people — and it applies to men just as much as women, even though male authors never seem to wonder why useless old guys who aren’t attractive, who can’t throw a spear or fight a lion are kept alive — anyway, where was I? Why have old people around. They’re the store of knowledge, the *only* store of knowledge during most of human history. In a world without wikipedia, without books, without records of any kind, who is there who knows what to do when a once-in-a-lifetime flood happens? How to fix the travois your mother left behind? And so on through a thousand examples. As a species whose whole approach to survival is thinking and memory, making sure old people are around is a real “Duh!” Much more chance of survival if each new generation doesn’t have to, um, reinvent the wheel.
Speaking of species that keep useless old types around – there’s evidence that Smilodons did just that. One of the fossils from Rancho La Brea shows osteo-I-forget-the-name in the pelvis, the result of a severe and long-lasting infection. Masses of the bone has been eaten away, and that would have taken months, at least. It would also have meant the sabretooth wouldn’t have been able to hunt. The conclusion is that zie was still getting food as part of the pack (pride?).
KITTEH POWER
(Sources: the fount of all knowledge, BBC docos.)
Angharad: thank you. I get sooooo tired of the “people were old at forty!” meme. It simply isn’t so.
@TKUP – I wonder if that is partly because we look at people who are 40 who don’t have the skin care regime, access to sufficient food, water etc that many westerners do and see ‘old looking’ wrinkled faces and assume that therefore their bodies must be as aged as their faces look to us?
I can’t imagine why men think they are so important that even female biology is caused by them.
Really. This has got to stop. How can the idea a woman is useless unless desired by a man still be getting credit?
I go with the personal survival strategy theory.
eilish, I’ve read articles (such as this one http://www.livescience.com/28537-ideal-penis-size-depends-on-height.html ) which suggests that penis size in males may have to do with female preferences.
It is interesting to note that both studies suggest that traits that are presently considered attractive would of course be selected for throughout all of human history, despite there surely being limited evidence of this.
Calculating historical life-expectancy is an immensely complicated field, for something that seems it should be straightforward. The Goldfish’s prediction of 42 in Victorian London does probably account for infant mortality, but probably only for ‘infants’ (under 1). (I should note Victorian Britain sees a dramatic increase in life-expectancy over the period, so this is complicated by that as well!). You need to take out the deaths for at least under 5s, and perhaps under 10, before you start to get the big jumps into the 50s and 60s. It’s weird science for this period really as basically for every year you live up to about age 30, you increase your chances of living longer past age 60. But it does make that ‘average’ number for life-expectancy in some respects meaningless, because the more people you take out of the calculation the less it reflects your real chances of getting to old age. The interesting thing is though that the potential to live to about 70 is always there; it’s just whether you get cut down by sickness, accident etc before you get there (ie people aren’t dying of old age at 40).
The relationship between the ‘signs’ of aging (wrinkles, grey hair etc) and life-expectancy is quite interesting. The few studies done on this now say that the relationships isn’t statistically significant, ie more grey hair at a younger age doesn’t mean you’ll die sooner (although no grey hair is associated with a longer life expectancy). However, historically, people used your physical appearance as a meaningful measure of age, especially in a period before birth certificates and where people weren’t as worried about counting age accurately. So, someone who looked old was assumed to BE old and also it was assumed they would die sooner. We can see this in some 19thC legal sentencing where people who looked older were given shorter sentencing because the judge assumed they would die soon, even when we know that they weren’t much older in years than other people given longer sentences. Because we can’t follow this up, it’s hard to know whether their predictions of death were true. What we can say is that poor people were often thought to look more aged than their older counterparts and they did have a shorter life-expectancy on average, so it may be that the signs of aging here are actually evidences of poor health (like TB or malnutrition). More generally, it seems physical appearance played a much larger role in how people ‘read’ your age, abilities, need for support, levels of wisdom, etc, than might be the case today. In some ways, this might be more equitable, because you didn’t have to reach a certain calender age to become ‘pensionable’ (before state pensions this is primarily poor law and charity support in UK) if your body was telling a different story.
I had a weird encounter with this sort of misreading not long ago. I was assumed to be my son’s grandmother, I’m guessing because I was using a mobility scooter. He’s 11 (and big for his age), and I’m in my forties with no wrinkles or visible grey hair.
I hope that doesn’t happen again Lauredhel, sounds very discomforting.
But I think you are right. Society seems to equate aging with disability or loss of ability, therefore if you are using a scooter you are elderly and obviously just use botox/hairdye/good genes to look younger.