Why Do People Have Periods When Most Mammals Don’t?

Why Do People Have Periods When Most Mammals Don’t?

[♪ INTRO] Humans have been getting their periods since,
well, forever. So you might think that’s just something
that comes along with being a mammal. But… it’s not. Few mammals get periods every month, or at
all. Like, that bleeding thing that female dogs
do? It’s not menstruation. It comes from the vagina, not the uterus,
and happens before ovulation, rather than after. As far as true menstruators go, there are
only a few non-human primates, some species of bats, elephant shrews, and
an African mouse. That’s it. Which just seems cosmically unfair.
And it begs the question: why? After all, you’d think bleeding once a month
would be a really good way to attract predators, not to mention a waste of perfectly good reproductive
tissue. Well, scientists have had several ideas over
the centuries. Some of which have been less great than others. But they seem to finally be getting somewhere
when it comes to understanding what menstruation really is and why it happens. And that’s not only great because it adds
to our understanding of our bodies, it could also help doctors treat conditions
that stem from the reproductive system. Now, part of the reason menstruation is so
poorly understood is very human. It’s that people throughout history have
been quite squeamish about periods. Even talking about “that time of the month,” like today, in the 21st century, is considered
taboo in some circles. So we still use euphemisms around the world
to describe the fact that half of all people between puberty
and middle age have blood and other fluids flow from their
vaginas for a few days each month. But we here at SciShow are not too squeamish
to talk about, well, most things. So we are going to talk about “that time
of the month”, Aunt Flo, the crimson tide, shark week, whatever
you want to call menstruation. Blood. Folks. There’s blood. And also some other stuff. And cramps. And weird poops. So thanks for that, nature. In humans, and those other animals we mentioned, menstruation is a month-long cycle regulated
by hormones. Technically, it starts with the bloody part. The lining of the uterus, or the endometrium,
is shed and expelled, so a mixture of blood, endometrial cells,
mucus, and other tissue flows out of the vagina for three to seven
days. Over this period, hormones slowly trigger
the ovaries to produce around five to twenty tiny sacs called follicles,
each of which contains an immature egg cell. And after a few days of maturing, just one or two of those follicles grows in
diameter and becomes dominant. It gets ready to release a mature egg cell,
while the rest disintegrate. At that point, a sudden surge in luteinizing
hormone prompts the dominant follicle to release its now-mature egg so it can begin
its journey down the fallopian tube. So, great, you’ve got an egg! Next, the body creates a nice place for it
to land. In this part of the process, the spent follicle turns into a tiny hormonal powerhouse called
the corpus luteum, which releases hormones that act on the lining
of the uterus. They tell it to differentiate into the types
of tissues needed for a fertilized embryo to implant and grow;
a process called decidualization. In non-menstruating mammals, this part of
the cycle only happens when a fertilized embryo attaches to the wall
of the uterus. But in humans, that process happens every
month, and it starts before the egg is even fertilized. Which is why scientists call what we do spontaneous
decidualization. And now, many think that’s ultimately why
we menstruate. See, after all that hard work, if the egg
doesn’t get fertilized and implant, the corpus luteum shrinks and dissolves. And since the corpus luteum stops producing the hormones that maintain that thick, decidualized
uterine lining, the uterus sheds the extra bits, kicking off
another crimson tide. Before we knew all this, of course, people had some interesting ideas about the
menstruating body. And by “interesting,” we mean “pretty
preposterous mythology” at the expense of, well, half of humanity. For example, scholars in ancient Rome thought
