Monster Girls

The Biology of Monster Girls Explained

Let’s learn about the science behind the popular monster girls within anime!

Japan is a country with a rich cultural history filled with many of its unique monster girls that do not seem to exist anywhere else in the world, like the Yuki-onna, or ice woman. 

This love of monsters continues to the present day with monsters like Godzilla and the other Toei monster movies I grew up watching.

Japan also has a popular culture that loves things cute, so it was only a matter of time until those two seemingly conflicting love combined to create the monster girl.

Biology of Monster Girls

The first of the modern monster girls could be seen as Lum from Urusei Yatsura, written by Rumiko Takahashi, which was a hit in the late 70s and early 80s. She was an alien Oni (Japanese for troll/demon), with a very human-like appearance.

Lum- note the small horns and pointed ears

Lum- note the small horns and pointed ears
Actual Oni

Now I like the monster girl version better, and I can’t have been the only one, as the number of half-human, half-monster characters has greatly expanded over the years, with a large number of shows from a variety of genres having at least one monster or half-monster character, usually female. Ĵ

This brings us to the 2010s when monster girls seemed to explode in popularity with ecchi comedies, slice-of-life shows, and family dramas.

A Centuar’s Life- 2011- slice of life

A Centuar’s Life- 2011- slice of life

Everyday Life with Monster Girls– 2012- ecchi harem comedy

Everyday Life with Monster Girls– 2012- ecchi harem comedy

Miss Kobayashi’s Dragon Maid– 2013- family drama

Miss Kobayashi’s Dragon Maid– 2013- family drama

Interviews with Monster Girls– 2014- slice of life

Interviews with Monster Girls– 2014- slice of life

Elf-san Can’t Lose Weight– 2016- ecchi comedy

Elf-san Can’t Lose Weight– 2016- ecchi comedy

That is almost one a year for the first half of the decade and those are only the ones that I have heard about.

The monster girls in this manga and anime run the gamut from traditional Japanese creatures like the Yuki-onna to monsters found worldwide, like vampires.

The real question in my mind is what the original inspiration for these monsters was. What phenomenon did our ancestors see that inspired the original monster myths, and what, if any, science is behind some of our favorite monster girls?

FYI- This time I will be sticking to the humanoid monster girls, and not the half-human half-animal monster girls.

Interviews with Monster Girls

The first monster on the list is a classic found in many cultures around the world, vampires, dating back to the cultures of Mesopotamia, even if the first official use of the word in the English language dates to 1743.



There are many anime vampires, but the one I will be using is Hikari from Interviews with Monster Girls. 

My choice of Hikari over other anime vampires is because Interviews with Monster Girls, as with several other anime/manga, goes to some lengths to explain her traits using real-world biology.

The specific vampire traits I will be examining include: Death by a stake through the hear, Pale skin, Reaction to sunlight, Glowing eyes, Dislike of garlic, and Drinking blood

The easiest of the vampire traits to examine is the idea that a stake through the heart will kill a vampire, which is rather hilariously and simply put to rest in Interviews with Monster Girls.


       Which is accurate for any living thing that has a heart.

Pale Skin and Reaction to Sunlight

The pale-skinned vampire has two different origins depending on who you ask. There is more than a little evidence that Bram Stoker’s Dracula was at least partially inspired by tuberculosis. This is due to the pale skin caused by the disease.

The other and probably far older inspiration is albinism, a genetic condition where the individual has a complete lack of melanin, the pigment that gives skin its color.

As to the bursting into flames upon being exposed to sunlight legend, that can include the previously mentioned albinism, but also sun allergy, pellagra, and xermodosa pigmentosa, all of which have a nasty reaction to exposure from sunlight.

Glowing Eyes

Some of the vampire myths state that they have glowing eyes, as does Hikari from Interviews with Monster Girls.  Despite how crazy it might sound, glowing eyes are a fairly common trait in nocturnal animals. 

The glowing eyes occur because nocturnal animals have an extra layer in their eye called the tapetum lucidum which is located behind the retina. 

This layer reflects light that has passed through the retina back onto it increases the amount of light that hits the retina and improving night vision.

Thus, the glowing effect that you see is light being reflected out of the animal’s eyes toward the viewer.

Dislike of Garlic

Vampires are by their nature nocturnal creatures being mainly active at night.  As such, they cannot rely solely on their eyesight for hunting.

