Could werewolves exist in a realistic world?

All that is required for an accepted answer is the ability to change shape over night. Bonus points will be awarded if you can explain a bite spreading transformation and a full moon causing the transformation itself. Could an animal evolve this ability?

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    $\begingroup$ So what you're really asking is "How can I make werewolves seem plausible in an otherwise realistic world?" because the answer to the question as originally stated is "no". $\endgroup$ Commented Sep 19, 2015 at 21:49
  • $\begingroup$ "Shapeshifting" is already quite difficult to be possible, I wouldn't really bother to ask all the extras. $\endgroup$
    – o0'.
    Commented Sep 19, 2015 at 21:52
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    $\begingroup$ I searched "werewolf" and found several relevant, related questions: worldbuilding.stackexchange.com/questions/5231/…, worldbuilding.stackexchange.com/questions/449/…, worldbuilding.stackexchange.com/questions/17206/…, worldbuilding.stackexchange.com/questions/15341/… $\endgroup$
    – zeta
    Commented Sep 19, 2015 at 21:56
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    $\begingroup$ What about illusionary werwolves? In other words, werewolves that don't actually transform, but become violent and release some airborne chemical that induces fear and hallucinations? Combined with taking on some wolf-life traits (howling, etc.) this might result in people seeing werewolves, as long as nobody brings a mobile phone and records it... $\endgroup$ Commented Jun 20, 2017 at 8:18
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    $\begingroup$ you really might want to read the INCRYPTID series by Seanan Mcguire which is about a family of biologist living a world with magic and fantasy animals, their take on werewolves is it is a spillover disease (like rabies), from a naturally biomorphic species, it hits all mammal with low transmission rate and only those with enough body mass survive the first change, and the cellular strain (repeated division and replacement) tends to kill even them within a few years from what is basically rapid aging. $\endgroup$
    – John
    Commented Feb 9, 2018 at 1:45

11 Answers 11


This is very unlikely to ever evolve, for an animal to have two forms there needs to be a reason for it to have both forms. Each needs to provide an advantage that compensates for the costs of changing. In the case of a werewolf the wolf form is clearly a superior hunter - but what would it gain by being in a non-wolf form for most of the time?

Also consider animals like caterpillars and tadpoles that undergo transformations in real life and look at how long those transformations take. It's not a simple matter to reshape your body.

Spreading by bite

This is clearly possible if it's a virus of some sort (or even nano machines) being transmitted in saliva.

Triggered by full moon

This is hard to explain, it would be easier to link it to some other cycle and have it coincide with the full moon. The one advantage of the full moon is that the nights are brighter so you could make some argument for it needing that but it's a pretty weak one.

The actual transformation

If mass changes then it's basically impossible by physics as we know it. That means you're going to have a really big wolf or really small person. Take a look at this question for a detailed discussion: Is there a credible way a shapeshifter could gain/lose body mass when changing forms?

So let's say a human turns into a huge wolf. In order to do that they need to reshape bones, re-align muscles, stretch skin, grow fur. That's not changes that can be done overnight. Any realistic biological system is going to need months.

If you had special transforming bones that could possibly speed that process up but would weaken the bones drastically in the process.

So in other words, no it's not possible.

The closest you could get is something which made more superficial changes. You would stay humanoid but you would sprout hair rapidly, claws normally sheathed would extend from the fingers, the jaw would hinge open wider and protrude, the teeth would also extend.

These sorts of changes could theoretically be done much faster, in minutes for the basic changes and in hours to grow the hair. In fact if the hair was always there but retracted inside the skin then that too could grow out very fast.

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    $\begingroup$ In a full transformation, I'd imagine reshaping the brain to fit the new skull form would prove to be the trickiest part, especially doing it when you want to retain all of the memories and faculties once the transformation reverts. $\endgroup$ Commented Sep 19, 2015 at 22:05
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    $\begingroup$ @KillingTime quite possibly, full transformation is impossible anyway so it's not really worth going into detail on things like that. $\endgroup$
    – Tim B
    Commented Sep 19, 2015 at 22:07

I actually thought of this answer while finishing my other answer; it's different enough that I think it works as a completely different option.

Things that cause rapid growth, altered mental state, and growth based on the phase of the moon actually exists in the real world - fungus! Perhaps, then, werewolves aren't wolves at all, but werefungi?


People are infected with this were-fungus through a scratch; it isn't small enough to get into the body by passing between skin cells, or similar, and it isn't strong enough to make it through the digestive system unharmed. Thus, to become infected, spores must fall into an open cut.

