Why do women have periods?

I got my first period when I was only 11.
In the first few years, my periods were not only a
source of occasional embarrassment, they were
also excruciatingly painful. I would lie curled up in
my bed with a hot water bag, unable to move or
breathe, and I would wonder: "why me?"

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 menstruation isn't just inconvenient and
unpleasant: it's also a mystery. Why do we
menstruate at all? And if it's such a good idea,
why don't other animals do it?
Menstruation is part of a woman's reproductive
cycle. Every month, in response to reproductive
hormones – mainly oestrogen and progesterone –
a woman's womb gets ready for pregnancy.
Women lose 30 to 90 ml of fluid over 3-7
days of menstruation
The inner lining of the womb, known as the
endometrium, prepares for an embryo to implant
in it. The endometrium thickens, divides into
different layers and develops an extensive network
of blood vessels.
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.
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.
On the face of it, this seems like a rather wasteful
process. As a result, many people have attempted
to explain why we do it.
"Some of the early thoughts on menstruation
were that it was removing toxins from the body,"
says Kathryn Clancy , an anthropologist at the
University of Illinois in Urbana.
Schick concluded that menstruating women
excreted toxic substances from their skin
Much of the research in the early 1900s was
coloured by deeply entrenched taboos against
menstruating women, some of which persist even
today .
For instance, Bela Schick, a popular physician,
conceived the term 'menotoxin' in 1920. He ran
experiments in which menstruating and non-
menstruating women handled flowers. Schick
concluded that menstruating women excreted
toxic substances from their skin that caused
flowers to wilt
.
These menotoxins, according to Schick, also
stopped the growth of yeast and prevented dough
from rising. Schick postulated that the
menotoxins might seep into a menstruating
woman's sweat as well.
Others corroborated his conclusions, concluding
that toxins from menstruating women could
wither plants and spoil beer, wine and pickles.
Most of these studies were very poorly
designed
"There was this idea at this time that women are
just awful and disgusting," Clancy says. "The
problem is that they tried to keep saying this all
the way to the 70s."
In reality, Clancy says that most of these studies
were very poorly designed, so they didn't really
show that menotoxins exist.
In 1993, a very different hypothesis about the
function of menstruation captured popular media
attention . Margie Profet, then at the University of
California Berkeley, suggested that menstruation's
function is to " defend against pathogens
transported to the uterus [womb] by sperm
".
"Instead of saying that it is women that are dirty,
she said that it is men who are dirty," Clancy
says. "We need to flush out the dirtiness of men
in order to reduce our chances of venereal
disease."
Scientists found no link between female
promiscuity and menstrual bleeding
Profet's idea quickly fell down for lack of
evidence. For instance, it predicts that there
should be more disease-causing organisms in
the womb before menstruation than after
. But this wasn't borne out. Some studies even
suggested that menstruation increases the risk of
infection, because bacteria grow well in blood,
which is rich in iron, proteins and sugars.
Moreover, during menstruation there is less
mucus around the cervix, making it easier for
bacteria to enter.
Profet made another prediction. If the females of
a species mate with multiple partners, they
should bleed more, because they would be at
greater risk of exposure to sperm-borne diseases.
But scientists found no link between female
promiscuity and menstrual bleeding.
One of Profet's leading critics was anthropologist
Beverly Strassmann of the University of Michigan
in Ann Arbor. In 1996, she put forward her own
idea.
Strassmann argued that if we want to understand
why menstruation occurs, we must figure out why
animals' wombs go through reproductive cycles
: not just humans, but other mammals too.
It costs a lot of energy to maintain a thick,
blood-filled layer
Other female mammals build up the inner walls of
their wombs just like women do. If they don't get
pregnant, they either re-absorb this material, or
let it bleed out.
It costs a lot of energy to maintain a thick, blood-
filled layer inside the womb all the time,
Strassmann argued. It might actually use less
energy to tear down the lining and regrow it. "I
used energy economy, not to actually explain the
bleeding, but to explain why there is this cyclicity
in the first place," Strassmann says.
Then it's a matter of whether the female can re-
absorb all the blood efficiently. If there is too
much, it might be simpler to menstruate. "The
fact that there is blood loss in some species is
not an adaptation, but a side effect of species
anatomy and physiology," says Strassmann.
Strassmann was not alone in proposing that
menstruation is a by-product, rather than
something evolution had specifically favoured.
Embryos have pushed deeper and deeper
into the mother's tissue
Colin Finn, then at the University of Liverpool in
the UK, suggested something similar in 1998. His
idea was that menstruation is a necessary
consequence of the way the womb evolved
, rather than a way to conserve energy as
Strassmann had suggested.
According to Finn, embryos have pushed deeper
and deeper into the mother's tissue, and the
womb lining has defended itself against the
embryo by thickening and forming layers. This
thick lining is perfectly receptive to the embryo,
but only for a few days. After that, if the woman
isn't pregnant, the lining must be got rid of.
Both these ideas are neat. To figure out the truth,
we need to compare animals that do and don't
menstruate.
Apart from humans, most of the other
menstruating animals are primates, the group that
includes monkeys and apes as well as humans.
Most monkeys living in Africa and Asia, such as
rhesus macaques, 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.
Other than our close relatives, menstruation also
evolved independently in two other groups: some
bats and elephant shrews.
Bats that menstruate belong to two families
called the free-tailed bats and the leaf-nosed
bats , says John J Rasweiler IV, a retired professor
from the State University of New York , and an
expert on bat reproduction.
