I understand estrus–from a survival standpoint it makes sense to control the timing and frequency of pregnancy and births. But I don’t understand how menstruation evolved. Using humans as an example (although most of these apply to all menstruating species):
Every time your body sheds the endometrium (lining to the uterus) it has to replace it. Not a big deal today when there is, more or less, unlimited food, but those extra calories would be a burden when resources are tight. (I note that species with estrous cycles instead of menstruation reabsorb the endometrium. Though that doesn’t explain why it wouldn’t be better to just leave it there all the time.)
Meanwhile–you’re bleeding! Predators can smell you and track the blood trail. In fact, it still happens every now and then to women who like to camp in bear country or swim in shark-infested waters.
Besides violent death, bleeding can also lead to anemia or dehydration. These are not things that help your survival.
Cramps. Personally, without the use of prescription meds I would be down for the count the first day of my period. So if it were 2 million years ago, that would be a day I couldn’t spend hunting or gathering–and thus might spend starving to death. I’d also be in no position to protect my offspring from wandering saver-tooth tigers.
Is that enough? I mean, this menstruation thing seems like a deathwish. How did evolution let this one slip through? And don’t give me any malarkey about how females in species with menstruation can have sex all the time and keep the males around better. Yes, that is a survival advantage, but in no way is shedding the endometrial layer of the uterus a requirement for that kind of behavior. Or if it is, why?
The idea that sharks target menstruating women is an urban legend and has long since been debunked. I suspect that the same is true of bears but I would be open to changing my mind if you have any evidenc whatsoever to support the idea.
The point to realise here is that animals are incredibly smelly creatures at the best of times. They not only stink but even prey species such as cats, rats and deer go around deliberately scent marking their territory. Blood is actually fairly odourless compared to the normal bodily secretions of mammals. As a result I doubt if the tiny amount of blood lost through menstutaion would increase human scent by a measurable amount.
Anemia aggravated by menstruation can be a problem in modern societies but is unheard of amongst hunter-hatherers. HGs invariably have diets very rich in vertebrate food which supplies more than enough iron to replace the amount lost in menstruation. You need to appreciate here is that humans evolved as HGs and have only been farmers for a blink of an eye, far to short a period for it to have any evolutionary effect.
In humans, at least, women do stop menstruating when resources are tight. That’s why women who have been starving or having nutritional problems stop menstruating, even if they’re young.
Blake, thanks for both the answer here and the extended answer you gave to the earlier question–but I’m afraid neither I nor Charles Darwin are satisfied. Imagine two women, identical in every respect expect that one sheds her endotrium monthly and the other one reabsorbs it. Which one seems like they would do better in the survival game? The thing about menstruation is that it evolved from a different system and then spread. If you believe in evolution then you have to believe that it was superior in some way; you can’t use the “because there is no detriment to it existing” argument. So why is it superior?
And on the subject of animals eating people :-), predators usually have keen ability to detect blood so that they can track injured prey. As an example, many–if not all–shark species become aggressive when exposed to blood (that’s where we get the phrase “smelled blood in the water”).
Okay, let’s not have any flame wars about evolution. I quote Wikipedia (Wikipedia: Evolution):
The basic mechanisms that produce evolutionary change are natural selection (which includes ecological, sexual, and kin selection) and genetic drift; these two mechanisms act on the genetic variation created by mutation, genetic recombination and gene flow. Natural selection is the process by which individual organisms with favorable traits are more likely to survive and reproduce. If those traits are heritable, they are passed to the organisms’ offspring, with the result that beneficial heritable traits become more common in the next generation.
Can a negative trait that appears in one member of a population spread to the entire population? Yes, when that trait goes hand-in-hand with an even nmore positive trait–see humans, standing upright, and lower back pain. But sprinkle on millions of generations and the negative aspects of the trait will slowly fall away as natural selection favors those individuals whose alleles result in fewer negative consequences.
So, if humans were the only species that experienced menstruation I might buy the argument that only 7 million years (max) of environmental pressure hadn’t been long enough to eliminate the negative aspects of menstruation (cramps, anemia, bleeding)–assuming you could adequately explain what positive traits menstruation is directly (i.e. genetically) tied to. Simply because humans have differences of sexual behavior with some other species doesn’t mean that the estrus/menstruation difference is genetically linked. However, menstruation is common to all (I believe) great apes, and that branch of the genetic tree is about 40 millions years old. That’s too long for negative traits to sit around undisturbed by the invisible hand of environmental pressure.
So, if you want to give me an evolutionary explanation for menstruation you need demonstate either: 1) that endometrial shedding itself is actually an advantageous trait such that the advantages outweigh the negatives, 2) that the negative aspects of the menstral cycle have actually been decreasing over time but haven’t had sufficient generations to breed out entirely, or 3) that endometrial shedding actually has no negative aspects (and then explain to me how cramps, bleeding, and anemia are not negative).
