If you have read Eragon, then you know of the Werecat: a race that can shapeshift between a 'large cat' and a 'child', almost certainly with no change in mass. The Magic System is based on the literal and the scientific, thus matter and material cannot be generated or removed without understanding how that could be possible.

I'd like to take this concept and evolve it into what might be considered a 'Kitsune'. Rather, let's say someone magically evolved a werecat into a reflection of a 'Kitsune'. A Werefox for convenience, but not necessarily a race that is part-fox, just somewhat mirrorsing the theme.

A mostly(or only) female race that could shift between a cat/fox-like animal with a long fluffy tail, a human with animal ear+tail features, and a human - but the mass has to be retained. Their evolution should require them to retain as small a figure as plausible, but be capable of safely 'mating with a human to produce children, which would need them to also have a pleasant appearance(if short?) to men in a medieval-type society. Probably closer to a physically attractive woman on a smaller scale in overall size rather than just being shorter? As a (probably unnecessary addendum) they would also prefer running (and probably walking) on their toes when in human form unless carrying significant weight(or would this be a lie?).

Specifics can be changed if it doesn't fit realistically, but this is the general outline. The only magical component to this is the intrinsic ability to shapeshift between working forms. As they would exist in a medieval-type society, the Humans there would typically be malnourished and generally shorter, and they might naturally reflect that if needed.

I would greatly appreciate numbers for both the werecat race and the custom race for comparison. Height: Human(-ears, +tail-mass), Hybrid(+ears, -tail-mass), Animal - also werecats too? Weight: how much (stays the same for each height) - also werecats too?

I'm mostly looking for general height and weight, but details are appreciated.

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    $\begingroup$ Please remember that we're here to answer specific, answerable questions. This site isn't build to provide general feedback of idea generation. it might be a good idea to edit your post to remove the add on questions and focus on the one question. $\endgroup$
    – sphennings
    Oct 24, 2021 at 1:22
  • $\begingroup$ Realistic compared to what? Magic doesn't exist on Earth and since you've created something magical (shape shifting) the solution can be anything you want. If you want to stick with science-based, you need to either explain to us where any extra mass can/could go or stick with even-mass transitions, in which case the high-mass of an arctic fox (Vulpes lagopus) is 21 lbs, and that's the maximum size of your human (the average weight of a 1-yr-old child). VTC:Needs Details, happy to retract the vote when you've added the details. Include @JBH to notify me. $\endgroup$
    – JBH
    Oct 24, 2021 at 18:53
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    $\begingroup$ FYI, cats and foxes are pretty close to the same size. At most, foxes are only slightly larger, on average. $\endgroup$ Oct 24, 2021 at 19:45
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    $\begingroup$ Isn't that either entirely up the Builder, or down to some averaging function of the difference between human and fox sizes? $\endgroup$ Oct 24, 2021 at 21:19

5 Answers 5


let's run the numbers. A quick google search shows that the average fox in the UK weighs 9 and 17 lbs. Then according to a child weight percentage chart (also provided by google), your fox in human form will probably be, at most, the size of a 8 month child, assuming mass is conserved. If you assume that all your fox/people weight as much as the upper bound of red foxes (30 lbs) then your fox folk can be as large as a 3 year old child, when in human form.

Or, and hear me out on this; unless your readers really like doing math and consulting statistical charts, you can just ignore conservation of mass altogether. You are already allowing for changing form, and you are saying that this is a fantasy race in a fantasy world. Skip the boring and science based limitations. Remember if your magic is sufficiently explained it ceases to be magic and becomes mundane technology. Magic is supposed to be the fantastic, by it's nature it breaks the rules of reality, don't ruin the fantasy and the wonder by binding it to what can be explained in our sciences.

