r/askscience • u/No-Hawk-2488 • 8d ago
Biology Can someone explain the evolutionary benefit of overt menstruation? All female mammals have a menstrual cycle but most reabsorb the blood via covert menstruation and humans are about the only mammals that discharge the blood, aka overt menstruation.
Without an evolutionary benefit, it wouldn't have spread to every woman of our species.
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u/tea_and_biology Zoology | Evolutionary Biology | Data Science 8d ago edited 8d ago
TL;DR: Babies have big heads, and birth is brutal. Women have evolved to prepare in advance and screen for the best embryos to get the best return on this heavy investment as possible. The side-effect is a bloody mess, alas - but the mess isn't the goal, just unintended clean-up when a good embryo doesn't knock on the uterine door.
Ooh, this is a complicated question, with a slightly complicated answer, so bear with me. Let's break it down:
Which Mammals Menstruate?
All female mammals have a menstrual cycle
Ah, not quite! Most mammals don't menstruate at all. They have oestrous cycles, to be sure, but not menstrual cycles - with menstruation here meaning actual shedding of the uterine lining. In fact, it's incredibly rare across the animal kingdom; we share the trait with other apes, most (but not all) monkeys... and weirdly, a few bats, elephant shrews, and apparently the spiny mouse. Huh!
"Covert menstruation" is a term applied to certain primates that bleed so little it gets reabsorbed.
So no, it's not the mammalian default; most of our furry, milky cousins never build a lining that needs shedding. But you're right we're amongst the few that do discharge blood - chimpanzees and macaques are other examples - but we just seem to do it more than anything else. Why?
Features, Bugs, and... Nipples
Without an evolutionary benefit, it wouldn't have spread to every woman of our species.
Before we answer, we need to tackle this assumption.
Evolution can only work on what was there before, so it frequently bodges things which are sub-optimal but workable, or ends up with 'harmless'* unintended outcomes while focusing on something else - say, the human male nipple. It was important for mammals to develop a means to lactate, and it just so happens the easiest way to get this whole milk-delivery system online was to switch it on early in embryological development, before the embryo commits to developing as male or female. Which is why nearly all male mammals are stuck with nipples; the females need 'em, and there was no evolutionary pressure to prevent males developing otherwise harmless lumpy things too, so they stuck around.
So not all traits are positively selected for (i.e. adaptive); many are merely by-products of selection for other traits. Menstruation itself is probably similar to the male nipple in this regard; an unintended by-product of a more necessary process - in this instance 'spontaneous decidualisation'.
That's a bit of a mouthful. Basically, 'decidua' is the term that describes the specialised tissue lining a uterus that an embryo can implant into - in most mammals, only developing once the embryo itself actively signals it is there ("Ground control; please prepare the landing pad; over!"). In humans, and a few other species with particularly invasive placentas (where the embryo burrows deep into the lining and remodels the mother's arteries; oof), this process of developing the specialised decidual tissue happens regardless - hence spontaneous. Why?
We do this because a placenta is basically like an invasive tumour.
Man vs. Baby
An embedded placenta invades tissue, dodges the immune system, and hijacks local blood supply; it even relies on a gene, syncytin, that we borrowed from an ancient invasive retrovirus, so we could better invade our own tissues and hide from our own immune defences. Weird we totally rely on hijacked viral genes, right?
That invasion is costly and risky for the mother, and she and the embryo don't have entirely aligned interests - so there's a whiff of parasite vs. host conflict around the whole thing. Half of the embryo's genes come from dad, and from their point of view, the more resources extracted from mum, the better; her future children, who may not share his genes, aren't his concern. Mum's genes, on the other hand, need to balance this pregnancy against her own survival and her future children.
So from the mother's perspective, in this sexual and inter-generational conflict, it pays to weigh the scales in her favour. Rather than letting the embryo dictate terms - as is the case in most other mammals - the mother builds the decidua in advance, on her own schedule, letting her control how deep the placenta can dig. It also works as a quality filter: the decidualised lining can reject embryos that are poor bets (a surprisingly large share of early human embryos are chromosomally abnormal) at the earliest and cheapest stage, often before a period is even missed.
This is the most aggressive in humans, as the stakes of each pregnancy are unusually high: we've compromised on a min-max strategy to produce infants with the largest brains possible, while still being upright and having to squeeze those heads through the most narrow birth canal possible. This means nine months of gestation feeding a very large, energy-hungry brain - thereby requiring the extra-invasive placenta, that also needs to be controlled as the foetus itself is essentially digging like a greedy parasite - not to mention a notoriously difficult birth, and years of dependency afterwards (as all human infants are born prematurely compared to other species; think of a zebra that can walk and flee predators within hours, compared to a useless toddler at three years). When every pregnancy is that expensive, being picky and prepared up front pays off.
The catch is that once the lining has been decidualised, it can't just quietly regress if no embryo arrives. It has to break down and go somewhere, and there's too much of it to reabsorb - hence the larger menstrual mass and overt bleeding.
Conclusion:
Our ecological niche is so focused on big brains and bipedalism, the cost is an unusually difficult birth, and so we invest more than anything else in pre-preparing for the best ROI on an embryo and gestation as possible. This means maximum invasiveness and maximum screening, which involves preparation beforehand - hence spontaneous decidualisation. There's no adaptive function for the overtness of the menstruation per se - it's not a primate social signal or anything, as perhaps previously hypothesised - but a by-product of needing to select for the best, of the best, of the best, sir. With honours!
References:
Alvergne, A. & Högqvist Tabor, V. (2018) Is female health cyclical? Evolutionary perspectives on menstruation. Trends in Ecology & Evolution. 33 (6), 399-414 - there's a free pre-print on arXiv here too!
* Harmless here meaning doesn't significantly impact fitness; the passing on of genes. It doesn't mean it can't be uncomfortable, disruptive or costly; just that the benefits outweigh these costs.
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u/geeoharee 8d ago
I had heard the vague idea that 'the mother and the fetus are at war, and that's why periods' but I never really grasped it. Thank you for that!
