Why Do Only 6 Species of Animals Have Periods?

We all get a visit from Aunt Flo once a month and it’s safe to say none of us really look forward to her visit. After the routine “Why God, why me?!” cries, we soothe ourselves with the thought that there are other animals that have periods too. But the question remains. Why do only 6 species have periods? Why not the rest?

Periods are a natural process that occurs in every woman’s body. Correction, every human woman. Correction again, every woman of 6 specific species.

For long, it had been assumed that menstruation was a purely human process. Soon, scientists discovered that other animals bleed too. As of today, apart from humans, there are 5 species that have been found menstruating:

Three are primates:

  • Chimpanzees
  • Orangutans
  • Monkeys

Two non-primate mammals:

  • Bats
  • Elephant Shrews

Now the question arises – why do only these 6 species have their periods? What’s the truth? This World Menstrual Hygiene Day, let’s try and find out.

Gynecology

The estrus cycle

Before we dive headlong into menstruation, we need to understand another biological process that is closely related to it.

A few decades ago scientists began observing female pigs, cows, cats, dogs and other non-primate mammals bleeding at periodic intervals. Stunned at the thought that they could be menstruating like human women, scientists conducted a thorough scientific inquiry.

What was thought to be menstrual blood, turned out to be vaginal discharge during estrus.

Estrus is the time of an animal’s life when it is in heat. Once a non-primate mammal female reaches sexual maturity and is physiologically ready to bear young, she undergoes a hormonal transformation which not only tells her body to get her womb ready but signals the males (through the release of pheromones) that the female is ready to mate.

During this time of estrus, due to the excessive production of hormones in their bodies, females display multiple external (visible) physiological changes in the form of a swollen perineum, changes in genital colouring and light bleeding.

Estrus dog
The estrus cycle of a dog (Image Source)

This bleeding is only an indication that the female is ready and willing to receive a mate. It does not indicate that she is menstruating. (An interesting fact to note is how non-primate mammal females mate only when in estrus .i.e. during bleeding. Unlike menstruating females who do not engage in sexual intercourse when on their periods.)

But the defining difference between menstruation and estrus is the way the endometrium is managed by animals.

To absorb or not to absorb

The endometrium is the main participant of the menstrual cycle of female animals. In females, the endometrium – which is the thick, jelly-like lining of the uterine walls – is the layer in which the egg will implant post-insemination.

In animals that undergo estrus, the endometrium is produced at the start of the estrus, in anticipation of the fertilized egg. But when insemination fails to happen, the body re-absorbs the uterine lining and keeps it in reserve for the next time the female is in estrus.

But, in menstruating animals, this endometrium layer is shed and expelled out of the body. During this process of shedding, the body also discharges blood.

Menstruation process
Menstrual process in humans (Image Source)

Why does this happen?

Scientists have been putting forth theories about why only certain animals shed their endometrium and menstruate, while others don’t.

Theory #1: Complicated gestation process and number/nature of offspring

One theory is that the gestation process of menstruating animals is more complicated and drawn-out than those that don’t menstruate. For starters, this theory does hold true for most of the animals on our list.

Orangutan
Orangutan mother and baby

Human pregnancies last 9 months, a chimpanzee’s and orangutan’s are around 8 months, a bat’s can go up to 6 months and a monkey’s is 5 months. These pregnancies are much longer than those faced by animals in estrus. Additionally, humans, chimps, orangutans and bats all give birth to just one baby (most of the time) during each pregnancy.

The explanation behind this theory is that menstruating animals require the best, most nutrient-rich and hospitable endometriums to support the birth of their babies. These babies, many of whom have immense intellectual capabilities, require additional support of nutrients in the womb, to develop completely.

Theory #2: Endometrium absorption expends too much energy which certain animals cannot spare

This theory connects with the first theory of the gestation process and the nature of the offspring. If we are to believe this theory, then we need to accept the fact that animals which have long gestation periods, which produce highly-intelligent young and which rear only single offspring during each pregnancy need greater amounts of energy than animals that are in estrus.

Chimp
Chimpanzees

This particular theory subscribes to the belief that absorbing the endometrium is just an energy-wasteful task; one that can be avoided and the energy re-directed to meet the requirements of complicated pregnancies. The body can save tons of energy by shedding the uterine lining and making a fresh one in time for the pregnancy (which is a better option than using a stale lining).

Counter evidence?

The only animal that doesn’t subscribe to either of these theories is the elephant shrew. The shrew has a gestation period of 1-2 months and produces multiple litters of 3-4 pups each pregnancy during the year. It doesn’t spend too much time or energy giving birth; yet bleeds during its period.

Elephant shrew
Elephant shrew

Additionally, these theories dampen a little in the face of evidence that highly-intelligent animals like elephants, whales and dolphins who all undergo very long gestation periods and bear intellectually mature young do not menstruate in their lifetimes.

What does this all mean?

Well, just like with their human counterparts**, things aren’t clear why or how chimpanzees, monkeys, orangutans, bats and elephant shrews menstruate. For now, scientists believe this to be a biological enigma, a riddle that remains unsolved, despite decades of study.

For now, we leave animal menstruation as it is – a blatant question mark, a brain teaser, a stumper of the animal kingdom.

**Here’s some extra info if you’re interested:

Why do women bleed?

For long scientists have wondered why the shedding of the endometrium happens in humans. Scientists believed that the reason women bleed and the endometrium sheds may be because there was something inherently wrong with women’s bodies. They thought that the complex biological processes in humans may produce unwanted chemicals in the body which needed to be thrown out as often as possible.

1920s physician Bela Schick believed women menstruated to get rid of poisonous chemicals called Menotoxins, which filled their bodies just before their period.

Dr. Schick conducted experiments on menstruating and non-menstruating women, asking them to carry flowers in their hands. He observed that the flowers held by menstruating women wilted faster and lost their fragrance within minutes. He even supported claims by other scientists that women on their periods sweated out these menotoxins from their bodies while menstruating. He claimed that these toxins were responsible for bread not rising and beer not fermenting when menstruating women touched them (reminded of persistent taboos, anyone?).

This belief in Menotoxins continued all the way to the 1990s when another theory was proposed – menstrual blood may be a way to get rid of pathogens, stale sperm and bacteria in a woman’s body, cleansing her in the process.

But this theory too did not hold good. If it meant that stale sperm was toxic to women’s bodies, surely it meant that stale sperm not ejaculated by men could be poisonous to them too. So why don’t men menstruate?

Till date, neither of these theories has been proven. Nor has there been any other theory that can explain the mystery that is menstruation.

-NISHA PRAKASH

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