Once infected, it takes roughly 29 days for the fungus to bloom.


Werewolves all have hair, of course. Long, shaggy hair that covers them head to foot. Were-fungus doesn't produce actual hair, but it does produce something similar - thin, hair-like strands of fungal growth. The tips of the "hairs" have spores in them. The night of the fungal bloom, every available pore of the host's body will erupt in hair, possibly inches long. The growth will take only a few hours. As it happens while the host is sleeping, the host never realizes it's happening.


Various chemicals exist that can alter the delicate chemistry of the brain; or, alternately, the fungus may be similar to Ophiocordyceps unilateralis; its grip on the human brain causes madness and blood lust. Either way, the fungus drives its host to attack others.


During a full moon, a victim is scratched, bitten, or otherwise injured, and comes in contact with werewolf spores. The fungus grows inside them for 29 days; at the next full moon, the fungus blooms.

As the fungus is blooming, it releases chemicals into the bloodstream; first, a local anesthetic, to mask its growth, which incidentally provides the ability to ignore injuries while in werewolf form. Next, chemicals to alter the brain of the host. And finally, chemicals that cause adrenaline to spike.

As the host's heartrate responds to the adrenaline, the internal painkiller is flushed; the host awakens, suddenly in great pain. Their mind is clouded; all they know is pain. They attack anyone who comes near, lashing out, and stumbling at a full run, trying to escape the pain, behaving like mindless animals. The fungus needs the host to run as far as possible, and injure as many people as possible, so as to spread.

Once the chemicals are flushed from the body, the fungus "hairs" will turn to sludge, leaving a slimy residue. The host will likely try to bathe as soon as possible, which washes away any evidence. The host, with no memory of the previous night, will return home and continue their life. However, 29 days later, at the next full moon, the fungus will bloom again, and the events will be repeated.

Reversion on Death

Something else mentioned with werewolves is that when they are killed, they revert to human form; this holds true with the fungus. When the human host dies, it no longer circulates blood to refresh the fungal bloom, which withers away. Visually, it will seem as if the incredibly hairy beast reverts to a human form.


Not really an answer, more a set of loose thoughts fit only for a production with no budget for an evolutionary biologist.

In short: all humans are slightly different from us, here.

Evolution: epigenetics? Homo sapiens lupus having the potential, but never manifesting it under normal circumstances?
Let us not assume complete transformation into exactly a wolf or hybrid. Instead - aside from very plausible behavioral changes and body hair growth - something approximating it. Maybe even wider joints allowing a greater range of articulation, or sliding further down the length for instance, making the joint both look and act different. More likely, though, that it would not be a change, rather, a "direction" one could grow in. Also bear in mind that the existence of such creatures might also alter wolves - if you were born in those circumstances - your point of reference for a comparison would be different.
Now... why? Mimicry? temporary environment-triggered "paradigm shift" (lifestyle, energy allocation)? Blending into, or taking control of (easier when you're bigger than them) very widespread wolf packs? I am not qualified to answer beyond claiming it does not so far feel very implausible compared to some other extant phenomena.

Moon triggered: Might be same as with wolves howling at the moon - a mistaken myth. Or might be that the time is seen as optimal for hunting in that form - Homo sapiens lupus having inferior low-light vision to Canis lupus and other competing predators. Or syncing with some prey animals. Again, I can't speak much of plausibility.

Bite: pathogen or hormone, to trigger the dormant abilities, maybe actually only accidentally activating, or hijacking, the "normal" process for a more extreme effect, like with cannabinoid receptors in the brain - those weren't put there specifically for pot.
Whatever it is, it need not wholly take away the ability to sometimes look and act like a "normal" human, but the "similar, but more so" effect could give the affected "werewolves" a specialized advantage, leaving, in the end, only them, and a society of "pure" humans, never manifesting any of these traits... until directly exposed to the former, restoring the victim's lost ability to shift , and possibly permanently changing their instincts.


The meme of "werewolf" is too vague to have a single answer, it depends on what you mean. If a werewolf is a superhuman being with acute eyesight, acute hearing, acute sense of smell, long fur/hair and extraordinarily powerful muscles, why sure, I can see some sort of genetic (virus) modifying a person over months and years to become a werewolf.

If you're talking about a being who changes from Bruce Banner to someone looking more like the Hulk in a couple of hours (overnight) then no (even if they don't turn green). The only realistic weight gain (mass gain) you're going to do in 8 hours is water gain, which won't do much for superhuman powers.