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    $\begingroup$ Yeah waste of time putting any pseudo science with this $\endgroup$
    – Kilisi
    Oct 24, 2021 at 4:20
  • $\begingroup$ Actually as I read it the conservation of mass would add a great comical value, in which for example their chest grows when in human form due to the missing ears and tail. The fox form however needs to be much larger then... $\endgroup$
    – trikPu
    Oct 25, 2021 at 14:07
  • $\begingroup$ Unbounded magic is boring; plots are driven by limitations. Forcing shapeshifters to preserve mass puts restrictions on protagonists and antagonists; not changing into a mouse and running down a hole or turning into a bigger dragon than the one you're facing. $\endgroup$
    – prosfilaes
    May 4, 2022 at 19:16
  • $\begingroup$ @prosfilaes If you limit your magic to what's physically possible it ceases to be magic. However there are plenty of ways to impose limits on your magic besides caving to the mundanities of physics. For instance OP's critters can change into a fox, hybrid and human. That by itself is enough to prevent the critters from changing into a mouse or a dragon. Remember that as worldbuilder all you need to do to impose a limit is to say it exists, it's just as much of a lift to say "transforming follows conservation of mass" as it is to say "Transforming follows some limit X" for all values of X. $\endgroup$
    – sphennings
    May 4, 2022 at 19:23
  • $\begingroup$ @sphennings And if you ignore what's physically possible, you get nonsense. Shapeshifters generally change into creatures that are distinctly mundane (like foxes) or that could exist realistically in our world (like fox-human hybrids). There's no reason for creatures to be continuous in normal space, but the exceptions are rare, and composed of a few pieces, never Cantor dust. "Transformation follows conservation of mass" is a rule that will be accepted by the reader; "the wearer is weak to yellow" is a rule that's widely mocked by readers. $\endgroup$
    – prosfilaes
    May 4, 2022 at 19:41

The average human female weighs around 53 kg and is 159.5 cm. The largest fox species is the red fox, Vulpes vulpes. The largest recorded red fox in Great Britain was a 17.2 kg 140 cm long male. They are normally 90-105 cm long and 5-7 kg. A 53 kg fox would be around 198-206 cm long. A wolf sized fox is far out of the ordinary and would be very conspicuous. If we reduce the human form to 145 cm and make it underweight at 32.5 kg then it would be around 168-175 cm long in fox form, Perhaps less if the fox is exceptionally fat.

The heaviest cat ever was 21.6 kg and 96.5 cm long. The longest cat ever was a 123 cm Maine Coon, which could have theoretically been 44.7 kg if extremely obese like the former cat, in which case it could be 161.3 cm tall in human form. If in human form it is only 32.5 kg and 145 cm like with the werefox, then it could either be an extremely obese 110.6 cm long cat or a moderately obese 123 cm cat.

For either of these, they are going to be disturbingly large in animal form and very short in human form.

  • $\begingroup$ How do you define "cat"? A mountain lion is still a "cat" in many people's view (and certainly a "large cat") and can easily have the mass of an adult human (of a healthy weight, anyway), let alone a child. I suspect Paolini had something like the Eurasian lynx in mind... You'd need a wolf-sized fox, though. Maybe one of these? $\endgroup$
    – Matthew
    Oct 25, 2021 at 15:37
  • $\begingroup$ I assume Felis catus, @Matthew $\endgroup$ Oct 25, 2021 at 20:05

While the other answers make clear the significant difficulties involved in conservation of mass one potential approach is to include other changes.


Achondroplasia is a genetic disorder whose primary feature is dwarfism.3 In those with the condition, the arms and legs are short, while the torso is typically of normal length.3 Those affected have an average adult height of 131 centimetres (4 ft 4 in) for males and 123 centimetres (4 ft) for females.

This chart puts the 10th percentile for 18 year old males at 25kg, and females at 35kg.

You can use this as a starting point, to get your human form weight down further. Significant sensitivity while writing a character like this would be required.

In the opposite direction for fox form, unbreakable bones have been observed in humans with up to 8 times the normal density, though I can't find any articles actually taking about the weight of the people involved or comparing that to others of their height and build.

Given how far apart the initial weight requirements are, you'd still need some handwavium (additional really heavy tails?) to make this work.


If they are goo-like shapeshifters, who only assume a form on the outside (not muscles or other biological material) that goo can just get denser or rarer as the form demands.


Cheat, anatomically.