This means that their other senses such as hearing and smell would be better developed than those of a creature that hunts in the daytime, as is indeed seen in other nocturnal animals.

As a result, strong-smelling foods like garlic could upset their enhanced sense of smell. The teacher in Interviews with Monster Girls discusses this, but Hikari mentions that she likes garlic anyway.

Despite this, I think the theory still holds because Hikari comes across as something of an oddball for vampires anyway.

Drinking Blood

The drinking blood part of the vampire myth was probably inspired by animals that feed on blood like certain species of bats, and insects like mosquitos. 

Some respiratory diseases like TB can cause an afflicted individual to cough up blood, making it look like they might have been feeding on blood as well.  In a theoretical biological sense, a vampire’s desire to drink blood is most likely inspired by anemia, which is a condition caused by a lack of red blood cells.

If we assume that a vampire’s base physiology is similar to a human, the vampire’s anemia is caused by a problem in absorbing the key nutrients needed for the body to form red blood cells, iron, and B vitamins.

These nutrients would be in higher amounts in blood, and one of Hikari’s favorite foods is liver and onions, which is a dish that used to be recommended to people suffering from anemia due to its containing iron and B vitamins.

Succubus in Anime


Also, from Interviews with Monster Girls, we have Sakie, who is a math teacher and more importantly a succubus.  A succubus is a demon that is usually evil and kills men through extremely vigorous adult activities. 

In the case of Sakie, we learn that she can have an aphrodisiac effect, which is stronger the more skin that she exposes.  I can’t think of any biological conditions that might have inspired the succubus myth, but there is a scientific explanation for Sakie’s powers.

Pheromones are chemical messages released by animals as a way of communicating with other members of the same species. This is most commonly seen in insects and is used for everything from giving directions to mating.

Some mammals also use pheromones, such as mice, which use them for mating purposes. While scientists have yet to discover sex pheromones in humans, there has been some interesting research and it’s not a huge leap to think that a succubus power could run on pheromones.



The Yuki-Onna, or ice woman, is a monster unique to Japan, and Yuki from Interviews with Monster Girls exhibits some of their classic traits: Low Body Temperature, Lack of Sweat, and Creating Ice.

Low Body Temperature

The obvious choice here would be that Yuki is cold-blooded, but that is not the case as she says that she is very similar to humans. In that case, she would be in a constant state of hypothermia (below 95F and probably the inspiration for the Yuki-onna myth) and that would impact how the rest of her body functions.

Interestingly enough there is a mammal called the Echidna which has a body temperature of 85-90F and it can go as low as 41F. So perhaps Yuki-Onnas is an evolutionary off-shoot of Echidnas that managed to breed with humans.

Lack of Sweating

This is an easy one as there is a real-world condition called Hypohydrosis, which is a lack of sweating, and it can be an inherited condition or caused by illness, drugs, or injury. This would make an individual more likely to overheat. Also, Echidnas don’t actually sweat so there is that as well.

Making Ice

Yes, I have a biological explanation for this one. Human sweat contains salt, and certain salts, when mixed with water, can cause an endothermic reaction that absorbs energy (heat) from the environment when they occur.

This can cause ice to form in certain situations. If Yuki’s sweat contained ammonium nitrate it could in the right conditions cause ice to form as the ammonium nitrate mixes with the water in the sweat.



There is not a whole lot I can talk about regarding Machi because let’s face it, there is no known organism living or extinct that ever existed as two completely separate parts.

I don’t know what inspired the Dullahan myth, but there is nothing in biology that I can think of. I can, however, explain how Machi got motion sickness during her outing with her teacher.

Humans have something called dynamic balance and that is how we stay upright while moving. Part of dynamic balance is essentially biological image stabilization so that our field of vision doesn’t bounce around like a shaky cam when we start moving.

This is normally done by the muscles in our neck and eyes. Machi doesn’t have a neck so her arms, which normally carry her head, have adapted to make those changes, and the teacher cannot make the minute adjustments needed to keep Machi’s field of vision stable.

Invisible Girl

Invisible Girl

The invisible girl is a modern monster only created in 1897 by H.G. Wells, and while an organism can’t be completely invisible, especially one as big as a human being, some smaller organisms have some degree of invisibility. For an organism to be invisible or at least somewhat translucent the following things need to occur:

1- low absorption of light by the body tissues

2- a lack of dispersion of light as it passes through the body

3- the minimal reflection of light when it hits the body

This is how it works in many translucent organisms and other organisms that bend or reflect light to camouflage their bodies.

