

Why do women have periods when most animals don't? - dataminer
http://www.bbc.com/earth/story/20150420-why-do-women-have-periods

======
madez
There is a highly complex biochemical relationship between the embryo and the
mother.

The mother actively tries to kill anything that might be in her uterus, she
even periodically cleans her uterus radically by ejecting a layer of it.

The embryo needs to produce hormones that manipulate the mother to stop that,
and deliver them into the mother.

If the embryo fails, it dies.

Source: [http://aeon.co/magazine/science/pregnancy-is-a-
battleground-...](http://aeon.co/magazine/science/pregnancy-is-a-battleground-
between-mother-father-and-baby/)

We do something similar in software development. If our program experiences a
segfault, we usually let it die, instead of figuring out at runtime what went
wrong and trying to do what we still can do. Then we change something and try
again.

Making it hard for the embryo in the beginning and starting over if it didn't
make it, is the biological equivalent to that strategy.

It might result in a heuristic whether it's worth investing more resources in
it. Saving resources gives an evolutionary advantage.

On the other hand, it makes evolutionary progress harder since more conformity
of the embryo is a condition to survive.

------
derefr
tl;dr:

What we know for sure: foetuses produce hormones to keep the endometrial
tissue from thinning. Once a zygote has implanted, the mother's hormones are
thus no longer in control of whether her uterus will support its growth or
not. Menstruation, when it happens, forcefully cleans out implanted zygotes,
returning the uterine hormonal balance to normal.

What we guess that that implies: menstruation is a side-effect of enabling
spontaneous abortion (miscarriage) of a still-live foetus, thus allowing
females of those species to not have to go through the "avoid males you're not
interested in during estrus" dance that most animals do. This confers social
advantages.

In humans specifically, estrus is hidden completely from both male and female
awareness. This confers further social advantages, but only could have evolved
to fixation if the sexual-selection advantage of applying a "proximity during
estrus" filter was completely outmoded/obviated by miscarriage.

~~~
ars
The problem with that theory is I don't know of any women who are actually
able to miscarry at will.

A garbled version of that theory got some politicians in hot water with the
whole "during rape, their body can shut it down" comment.

~~~
derefr
It's not in the control of the conscious mind. It _is_ controlled by the body
(or, you might say, by an evolutionary adaptation instantiated in the linkages
of hormonal pathways in the brain.)

Note that miscarriage and spontaneous abortion are separate things.
Miscarriage happens because the foetus dies in the womb, or the womb is
ruptured, and the mother's body detects this. Any other cause is spontaneous
abortion—effectively "something the mother['s body] did."

In humans, at least the experience of the emotions of extreme stress, and
extreme fear, can cause spontaneous abortion. In a number of other animals
(with a large subset of those being the ones that menstruate), there is the
Bruce effect[1]: when the detection by the vomeronasal organ of an unfamiliar
male's pheromonal signature precedes/causes spontaneous abortion.

[1]
[http://en.wikipedia.org/wiki/Bruce_effect](http://en.wikipedia.org/wiki/Bruce_effect)

~~~
XorNot
That is a poor explanation given that extreme stress is known to have all
sorts of physiological effects and is noted to negatively effect reproductive
success in numerous species. But not all those species menstruate as noted by
the article.

~~~
derefr
The thing muddying the water with stress, is that most things that _trigger_
stress are also bad for the body, and thus for foetal development. Making rats
run until they collapse from exhaustion might cause miscarriage, but it's
unclear whether that is due to stress-hormone-induced spontaneous abortion, or
due to the mother's body using up some micronutrient necessary for the foetus,
thus causing a developmental defect which either kills the foetus or stalls
its metabolism enough to be detected.

I'm unaware of whether there has been the study of the effects of _purely
psychological_ (or synthetic, using injected epinephrine) stress on
spontaneous abortion rates.

It's much clearer with fear, though; things that cause a fear response don't
tend to harm you (in fact, in an adaptive environment, to the degree that a
fear response is created, you tend to avoid harm you would otherwise succumb
to) but do trigger spontaneous abortion anyway. This would be much simpler to
prove/disprove a purely-hormonal effect.

------
ars
To answer in short: No one knows.

All sorts of theories in the article, none with any real evidence, except
"this makes sense to me".

------
acqq
"most animals don't"? What do they mean? Dogs definitely do have a kind of
"bleeding" as (hopefully) any dog owner knows.