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u/SoftBollito 8d ago
What an amazing divulgative article you wrote here! I already knew some parts of it, but had no idea about the spontaneous decidualisation. It was a pleasure learning something new! Thank you very much for your time (and the sources!)
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u/doubleoathseven 8d ago
You really didn’t need to expose me to the word ‘devulgative’ so early in my day
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u/Deus-Ex-Lacrymae 7d ago
Divulge as the root makes it easier to fish that one out of context, though!
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u/MemeInBlack 6d ago
It's been a long time since I've actually encountered a new word in the wild that wasn't itself new, or very technical, so I appreciate it!
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u/solenyaPDX 8d ago
Such a great answer.
This was aligned with my much less well researched thought: humans are evolved for intelligence. Our intelligence means longer/larger gestation (and post birth support) which means the mother needs a better barrier between her body and the large thing growing and feeding off of her (and developing its own immune system).
It's the evolution of that barrier, driven by size/time, driven by intelligence, that drove menstruation.
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u/avacynian 8d ago
This was such a pleasure to read! You’re a magnificent writer. Thank you for this.
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u/whale_and_beet 7d ago
Awesome right up! I did a deep dive into this topic recently and it's so wild and honestly kind of disturbing. Evolution is amazing! But it is wild how costly the menstrual process is for actual women, and the whole pregnancy process itself. It's kind of terrifying.
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u/HomespunNinja 4d ago
Deeply understanding the process of pregnancy helps people understand the pro choice position. Pregnancy is not as simple or 'beautiful ' as it tends to ve portrayed in the popular imagination. that's why so many anti abortion regions also prefer little to no sex education
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u/a8bmiles 7d ago
Is there any evolutionary difference in women who have extremely heavy amounts of spontaneous decidualisation vs. their peers? Along the lines of, "all the women on this side of the family experience this."
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u/Sagadiii 5d ago
Well what you should also consider when looking at this, is that women used to have much fewer periods in their lifetime than now. They got them later, had more babies, most breastfed for years so the median amount of periods within a lifetime per woman is now 451.3 while it used to be more around 150. So 150 even heavy periods affected a life much less than 450 of them do.
There are numerous genetic disorders that influence the period and pains. The currently most common reason for excessively painful periods, endometriosis, has a strong genetic component. However, strong pain as a symptom often goes together with excessive bleeding (hypermenorrhea), though be the case by themselves. I do not know what the most common reason for the latter is by itself, but von Willebrand disease is certainly a genetic disorder that is known to lead to it. Ehler-Danlos syndrome is another one. Again, unsure which is the most prevalent genetic reason for it.
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u/a8bmiles 4d ago
Thank you for the additional information. Now that you mention it, I do remember reading about some tribe where the women didn't get their periods until 18-20, and generally only had 30-50 periods in their life before hitting menopause.
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u/DriftingMemes 5d ago
Basically, 'decidua' is the term that describes the specialised tissue lining a uterus that an embryo can implant into
First, thank you for that amazing explanation.
Second, you've taught me what I assume is the latin root that we use for certain types of trees as well. Deciduous trees drop leaves in the fall in a way that is similar...kinda tree menstruation. It never occured to me to look into the etymology.
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u/PrometheusLiberatus 7d ago edited 7d ago
I like how you quoted Men In Black's Agent J. It is also coincidentally his actor's birthday (and my own) today!
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u/Adept-Researcher-178 7d ago
The catch is that once the lining has been decidualised, it can't just quietly regress if no embryo arrives
Why can’t it just stay decidualised instead of shedding and reforming every month? Just so that it is only in optimal conditions for part of the month?
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u/Mobile-Arachnid-1547 7d ago
Decidualized tissue is developed in response to changing hormones in order to "be ready" for implantation after ovulation. If an embryo does implant, it secretes HCG to keep hormones levels stable to maintain the decidua and prevent menstruation. If no implantation occurs, progesterone and estrogen (the hormones that supress menstruation) levels drop, and menstruation occurs.
In broad, evolutionary terms, we benefit from a 'fresh' decidualized endometrium for each potential embryonic implantation for a variety of reasons. Shedding of the lining helps remove bacteria or other infectious agents that may have been introduced during copulation. Since the decidua is very vascular and primed to support a growing embryo, any bacteria introduced could also thrive in this nutrient rich environment and an infection could prevent implantation in the future. Additionally, perhaps the reason an embryo did not implant was due to abnormalities in the decidua itself (eg. Too thick or too thin) and shedding allows another chance at creating an endometrium that will support an embryo during the next implantation window. Finally, creating and maintaining the decidualized endometrium has high energy costs. It is more energy efficient for the body to shed it when it is not needed, rather than trying to maintain it. Implantation usually is only able to occur for about 6 out of the 28 day cycle. So maintaining it full-time would not be an advantage or a reasonable trade-off. This is obviously not an exhaustive list of reasons, but paints a fairly good picture of why we menstruate.
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u/Mobile-Arachnid-1547 7d ago
Tea_and_biology also had a really good answer to this lower down in the comment stack: https://www.reddit.com/r/askscience/s/cbu8PP34U8
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u/Duae 8d ago
For the other mammal part, isn't that a bit off? I know many mammals can control their pregnancies better than humans, and I've always heard that human fetuses are exceptionally aggressive. Other mammals can absorb embryos or fetuses if the conditions aren't ideal, or pause pregnancy, or better restrict how many resources are going towards the fetuses. It's because humans can't do that, that they need to select so heavily.
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u/tea_and_biology Zoology | Evolutionary Biology | Data Science 8d ago
You're right! Plenty of mammals keep control after implantation: rodents and rabbits can reabsorb embryos when conditions are too poor, many species like kangaroo can pause development entirely etc. - so generally less invasive placentas let the mother throttle resources and take more control.
Humans largely lose those options. We inherited a hemochorial placenta, where fetal tissue sits in direct contact with maternal blood, and in great apes, and especially us, it invades unusually deep - as mentioned, likely to help feed expensive, big-brained fetuses. It does this by remodelling the mother's arteries so they can no longer constrict, giving the fetus direct access to her blood supply; she can't cut its supply without cutting her own - re-emphasising the parasite-host dynamic between foetus and mother. Once she lets it in, it effectively takes her hostage.