that contact with menstrual blood could turn crops barren, kill hives of bees,
and drive dogs crazy. Even when people who considered themselves
scientists first started studying menstruation in earnest,
they did a pretty lousy job of it. Like, in 1920, a doctor coined the term menotoxin to denote the supposedly toxic substance in
menstruating people’s sweat. He, because, of course it was a he, observed that flowers wilted when handled
by a menstruating nurse, so clearly, she was emitting some terrible
stuff. That led to the actual hypothesis from scientists
that that’s why people menstruated: to get rid of awful toxins that had built
up inside them. Research into menotoxin continued for decades. Except … there’s no such thing as menotoxin. Scholars today think that the whole idea came
about and persisted simply because of ingrained
misogynistic biases. Still, this whole idea that menstruation was
related to something toxic could be one reason why the first sort-of
plausible, but ultimately-debunked, hypothesis was that
menstruation evolved to flush out pathogens. Though, this time, they were thought to be
sperm-borne pathogens. The idea, proposed in a 1993 paper, hinged
on the fact that bacteria from the genital tract are known
to cling to the tails of sperm cells. And if sperms are squirmy little pathogen
carriers, then menstruation could have evolved to dislodge those hitchhikers and flush them
out of the uterus. But, sperm-borne pathogens are a thing in
species that don’t menstruate, too. And in 1996, a scientist analyzed all of the
data they could find regarding pathogens in the uterus and periods,
and found that neither the number nor abundance of them decreased
following menstruation. So that kind of put a nail in that coffin, and the entire notion that menstruation has
anything to do with “cleaning” the uterus. Instead, that scientist proposed that monthly
menstruation evolved for a much simpler reason: because it’s more energy and resource-efficient. The idea here was that we know the uterine
lining has to be somewhat ready before implantation
can happen. And species could keep their linings in a
perpetual state of readiness, and therefore be ready to get pregnant whenever. Woo hoo! The problem is, maintaining that lining all
the time would take a lot of energy. It’s actually less costly, from an energy
standpoint, to grow a new, implant-ready lining each month. So, we evolved to do that instead. That does sound logical. But it’s ultimately
not a great explanation for menstruation, mostly because no species of mammal we know
of has a constantly-ready uterus. As far as we can tell, every non-egg-laying
mammal that has ever lived has had a menstrual-like cycle where the lining
builds and recedes. Others are just way better at absorbing the
extra tissue they make. Presumably because, without decidualization,
they don’t make as much of it. So, that brings us back to the big question: Why do humans have periods? Why did we go from decidualizing when an egg
implants to doing it spontaneously? The current hypothesis is this: that menstruation is a byproduct of the evolutionary
struggle between the fetus and its host. And yes, the word “host” kind of makes
it sound like fetuses are little parasites… because they kind of are. They’re a somewhat genetically-distinct
organism that’s dependent on another, and whose wants and needs might not line up
perfectly with those of the parent they’re inside
of. And depending on the species, fetuses vary
in just how parasitic they are. There are around four thousand mammals, including
us, that deliver nourishment to their young before
birth through a placenta, an organ attached to the wall of the uterus. All placentas are temporary. They grow at the beginning of pregnancy, and
are expelled immediately following birth. But that’s pretty much where the similarities
end. More to the point, there’s a lot of variability
when it comes to the invasiveness of a placenta, or how deeply it attaches itself to the uterine
wall. A horse or pig’s placenta, for example, only superficially squishes up against the
uterine wall. Dog and cat placentas go a bit deeper, but still don’t make direct contact with
the maternal bloodstream. But lucky us, we have what’s called a hemochorial
