Why do humans menstruate and do animals have periods too?


Why do humans menstruate and do animals have periods too?

What do monkeys, some bats, and women have in common? Period. Bad joke? Not exactly. Hop in on a journey through the world of biological hypotheses related to menstruation. We’ll ask “Why?” until we find a sensible answer :)

Cezary Polak

Published: 1.02.2021 5 minreading time

Why do women menstruate illustration by Jarek Danilenko

Illustration: Jarek Danilenko

This we know

Every month, as the reproductive hormones’ — mainly estrogen and progesterone — levels rise, the uterus prepares for pregnancy. The innermost uterine lining, called endometrium, thickens, divides into layers, and grows a vast net of blood vessels to receive a fertilised egg. If pregnancy doesn’t happen, the levels of progesterone start to drop. The thick endometrial tissue together with the blood vessels begin to break down and flow out through the vagina. And this is menstruation.

Do animals menstruate?

Apart from humans, menstruation has only been observed in other primates, e.g. Old World Monkeys and apes (inhabiting mainly Africa and Asia), 3-5 species of bats, and the elephant shrew. The length of cycles differs from one species to another but, in short, they may last 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. Other mammals, such as dogs, cows, horses, or whales don’t have periods. Instead, their reproductive hormones induce “the heat” (i.e. oestrous cycle) which results in bleeding only in dogs (this being the source of a common misconception about menstruating dogs).

This we don’t know

Why do women menstruate when most other species which give birth don’t?

Removing toxins?

This was one of the first ideas about menstruation. Much of the research at the beginning of the 20 century was tainted with deeply rooted superstitions about menstruating women, some of which are still alive today.

Béla Schick, for instance, a popular Hungarian-born paediatrician came up with the term “menotoxin”. In 1920, he conducted experiments in which menstruating and not menstruating women were to handle flowers. Schick came to a conclusion that the former secrete a toxic substance from their skin, which causes the flowers to wilt. According to Schick, menotoxins stopped the yeast growth which prevented the dough from rising. It was impossible, though, to isolate those chemical compounds or to determine their chemical makeup.

Obviously, such claims were extremely harmful to women who were, from then on, treated as lesser or even disgusting. Attempts at studying the menotoxin were made until the 70s!

Protection from men’s pathogens?

In 1993, Margie Profet, then from the University of California, Berkeley, suggested that menstruation functions as a defence mechanism against pathogens transported to the uterus with sperm. Ergo — contrary to the previous hypothesis — she deemed men “impure”. In her view, menstruation was a means to reduce the chances of contracting venereal diseases. Profet’s idea quickly fell due to lack of evidence.

 Energy conservation?

In 1996, Beverly Strassmann, an anthropologist from the University of Michigan claimed that keeping a thick, blood-filled layer in one’s uterus takes up a lot of energy. For the organism, it is much more efficient to get rid of the excess blood and rebuild the defects in the uterine wall. “The fact that there is blood loss in some species is not an adaptation, but a side effect of species anatomy and physiology”, said Strassmann.

Evolutionary consequence?

Colin Finn, then from University of Liverpool, had a similar suggestion in 1998. His idea was that period is a necessary consequence of the uterus’ evolution, and not a way of conserving energy as Strassmann claimed.

According to Finn, the embryos grow deeper and deeper into the mother’s tissue and the uterine wall defends itself against it by thickening and building up layers. This thick lining is able to host the embryo perfectly, but only for a couple of days. Then, if pregnancy doesn’t happen, the lining must be removed.

Deena Emera, from Yale University in New Haven, in her 2011 article has noticed that in the majority of mammals the changes in the uterus are prompted by signals coming from the embryo. As a result, the uterine lining thickens in response to pregnancy. This ability evolved in order to protect the mother from an aggressive foetus.

In horses, cows, and pigs, the embryo lies directly on the uterine lining. In dogs and cats the foetuses grow in a little more. However, in humans and other primates the foetus digs through the lining in order to completely unite with the mother and literally bathe in her blood. Elizabeth Rowe from Purdue University in West Lafayette, Indiana, called it “an evolutionary rope pulling” between the mother and the foetus.

During pregnancy, the organism wisely chooses appropriate measures to design a foetus-friendly environment and creates conditions for the mother to feed the growing baby. And this is the bright side of pregnancy. Unfortunately, it also puts the two into conflict. The mother’s job is to ensure equal chances of survival for all her offspring.

The mother wants to ration the amount of nutrients she gives each child so that she is able to have more children. On the other hand, the growing child wants to take as much energy as possible from its mother. Through the placenta, the foetus pumps hormones into its mother’s arteries, causing them to dilate which provides it with access to nutrient-rich blood. This may cause the increase of blood sugar levels, pupils’ dilation, and increased blood pressure in the mother. Most mammals can remove or absorb embryos but in humans, breaking the connection between the mother and the foetus, which is linked to the cardiovascular system, may result in bleeding. If there are anomalies in the foetus’ growth or it dies, the mother’s health is also in jeopardy. The foetus’ constant demand for nutrients and oxygen may lead to fatigue, increased blood pressure, diabetes, and pre-eclampsia.

Because of all the threats, pregnancy is always a huge, and sometimes dangerous, investment. This is why the organism checks the embryos thoroughly to make sure which ones are worth the risk. When an embryo dies, it exposes the mother to infections and may still release hormones damaging the mother’s tissues. The organism tries to avoid such situations by eliminating the potential risk. If the ovulation doesn’t end with a healthy pregnancy, the uterus breaks down the lining and disposes of it together with an unfertilised egg, a damaged, dying, or dead embryo. This defensive process is called menstruation and results in bleeding.

What is menstruation then?

In a nutshell — it’s a biological quality that allows humankind to survive. It’s a really wide perspective, yes. But perhaps thinking about period that way will make this monthly strenuousness a bit easier?


https://carta.anthropogeny.org/moca/topics/menstrual-cycle-duration [Accessed: 26.01.2021]

https://academic.oup.com/biolreprod/article/102/6/1160/5775593 [Accessed: 26.01.2021]


Cezary Polak
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