[https://answers.yahoo.com/question/index?qid=20060612224917A...](https://answers.yahoo.com/question/index?qid=20060612224917AADVpCu)

It's however not "the same" as the one in primates:

[http://en.wikipedia.org/wiki/Menstruation_%28mammal%29](http://en.wikipedia.org/wiki/Menstruation_%28mammal%29)

"Some species, such as domestic dogs, experience small amounts of vaginal
bleeding while in heat; this discharge has a different physiologic cause than
menstruation."

And if we talk about "periods" (like in the article title) and not just
"bleeding," the fertility "periods" are everywhere.

[http://en.wikipedia.org/wiki/Spawn_%28biology%29](http://en.wikipedia.org/wiki/Spawn_%28biology%29)

Even corals have spawn cycles.

Most animals do have fertility "periods" they just don't bleed. But again, we
are primates, and a lot of primates do have the same menstruation that the
humans have. So humans certainly aren't special in that aspect.

~~~
itsybitsycoder
So you agree that most animals don't menstruate... what's wrong with the
article's title?

EDIT: Oh, I think I get it now. "Period" in this context does not refer to the
general fertility cycle. It is specifically a euphemism for menstruation. That
is why you'll hear people saying things like "I'm on my period", "my period
started late/early", etc. These phrases would not make sense if it referred to
the fertility cycle in general, since from puberty to menopause, most healthy
women are in some phase of a fertility cycle.

~~~
acqq
Yes, and even if the topic is just bleeding, humans aren't special, so it
still shouldn't be "why do women bleed" but "why just primates, bats and one
kind of shrew bleed during estrous cycle." Humans simply aren't special in
that regard. And the whole "mystery" doesn't appear so dramatic anymore. It
becomes obvious that that "optimization" developed more than once.

------
dhimes
Please fix the apostrophe in "animal's" It is annoying and leads non-native
speakers on the board astray. Thank you.

To non-native speakers: apostrophes are for possession, not plural.

~~~
jacquesm
> To non-native speakers: apostrophes are for possession, not plural.

You could easily repeat that for plenty of native speakers. It's one of the
most common mistakes in written English.

~~~
profinger
Isn't it sad? It's such a frustrating mistake.

There's a bar that we drive past frequently that does it with everything.

They're "Now accepting application's" and their "Burger's are $2"

Sad.

~~~
drostie
Since I moved to Missouri, I've been upset by a similar grammatical error:
quotation marks used for emphasis. You would expect, say, all-capitals or an
underline or italics, but you instead see 'Come taste the "best" burger in
Missouri,' which is awful.

In New York, where I grew up, that use of quotation marks is called "scare
quotes": quotation marks in all cases signal that you're detaching a text
fragment from its context; with scare quotes you detach a phrase which reads
just fine in that context to either bring focus to the phrase (like when
you're defining it) or to indicate that the phrase itself is somehow
problematic (like when you want to criticize the phrase, e.g. 'these
"businessmen" are stealing our money,' where you're implying that they are
either not truly men or not truly doing _business_ per se.)

When you say that you have 'the "best" burger', to me it sounds like you're
implicitly adding at the end, 'well, at least, it's the best at being an awful
burger.'

~~~
panglott
This is a common practice in signs. It's wrong in literary English, but is it
wrong in signs?

My pet theory is that quotes for emphasis were popularized in 20th century
movie posters, where it was common to surround the movie title in quotations
(this is still the accepted citation style in, say MLA style). This is
probably wrong, of course.

Italics or bold for emphasis can be hard to hand-write, and the quotation
marks for emphasis probably look more legible on chalk signs.

------
amyjess
Am I the only one bugged by the apostrophe in "animal's"?

~~~
herbig
I'm more curious as to whether the original article was published with an
apostrophe, or the HN poster actually typed the title out from memory rather
than cut/paste.

------
daktanis
Weird thing for hacker news

~~~
dreamdu5t
Downvoted but you have a point. Why is there _no_ moderation with regards to
topical content on HN? Why should HN be a place for pop-sci articles and
repostings from the likes of the Economist, LA Times, BBC, etc. r/startups has
more content pertaining to - you know - actually starting a technology
business than HN does. HN lately reads like newsclippings sent to me by my
grandma.

~~~
jacquesm
A glance at the homepage makes me wonder if your grandma is a former co-worker
of Grace Hopper or something like that.

~~~
kzhahou
You should understand that "my mom" and "my grandmother" are commonly used to
denote technical illiteracy/incompetence and inability to keep up with the
latest technological advances. This usage remans an unchallenged bastion of
both sexism AND ageism -- a double whammy!

Proper usage:

* This app is so easy, my mom could use it!

* This article is easy to follow and avoids too much jargon -- my grandma would even understand most of it!