Consider bamboo: some species can grow over 3 cm/hr. So, there's just no reason why hair couldn't grow just as fast (as well as thickening). But we are talking about growth, not magical appearance. Similarly, it's not going to magically return back to the well manicured state it was in when the Sun comes up. At best what could happen is that it all fall out (over quite a few hours) and then maybe the "original" pattern of hair (eyebrows, head, pubic, armpits, legs, arms, etc.) re-grows at a similar accelerated rate. But you're going to have a lot of fur on the rug to clean up.

Muscle strength varies quite a bit for the same person depending on fatigue and other factors (including hormones like epinepherine (adrenaline)). So there's no reason a werewolf couldn't seem to be as weak, blind, deaf, etc. as a 'regular' person when a certain hormone was low, and couldn't suddenly be superhuman with the release of the hormone, and the hormone could be cyclic, every 27 days with reinforcement from a certain intensity of moonlight.

This is a bit of a problem, since you're not going to get it if there's cloud cover or if the werewolf wasn't exposed to it. So, the coupling to the full Moon couldn't be very high.

So, here's what could happen: A virus or a series of viruses which improve eyesight, hearing, and smell.

You're still going to be limited to the size of the skeleton, which would take months to change substantially. Muscles aren't going to change in size overnight, but pound for pound humans are extremely weak, so there's no reason why better muscles couldn't replace our puny ones (better on a pound for pound basis), maybe an improvement of 3X or even 5X.

So, perhaps you have to consider how to "dampen" their powers for 20-24 out of the 28 day cycle, IDK. Seems possible. And there are cases in the literature where experimental drugs induced homicidal rage in human test subjects. So if your werewolf is homicidal, when in its "high" state, I see no problems.

  • Claws? Like bamboo. Say something like 2 hours to grow ~5 cm, another couple to "dry" and harden, so sure.
  • Teeth? No. Not going to happen.
  • Retractable teeth? Well, sure, as long as the werewolf doesn't ever have to appear human again (although perhaps a beard could disguise the deformity).
  • Retractable claws - not if hands need to ever again look human.
  • Change in posture? No. I can imagine slight differences in posture and gait (walking, running) due to changes in various muscle tensions, but any substantial changes in speed, etc. would require drastic changes in skeletal structure, and the werewolf would look inhuman, even when in "low".
  • Sense of smell - well there is only so much you can do given the small nasal cavity we have, so all you can hope for there is increased olfactory density- but that requires increased amounts of brain dedicated to it, so either you're going to have a deformed head, or you're going to be pretty limited.
  • Similar to eyes. Getting very low light vision would require some changes to the retina, but again you're limited to the size of our eyes and the amount of brain (a lot, currently) we'd need for this extra vision.
  • Similar limits to hearing, but that would probably be the hardest to improve, without scooping out some of our brain. ...

One (obvious) problem is muscle tone/conditioning. If for 3 weeks a month, the werewolf isn't using much of his/her extra capacity, they're going to lose a lot of that extra capacity. Regaining it might take the entire week (at least) with a high activity level (and needed rest).

To sum up: if they're to look human most of the time, then they'll have to be able to pass for human all of the time, there's not going to be any "yesterday he had a muzzle and 3 inch long canines, and today he looks completely normal".

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    – sphennings
    Commented Jun 19, 2017 at 18:54
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    – Secespitus
    Commented Jun 19, 2017 at 19:13

The first question is what your werewolf looks like. The traditional werewolf is a huge, powerful figure, covered in shaggy fur, with a distinctly canine facial structure: pointy teeth, long jaw, nose at the end of a snout, pointy ears, and so on. They might even have a tail, paw-like hands, or reversed dog-like 'knees'.

Almost all of that would be impossible, unless the soon-to-be-werewolf were to undergo substantial plastic surgery, which surely wouldn't be an overnight change. So, we'll have to make our werewolf a little differently. That isn't actually going to be hard; normal humans in little more than face paint can look so very different that people will attribute all sorts of terrible things to their features. Thus, the long face, pointy ears, tail, paws and reversed knees can be done away with.


The most obvious difference is going to be body hair. Our werewolf is going to need some serious hair growth. Two things will affect that: hair color, and hair length. Dark hair will show up much better than light; the darker the better. Second, hair length. For a furry coat, our werewolf will need at least an inch of hair on his arms and face. Other locations, like back, chest, legs, etc. would be helpful, but not absolutely required, especially in climates where clothing covers most of the body. Some people claim hair growth of over 3 inches in a month; pushing the human body to its limit could result in inch-long hair in a matter of hours, especially if supplementing existing hair. Even if it isn't an inch long, hair grown so quickly will be very unhealthy, twisted and matted, and will give the appearance of long hair.