placentation. It’s the most invasive kind of placenta,
where the fetal tissue burrows through and actually erodes some of
the maternal endometrial tissue to make direct contact with the host’s bloodstream. And scientists think that spontaneous decidualization could have evolved to mediate this relationship. There are two main hypotheses for how that
would work. The first is that, since spontaneous decidualization is driven by the host’s hormones instead
of signals from the fetus, it could help the host keep a bit of control
over their resources. If the uterine lining is wholly responsive
to the fetus, there’s not much standing in the way of
that little parasite becoming too attached, taking too much from the host, or destroying
too much of the host’s tissues. And that’s especially important if you have
an already-invasive placenta. The second hypothesis is that instead, or in addition to preventing the fetus from
exploiting its parent, decidualization helps people deliver more
healthy babies. See, researchers know that human embryos have
particularly high rates of aneuploidy, which is when there are too few or too many
copies of any specific chromosome. Aneuploidies don’t often keep an embryo
from implanting and growing into a full-term baby, but most of them would prevent that baby from
surviving. It therefore makes sense for the uterus to
have some way of detecting that kind of genetic
red flag early on, so it can terminate the pregnancy earlier
and save resources. At least, from a purely evolutionary perspective. Miscarriages can be so hard, no matter what biology says. But ultimately, spontaneous decidualization
may help make those calls. This is what’s known as the choosy uterus
hypothesis. And there does seem to be some evidence for
it. Like, research suggests that shortly after
they burrow into the uterine wall, embryos with lethal aneuploidies are more
metabolically active than healthy ones. Probably since they’re doing their best
to stay alive despite having a lethal number of chromosomes. It’s thought that there’s something special
about the decidualized cells in the outer layers of the endometrium that
allows them to sense this overactivity. And when they do, they basically slough off,
leading to early miscarriage. The remaining piece of the puzzle is understanding
exactly how the endometrium being decidualized prior to
implantation makes a difference when it comes to choosing embryos. And if we figure that out, it wouldn’t just
explain why we menstruate, it could help treat many cases of infertility
and recurrent pregnancy loss. In the end though, there’s more work to
be done, scientists are somewhat confident that our monthly crimson tide is a side-effect
of spontaneous decidualization. And that, in turn, evolved because of the
delicate evolutionary balance between being able to get and stay pregnant and not
get totally ransacked by the fetus. Now, they want to understand how spontaneous
decidualization actually evolved. That should help quench our curiosity about why the heck half of us get a monthly visit
from Aunt Flo. Nobody really likes her, we just tolerate
her because we have to, am I right? And it should also help us better understand
how the reproductive system works in general, so we can find ways to help people whose uteruses
and ovaries don’t work as well as they’d like or make
them sick. Thanks for watching this episode of SciShow! And thanks to Rolando, who asked about this
through our Patron Quick Question Inbox. It turned out the answer wasn’t so quick,
but that’s OK! We enjoyed answering it all the same. Access to our QQ inbox is just one of the
many joys of being a SciShow patron on Patreon. Patrons also have access to our discord, where
they can talk about science and YouTube and the like with other patrons
and members of our team. Most importantly, though, being a SciShow
patron means you’re a part of a wonderful community
of people who think free, educational science content
is worth supporting. And I may be a bit biased, but I think those
people are really awesome. And if you’re curious, you can learn more
about our patron community at Patreon.com/SciShow. [♪ OUTRO]