A final note: even today, with knowledge of argriculture, herding, refridgeration, and transportation firmly within our grasp, 60 million people–1% of the human population–starve to death annually. So for animal species and early humans without those tools, any bit of extra nutrition your body required could be the difference between life and death.
I’m afraid I have to differ with regard to Blake’s point about hunter-gatherers never becoming anemic from lack of vertebrate food. The evidence is indirect, but compelling. A mutation occurred about 20-30,000 years ago in the Celts which allowed people to harvest all the iron possible from a very iron-poor diet. The Celts at that time were not limited to the British Isles, but lived across an arc from Ireland to Germany and points south and east of there. The mutation has proved possible to track back to one small location of origin.
The mutation, which was accidental, spread very rapidly, consistent with its conferring a survival advantage. It remained a point mutation in a singe gene, which is why it is easy to diagnose today. Compare to Wilson’s Disease, the disease that causes a person to retain copper; this is a very rare disease, and all its mutations are random; there is no screening tool because there are so many different mutations that cause it; the only way to screen for it is if a patient has been diagnosed by recognition of the Kayser-Fleischer rings or other physical signs, one can then screen that person’s kindred for carriage of the same mutation. That is a typical presentation for a truly random gene carrying no survival advantage.
As many as one in four people who are of Irish or German descent carry the iron-sucking gene, and one in 200 Celts (a very high number for a genetic disease) are sufferers. The disease is named hereditary hemochromatosis. If you ilve in an environment where there is a great deal of vertebrate food and iron available, and you have HH, you will die from it.
I understand but have not myself evaluated the evidence that the skeletons of large and small mammals over the same 10,000 year period are scarce, consistent with a very difficult period for European humans.
This genetic evidence strongly suggests that there have been long periods when there was not enough vertebrate flesh available in hunter-gatherer diets to provide enough iron.
Why do we menstruate? I don’t know. Shedding the endometrial lining has been proposed as one method of combating disease. It is true that in mature females, gonorrhea and some other sexually transmitted bacterial diseases tend to colonize the vagina and/or the Fallopian tubes, but do not get a foothold in the uterus; whereas in sexually molested girls who have not yet begun to menstruate, gonorrhea colonizes the uterus as well.
My WAG is that the ‘current system’ of bleeding cramps ect. give a increased chance of conception, which overall the chance of more conceptions outweighs the downside. It’s a trade off like anything else.
If you’re considering the evolution of menstration it’s important to realize that most fertile females probably got pregnant fairly often and were pregnant a good deal of the time. So it’s not like most females menstruated like clockwork for years and years punctuated by 1-4 pregnancies during their lives as is the case in the majority of instances today.
Remember Edith Bunker telling Archie that her Catholic friend called menstration a blessing?
Most animals only copulate when the female has a high likelyhood of getting pregnant. That is when the female goes into estrus. The male can tell she’s in estrus because he can smell the difference. Females “know” they’re in estrus because they allow themselves to be bred. Humans don’t have noses that good, and females are receptive any day of the month.
What is early homo sapien to do? How does a male know when he is wasting his seed by copulating with females that have not yet reached puberty or are pregnant or in such a poor nutritional state that they are not going to concieve? He can’t smell a female in estrus, but he does have excellent vision and can see all colors in the spectrum. How about some unsubtle visual cue that a female is of appropriate age and can be expected to be fertile? Menstration does that.
The problem with this line of reasoning is that it presupposes that there were ever humans who did not menstruate. For such a mutation to win out, it has to have been present at some point, and there’s no guarantee that it ever was. Is there any evidence that it was? That there are other animals who do not menstruate doesn’t imply that there was ever a human or direct human ancestor who did not. Maybe that mutation only occurred in other species, or the carrier was unlucky enough to get wiped out for other reasons before breeding.
Unlikely. There are plenty of clues besides bleeding that a woman is in her fertile years.
Besides, menstruation evloved long before the genus *Homo *appeared on the scene. I know that the other great apes menstruate, but do any other primates or other mammals?
I don’t think there is any question as to whether there was an ancestor of modern humans that did not, in fact, menstruate. There’s a very good chance the last ancestor to not menstruate might have even been a mammal.
A mutation such as menstruation need not confer any advatange just by the virtue of existing, however the OP detailed specific disadvantages and that necessitates an explanation.
If I understand physiology correctly, and I hope I do, menstruation is probably the worst time to copulate, so I sincerely hope it’s not a misplaced signal. Using it as a signal of puberty is also not particularly good reasoning since although this would work, I do not see what advantage it confers at all. I really don’t think any mammal males will sit around waiting for the first period as a sexual charactiristic.