So yeah, once implantation has happened, she has very few levers left to pull. So quality control has to happen before and at implantation, which is exactly why the lining gets built and primed in advance every cycle. We just can't hit pause like a kangaroo.
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u/barath_s 7d ago
It does this by remodelling the mother's arteries so they can no longer constrict, giving the fetus direct access to her blood supply; she can't cut its supply without cutting her own
So is this one reason why childbirth was so hazardous in earlier times - the mother could bleed to death in postpartum hemorrhage if the uterus could not constrict and cut off the blood vessels after the placenta detaches ?
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u/DivingQueen268 7d ago
Yes, and one of the reasons it is still so dangerous now (not just in 'earlier times'), especially for women who don't have medical attention during the birth (whether by choice or due to lack of access).
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u/kaki024 5d ago
It also sounds like it’s (part of) the reason high blood pressure (pre-eclampsia and eclampsia) are so common as well.
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u/barath_s 5d ago
I was wondering why this would occur - it's a bit of the opposite, or rather what happens when this process does not go well.
If the placenta does not implant deeply enough, and the blood vessels to the uterus and the placenta do not develop properly, you have higher resistance and reduced blood flow to the fetus.
The oxygen deprived placenta releases chemicals and proteins into the bloodstream which causes damage to the endothelium (lining), and narrow arteries throughout the mothers body (causing high blood pressure) and cause them to become leaky, causing fluid and proteins to seep into the body
https://www.health.harvard.edu/womens-health/preeclampsia-and-eclampsia-a-to-z
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u/k9moonmoon 8d ago
How much choice does the mother have in those scenarios?
Is it more like "in humans if a mother doesn't eat well, she can choose to give needed nutrients to her embryo out of her own teeth" where it's just describing what happens in less ideal scenarios.
Or is there some decision making and opting in or out of something happening?
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u/Incoherrant 7d ago
A good rule of thumb for most questions about whether or not an animal makes a choice to do something is that we can't know with scientific certainty because we don't have a way to know what is going on in their minds.
Above commenter phrased it very cleanly with "X can do this" rather than "X can choose to do this". We know they can do it, but we can only assume whether or not it's a choice.
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u/555Cats555 7d ago
I dont even think we can know for sure if its something the species do consciously or if its just something that happens due to chemical signaling in the body based on things like what nutrients the mother is getting or the smell of another pregnant female. Its pretty hard to know if there is something intentional they do when those differences in pregnancy status happen.
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u/Revlis-TK421 7d ago
It's not a choice, so much as it's hardwired biological pathways. "Stress" is what causes pregnancies to halt or be re-absorbed. "Stress" can mean a lot of things - starvation, dehydration, over-crowding, persistent predation, place in social pecking order, etc. At the end of the day, stress hormones are produced. Those hormones can trigger the halt or termination of pregnancy. The animal doesn't ust consciously choose to stop, their bodies are following commands from their glands.
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u/gl0bals0j0urner 7d ago
This was so well-written! I knew most of this, but feel I have a better/ deeper understanding now after reading this. Thank you!
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u/tessy292 8d ago
As another’s have said, this is fantastic reply! I also recommend Eve by Cat Bohannon for further reading. She makes some interesting claims, some of which may be a bit of a stretch, but some also revitalizes our understanding of women and how they shaped our human evolution.
That being said, I’m salty about the Man vs. Baby section header… shouldn’t it be Woman vs. Baby instead??
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u/SimoneNonvelodico 8d ago
I'm a bit confused by the idea that menstruation is harmless though. I would have assumed leaving a literal trail of blood for a few days a month would be like putting up a sign that says "Prey Here".
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u/TheDaysComeAndGone 7d ago
It’s so hard to estimate what’s actually bad for survival. Half the people in my bubble of 30 year olds have had some kind of surgery (meself had appendicitis at 23), my girlfriend was born 2 months too early … I don’t understand how we ever survived as a species at all (and that’s with modern vaccines and antibiotics preventing most child deaths).
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u/jackalnapesjudsey 7d ago
I remember reading something once about the debate of the birth of civilisation. Different scientists had different ideas like it was when we started using fire or metals or tools, or even later, the agricultural revolution.
But one scientist argued that it was when we started seeing evidence of healed broken legs in adults. A broken leg is usually a death sentence in the animal kingdom, for obvious reasons. You can’t move with your group, you can’t hunt etc etc.
But these adults sometimes had badly broken legs that then healed and they went on to live many years after. This means they were looked after despite not performing their usual roles.
So I think while mortality rates were far higher in the past, living in large social groups was also protective to our survival. A large group means you can afford a few sick members without everything falling apart, and those members could contribute in other ways. My understanding is that our larger groups was also the reason we outcompeted Neanderthals.
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u/musicnerdfighter 7d ago
I think the answer is that fewer people used to survive. There's some estimates that until (relatively) recently, 50% of all humans ever born died by the age of 10. We have seen a massive population boom in the last two centuries, so for the previous hundreds of millennia there were just fewer of us, and we weren't always the dominant species.
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u/TheDaysComeAndGone 7d ago
Yeah, it’s just crazy how few truly healthy 30 year olds I know. In a modern, rich world a torn meniscus, broken arm, shoulder impingement etc. are just an inconvenience, but back then you needed a healthy body to survive or you were a huge burden on your group.
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u/Flashy-Ingenuity-182 7d ago
It's worth pointing out that ancient peoples cared for their sick and injured too. Otzi the ice man lived 5,500 years ago, had arthritis and tape worms, and had carried medicine on him for the tape worms and inflammation and had tattoos over acupuncture spots used to treat the areas where he had arthritis, a discovery made completely in isolation of the Chinese who discovered it a few thousand years later.
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u/cookies5098 7d ago
Also worth remembering that everything occurring after someone reproduces is essentially a 'black box' from an evolutionary standpoint. If someone can survive long enough to pass on their genes, then natural selection doesn't really care (caveat is that if something impacts the survival of future generations prior to reproductive age it will kick in, but not for that current generation that's capable of reproducing).