  1. Now why would I, the host be in crippling pain before (PMS – migraine, bloating, mood swings from hell) which advance to acute cramps that feel like some 😈 "internal parasite" is poking me with gazillion needles, sawing me open and allowing shock therapy with an electric current (acute abdominal cramps, lower back ache, migraine on the left side of my temple, eye, and jaw. Vomiting and diarrhoea) then finally after my morbid menses are done on the third day… I still suffer from acute migraines. Now I am on narcotics to manage the pain… My menses are my phobia😑😞😕😖😲😩😥😫

  2. ^gratefulness for iud intensifies^

    (despite it being one of the more painful things I experience) oh the joys of being a uterus owner. 🙃

  3. I am a male and medical professional(not dealing with that aspect directly) and I was always squirmy with this stuff in school and it sucks that people like me have held back needed research in woman’s health. We need to talk about this more so that is less scary. Here here.

    I wonder if there is any difference in some of these proposed features in young vs old and regular periods versus being on the pill or some other system with few or no periods … could it be a preparation/ conditioning of sorts.

    I wonder what things we could help with like endometriosis and infertility issues looking down this avenue.

  4. Would love to see more SciShow videos about these sorts of topics! So interesting, and also important for reducing taboos and starting positive conversation!!

  5. For all the people complaining about how expensive a period is>>> https://putacupinit.com/quiz/ and https://www.google.com/search?q=cloth+pads&sxsrf=ACYBGNRGsOAC4laJVUeTEO9uhkOhm72rUA:1581440693009&source=lnms&tbm=shop&sa=X&ved=2ahUKEwibzfXV_cnnAhWOFMAKHYU_B6gQ_AUoAXoECA0QAw&biw=1777&bih=876

    Go eco and savin money lol

  6. A fetus is not a parasite for several reasons.
    1. A parasite is a different species
    2. A parasite is not something that your body and the body of your partner created.
    3. A fetus is created by the body so that the species can live on and so that your genes can be passed on.
    4. A parasite takes from the host usually without the host doing anything on purpose to help it. A fetus takes nutrients because nutrients are being given to it by the mother.
    5. A fetus is literally made using half of the mother's chromosomes.
    6. Helping a child even when it's not convenient for the parent is called parenting and even though it may not be helpful for the mother her body is still supporting the child so that her Gene's are supported.
    6.You can't go through the process that's makes the baby and then blame the baby like it came out of nowhere.

    Calling a fetus a parasite turns my stomach because it's like calling an infant or a child a parasite. An infant cannot live on it's on and neither can children but calling them parasites would be looked down upon by society.

    You may not have wanted the baby, but your body did and you chose to have sex so you have to deal with the consequences that you created a human child.

  7. I can't believe instead of "mother and child" it's now "host and invasive fetus."
    So I was a "invasive fetus parasite", hu?
    Sorry, you're just wrong.
    My host said so. 😂

    (But in all seriousness, my parents would be horrified to hear parenthood seriously spoken of in that way. I'm pretty disgusted myself.)

  8. For those who think an infallible god created us, this video should be "must" viewing.
    Add this unnecessary monthly cycle to a decidedly inadequate and inappropriate birth canal and ya gotta observe that he either sucked in the omniscient department or was misogynistic from before the beginning, or both.

  9. I find menstruation as an unfair punishment just because wamen don't want to get pregnant like, /now/, right when the uterus feels like it, and then after they do have one baby or more it keeps pushing their bodies to get ready to get pregnant, it's very cruel from nature, I've heard that men go through something similar but idk if that's true somehow, there must be a process related to sperm like to renew it for a better stock of them and expulsing the old and not so good ones or something

  10. In my country, some people still have the belief that a menstruation woman can't be around plants. My mom doesn't let us sit in our balcony or get around her plants around the house when we are on our period lol

  11. Wow so much science mambo jumbo… I'm not sure even the speaker understand fully what she is talking about. You must be doctor or something to understand this.

  12. I thought this would mention the fact that we have to prepare our bodies for fertilisation because it can happen at any time. While most other mammals have seasons. So their bodies know that this time fertilisation will happen. It doesn't over prepare so its easily absorbed. We have to flush ours out each month and go through the process of will she won't she each month. A lot of energy goes into keeping up this frantic pace so it terminates the whole process rather than keep it going perpetually

  13. Millennials: "Babies are parasites."


  14. I've been waiting for my period for the past 2 days. A few hours I finally had my period poop and now I've just started bleeding. Only a few days and I'll be all good for the next few weeks which I'm looking forward to

  15. Thanks for the info! May I point out that your use of the term "parasitic" in reference to a human fetus is incorrect as parasites are scientifically different species.

  16. I could really do without the retelling of incorrect ideas. I don't think we should ignore/forget our misogynistic past, but maybe let's keep blatant falsities separate from videos meant to educate us about reality. Otherwise this vid is AWESOME.

  17. 12 minute video about menstruation and it's obvious how careful you were not to day out loud that it's women who menstruate. Sad how the crazy ppl now force supposedly science- sites/channels into talking in code about even self evident facts. Just to avoid attacks from the crazy ppl on the far left.

    Also, superstition was a big thing back in the day. It wasn't just about menstruation. You can stop trying to play a victim now. Everyone was subjected to superstition back then.