The rough lifestyles of apes in general might require a signal for a female that she is pregnant. Not menstruating is very noticeable signal and might prompt a change in behavior that would increase the chances of carrying the baby to term.
Aren’t you confusing menstruation with estrus? I’m still not convinced that non-apes menstruate. Does anyone have a cite? Wikipedia says they don’t, but I’m not sure I trust them on that point, although they tend to be pretty good on science topics.
I didn’t imply non-apes menstruate. I was being somewhat sarcastic by “might have been a mammal”. As far as I remember college bio apes are the only ones who menstruate.
Some of the side effects of the menstrual cycle can be reduced with lifestyle changes. Getting regular exercise reduces bloating (through sweating and resultant salt loss) and reportedly reduces the severity of cramping. Information on female health that I’ve read mentions that maintaining or even increasing your level of activity helps reduce the symptoms of PMS. Obviously, getting enough exercise was never a problem for our ancestors.
Severe cramping is thought to be caused by prostaglandin imbalances. Fatty foods, especially those with partially hydrogenated fats, are known to contribute to the symptoms. Large swings in blood sugar cause problems too. Eating foods with a low glycemic index value can help you avoid rapid changes in blood sugar. Again, eating “healthy” by eating unprocessed high-fiber, low-carbohydrate food with a blend of fats (with low or no trans fats) wasn’t really an option for humans in the past; it was the only way to eat.
Onset of first menses is later in many traditional societies than it is in the developed world, due mostly to diet. They don’t have 9 year olds with periods (typical is between 14 and 17) and people get married/start reproducing pretty young anyway. Getting pregnant will do away with that pesky bleeding problem too, and as other posters have pointed out, women didn’t have all that many periods in between pregnancies and nursing a lot of the time. It’s likely that women had fewer side effects from their menstrual cycles than modern women simply because of lifestyle differences. (Of course, even if they did have problems they probably had other concerns that were more pressing.)
So really, because in the past there were probably fewer actual problems due to menstruation than now, it wasn’t that huge of a detriment. As Blake pointed out in the other thread: it doesn’t have to work perfectly, it just has to work well enough. If you want to pick on other reproductive flaws, the tradeoff between big heads and an upright posture with a necessarily narrow pelvis means that a certain number of women die in childbirth and our young take a long time relative to other species to develop. Possible death and having to put years into child care are pretty big things. Definitely worse than losing a bit of blood and dealing with pain (sometimes even significant pain) and discomfort for a few days a month.
My original post was never meant to imply that there can’t be any negative consequences to positive mutations. In your example, the disadvantages of the big head/small pelvis combo are obviously outweighed by the increased chance of survival gained by high intelligence and free hands. But if humans’ large heads were filled with air and we walked on all fours sans opposing thumbs, it would make sense to question how the big head/small pelvis thing was a positive adaptation.
Likewise, in your example the negative aspects of the mutation are directly related to the positive aspects and can’t be separated. Most explanations for menstruation are based on behavioral differences amongst apes. But so far only Gabriela has sugested a possible reason why these behavioral differences are tied directly to the physical mechanism of menstruation. If menstruation is a protection against STDs we’d probably expect species who have sex more often with more partners–say, bonobos and humans–to have shorter cycles so that they menstruate more often as well. Does anyone know if this is the case?
I think your point that many of the modern negative aspects would have been diminished for our ancestors is well made. I’m not enough of a geneticist (i.e. I’m not one) to know what the standard ratio is between the amount of “badness” of a mutation and the length of time it will require to breed out, although I’ve heard that the larger the population the faster it occurs.
That’s certainly true, but that’s the problem isn’t it. It might work well enough or it might work perfectly but we can’t even determine that unless we know what it does in the first place. And that’s what the OP’s question is all about - what reproductive advantage is provided either direcly by menstruation or provided by something of which menstruation is a side effect.
It doesn’t have to do anything at all. It doesn’t have to confer an advantage, it just has to be a bearable disadvantage. As long as humans can reproduce and survive long enough to raise their offspring to independence, it works well enough from an evolutionary standpoint.
You’re probably not going to get a definitive answer to this question because we don’t exactly know what purpose, if any, menstruation serves, not for sure anyway. Besides the proposed advantage gabriela mentioned, she also pointed out a mutation which was advantageous in one area and time that now is a problem. Menstruation could be one of those things; a leftover from a time when it was useful that never went away because it’s not enough of a disadvantage to kill outright, significantly endanger survival, or prevent reproduction.
Or, it might still be useful. We just don’t know yet.
(BTW, the article on the proposed advantage of endometrial shedding gabriela was talking about is probably the same cited here.)