Also, it’s pretty obvious that a human body is too big and thick for such methods to work; even if the skin is invisible, the rest of the body isn’t, and this is shown in the side character of Venture Brothers. Chameleons use a different method of changing the color of their bodies by using crystals to reflect light.

So theoretically our invisible girl could use microscopic crystals in her skin cells to reflect light around her body rendering her invisible.

This would only work when she is standing still, as the crystals would not be able to react fast enough to accurately reflect light around herself while moving.

Everyday Life with Monster Girls


Cyclops 1

The defining characteristic of a cyclops is its single eye in the center of its forehead and there is a technical term for this, cyclopia. Yes, being a cyclops is a rare but real-world condition seen in animals, like goats. It has yet to be seen in humans, but it is thought to be involved in some miscarriages.

The condition occurs when the brain does not fully develop and forms the left and right lobes of the brain. Additionally, if a cyclops were real, it would have trouble with depth perception, as two eyes are needed to create parallax, allowing the brain to accurately determine just how far away an object is.



An ogre is a large humanoid beast that may also have a horn on its head.  Size-wise ogres are much larger than your average human, which could be caused by a condition called gigantism.

People with gigantism have a pituitary disorder causing the over secretion of growth hormone, and this leads to individuals with gigantism being well over 7 feet in height and some may be over 8 feet tall. This leads to several health issues, as the human body isn’t meant to be quite that tall.

The next question is, of course, the horn, which is also possible, as there is a condition called cutaneous horn. The cutaneous horn is an overgrowth of the epidermis, and it can take the shape of a horn and can be located anywhere on the body including the forehead.  It is cancerous in some cases.



Yes, I have a biological explanation for zombies, and it might not be what you think. There is a very rare but real form of brain damage to the fusiform gyrus, which is involved with recognition.

Damage to this one specific area in the fusiform gyrus causes Cotard’s syndrome, or delusion, where the patient believes they are dead. There have only been a few cases since it was first described in the 1880s and there are some treatment options.

Elf-san Can’t Lose Weight


Alraune 1

The Alraune, or plant woman, first appears in a 1911 novel, but there are some German folk stories with similar types of characters.

Once again there is a rare genetic condition that might be involved in inspiring tales of human-like plant creatures. Epidermodysplasia verruciformis, or tree man disease, is a skin condition that leaves the individual more susceptible to human papillomaviruses (HPV).

The virus infects the skin and causes the excessive growth of warts. These warts completely cover the skin and begin to take on a tree bark-like appearance even though it is an overgrowth of skin cells.



This is another fun one I like to use in my classes sometimes, as there is an actual genetic condition called Hypertrichosis, or wolfman syndrome. In this case, the condition causes extremely overgrown facial hair, which covers the individual’s entire face.

It was first described scientifically 300 years ago and it is extremely rare. As to werewolfism being transmitted by a bite the answer to that is rabies. The description of a dog or person afflicted with rabies matches some of the early descriptions of werewolves.

A Centaur’s Life

6 limbs

6 limbs

I know I said that I would cover the half-human, half-animal monster girls like many of the ones we see in A Centaur’s Life in another installment, but there is one thing that I wanted to mention here.

Many of the monster girls in A Centaur’s Life have 6 limbs in one way or another, which is explained by saying that evolution took a completely different path which I will dive into another time.

What I did want to point out is that there are a few cases of humans having more than 4 limbs. This occurs through a rare event during embryonic development.

Normally a fertilized egg develops into a baby, but sometimes during the early stages of development, the embryo splits in half, leading to identical twins. In rare occurrences the split is not complete, leading to Siamese or conjoined twins.

In extremely rare cases the split is not equal and isn’t complete leading to a parasitic twin. This twin is connected to the larger sibling and never fully develops.

The results can vary widely depending on how and when the twins began to separate and when the smaller twin ceased developing. In two recorded cases this resulted in the individual being born with extra arms and legs.

In my research on parasitic twins, I ran across a genetic condition called Dipygus where the body essentially becomes confused (for a lack of a better term) during embryonic development. This results in the body growing duplicate parts and in one case this involved a woman growing an additional pelvis and set of legs.

In both cases, some of the extra limbs are less functional and they can impair the function of the stronger limbs. The extra limbs can be removed with surgery to return normal function to the remaining limbs.


I hope you have enjoyed this overview of the various monster girls that have shown up in anime over the last few years.

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