The uterus prepares for pregnancy each month as levels of reproductive hormones, primarily estrogen and progesterone, rise. In preparation for receiving a fertilized egg, the endometrium, the innermost layer of the uterus, thickens, separates into layers, and develops a vast network of blood vessels. Progesterone levels begin to decline if pregnancy is not successful. Blood vessels and the thick endometrial tissue start to break down and flow out through the vagina. Also known as menstruation.

A pair of opposable thumbs, the ability to speak, the capacity for thought and reason: there are many reasons to relish being human. Not one of them is having regular vaginal bleeding.

As a species that reproduces sexually, we are taught that menstruation is a normal aspect of the reproductive cycle. Reproductive hormones like progesterone cause the lining of the uterus, known as the endometrium, to thicken and fill with blood vessels before an egg is released from the ovaries each cycle. In the event that the egg is fertilized, the uterus is ready to receive it so it can implant and begin to develop. If not, the endometrium starts to shed and progesterone levels drop quickly. You can expect your usual crimson tide, which will stain your underwear and give you cramps.

It turns out that menstruation is extremely uncommon in the animal kingdom, even in mammals. A few species of bats and elephant shrews also menstruate, though not as frequently as humans. That’s it. The number of myths, taboos and misconceptions surrounding this phenomenon are a testament to how rare and alarming it is – the word taboo itself likely derives from the Polynesian word for menstruation, tapua. Menstrual blood, according to Pliny the Elder, could prevent seeds from germinating, cause plants to wither, and cause fruit to fall from trees. Conveniently, if the menstruator roamed the field naked, it would be free of pests. Due to concerns that their menstrual cycles would contaminate food or reduce men’s virility, people have historically been forced to isolate themselves. Because periods are still widely stigmatized and discussing them openly causes discomfort, menstruation is still poorly understood. The term “period” has more than 5000 euphemisms, according to a recent survey by the developers of the cycle-tracking app Clue. Among my favourites: ペリー来航, Japanese for ‘the arrival of Matthew Perry’ – the naval commander not, sadly, the Friends actor; les anglais ont débarqué, French for ‘the English have landed’, in reference to the red coats the English wore in the Napoleonic wars; and, inexplicably, kommunister i lysthuset, Communists in the gazebo is Danish for it.

It seems like menstruation uses a lot of energy and is wasteful. It would be comparable to thoroughly cleaning your spare bedroom each month in preparation for a visitor you might not even want to have at your home. However, menstruation must have some evolutionary advantage since it has undergone at least three independent evolutionary waves.

It’s common to think of pregnancy as a special period in a person’s life when they can form a close bond with their unborn child. Pregnancy is, in fact, a full-scale evolutionary conflict. The embryo and its genes are on one side of the battleground. Human embryos are among the most intrusive during pregnancy because they try to take as much nutrition from the parent as they can in order to grow and spread their genes. We have the parent on the opposing side. In order to have numerous offspring to pass on their genes, the parent wants to conserve their energy. There is some evidence to suggest that the embryo receives some genetic support from the father, whose genes compete with the parent’s during the early stages of embryo development. The outcomes of each belligerent’s strategy may have been advanced by natural selection.

Menstruation must be viewed as a byproduct of spontaneous decidualization in order for us to comprehend how and why it evolved. The embryo controls decidualization, or the thickening of the uterine wall, in the majority of mammals: it occurs in response to fertilisation rather than in preparation for it. In species that have menstruation, such as humans, spontaneous decidualization is one strategy used by the parent to reclaim control of their uterus from an encroaching embryo. Now that only the parent’s hormones still affect the uterine lining, the parent now has complete control over whether or not they become pregnant. By cutting off the main blood supply from the endometrium before the embryo implants there, they proactively erect defenses. Not satisfied with this, the embryo evolved to penetrate the endometrium and travel to the arteries, where it rips through the wall and rewires the blood vessels to allow it to bathe in the parent’s blood directly. The allegedly ungrateful parasite releases hormones that cause the arteries to widen around it and become paralyzed, preventing the parent from cutting off its supply. It produces more hormones, which directly affect the parent to keep the pregnancy going and make more nutrients available. The parent offers the following defense, doing their best: their endometrium fights against the embryo’s invasive proteins, their immune system attacks the invading cells, and their own hormones try to counteract those of the embryo. The tug-of-war is still going on.