    And referring to unborn babies as parasites. Well, if you read the actual definition you'd have known that parasites not only luve on or in the host body, they also give nothing back. Remembering that a big part of having children is to get help when you become old, refuted the proposition that babies are parasites. They just give back with a delay. Unless ofc you consider that many women do enjoy being pregnant and therefore get that back already.

    You on the other hand, sound like you hate being a woman altogether.

  18. I read something else in the thumbnail and I thought Scishow was about to give me some good reasons for self justification.. sure enough, it said "menstruation" 🙁

  19. Also, not one mention of mother. Would it kill you to actually refer to the 'host' as a mother??? 'People' who get pregnant are women. Women, you know, those who have xx genes? Sheesh.

  20. It is amazing the wonderful way that we are made. Science does such great work in understanding the what and how of our bodies. Too bad the why is dismissed with not yet knowing.

  21. She lost me when she called unborn babys parasites, like is she serious. Also anamils that care unborn of their own kind don't get periods.

  22. If you handle the hormones from the BC pill pretty well, and you don't want to have kids, you can talk to your Dr. about a daily low dose BC that is truly daily. I get mine in 90 day packs and just take it every day with no gap weeks. I haven't had an actual period in years. After a while, even the breakthrough bleeding stops. My horrific, debilitating cramps are only a faint memory and I don't even remember the last time I needed to buy sanitary napkins. It is a feeling of true freedom and makes traveling or camping so much less of a pain.

  23. The video is interesting. I just find the choice of terminology of "host" and "parasite" odd and off-putting. Not a good frame of reference, gor example for people, who are already afraid of or anxious towards pregnancy. I understand why they did it. But it would have sufficed to use and explain the terms once instead of sticking to them for the rest of the video.

    The logic of the theories would have come across anyway, even just using the terms "mother/parent" and "child/fetus/baby".

  24. I've literally been saying for YEARS that it makes NO SENSE from an evolutionary/natural selection stand point for us to bleed all over the place once a month just to leave our sent and set a trail for predators to come kill us

  25. Let's celebrate Bleeding Person's Rights while I call my host and tell them I love them and thank them for not exterminating me, as a parasite

  26. So I imagine the nose piercing was a personal aesthetic choice. However on a YouTube video you have tiny boogers just hanging there constantly, Very very distracting.

  27. This still doesn’t explain the pain during periods (unless i misheard something). When I had mine, I couldn’t move. I couldn’t eat. I couldn’t sleep. I couldn’t go to the bathroom. I couldn’t study. I once fainted due to the amount of pain I experienced. I would describe the pain as something close to a person jumping on my stomach while having a knife shoved into me. Why the hell do some of us experience severe pain during periods that would literally incapacitate us for 3-7 days? (flashback to that time when one of mine lasted for 10 straight days 🙃)

    I’m very glad i’m on the pill now as I was very close to just removing my uterus myself.

  28. I have pmdd and right before my period I become crazy. Suicidal, anxiety feeling like I can’t breathe, crying nonstop, literally hitting myself due to how out of control I feel. I’ve been trying to get on the pill but the ones I’ve tried have caused me severe migraines. I really hope I can find a cure, I can’t live with this every month. 😥

  29. Minor correction: Even in humans, the parent and fetus never actually mingle their bloodstream. They pass very close, close enough to allow nutrient/waste exchange, but each maintains their own distinct circulatory system.

  30. Yup totally parasites, and yet misogynists in government wish to take away the woman's right to choose whether or not to keep the parasite inside her body!… I'm glad I've had a full hysterectomy (took ovaries too) Periods are terrible, at least mine were!

  31. Just one (not actually a complaint) complaint: the fact that other mammals also experience sperm based hitchhikers (forget actually wording, sorry) doesn't mean we'd all evolve to deal with such invasions in the same way so that's not a good reason to declare this hypothesis dead.

    The reason(s?) given immediately after that part tho, great reason to declare that hypothesis dead!