Werewolves are tremendously powerful; they fight with their claws and teeth, even against powerful weapons like guns. They can take a lot of damage without going down.

This is actually the easiest part to build. When fueled by adrenaline, normal humans can do superhuman feats, like lifting cars or smashing through concrete walls. Drugs exist that can raise the strength of humans, and many more drugs exist that simply turn off limiting switches in our brains, letting humans use strength even when it does incredible damage.

The right concoction of pain reduction and adrenaline could make even a couch potato into a world-class athlete for a day. After that, of course, they would be bed-ridden for weeks, trying to heal the torn muscles and ligaments, but for a day, they would be monsters. And speaking of monsters...


Werewolves aren't known for their thinking ability. When they "turn", they go on murder-filled rampages, killing and maiming. This, too, is easy to replicate; just search for "bath salts zombie" and you'll have real-life examples of mind-altering drugs turning regular people into horrifying monsters.

Teeth & Claws

It's not that hard to get pointy teeth, but reverting after a night of murder will certainly be difficult. For this, we'll have to stick with the imagination of the masses, rather than real pointy teeth. Swollen jaw muscles can cause the jaw to hang open; in that case, some drooling can be expected. Blood from either an internal mouth injury, or from an outside source, can make the most pristine set of teeth look wildly nightmarish.

Claws, on the other hand, can be grown fairly quickly, especially if the body isn't growing them as it ought. The data I could find on fingernail growth suggests something like 3.5 mm per month, which is hardly claws; however, a rush of growth could turn otherwise normal fingernails into yellowed claws in a matter of hours. The fingernails would be weak, jagged, and horribly ugly, but sharp and nasty nonetheless.

Putting it all together

A bacteria invades a host's body. Over the course of a month, the bacteria causes the host's hair to grow darker and more quickly. Finally, after almost exactly 29 days, the bacteria causes pockets of chemicals in the body to rupture.

The chemicals have several effects. The first chemicals cause rapid hair and fingernail growth. The hair and nails are unhealthy, twisted, and ugly because of its rapid growth. Next, the bacteria floods the body with a concoction of chemicals that reduce pain, increase adrenaline to borderline deadly levels, and cause swelling throughout the body.

The infected person flies into a homicidal rage; blinded by drugs and pain, the host attacks anyone who comes near, unable to comminucate in anything beyond growls. The infected's arms and face are covered with coarse hair; their fingernails are jagged claws. They feel the extreme urge to bite and scratch, because that causes the sharp pain in their mouth and fingers to numb.

After their episode (usually only a few hours), the concoction of chemicals causes the excess hair to fall off. They will be weak, fevered, and tired; additionally, any injury they sustained will remain. They likely have no memory of the previous events.

Those that the scratch will become infected; since the disease takes roughly 29 days to mature, the exact time of maturation is roughly the time between full moons, leading to the myth that werewolves only come out at the full moon.


Biological Transformation: The ability to rapidly transform tissue comes at a tremendous metabolic cost if we are talking about the synthesis of new cell structures. The 'transformation' would really be the change of state of a dynamic organ or organ system. Hair, canines, claws would need to telescope or sheath, as others have said.

Transferring blood flow to engorge body parts is not feasible because A) it will cause tissue damage after a few hours see Viagra warnings and B) we don't have enough blood to change the body at a large scale without depriving vital organs resulting in lethal damage.

A far more realistic mechanism of bodily shape shifting would be bones that grow concentrically rather than in parallel that can telescope through tendons and muscles with some kind of interlocking grooves that allow them to hold their positions without active muscle flexion.

Bite Spreading/Lunar Triggered Transformation: My money is on Protozoa. Malaria is a blood-borne disease of plasmodium parasites, so there is already a natural precedence for oral-hepatic transmission of Protozoa, and it has the advantage of already manifesting its symptoms in a cycle, albeit one of hours rather than days.

I see the lunar triggering of symptoms being hormonal, as the body already follows hormonal cycles with the moon. These Protozoa would release chemicals into the blood stream in response to a particular phase of the human body's estrus/circadian cycles to trigger the overall response. (I'm no endocrinologist, so be gentle.)