The aggressiveness of human embryos is matched by their high genetic susceptibility. Around 70% of IVF embryos have complex chromosomal abnormalities, such as aneuploidies, according to genetic analysis of morphologically normal, high-quality embryos. The majority of aneuploidies are fatal, and they are caused by cells that have either too many or too few chromosomes. The rate of aneuploidy in human embryos is thought to be ten times higher than in other mammalian species, though the reason for this is unknown. The parent has the opportunity to discard damaged embryos during menstruation, which is the second benefit of spontaneous decidualization. We’ve already established that raising a child puts a significant strain on the parent. They are able to choose the embryos with the greatest potential for success thanks to spontaneous decidualization. Lethal aneuploid embryos have higher metabolic activity than healthy embryos, which may be related to the higher energy expenditure necessary for them to survive despite their abnormal genotype. The endometrium’s cells develop the capacity to detect this metabolic activity as they get ready for a potential pregnancy. When an embryo is deemed unworthy, the endometrium actively prevents it from moving through the endometrium. The embryo then passes through the uterine lining while it is being shed. This may help to explain why humans are less successful reproductively than other species: it has been estimated that only half of human conceptions progress to a full pregnancy. The human endometrium may be able to adapt and advance due to the repeated cycle of regeneration, it is also hypothesized. Humans get a monthly practice run, whereas in other mammals this renewal only happens at the end of a pregnancy. This may be the case for the majority of women who experience recurrent miscarriages and go on to have a live birth.

We may have more periods now than ever before, which may be why menstruation bothers us so much. It would have been unusual to have a period for the majority of human history. Other menstruating mammals and so-called “natural fertility” populations of humans (those who don’t use contraception) devote the majority of their reproductive lives to being pregnant or nursing, during which time they cease menstruating. The Hadzabe people of Tanzania typically have 6 children, which they breastfeed for 4 years. They likely have a couple of tens of periods at most. Contrarily, those who menstruate in our society today can anticipate 300–500 periods throughout their lifetime. It’s shocking that we know so little about it and talk about it in such a dishonorable manner for something that happens so frequently.

Do Animals Have Periods?

Menstruation has only been noted in other primates besides humans, such as Old World monkeys and apes (mostly found in Africa and Asia), three to five different types of bats, and the elephant shrew. Cycles can last anywhere from 24 to 37 days in primates, 21 to 33 days in bats, and only once at the end of the breeding season in elephant shrews, though the duration varies from species to species. Whales, dogs, cows, and other species of mammals do not have periods. Rather, their receptive hormones cause “the heat” (i.e. oestrous cycle) which results in bleeding only in dogs (this being the source of a common misconception about menstruating dogs).

So What Exactly Is Menstruation?

To put it briefly, it’s a biological trait that enables humankind to survive. Yes, it has a very wide perspective. However, maybe doing so will help this monthly rigor become a little bit less difficult.

Why Do Women Get Their Periods?

Removing Toxins?

One of the original theories regarding menstruation was this one. Some of the deeply ingrained superstitions about menstruating women that permeated early 20th century research are still prevalent today.

The term “menotoxin,” for example, was coined by Béla Schick, a well-known pediatrician of Hungarian descent. He performed experiments in 1920 in which menstruating and non-menstruating women handled flowers. Schick came to the conclusion that the former cause the flowers to wilt by secreting a toxic substance from their skin. Menotoxins, according to Schick, stopped the growth of the yeast, which prevented the dough from rising. However, it was impossible to separate those chemical compounds or to ascertain their chemical composition.

Such statements clearly had a negative impact on women, who were then viewed as inferior or even repulsive. Up until the 1970s, attempts to study the menotoxin were made.

Protection From Male Pathogens?

Margie Profet, a researcher at the University of California, Berkeley, proposed in 1993 that menstruation serves as a defense mechanism against pathogens brought into the uterus with sperm. Therefore, she believed that men were “impure”—contrary to the previous assumption. She believed that menstruation served as a protective measure against contracting venereal diseases. Lack of supporting data caused Profet’s idea to fail quickly.

Energy Conservation?

According to Beverly Strassmann, an anthropologist from the University of Michigan, maintaining a thick, blood-filled layer in one’s uterus requires a lot of energy. It is much more effective for the organism to get rid of the extra blood and repair the uterine wall defects. “According to Strassmann, the fact that some species lose blood is not an adaptation but rather an unavoidable consequence of their anatomy and physiology.

A Result For Evolution?

In 1998, Colin Finn from the University of Liverpool proposed a similar idea. His theory was that, contrary to Strassmann’s assertion, menstruation is a necessary byproduct of the evolution of the uterus.

The uterine wall defends itself against the embryos by becoming thicker and accumulating layers, claims Finn, as they burrow deeper and deeper into the mother’s tissue. The embryo can be perfectly housed in this thick lining, but only for a few days. The lining will then need to be removed if pregnancy is not successful.

In her 2011 article, Deena Emera of Yale University in New Haven observed that signals from the embryo generally cause changes in the uterus to occur in mammals. As a result, pregnancy causes the uterine lining to thicken. Due to an aggressive fetus, this ability evolved to defend the mother.