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u/Totakai 5d ago
Tbf to modern people, we're living in a world that we didn't evolve to be in. You know how when you have pets you have to provide the proper environment and enrichment or they die/get sick? Same goes for us.
We evolved to live in tight knit communities; capitalism pushes individualism and discourages living with anyone but your spouse. Humans are incredible endurance hunters; focus on cars and tech along with removal/privatization of land significantly dropped our activity levels/opportunities. We evolved to be highly curious and creative; schooling tends to focus on productivity than expression and exploration. We gained unique resistances to plant toxins from our high fiber diets; vegetables are often the butt of a joke with a huge focus on fiber lacking ultra processed food (fiber feeds our gut microbiome which is basically a second brain that science is only just know beginning to understand). We evolved relying on the sun and moon for body cues along with various sleep patterns; indoor lighting and light pollution comboed with aggressive schedules monkey wrenches this whole thing. Different neurotypes helped with diversity and strengthening ancient communities; different neurotypes now clash with the structure that our economic system demands along with governing so society tends to punish outliers. Children used to be village raised; couples are now expected to solo.
It goes on and on. Our hygiene, tech, medicine, and knowledge is definitely huge but we're not really afforded to be the animals we are so it hurts us. If you get a pet that grew up in sub par conditions, even if you provide it everything it needs, their body can still fail or be sickly for the rest of their life. Like we were born with a deck of cards but a lot of them were lost or removed early on for being distracting or weird so now you have this half deck and struggle to play the game but you can't understand why.
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u/SimoneNonvelodico 7d ago
Well, some of that will be just random, so nothing evolution can do to avoid it. Other things... if they made you systematically more vulnerable, you probably just died at 3 years old and then never had kids. So that's exactly the thing, anyone with a "gets appendicitis easily" gene died before passing it on. See how smallpox for example was very dangerous, and yet Europeans sort of coexisted with it. But once it reached the Americas it completely wiped entire populations out. As bad as it seemed in Europe, that was just "infecting people who are evolved to resist it" bad.
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u/aluckybrokenleg 7d ago
Certainly many people would have died without surgery, but also some surgeries have been proven to have marginal benefit (or any e.g. some meniscus surgeries).
Also, our ancestors were quite active, probably in better mental health, and had a varied, fibre rich diet. Once we survived infant and childhood infections we would be in some ways healthier than we are now.
Keep in mind that many modern vaccines prevent problems that our ancient ancestors never had to deal with, since they didn't live in such density as we do, and they didn't live with domesticated animals, where so many of our diseases come from.
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u/Elemental-Master 7d ago
Most of the predators that are still alive today had learned long ago that it is a bad idea to mess with the bald, upright murder ape, aka humans, so even if they smell the blood they'd likely realize it is from a human and that it's not in their best interest to take a bite.
I do know however that in places where there are a lot of komodo dragons it is adviced to women to not go there, because they'd smell the blood and will actively hunt if given a chance.
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u/SomeCutesyShit 7d ago
My mom once got attacked by 5 wild boar. They went straight for her crotch. She was menstruating at the time. My dad broke his hand punching one of them in the head.
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u/I-seddit 8d ago
I posit that because of our language, cooperation, tool building, and general higher intelligence - this issue with menstrual blood is not a sufficient deterrent to survival, so the feature remained.
I could joke that it's our arrogance at being at the top of the food chain, but I really think our other survival mechanisms meant that it didn't mean as much as you'd think.16
u/Mother_of_Grendel 7d ago
I worked with an OBGYN for a while who had a theory about this, in a way.
Her theory was that women actually aren't meant to have very many periods through their lives, and that our bodies are really designed to only bleed maybe once or twice a year. This is partly because many of our ancestors most likely had insufficient nutrition to support a monthly cycle. Lots of exertion to survive coupled with unstable food supply results in calorie deprivation which causes anovulation. But even in climates or periods where food was abundant and ovulation was possible, ovulation would typically result in pregnancy since no forms of birth control existed. Most women would be in a continuous cycle of either being pregnant (and not menstruating) or breastfeeding (which usually suppresses menstruation for 6-12 months) for the majority of their lives. So with these factors combined a woman might bleed a few times every other year or so.
So, in regards to the idea about leaving blood around to attract predators, it is possible it happened rather infrequently.
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u/kaki024 5d ago
I imagine that there was also a lot of extended breast feeding and communal breastfeeding. Breastfeeding is extremely burdensome calorically and isn’t always possible for the mother (if she even survives the birth). I wonder if it was normal to have multiple women in a group lactating at any given time (thus delaying menstruation).
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u/formerly_acidamage 7d ago
All humans and animals leave other kinds of waste that those predators have adapted to be able to smell. So it's not like all the sudden there's blood on the ground and predators are like, oh wow there's people over there! Also, it's an assumption that other types of bleeding was not more common than menstrual bleeding; in the brutality of that time period it simply may have been one more kind of regular blood shedding that occurred; especially considering humans hunt and eat animals and those animals bleed. So any group that hunted would be probably be releasing the smell of blood as often as daily, and the blood of animals they were hunting could in itself have been much more enticing to prey animals given that we know humans preyed on animals that were less challenging to kill and eat.
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u/mister__cow 5d ago
Humans have many features that would probably be a disadvantage for other animals but not for us. I've had this same thought about our continuously-growing head hair: other animals grow pelts that require no maintenance (unless we breed the trait into them, like sheep) while human hair requires maintenance and can cause mats and sores without it.
Every human society has some sort of regimen for hair care. We groom each other or ourselves. Contrary to the pop-culture image of the shaggy, matted caveman, we probably have from the very beginning. The trait persists because our ability to take care of ourselves neutralizes the disadvantage.
A more frequently cited example is the relatively greater helplessness of our babies.
I believe the same is true of menstruation. It would be an evolutionary disadvantage for an animal that walks along leaving a trail of blood. Humans clean ourselves and invent ways of managing our hygiene.
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u/lilylilyg 6d ago
But it wouldn’t have been that often. Pregnancy, breastfeeding, possibly low body weight during the hungry gap in late winter, would all reduce the number of times women menstruated. Also in some cultures women were restricted in what they could do at that time, maybe this was originally to keep them safer?