... you get the picture.

~~~
7Z7
You should understand that jacquesm made a pretty sweet joke.

~~~
kzhahou
Jacquesm indeed made a sweet joke. No, he wasn't the one who used the "my
grandma" line. It was the person he was replying to, who wrote:

> HN lately reads like newsclippings sent to me by my grandma

which continues to propagate the insidious, ageist and sexist assertion that
older women don't get high technology.

------
ttflee
A little OT, but I am always tempted to think whether the lunar calendar has
something to do with menstruation or not.

------
ripter
> It seems the list of animals that menstruate is quite short: humans, apes,
> monkeys, bats and elephant shrews. What do these seemingly disparate animals
> have in common?

My unfixed female dogs have periods. So either the article is poorly written,
or they are ignoring animals that don't fit the theory.

~~~
rosser
Dogs don't have periods; they have estrous, commonly called "heat".

[https://en.wikipedia.org/wiki/Estrous_cycle](https://en.wikipedia.org/wiki/Estrous_cycle)
\- note specifically #Differences_from_the_menstrual_cycle

~~~
ripter
Dogs bleed. It might not be a 'period', but the article is talking about why
some animals bleed and most don't.

~~~
rosser
Well, if you're going to attempt to be pedantic about it, the word "bleeding"
isn't exactly correct to begin with. The discharge is endometrial tissue, not
blood.

~~~
ripter
Before calling names, let's look at the article in detail.

"Of course, I wasn't the only one: most women menstruate. But most other
female animals don't bleed outwardly like us. Even among those that give birth
to live young as we do, only a handful of species menstruate."

So the term menstruate is used to mean to "bleed outwardly like us".

"If the woman doesn't get pregnant, progesterone levels begin to fall. The
thick endometrial tissue with its blood vessels then begins to slough off, and
passes out through the vagina. This bleeding is menstruation."

So they are expanding on the term menstruate. Now it's both "bleed outwardly
like us" and "The thick endometrial tissue with its blood vessels then begins
to slough off, and passes out through the vagina"

"On average, women lose 30 to 90 ml of fluid over 3-7 days of menstruation. We
know, because scientists have given women pre-weighed pads and tampons, and
weighed them again after use."

Clarifying the term menstruate and showing how to measure it. I'm going to
paraphrase the definition of the term. So at this point, when they talk about
menstruation, they are talking about a blood like fluid, that can be measured,
and that happens when the the tissue and blood vessels pass out of the vagina.

The article has a break to talk about the history, theories, and research that
has happened over the years. After that they get to the main point. "To figure
out the truth, we need to compare animals that do and don't menstruate."

"Great apes do it too. Menstrual bleeding is easily detectable in chimpanzees
and gibbons. However, gorillas and orang-utans bleed less copiously, so
menstruation is only visible on closer inspection. Other primates, such as
tarsiers, may also menstruate, but there is little hard evidence."

They re-iterate key parts of the term. When this article talks about
menstruation, they are specifically talking about detectable bleeding.

Now we can get to the heart of my objection. They spent most of the article
defining their usage of the term menstruation, and how it relates to the
research.

The article claims "It seems the list of animals that menstruate is quite
short: humans, apes, monkeys, bats and elephant shrews. What do these
seemingly disparate animals have in common?", remember the definition they use
for menstruate, blood and tissue passing though the vagina.

If you've ever owned an unfix female dog during heat, you'll be quite aware
that they do, in fact bleed out their vagina for a period of time during their
reproductive cycle. This fits with their definition.

The only mention of dogs is: "In dogs and cats, the foetuses dig in a little
more. But in humans and other primates, a foetus will dig through all the womb
lining to directly bathe in its mother's blood." This does not specify why
dogs do not fit into their term. In fact this seems to support that dogs have
a very similar cycle to humans.

So my question remains, Why are dogs excluded? They never acknowledge the
contradiction. So either the article incorrectly sets up the terms, or they
don't want to talk about dogs because it doesn't fit the narrative.

~~~
rosser
Look, your quibble isn't with me; it's with the biologists and the terms
they've chosen to use for the different (and they _are_ different)
physiological phenomena. I'm using _their_ terms here, in the ways they've
defined them.

If you're utterly convinced that estrous and menstruation are the same, on the
basis of some blood-like discharge occurring in both, then you might want to
explore a career in academia. I'm sure there's a journal somewhere that would
take your well-reasoned and compelling thesis on the subject. (You might need
to pay them for the privilege, but as assiduously as you're defending your
position here, that doesn't seem too unreasonable.)

~~~
ripter
> If you're utterly convinced that estrous and menstruation are the same

You've completely missed my point.