Setting aside the fact that transforming from a human into a fox is so wildly physically impossible that "conservation of mass" is the least of your problems, remember, they're not real foxes or real people. So neither body has to be exactly like those of the corresponding species, only enough so to be convincing from the outside.1

Otherwise, since you're already deeply altering the internal organ structure to make room for more brain tissue, alter the organs even more.

The gap

Let's start with the baseline of:

  • A small and under-weight human, 4'10" and 35 kg, BMI 16.1
  • An uncommonly large and powerfully built fox, 10kg in mass We need to bring that 35 number down, and that 10 number up.

Bridging it

Let's start by making the fox male. We're transforming back and forth between species, so transforming gender shouldn't be a problem, and gets us to the size of a very large male fox. Now, we're at 15kg for the fox, without even doing anything too weird!

Next, let's change up the proportions a little.

Foxes are also deceptively light canids; thanks in part to their very slender leg bones, they weigh about 30% less than you’d expect for a dog of their size. (link)

Excellent. We're up to 19.5 kg.

We can actually do the rest with bone density.

  • Give the fox form extra-dense bones. Bone mass proportion goes down as total mass goes down, but let's cheat and use the human numbers of 10-15% That gives us 2.5±0.5 kg of bone mass to start with. Terrestrial mammal bones are roughly 1g/cm in density. Whale bones go as high as 5 g/cm. Let's bump that up a notch to 6 g/cm,and make your entire skeleton out of bone that dense, which will add a rad 12.5 extra kilos. (if you were willing to make them out of steel, at 8~g/cm, you'd actually be done with room to spare).
  • Give the human form extra-rare bones. We'll reuse that 10-15% number, which gives us 4.375±0.875 kg of bone. Reduce bone density from 1 g/cm to 0.75 g/cm, and we get to drop the human's mass by a kilo!

Now we're at a 34kg human (it's hard to lose weight, bro) and a 32.5kg fox.

At this point, by fiddling around with some of the numbers, we can come up with something to close that 1.5kg gap, or just blame it on the tail.

Or be cool as hell, and give your foxes steel bones. You're shapeshifting anyways, so why not?

  1. You already have to do this to make the brain work, since fundamentally, human brains are vastly bigger than fox ones. Grey wolves, who are considerably bigger than foxes, have a brain 10x less in mass than a human brain.

Actual foxes weigh, at most (according to another answer), ~17kg (38lb). According to the CDC, that's roughly a 4½- to 5-year-old child, or ~110cm (43in). A more realistic "typical" number is probably 12kg (26lb), equivalent to a 2-year-old, ~90cm (35in). That's actually not entirely implausible for a meaningful character, but they're going to have a lot of difficulties interacting with "mundane" humans due to their perceived age.

Paolini's "large cats", equivalent to "children", might well have had animal forms closer to a Eurasian lynx. At ~20kg (44lb), such a cat would have a human form closer to that of a 6-year-old, about ~115cm (45in) tall. They would definitely seem like "children", but they'd have a much easier time than the toddler-sized shifter you get using a realistic mass for a fox.

That said... what constitutes a "fox"? If you're looking for something that looks the part, but is allowed to be larger in animal form (more like Paolini's werecats)... have you considered the maned wolf?

maned wolf

These beautiful animals look a lot like foxes, except in size, with an adult weight range of about 20-30kg (44-66lb). A human child of similar weight would be between 6 and 9 years old with a height range of about 115-135cm (45-53in). It wouldn't be too unbelievable to stretch that a bit and have your characters be roughly equivalent to children in their early (pre-pubescent) teens. (Except your females are going to be, ah, "fully developed". That's likely to make for some interesting reactions from "mundane" humans.) For that matter, you transplant the maned wolf's colors onto a Eurasian wolf, for which particularly large specimens can reach almost 80kg, which is easily within the realm of weight for a normal adult human male.

So, in short, if you want your fox-people to be fox-sized, they're going to have to be really small humans. If you're willing to let them be more wolf-sized, they can be much larger as humans while still looking like relatively "normal" animals.

Note that all numbers are taken from the mean curve; actual correspondence between age and weight/height can vary considerably, but since we're talking about adults that look like children, that variation isn't all that meaningful for our purposes. The weight/height correspondences are less variable.


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