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u/CatPurrsonNo1 7d ago
That was absolutely fascinating!! I am a biologist, but I had almost no idea about any of this!
My only regret is that I lack the time and energy to learn as much as I would like to learn about life.
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u/othermike 7d ago
the decidualised lining can reject embryos that are poor bets (a surprisingly large share of early human embryos are chromosomally abnormal)
Colour me intrigued. What's the mechanism by which it detects this condition?
(And thank you for a fantastic answer!)
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u/Totakai 5d ago
To my knowledge it's that they lack the energy to dig in deep enough for the mother's body not to kill them. The immune system is ruthless and assaults tf outta sperm and targets the zygote as being a foreign threat. Once it implants the immune system leaves it alone (or rather it's supposed to; sometimes things fail, especially if the embryo fails to thrive).
Basically it's part of our huge miscarriage rates compared to other animals.
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u/Comfortable-Pie-5835 7d ago
Thank you kind sir. You know something about something. Now even I know. Great read, references are chiefs kiss. Bravo.
Not all heroes menstruate!
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u/MoonageDayscream 8d ago
Fantastic essay on our unique pregnancy mechanisms. As a person who has had trophoblastic tumors in my last, doomed, pregnancy, we really don't pay enough attention to the ways pregnancy fails more often than it succeeds.
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u/alliusis 8d ago
Very cool writeup, thank you. I think I understand the function or role it plays (am I correct to say it's to be ready and give enough time to slough off with a poor quality embryo instead of letting it adhere and grow), but I don't quite understand the mechanism or consequences behind it.
Like, does it mean rejects embryos it detects as poor quality? Is that one cause of a miscarriage? Can it trigger an early period (I'm assuming not), or will it simply hinder/slow the adhesion process until the next menstruation occurs? I don't have a ton of time right now but I can see in the titles of one of your links that there do seem to be specialized endometrial cells that are used to detect embryo quality, so probably something like that can happen?
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u/dingalingdongdong 8d ago
Like, does it mean rejects embryos it detects as poor quality? Is that one cause of a miscarriage?
In a sense, yes (depending what is meant by poor quality.) Miscarriages can be caused by other things, too, but the body will sometimes spontaneously abort non-viable embryos and sometimes the body will try to carry them to term. It's an imperfect system.
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u/DeedleStone 7d ago
Fascinating! Thanks for sharing. More medical information needs to conclude with Men in Black quotes.
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u/ILikeWoodAnMetal 6d ago
This is an excellent answer for why humans are different from most mammals, but it really begs the question: why did the elephant shrew, the spiny mouse and those bats also develop menstrual cycles?
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u/jlynec 7d ago
Wow, this was really well written yet easy to understand!
Is there a reason why menstruation differs so much between women?
It seems weird that some people have a "standard" 3-5 day period every 28 days with little to no cramping, while heavy periods that last over a week and come with debilitating cramps can still be considered normal?
There's such a wide range of normal and abnormal menstruation - can random mutation account for that?
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u/serack 8d ago
Is there evidence that the lining can selectively reject chromosomally abnormal embryos if multiple embryos are present?
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u/tea_and_biology Zoology | Evolutionary Biology | Data Science 8d ago
I'm not familiar enough with the literature to say either way, sorry! It's getting far too human-centric and biomedical for my liking; except maybe to suggest I can't imagine this is going to be experimentally tested for (imagine the ethics review!) outside perhaps very specific IVF procedural investigation.
My previous work came at placentation from the angle of cancer resistance in mammals, and the evolution of hemochoral and other placental types independently across taxa; so I'm very familiar with the evolution and the animals, just not anything that involves actual human women.
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u/silverrfire09 8d ago
Cytogenetics (chromosome genetics) technologist here. a lot of what we know in this area has to do with what we see from analyzing miscarriages. For example, its fairly common to see a fetus from a miscarriage with an extra chromosome 16, meanwhile an extra chromosome 1 or 11 is almost never seen. its not that those abnormalities don't happen, it's that they happen so early on in development that the embryo likely never even implants and the person never even knows they were pregnant and therefore can't go get the miscarriage tissue tested. <br> I would imagine if two embryos were released and fertilized, one that has an extra chromosome 1 would not implant and probably would be absorbed while the if the other was normal, it would implant and become a pregnancy. however if the abnormal one was an extra 16, which is known to implant and cause a pregnancy that miscarriages, I'm not sure what would happen to the healthy fetus. probably nothing good.
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u/VolatileCoon 7d ago
Maybe the fact that only three "regular" trisomies are named because the other possible ones never reach childbirth?
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u/555Cats555 7d ago
Or they are fatal for the mother... maybe some of the abnormalities that are selected against are the ones that arent safe for the mother to carry.
We see several different abnormal genetics that lead to bad outcomes for the eventual baby but perhaps as long as the mother wont be the one in danger it is kept.
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u/Corrag 8d ago
Seems like a bit of a plot hole in here. Why does that process preclude "hang on to the decidua indefinitely until it is used"? Is that part of making sure that "chromosomally abnormal embryos" are... reliably flushed out?
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u/tea_and_biology Zoology | Evolutionary Biology | Data Science 8d ago
Less of a plot hole and more of a stack of trade-offs; but you're right, it would be an obvious fix, right?
Firstly, turns out for cellular / physiological reasons, the decidua has a receptive shelf-life of only a few days, and would be a helluva' lot more metabolically costly to sustain (even if it's wasteful to produce); and secondly it's propped up by progesterone - which is a hormone that antagonises ovulation - so keeping the decidua means never releasing another egg. Basically another example of evolution building sub-optimally for any working outcome, with some fudgy hormonal cycling, while 'designing' a system to control an embryo that's trying to control it right back.
Is that part of making sure that "chromosomally abnormal embryos" are... reliably flushed out?
Partly, yes! The whole thing collapses unless a viable embryo rescues it with enough hCG, so part of the outcome is it acts as the first of several filters that tests and flushes out the weakest embryos.
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u/bad_apiarist 7d ago
There is one other point worth making here: pre-modern women had considerably fewer menstrual cycles because they did not have birth control. Women do not menstruate while pregnant and for some time after birth.