The list of animals that menstruate is quite
short
In these species, menstruation is similar to that
of humans. For instance, short-tailed fruit bats
have cycles lasting 21-27 days
, almost as long as that of humans, says
Rasweiler. Menstruation also appears to last a
similar length of time.
However, bleeding in bats is not as clearly visible
as in humans. "This is understandable because
the menstruating bats are much smaller animals
and have smaller blood vessels vascularizing their
uterine lining," says Rasweiler.
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?
It all comes down to how much control the
mother animal has over her own womb, according
to Deena Emera of Yale University in New Haven,
Connecticut. In a paper published in 2011, Emera
and her colleagues pointed out that in
menstruating animals, the transformation of the
womb wall is entirely controlled by the mother
, using the hormone progesterone.
These changes to the womb are triggered by
signals from the embryo
Embryos can only implant in the womb wall if it is
thick and has specialised large cells, so this
means the female is effectively controlling
whether or not she can get pregnant. This ability
is called "spontaneous decidualisation".
In most other mammals, these changes to the
womb are triggered by signals from the embryo.
In effect, the womb lining thickens in response to
pregnancy.
"There's a nice correlation between species that
menstruate and species that exhibit spontaneous
decidualisation," says Emera.
Assuming this pattern holds, Emera seems to
have identified the key question. Why do some
females control their own womb linings, while
others allow their unborn embryos to control
them?
"We argue that spontaneous decidualisation likely
evolved because of the conflict between the
mother and the foetus," Emera says.
A foetus will dig through all the womb lining
to directly bathe in its mother's blood
"We put forward two possibilities, especially in
primates." The first is that spontaneous
decidualisation may have evolved to protect the
mother from an aggressive foetus.
All foetuses burrow into the linings of their
mothers' wombs, in search of nourishment. But
some do this more than others.
In horses, cows and pigs, the embryo simply sits
on the surface of the womb lining. 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.
That's because mothers and babies are engaged
in an "evolutionary tug-of-war", says Elizabeth
Rowe of Purdue University in West Lafayette,
Indiana.
The mother responded by putting up her
defences
The mother wants to ration how much nutrients
she gives to each baby, so she has some left and
can have more babies. On the other hand, the
developing baby wants to get as much energy
from its mother as it can.
"As the foetus became more aggressive, the
mother responded by putting up her defences
before the invasion actually began," says Emera.
The second possibility is that spontaneous
decidualisation evolved to get rid of bad embryos.
Human embryos are very prone to genetic
abnormalities
, which is why so many pregnancies fail in the
first few weeks. This might because of our
unusual sexual habits, says Emera.
An egg may be several days old by the time
it gets fertilised
"Humans can copulate anytime during the
reproductive cycle, unlike many other mammals
that copulate right around ovulation," says Emera.
This is called "extended copulation". Other
menstruating primates, some of the menstruating
bat species, and the elephant shrew all engage in
extended copulation.
As a result, an egg may be several days old by
the time it gets fertilised, says Emera. Ageing
eggs may result in abnormal embryos.
Once the womb lining has thickened and changed,
its cells develop the ability to recognise and
respond to defective embryos. So spontaneous
decidualisation may be a way for the mother to
save her resources, says Emera. "It prevents her
from investing in a bad embryo, lets her get rid of
it right away, and primes her body for another
successful pregnancy."
This makes a fair bit of sense. Almost all
menstruating mammals have long pregnancies
and invest a lot in producing one or two babies at
a time. As a result, losing even a single baby
comes at a high cost, so evolution would favour
anything that helped avoid doomed pregnancies.
Human menstruation is an accidental by-
product of how our reproduction evolved
In line with this idea, a study published in 2008
found that rhesus macaque embryos are also
prone to genetic abnormalities
. But we don't have similar data for many other
species, says Emera, so this idea can't be
properly tested.
While we can't yet be sure why spontaneous
decidualisation evolved, we are still closer to
answering the riddle of menstruation. The ideas of
Strassmann, Finn and Emera all suggest that
human menstruation is an accidental by-product
of how our reproduction evolved. It could be a
consequence of our aggressive foetuses, or our
habit of mating regardless of whether women are
ovulating, or both.
In species that reproduce differently, menstruation
never had to happen. In fact, menstruation itself
used to be a rare event. In the wild, and in some
human societies, it still is.
This is because wild mammals that menstruate
spend most of their time either pregnant or
nursing a baby. It actually takes considerable luck
to catch one of them menstruating, says
Rasweiler.
Menstruation is also rare in human societies that
don't use any form of contraception. There are a
few such "natural fertility" populations even
today, and women in these societies spend most
of their reproductive life either pregnant or
breastfeeding.
Among the Dogon, a natural fertility population in
Mali, Strassmann has found that women have
about 100 periods over their lifetime. This was
probably fairly typical for much of our species'
history.
By contrast, most modern women have 300-500
periods. "What we are experiencing, as part of our
evolution, is very unusual," says Strassmann.
"There are women who are sometimes afraid of
not having a period," says Clancy. "I think
understanding the origins of how our body works
helps us realize that the boundaries of normal are
much wider than we might think. Rather than
pathologising every little tweak and every little
difference we have, maybe we should leave it
alone a lot more."
What would 11-year-old me have made of these
ideas? None of them would have made my first
periods any less painful. But I might have felt a
bit better if I had been able to see my discomfort
from such a broad perspective.