I am aware of these previous questions

How quickly could a "shapeshifter" lose mass, realistically?

Is there a credible way a shapeshifter could gain/lose body mass when changing forms? -

However I want to preserve mass, so my question is different.


Shapeshifters may have to simulate a variety of people/animals/objects in order to fool humans. I want them to be able to change back and forth rapidly.

I can account for a change in size in varying ways, but here I will focus on asking about the following method. Large objects that are solid in real life can be made to appear solid from the outside by a shifter making themselves hollow on the inside.

The problem comes when simulating different Earth animals.


A shifter that has the mass of a Border Collie dog, needs to simulate a large horse. There is conservation of mass and they have to change quickly. Suppose that the "horse" is hollow.

enter image description here


A balloon-like horse will tend to act more like a balloon than a horse. A shape-shifter horse will have a great deal of trouble galloping. It may leap too high, not get enough traction, or just get blown away in a stiff breeze.

Is there any way to solve this while preserving the mass of the shifter?


  1. The fake animals do not have to simulate bodily functions such as breathing internally. That is they could imitate the rising and falling of the chest but there are no lungs inside the simulacrum.

  2. They have to move realistically enough to fool a human that sees but does not touch them.

  3. Their biology is different from Earth creatures. They don't eat or breathe like we do. They have a form of photosynthesis.

  4. They can assume the surface colour and texture of the animal they are simulating.

  5. Please ask for necessary clarifications before answering.

  • 1
    $\begingroup$ Do they absolutely need to be able to have such a large change? It seems more efficient to change to items/animals within a certain range where the hollowness isn't readily noticeable. Dog-> goat. $\endgroup$
    – Trioxidane
    Commented Sep 3, 2020 at 11:29
  • 2
    $\begingroup$ @Trioxidane - For the purpose of this question, I particularly want to focus on sizeable changes. Even if the changes weren't so great, I suspect that the difference in mass is going to make a noticeable difference to gait. $\endgroup$ Commented Sep 3, 2020 at 11:33
  • 1
    $\begingroup$ @JBH That question sounds way more specific than Chasty's here. $\endgroup$
    – KeizerHarm
    Commented Sep 3, 2020 at 14:46
  • 1
    $\begingroup$ @KeizerHarm That's one of the reasons I didn't vote to close as a duplicate, but simply brought it to Chasly's attention. $\endgroup$
    – JBH
    Commented Sep 3, 2020 at 15:03
  • 2
    $\begingroup$ +1 for a horse getting blown away in a breeze $\endgroup$ Commented Sep 3, 2020 at 20:42

7 Answers 7


Hydraulics, anyone?

I would think the fastest way for an animal to change mass and use that mass to effect would be to slurp up water. Hero of Alexandria, thousands of years ago, was already demonstrating how air and water pressure changes could create motion. Internal muscle arrangements combined with variable bladders would allow rapid shifts in shape, while drinking or "Peeing" excess fluid (separate fluid, not enriched body fluids) would allow any shape shifter who has access to water to rapidly alter shape and mass.

Hydraulic motion is certainly possible, and more radical designs than those in tuna and jellyfish have been suggested in speculative evolution. Hydrostatic Skeletons can and do exist biologically, allowing a semi-rigid structure (especially if there was a way to rapidly and selectively gel and un-gel the fluid to increase the rigidity).

Combined with air pressure to create rapidly changing shapes of some body parts, hydraulics would give your organism the ability to weigh whatever they wanted, and shift weight around to more accurately simulate shapes and motions. I imagine each new form would require considerable practice and tweaking to get right. Further, your organism would have an appropriate weakness (needing access to water), but super organisms are really not much fun in stories.

I don't know HOW alien your shape changer's biology is, but if they have a wide range of internal temperatures, they can freeze ice and vaporize water to steam, and at that point the sky is the limit. I don't think that one even needs explanation.

  • 1
    $\begingroup$ So besides the air pressure you're also taking up other stuff like water to just be heavier? Great idea. $\endgroup$
    – Trioxidane
    Commented Sep 3, 2020 at 13:09
  • $\begingroup$ This is a great idea. I'll just say that they can't boil or freeze water inside themselves although I suppose they could use human form and human equipment to do this. If they are out of doors in winter and the only available water is frozen they could have a big problem. They could however "eat" snow and store it the way you suggest. $\endgroup$ Commented Sep 3, 2020 at 17:24
  • $\begingroup$ Nice one. Depending on their strength, they could also/instead compress the air inside them to gain mass and most importantly - suppress bouyancy. $\endgroup$ Commented Sep 4, 2020 at 14:22
  • 3
    $\begingroup$ After reading this I get a mental image of a horse peeing A LOT while slowly getting thinner and slightly smaller before suddenly transforming in a dog.. weird but possibly suits the story needs of OP. $\endgroup$
    – Peteris
    Commented Sep 4, 2020 at 16:10
  • $\begingroup$ @Peteris LOL but yes, that's pretty much what I was envisioning. $\endgroup$
    – DWKraus
    Commented Sep 4, 2020 at 18:29


Since shapeshifters are able to assume a new shape at will, they can shape parts of their body in a way that they connect to each other (think of the connections and fittings of lego parts, electrical plugs and sockets, or private parts). One single border collie does not have the same mass as a horse, but fifty border collies will.