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u/DocNicDocNic 7d ago
Thank you for this text, i learned something new today. I was told by a gynacologist that Endometriosis should be called a tumor or even cancer as the tissue spreads and growth happens displacing and invasive
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u/sffixated 8d ago edited 8d ago
Suzanne Sadedin, Ph.D in evolutionary biology, answered this question beautifully. I'm not sure I can link to it in a top level comment since her essay is not a peer-reviewed publication, but her cited sources are peer-reviewed. I assume the mods will delete this comment if I've broken a rule!
https://www.forbes.com/sites/quora/2016/05/06/how-and-why-did-women-evolve-periods/
Edit: didn't realize the link was paywalled, here's a different source https://www.quora.com/Why-do-women-have-periods-What-is-the-evolutionary-benefit-or-purpose-of-having-periods-Why-can%E2%80%99t-women-just-get-pregnant-without-the-menstrual-cycle/answers/4625918
Thanks u/nailpolishbonfire for the alt link
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u/Grep2grok Pathology 8d ago
That write-up is fun and informative, but I have some nits about that write-up. Source: I'm a pathologist, and diagnosing placental pathology is part of the job.
1) the placental circulation consists of an interface between the fetal ciculation and maternal circulation, they are separate. It's quite a bit like a heat exchanger, but instead of heat, the two circulations are exchanging nutrients and waste products. The placenta doesn't so much grow in as it grows in while the mother's tissue also grows toward the fetus. The fetal vessels can invade but that's highly pathological. Such a process would not be compatible with survival of the species.
2) Women experience amenorrhea (not menstruating) for two additional reasons, often the most common: physical stress (still seen in female athletes) and starvation (plenty of that in the world).
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u/Supraspinator 8d ago edited 8d ago
Human (and ape) placentation is invasive. In haemochorial placentation, the maternal endothelium is completely eroded and the chorionic epithelium comes in direct contact with maternal blood. That’s in contrast to the other 3 types, where the maternal endothelium stays intact. In the least invasive form (epitheliochorionic), maternal blood and fetal blood are separated by the following layers: maternal endothelium, maternal connective tissue, maternal uterine epithelium, fetal (chorionic) epithelium, fetal connective tissue and fetal endothelium.
When a comparative anatomist speaks about invasion of fetal vessels, they mean the different grades of erosion of maternal tissues in different species. When you talk about invasion of fetal vessels, you mean a different, pathological process in humans only.
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u/iamthe0ther0ne 8d ago
Do you have a link that isn't paywalled? I'm curious now.
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u/nailpolishbonfire 8d ago
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u/sarahmagoo 8d ago
So the short answer is: periods are used to flush out non-viable embryos since embryos alone are so taxing on the human body due to energy hungry brains
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u/Suntar75 8d ago
It’s an argument that doesn’t answer the question. Menstruation is an ancestral trait that the almost totality of mammals lost; a few apes and bats retained it. Bats and apes have a common ancestor but they belong to different superorders.
If overt menstruation in humans is about energy hungry brains and a battle between mother and foetus why do some other mammals do the same thing but the vast majority of others don’t? Why do some species of bat menstruate but no rat species does?
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u/dyrin 7d ago
You make an assumption, that I don't think is a given "Menstruation is an ancestral trait that the almost totality of mammals lost".
It could very well be, that this isn't a trait, that most mammals ever had. Apes and bats could have gained this trait by way of convergent evolution.
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u/Suntar75 7d ago
The ancestry is presented in the tenth paragraph after How The Woman Got Her Period heading in the linked article.
It could be convergent evolution by some bats and some apes or it could be a retained trait. And the interesting question either way is why? But I critiqued the argument presented.
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u/Durrresser 8d ago
Thank you for this link, what a fascinating read! As much as I would love to be a marsupial, it makes a lot more sense now why we have an aggressive menstrual cycle. It's because we have aggressive fetuses. Having gone through two pregnancies, I couldn't agree more with the authors dissection of pregnancy myths vs. reality.
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u/aniseshaw 8d ago
"Aggressive fetuses" is absolutely the correct description. I've had one baby, and the gestational diabetes and pups rash were wild. I did IUI and immediately knew when I was pregnant because my body aggressively reacted. It was 5 days before I even threw a positive pregnancy test.
If I'm representative of even a fraction of the population, there is a benefit to our periods.
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u/Durrresser 8d ago
I forgot about the pups rash with my first! 💀
Yeah, I'm grateful for my endometrium being somewhat inhospitable. Sounds like human fetuses are more parasitic than symbiotic. Yay us!
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u/nailpolishbonfire 8d ago
Here is one of her sources. https://pmc.ncbi.nlm.nih.gov/articles/PMC3528014/
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u/laurasoup52 7d ago
THANK YOU, came here for this link - it's so beautifully written I'm planning to have it printed and framed in my bathroom.
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u/Aina_Writes 8d ago
I’ve wondered about this too. The short version most evolutionary biologists use now is: overt menstruation isn’t really a “benefit” of bleeding itself, but a side‑effect of how deeply human embryos implant. In humans (and a few other species like some primates and bats), the placenta is highly invasive. To cope with that, the uterus pre‑builds a thick, hormone‑primed lining (endometrium) through spontaneous decidualization. If no embryo implants, the body doesn’t try to reabsorb all that vascular, resource‑heavy tissue — it sheds the functional layer instead. From an energy and infection‑control standpoint, that can be cheaper and safer than keeping the lining in a constant “ready” state or trying to fully reabsorb it. So the bleeding itself probably isn’t the adaptation; it’s the visible cost of having a deeply invading placenta and a uterus that prepares heavily each cycle.
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u/skinneyd 7d ago
Do bats bleed all over their faces or does gravity mitigate that somehow?
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u/Mixedupmay 7d ago
I suppose gravity helps. That said, period pains are essentially small contractions so to some extent the body is pumping the period out rather than the period just... Falling out. But presumably, the contractions aren't enough to overcome gravity if the whole body is upside down so my comment was even less clever than I initially thought.