The embryo in pigs, horses, and cows lies directly on the uterine lining. The embryos grow slightly more in dogs and cats. But in humans and other primates, the fetus burrows through the lining to join the mother completely and figuratively take a bath in her blood. Between the mother and the fetus, Elizabeth Rowe of Purdue University in West Lafayette, Indiana, referred to it as “an evolutionary rope pulling.”

An environment that is friendly to the developing fetus is created during pregnancy by the organism, which also makes it possible for the mother to feed the child. And this is one advantage of being pregnant. Unfortunately, it also creates a conflict between the two. Equal survival chances for each of the mother’s children are her responsibility.

In order to be able to have more children, the mother wants to ration the nutrients she gives each child. On the other hand, the developing child wants to absorb as much energy as it can from its mother. The fetus sends hormones into its mother’s arteries through the placenta, causing them to widen and giving it access to nutrient-rich blood. This could lead to the mother’s blood pressure rising, blood sugar levels rising, and dilated pupils. Most mammals can remove or absorb embryos, but in people, severing the link between the mother and the fetus, which is connected to the cardiovascular system, may cause bleeding. The mother’s health is also in danger if the foetus experiences abnormal growth or passes away. Fatigue, elevated blood pressure, diabetes, and pre-eclampsia can result from the developing fetus’ constant need for nutrients and oxygen.

Pregnancy is always a significant, and occasionally risky, investment due to all the risks. To determine which embryos are worth the risk, the organism carefully examines each one. When an embryo dies, it exposes the mother to infections and may still release hormones that harm the mother’s tissues. By removing the potential risk, the organism tries to steer clear of such circumstances. The uterus destroys the lining and discards it along with an unfertilized egg, a damaged, dying, or dead embryo if ovulation does not result in a healthy pregnancy. Menstruation is the name for this protective process, which produces bleeding.

Why Human Bleed Way More Than Other Animals During Their Period？

It seems unfair that the only other animals with menstrual cycles, aside from humans and our close relatives the primates, are elephant shrews and some types of bats.

A menstrual cycle, which takes place about once a month, is the uterine lining shedding, as you likely recall from health class years ago. A “fluffy and plush lining of blood vessels and stuff for the [egg and sperm] to implant into,” according to Dr. An OB-GYN explains, Suzanne Gilberg-Lenz. “The uterine lining sheds when this does not occur and a female releases the egg but does not conceive; this is known as the period.”

Your vagina starting to squirt blood, cramps, PMS, and pimples are all lovely symptoms of this monthly process. While dogs may have a small to moderate amount of discharge, mice, and mares do not bleed at all, whereas humans bleed a lot relative to their body size.

Although there is no universally accepted explanation for why human menstruation is so incredibly bloody, one theory postulates that the bleeding may be intended to prevent complications. “It’s similar to an evolutionary balance, but the tissue lost during menstruation is a mother’s method of controlling the fetus, according to Dr. Menstruation, the uterus, and genetics are the focus of research by anthropologist Elizabeth Rowe of Perdue University.

“The fetus in animals that bleed during pregnancy burrows deeply into the mother’s uterus to gain access to her blood supply, according to Rowe.

This provides nutrients for the developing young creatures, but it may not be good for mom. “The issue with that is that, if you’re a mother mammal, letting a fetus eat your tissues at random could ultimately kill you, says Rowe. A period prevents this sort of thing from happening by acting as a preemptive strike.

“Between the mother and the fetus, the tissue that is lost during menstruation acts as a kind of barrier. Rowe, who used data from primates to test this theory, calls it “pre-gaming for pregnancy.” This idea explains why some mammals bleed while others do not. “In species without an aggressive fetus, it turns out, menstrual bleeding is not observed, says Rowe.

Rowe wants to compare brain size to blood loss in order to advance her research. “In order to better understand our species, I want to examine how different brain sizes compare to body sizes. In relation to our body sizes, we have enormous brains. “If you have a big brain, you have a big-brained fetus, and that big brain is greedy for nutrients and oxygen,” says Rowe. “They tear into mom’s tissues so much because of this.”

What about cramps and PMS, two less-than-luxurious period symptoms, now that we have a firm understanding of why we are destined to bleed? Do other animals also go through them? Although biologists did note PMS symptoms in baboons in the 1980s, it is regrettable that it is still unknown how common these symptoms are in animals. The validity of PMS in humans is even a topic of discussion today. The uterus functions in a mysterious way.

But we are aware that some animals, like cats, have even been observed to behave more amiably when they are bleeding. “When they’re on the cat-rag, cats will be incredibly friendly and will “rub against objects, knead their paws on things, vocalize randomly, and even posture with their hind end in the air,” according to Dr. “The Southern Pet Vet,” Ashley”

Personally, I prefer to use cannabis suppositories and ice cream during my period, but now I can feel less resentful knowing that my body is just trying to protect me from the threat of an aggressive, greedy, large-brained fetus.

animal human period

Lucky Wong