You may think that coordinating the movements of 50 shapeshifters for galloping may be hard, but they might also connect their nervous systems when they all form a single creature. Or they might have rehearsed a lot, thus it becomes second nature to them.

  • 1
    $\begingroup$ In Jennifer Fallon "Hythrun" universe, the demons that live with the Harshini have the ability to shape change and will "blend" together to form larger objects. They have to train for blends, and younger demons tend to cause instability in blends leading to their collapse. $\endgroup$ Commented Sep 4, 2020 at 8:33

Hold Still and Hide Flesh while Pretending to Be Small

Say you look like a horse. You need to look like a dog for a while, and your onlookers don’t have direct eyes on yet.

Shove most of your mass in a psuedopod under a bush, or into the ground, or some other place they won’t look.

Then wait and have the pleasant encounter but don’t move significantly, so that they don’t notice that the dog is tethered to the bush.

  • 1
    $\begingroup$ Well I suppose the pseudopod could actually be a bush or part of the scenery. It could be difficult if a human put a lead on the dog and tried to take it home or to a rescue centre! $\endgroup$ Commented Sep 3, 2020 at 22:59
  • 4
    $\begingroup$ Or even better, you could potentially fully split off a portion of yourself and come back later to merge with it. If they have such advanced biological control over their shape and form they could reasonably be able to do that as well $\endgroup$
    – Kevin
    Commented Sep 4, 2020 at 16:19
  • 1
    $\begingroup$ @Kevin Wells - In an emergency they can slough off certain parts of themselves in the same way that lizards can lose a tail and grow it back. However they cannot merge with the lost pseudopod - it will start to die immediately. They have to grow back slowly. Only certain parts can be lost - they can't lose the brain for example. $\endgroup$ Commented Sep 5, 2020 at 9:09

Do Nothing

All things fall at 9.8m/s^2 and it takes a very significant increase in surface area for a shape shifter to be so affected by air resistance that you would get a slow fall effect. This is because your horse never "falls" more than a few inches per step. Since air resistance compounds exponentially with speed, your vertical air resistance would be next to nil.

Where you would see the most air resistance would be against the Coronal plane of the horse at full gallop.

The Air Drag equation is F = 0.6⋅c⋅A⋅v^2

F = Air Resistance
c = Drag Coefficent
A = Frontal Area
v = Air Velocity

To figure out how Ballon like your horse will be, I will solve for a border collie, a horse the weight of a border collie, and an 11" party balloon filled with air.

Since I can't find drag coefficients on dogs and horses, we'll have to look at humans instead. A human has an average c of 1.0-1.3. Horses have a slightly sleeker body design than a human; so, I will assume the low end of that at 1.0 and border collies have a lot of shaggy fur which creates more drag so I will assume they are at the high end at 1.3. Balloons are about a 0.5.

To find A I took a histogram of the front profile of an average sized horse, border collie, and balloon where each pixel is at the scale of 1/2".

enter image description here enter image description here

  • For the horse @ a Mean of 114.35 this silhouette is 55% black so if you take the 8055px each representing 1/2" by 1/2" your get a total Coronal plane surface area of 3612px = 903in^2 = 0.5826m^2.

  • For the dog @ a Mean of 82.55 this silhouette is 68% black so if you take the 1012px each representing 1/2" by 1/2" your get a total Coronal plane surface area of 684px = 171in^2 = 0.1103m^2.

  • For the ballon @ a Mean of 73.06 this silhouette is 71% black so if you take the 374px each representing 1/2" by 1/2" your get a total Coronal plane surface area of 266px = 66in^2 = 0.0426m^2.

v is just the top speed which for a horse is 12.5m/s and for the border collie 13.4m/s. For the ballon I will use the horses speed to simulate the air resistance at our target speed.

I will also use a mass of 17kg for the horse and dog, and 2g for the ballon filled with air.

So the force of friction from airdrag is:

So the air resistance of a horse is (0.6)(1)(0.5826)(156.25) = ~54.6N.
And for the border collie is (0.6)(1.3)(0.1103)(179.56) = ~15.4N.
And for the balloon is (0.6)(0.5)(0.0426)(156.25) = ~2.0N.

From this we can determine the coefficient of friction using μ = F/(M⋅v)

So the coefficient of friction of a horse is (54.6)/((17)(12.5)) = ~0.2569
And for the border collie is (15.4)/((17)(13.4)) = ~0.0676
And for the balloon is (2.0)/((0.002)(12.5)) = ~80.