    I paused at 5:47 so the part I'm referring to happened shortly before then.

  32. Omg, I totally started crying when you called the embryo a parasite. I mean, that's what it is, right? A good little parasite which we'd love to have. But which could be also deadly, if the balance between mother's and embryo's needs is broken.
    It's remarkable how intelligent our bodies are. And it's very sad that even today people fear saying the word "menstruate". Yesterday in an ad, they were talking about "those days". What the hell?! Female physiology isn't dirty or toxic, it's what gives life, selects life and preserve life. And menstruation is f*ing bleeding, what's so horrible about bleeding. If you cut your finger, you bleed, not such a big drama.

  33. I'm not female but I've two questions:
    1) Why do some people have troublesome periods? What's the cause(s) of that?
    2) Did you choose the colour of that sweater just for this video? 😉

  34. I'd love for someone to explain why during the period (mine, at least) digestion hours all nuts for that week. I asked my gynecologist once and he just shrugged.

  35. It has been noted that most mammals have estrus periods at certain times of the year but the human female can be receptive to males and can get pregnant on a monthly basis.

  36. I call it Death Week, not because I find it gross to say period or talk about it. I just hate it and its like death of my lower half because of the pain and heavy flow I have. I'm also gender fluid, so when I'm bleeding and theres bodily focus on my female parts, and I'm not feeling feminine, then it confuses my head and I feel very out of it

  37. The language used by the host is disgusting. Calling fetuses "parasites" has nothing to do with science. Of course pregnancy has "costs" for the mother, but "parasitic" is not an appropriate term to qualify the mother-fetus relationship.

  38. Thanks for sharing this video though. As much as I hate saying it.. but it's true… I've learned more from this video than I did in school from our politically "correct" public education system. Not to mention having a family that somehow managed to view nudity/sex as something super evil.. but were fine with us watching hack and slash movies. American society at it's finest! (Sarcasm)

    I think the biggest mystery in all of human nature is how a lot of other men in the world act so tough and endorse hunting or violent media/content…. But are grossed and weirded out by a woman's menstruation cycle. I'm sure as well the small portion of other men out there who also realize that ridiculous logic of our male kin are very rare. And props to any other dude who agrees with me.

    Society is backwards as hell. From what I've just mentioned… To mental health stigma… I really believe that if humanity focused on trying to understand things better rather than jumping to conclusions. We'd be way further in reaching our goals and even the advancement of technology.

    Ignorance.. From sexism, to racism, to mental health stigma… It's the property of humanity that has been around since the beginning of the age of humanity… I'm surprised it's 2020 and people still judge others based on skin color, gender, or disability. Technology advances and changes.. But human ignorance always stays the same.. Lol ironic.

  39. I don’t appreciate when you guys say people or humans have menstrual cycles. Until this day women are the only ones who has menstrual cycles is not humans or people because if it was like that, that would include men and men don’t have menstrual cycles.

  40. So abortion is the removal of a parasite from your body. Works for me. (and here come the haters in 3… 2… 1…)

  41. Funny how there seem to be so few comments left by men and almost all of the ones I've seen are about how "the title should say women, not people😡😖😠"…

  42. 4:30 when I was a small child, in the early 1970s, I found my mother's pads, asked her what they were all about, and then of course asked why women had to bleed every month. She said it was to get rid of the bad blood. That was my understanding of it until sex ed in school.

    For a while, I looked back on that and thought that she had merely said that because it was an easy answer, easier than explaining it. I've come to realize that it probably was her understanding of it. She was born in 1941. Based on this video, I'm now convinced that this was the explanation that she was given as a child.

    Note: with some Googling, I found that this continued to be a belief with people doing research into it right through the 1950s, when people started disproving it. My mother would indeed have grown up believing that the body was purging itself of toxins.

  43. I talk pretty openly about menstruation and pads and cups and stuff like that… It didn‘t gross out anyone yet. I‘m so glad I live in a time and place where that’s possible.

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