Naturally Evolvable: This parasite works on naturally evolved 'lycans'. The disease involuntarily tirggers and sustains all of their predatory/aggressive adaptations, sending them into a slavering homicidal state due to hormonal overrides. These creatures could, otherwise, turn these adaptations on or off voluntarily or temporarily through something like an adrenaline response.

What is the advantage for something like this to evolve? The ability to shift between two states has some obvious advantages: metabolic efficiency, behavioral prioritization, & environmental adaptability. (e.g. you can use less effort and fewer calories when limbs and joints are withdrawn/shorter, move with prioritized agility vs straight line speed, dart between trees or lope across a plain.)

In order to turn a human into a werewolf, however, you would need to put the poor fictional bastard into a cocoon (?for the preceding three weeks?), and that would have to be triggered by something far more contrived and artificial than any disease.


my approach on this is a reversible zombification-type virus.

spreading by bite

this is fairly easy, the virus is transmitted orally and requires body fluids to exchange, meaning just getting drool on you won't work, the saliva needs to penetrate into the skin and contaminate the bloodstream.

the transformation

the transformation is actually much simpler, the muscles swells through inflammation, the skin color transitions into a grayish black, and the infected would suffer from high adrenaline and dopamine secretions, tunnel vision, hallucinations, intoxication, severe hunger and thirst during this phase.

triggered by nightfall

triggering by fullmoon is unlikely, but the virus can instead become dormant during the day, it synchronizes with the body's circadian rhythm, this way the infected would transform during the night.


Hmm. So, werewolves? How would we do this? For starters, we aren’t going to be able to transform into a bloodthirsty beast overnight and back, and if we did the results would be... disappointing, to say the least. So let’s change the way werewolves go through transformations. Don’t you think it’s awfully prejudiced that a werewolf spends lots of time as a human, so it could spend equal amounts of time as both species. It could go through a cycle of a year: What is in my mind is that it spends two months as a human, spends four months gradually becoming lupine, spends two months like that, and spends four months going back. First, it works up a huge appetite and eats lots of meat and food. It has the urge to exercise its muscles, as well.

This leads to huge muscle and height growth so as our wolf doesn’t resemble a terrier in size. 😕 Also, the senses gradually become acute and the skull and bones start lengthening and transforming. This has to begin early on so as it has time to happen, as claws and teeth are forming in the hands and gums. Then, the joints start becoming more wolf like and you become aware of great pain at all times. Gradually, hair starts growing- this is the least complex part of the transformation and starts last. At the end, you’re officially lupine! 😀 The, SNAP! Ginger snaps back and you gradually start becoming a human again. Does that sound plausible?

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Re teeth: One possibility for a werewolf might be a retractable jaw, a bit like a goblin shark or the slingjaw wrasse. It wouldn't be anything biologically like a wolf, but it would give a wolflike appearance while extended, and the teeth might be more pronounced / visible when extended like that. Most animals that do that are ambush predators and do that while striking or attacking, but it could have developed that feature for that reason and adapted it to some other purpose as well.


well, first of all it would be more probable that the state of mind would not change. for example a kind peaceful person will still be a peaceful person. If the frame of mind change it could relate to being drunk or high. so whatever is making the person a werewolf also makes the drunk or high.

Another question would be: how did the person become a werewolf? How is it spread? It could be a disease not unlike cancer; which is occurs when a mutated cell is not destroyed by the immune system. it is formed in the body. or take AID's. its a virus that target the t-cells which is the immune system. Aids is spreadable by bodily transfer unlike cancer. werewolf disease can be spread like aid's or it just occur like cancer.

How does it transform? A werewolf transformation would be a longer process then just 30 secs. it would take weeks even months! you would need abnormal hair growth which is entirely possibly but you'd look like a strange ape. then you would need a snout which is also totally possible. Cancer causes tumors. it could be some sort of bone and skin tumor were it is pulled out. tumors on the ears can give a pointed apearance as well as a tale. scabing or some sort of drying process on the hands as well as tumors on the fingers would make paws. nails would do the same thing as the hair grow. walking and using the nails would erode them down. The werewolf would be very disfigured, in pain and probably die in the process. but if the werewolf didn't they would still die for the fact they would be primarily immobile.


A werewolf could be an ecdysozoon, which sheds its skin at the full moon, and then again a short while later. This would be so that it could display urticating hairs during the full moon, to protect itself then, but not normally, to make mating easier. It needs protection during the full moon, as that is when it uses the long ovipositors in its mouth to inject its eggs into hosts, which then grow and burst from the host.


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