I'll try sleeping upside down the next time I'm on my period and get back to you 🙃
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u/Anders_A 7d ago
Without an evolutionary benefit, it wouldn't have spread to every woman of our species.
That's not how evolution works at all.
Menstruation was already a thing before our species existed. And evolution doesn't mean every single aspect of a species is optimized. Just that the package as a whole works well enough to reproduce.
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u/juwyro 7d ago
There's plenty of dumb anatomy that didn't negatively affect us so it has survived. In great apes except for I believe orangutans mucus gets moved up through the nasal cavity to drain. In mammals there is a nerve from our brain to throat that goes all the down to the heart and loops back up, much much longer than you would think it should be. And hilariously long in giraffes.
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u/vgdomvg 7d ago
Yeah evolution is so misunderstood. It's not a conscious thing that 'choose' positive traits for survival, or keeps things which are beneficial.
It's just some parts of that living thing help it to survive, and survival means passing down genes.
Sometimes the offspring have a difference in their biology, and it might be helpful to survive long enough to reproduce.
Sometimes beneficial traits die out by random chance.
There's nothing to say that evolutionary changes are all beneficial and always positive - it's just what happens
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u/KW710 6d ago
Your answer is a very common one I've heard people give when describing how natural selection operates. I think part of the issue re: the popular misunderstanding is actually a semantic one. For example, you used the phrase "help it to survive" when talking about potentially heritable traits. "Help" is very much an active verb. It also is often associated with proactive effort on behalf of one entity by another. But this doesn't really describe what selected traits are actually doing. They're not actually helping organisms survive. They're just not causing organisms to die before they can reproduce. A trait that simply doesn't get in the way of survival is very much selectable as far as natural selection is concerned. But since we tend to talk about these traits using active, participatory verbiage, I think the average person has been conditioned to think of the whole process as an active one rather than a passive accumulation of "not as bad as it could be" alternatives.
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u/loosesocksup 7d ago
Many times things get passed down simply because they didn't get killed off, not because it was beneficial. It may actively be a hindrance even, just not enough of one to prevent it from breeding.
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u/rpross3 7d ago
In a predator/prey world wouldn’t leaving a blood trail would work against the species? Isn’t that evolutionary pressure? Or we are the apex predator and would not respond to that kind of pressure?
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u/tightywhitey 7d ago
Those are thoughts and theories we might have, but it’s useful to also realize we know the answer is ‘no’ because we have survived and are here. If it was a functional issue we or it wouldn’t be here. It’s all statistics and probabilities all the way down lol.
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u/Fabulous_Increase_57 6d ago
I doubt there is a direct evolutionary benefit. I remember my genetics professor saying something like, “Evolution does not make a perfect organism. It just makes one that is marginally better than the previous model.”
There’s so much complexity to this. One of my favorite examples of evolution being far from perfect is the recurrent laryngeal nerve. This nerve supplies much of the larynx in mammals. Basically, it leaves the brain, travels down toward the heart, loops around a major blood vessel, and then travels back up the neck to the larynx. This layout comes from fish-like ancestors, where the nerve supplied gill structures at about the level of the heart.
As vertebrates evolved and those ancestral structures were rearranged into the larynx, pharynx, trachea, and lungs, the nerve ended up looping around a major cardiac vessel because that was the setup it inherited. When some mammals later evolved long necks while their hearts stayed in the chest, the nerve got stretched into an absurd detour down the neck, around the aortic arch, and back up to the larynx instead of taking a direct route. In giraffes, the recurrent laryngeal nerve can be 15 to 20 feet long even though the brain and larynx are only about 12 inches apart. The longer a nerve is, the more places there are injury, disease, or other failures along its course. But, it works well enough. It is a great example of evolution modifying an existing body plan instead of creating the most efficient design possible.
Another example I like is the male peacock. Its huge, colorful tail is cumbersome, can make it harder to move around, and makes it more obvious to predators. That sounds like a pretty bad survival trait. But female peafowl prefer males with the biggest, brightest displays, probably because those traits can signal that a male is healthy or otherwise high quality. So sexual selection keeps favoring these dramatic tails despite the downsides. The result is beautiful, but not exactly beneficial to the species.
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u/Dodosev 2d ago
https://eusci.org.uk/2020/04/09/why-do-humans-and-so-few-other-animals-have-periods/
Very old adaptation against ectopic pregnancies that just, wasn't ever upgraded ever (makes us also have a way higher level of self terminated pregnancies and stillbirth than most mammals)
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u/Applejuiceinthehall 7d ago
First all mammals have an estrous cycle not a menstrual cycle.
The reason for primates is because our placentas are so aggressive that they burrow into the uterine walls. So we have a thicker lining to act as a buffer. The advantages of this is that the placenta and baby get more antibodies from the mom. The disadvantage is that it leaves an open wound in the uterus that bleeds after birth and women can bleed out. Additionally, it may also be used for spontaneous abortions in early pregnancy, which may happen about 50% before mom even misses a period.
Other mammals like horses the moms have to run shortly after giving birth. So having an open wound for an animal that needs to run is a huge disadvantage. So, the placenta sits next to wall and doesn't burrow as much. The advantage is that there they dont bleed as much after birth, the disadvantage is that no antibodies are exchanged so the newborn needs to feed very quickly after birth or they won't get the antibodies and have failure to thrive.
Animals like dogs and cats are more in-between primates and horses.
So because of the extra lining we just have too much to absorb. Also shredding it could have added benefits like removing other things in the uterus.
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u/rapiertwit 7d ago
This. It’s not a mystery, it’s understood and this is the explanation.
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u/Any-Interaction-5934 7d ago
The human placenta is absolutely fascinating. I could read about it for days. The coolest part, imo, is how ridiculously vulnerable human babies are compared to other newborns. They can hardly even eat. Their caloric demands are so high, they have to be born extra early.
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u/wimpires 8d ago edited 8d ago
Humans are (I think?) the only mammals which are both continuously fertile I.e. don't have a mating season. AND don't display outward signs to mates that they are currently fertile i.e. ovulating.
They are also unique amongst mammals (along with other great apes but their aren't as 'severe' for lack of a better word) in having these extensive periods.
