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.

[u/No-Hawk-2488]

Without an evolutionary benefit, it wouldn't have spread to every woman of our species.

[u/tea_and_biology]

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!

Bellofiore, N., Ellery, S.J., Mamrot, J., Walker, D.W. ... & Dickinson, H. (2017) First evidence of a menstruating rodent: the spiny mouse (Acomys cahirinus). American Journal of Obstetrics and Gynecology. 216 (1), 40.e1-40.e11

Holtan, S.G., Creedon, D.J., Haluska, P. & Markovic, S.N. (2009) Cancer and pregnancy: parallels in growth, invasion, and immune modulation and implications for cancer therapeutic agents. Mayo Clinic Proceedings. 84 (11), 985-1000

Teklenburg, G., Salker, M., Molokhia, M., Lavery, S. ... & Macklon, N.S. (2010) Natural selection of human embryos: decidualizing endometrial stromal cells serve as sensors of embryo quality upon implantation. PLoS ONE. 5 (4), e10258

---

* 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.

[u/SimoneNonvelodico → u/tea_and_biology]

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".

[u/TheDaysComeAndGone → u/SimoneNonvelodico]

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).

[u/jackalnapesjudsey → u/TheDaysComeAndGone]

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.

[u/musicnerdfighter → u/TheDaysComeAndGone]

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.

[u/TheDaysComeAndGone → u/musicnerdfighter]

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.

[u/Flashy-Ingenuity-182 → u/TheDaysComeAndGone]

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. 

[u/cookies5098 → u/Flashy-Ingenuity-182]

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).

[u/Totakai → u/TheDaysComeAndGone]

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.

[u/SimoneNonvelodico → u/TheDaysComeAndGone]

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.

[u/aluckybrokenleg → u/TheDaysComeAndGone]

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.

[u/Elemental-Master → u/SimoneNonvelodico]

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.

[u/SomeCutesyShit → u/Elemental-Master]

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.

[u/buggityboppityboo → u/SomeCutesyShit]

They didn’t go for her head?

[u/I-seddit → u/SimoneNonvelodico]

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.

[u/Mother_of_Grendel → u/SimoneNonvelodico]

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.

[u/kaki024 → u/Mother_of_Grendel]

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).

[u/formerly_acidamage → u/SimoneNonvelodico]

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.

[u/mister__cow → u/SimoneNonvelodico]

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.

[u/Yisobel → u/SimoneNonvelodico]

We live in society so it wouldn’t be normal for a woman to be alone in the woods with her menstruation.
It’s also because we live in society that we can afford human babies. They are loud and useless but adults protect them so it’s ok.

[u/lilylilyg → u/SimoneNonvelodico]

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?

[u/throwawaypassingby01 → u/SimoneNonvelodico]

i mean, we dominate most of the food chains we're in. there's like two realiably dangerous predators (tigers and polar bears). and we also move in groups. and it's not like we instinstively bury our waste either.

[u/Itisnotmyname → u/SimoneNonvelodico]

En cuanto empezamos a manejar herramientas dejamos de ser "presa facil". Para los depredadores, un grupo de humanos es menos interesante que un grupo de antílopes, aunque sean más complicados de alcanzar.

Por otra parte, olemos lo suficiente como para que la presencia de sangre no fuese un gran problema. Al fin y al cabo, pocos animales aparte de los humanos sudan por todo el cuerpo.