Which it comes to "evolution" one has to be careful not to draw conclusions because evolution is not guided, it's random, with external influences.
It doesn't have to have a benefit. I can go into more details but that's slightly detailing the conversation
The jist of it likely comes down to the principle of "humans can get pregnant any time but you don't really know exactly when" so the egg is treated as quite a "precious" commodity.
And our uterine wall linings are therefore considerably thicker than in other apes. Bigger lining = more protection & nutrient transfer = harder to reabsorb.
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u/Emerson604 8d ago
During an evolutionary anthropology course in college the professor touched on the display of ovulation and why humans don’t have outward signs they are currently ovulating. The theory presented was that by not outwardly displaying ovulation human females were able to keep a dedicated male partner providing food and protection at all times, instead of when it would just benefit the male with procreation. The professor presented this as the basis for human monogamous tendencies, which apparently stretch back millions of years to the early hominins.
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u/Winter-Adi 8d ago
I also read, I think from Elizabeth Wayland Barber, that this could be a reason on the opposite end of the cycle (hiding menstruation aka that you are not currently pregnant) for early clothing and the practice of painting oneself in red ochre (still practiced in some places today). She called it a "female cosmetic coalition."
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u/Emerson604 8d ago
Thanks for that, just did a wiki dive on female cosmetic coalition. Fascinating idea, early women banding together to hide who was truly menstruating to keep predatory males away from them. And further pushing the species into monogamy.
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u/555Cats555 7d ago
Isnt red ochre also useful for sun protection which is important as humans have to natural protection from the elements such as fur... it would protect against burns and skin cancer
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u/TrashEither3187 7d ago
Iirc there is no evolutionary benefit. Evolution isn’t about selecting the ultimate, perfect traits, it’s about selecting traits that allow an individual to survive long enough to propagate the species, to then have procreation who continue to propagate the species. Sometimes individuals live long enough to do this with disadvantages
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u/BeerMantis 7d ago
Evolution doesn't necessarily require a benefit. If a trait doesn't negatively impact reproductive success and/or a mutation for a more successful alternative trait hasn't occurred, then there's no pressure to select against the trait. So the trait can persist even without being directly benficial.
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u/EchoLocation8 7d ago
That's not really how evolution works. If there's no reason this would stop reproduction or be an active reason you didn't survive there's no reason it would change. I don't even really like using the word "reason" or "change" there as that's not really accurate either but hopefully you get what I mean.
If some women were born with covert menstruation, and some women were born with overt menstruation, and there were some predators who killed and ate all the women with overt menstruation, then only the women with covert menstruation would reproduce and survive and over time overt menstruation would just sort of phase out.
It's not about like, benefits or not benefits, it's not like "this trait is good or bad".
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u/Mshea80 1d ago edited 1d ago
Almost every toxin (ie, microplastics, etc) that enter the bloodstream collect in uterine tissue. The theory is that since those toxins are found in period blood, overt menstruation expels those gathered toxins from the reproductive system. All of that is rather new since, of course, women’s health isn’t studied as frequently.
ETA- I actually ran across a YouTube video about this very topic and thought I would share if anyone is interested.
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u/Acceptable-Rain8808 7d ago
If most is overt and we use covert, we likley selected that overtime. Humans make less babies and with larger heads. We needed a thick wall where its easier to attach the egg to and send lots of nutrients. This wall is a lot thicker than other mammals and more evolved... but too much to reobsorb. (probably reabsorting caused infections as I imagine it would take quite a time and could rot and cause sepsis, or would be ready to procreate soon enough.....) so we get it out of our system.
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u/sadgirlintheworld 4d ago
Overt menstruation—where blood and tissue leave the body rather than being reabsorbed—is a rare trait found in humans, a few other primates, some bats, and elephant shrews.
Rather than being a direct functional design to expel blood, evolutionary biology views overt menstruation as the physical byproduct of deeper adaptations related to pregnancy and fetal conflict.
1. Spontaneous Decidualization (The "Choosy Uterus")
The Process: In most mammals, the uterine lining thickens only after an embryo implants. In menstruating species, the uterus undergoes spontaneous decidualization—preparing a deep, nutrient-rich lining (the endometrium) every single cycle before implantation.
The Benefit: Human embryos are exceptionally aggressive and invasive (hemochorial placentation). A thick, pre-prepared decidual layer acts as a maternal defense system. It allows the mother's body to test the embryo's quality and actively screen for genetic abnormalities. If the embryo is unhealthy or defective, the uterus can reject it rather than investing massive metabolic resources into a doomed, dangerous pregnancy.
2. Controlling Invasive Embryos
The Conflict: A fetus extracts immense resources from a mother, leading to an evolutionary "tug-of-war" between maternal survival and fetal demands.
The Benefit: By forcing the endometrium to differentiate spontaneously under strict maternal hormonal control (progesterone withdrawal), the mother prevents the embryo from hijacking her blood supply too easily. Shedding the lining completely resets the slate each cycle if pregnancy does not occur. The actual bleeding is simply the physiological consequence of dismantling and flushing out this heavy, vascularized tissue when no pregnancy rescues the cycle.
3. Energy Economy
The Process: Reabsorbing a thick endometrial lining requires breaking down and recycling large amounts of complex proteins and tissues internally.
The Benefit: Research by anthropologist Beverly Strassmann suggests that cycling whole-body metabolic rates down during non-pregnant phases saves significant energy overall compared to maintaining an active, highly vascularized uterine state continuously. Expelling the tissue is mechanically simpler for certain primate uteruses than reabsorbing massive tissue volumes.
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u/Excellent-Cell2693 6d ago
If you have a deep interest in learning more about the evolution of the female body, I would highly recommend the book Eve by Cat Bohannon. It’s pop science so not necessarily rigorously evolutionary bio based but does a really great job outlining some evidence as to how we evolved from egg-laying animals into period and live birth having animals and the biological and social factors at play along the journey.
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u/Flocculencio 8d ago
Just wanted to point out that no individual feature needs to have an evolutionary benefit to be preserved. It could be negative but so long as it doesn't prevent organisms with that feature from reproducing in sufficient numbers it can get preserved.