This can then be plugged into a stopping distance equation d=v^2/(2μ)

So the stopping distance of the a horse is (156.25)/(2(0.2569)) = ~304m
And for the border collie is (179.56)/(2(0.0676)) = ~1333m
And for the balloon is ((156.25)/(2(80)) = ~0.98m.

What about Buoyancy

Going back to the average mass of a border collie at 17kg and comparing it to the average mass of a horse 380-1000kg, we see that the horse would have to be 22-59 times less dense than the dog, assuming that both animals have a density of about 1000 kg/m3, this gives a final density of somewhere in the range of 16-45 kg/m3. Since air has a density of 1.225kg/m3 the effective downward force of gravity - buoyancy would be somewhere between (17 - (1.225 / 16 * 17)) = 15.7kg and (17 - (1.225 / 45 * 17)) = 16.5kg.

This is not a significant enough reduction to cause a noticeable change, again, because the up and down motion of the horse is not very significant, one would no notice such a small change in downward acceleration unless they were extra-ordinarily observant and deeply familiar with horses.


As it turns out, density does not significantly affect how well an animal can move to nearly the degree we think it would. You can see that the horse experiences much more air resistance than the dog, but it's still so little that it would take 304m to come to a complete stop from air resistance alone. This is not at all like a balloon that would come to a stop in just under a meter at the same speeds. Since the horse makes ground contact about once per meter while at full gallop, this means it's total slowdown from air resistance between steps is still going to be too small to be perceivable, even if it is technically more than a dog or horse would normally experience.

As for comments about traction, the total difference in downward force between a dog and a horse sized dog from buoyancy is only 3-8%. However, the increased surface area of the hooves compared to the dog's paws is over 1000%. So, the horse sized dog would actually have WAY better traction as a horse than it did as a dog thanks to having so much more gripping surface to work with. This means the horse would not slip and slide at all like some answers have predicted. The bigger give-away would be its ability to stop too quickly; so, hopefully the shapeshifter has made it's hooves adequately slippery to properly emulate the right level of traction. But if the horse were to panic and have to stop of a dime, it could, which would give away that something is off about it.

The other real giveaways would be if a really observant person notices that the hooves aren't sinking into mud the way they should or if it runs into something it should be able to push through (like getting knocked over running into a small branch.)

  • 1
    $\begingroup$ My apologies to anyone who read this before. I originally plugged the stopping distances into the wrong kind of stopping distance equation. Reworking it using an equation meant for air drag, I've come to much larger stopping distances invalidating my original conclusion that you might have to do anything at all. $\endgroup$
    – Nosajimiki
    Commented Sep 4, 2020 at 21:26
  • 4
    $\begingroup$ What they will notice is the horse either slipping on flat ground because it can't get enough traction, or bouncing into the air when they walk. Buoyancy in air will be noticeably different. $\endgroup$
    – John
    Commented Sep 5, 2020 at 3:49
  • $\begingroup$ Air resistance does not apply to just the front profile of an object, wind comes from the side as well and you have skin friction. $\endgroup$
    – John
    Commented Sep 5, 2020 at 13:01
  • $\begingroup$ @john Mammals are about 1000 times as dense as the air around us so we experience negligible buoyancy. Even with 15-20 times the volume of a dog, the horse will still be 50-67 times the density of the air. For comparison, a blown up balloon is no more than twice the density of air. $\endgroup$
    – Nosajimiki
    Commented Sep 21, 2021 at 18:55
  • $\begingroup$ see my answer, the horse is less dense than Styrofoam, which drastically alters how it behaves in the air. $\endgroup$
    – John
    Commented Sep 21, 2021 at 21:21

Ghost Busters!

Shape shifting involves occult/magical powers. Any such powers manifested in the 'normal' world involve the drawing of power from the aether. Shapeshifters are simply magically endowed humans with a very specific talent.

By concentrating they can draw magical power from the aether which manifests as ectoplasm. They can then draw this around themselves to form specific shapes and masses.

As long as they visualize and concentrate on the 'form' they need they can draw the needed mass from the aether. So if for instance visualize a horse ectoplasm will start to form around them flowing into the desired shape. Their human form changes form of course as well, flowing into an merging with the extra mass.

The effect only lasts as long as they concentrate on maintaining it. As soon as they stop doing so the spell collapses and;

  1. they instantly revert back into human form;
  2. all the extra mass they gained dissolves back into unshaped ectoplasm which immediately 'slimes' them and everything in the immediate vicinity before it dissolves back into the aether.

This means of course your shapeshifter can form shapes larger than him/her self but not smaller. So horses and elephants etc are a goer but not mice or anything with less mass.


Wind and magnets

Weight is a force downwards due to gravity. What needs to be done is simulating gravity. I'm proposing 2 solutions, although they are far from perfect.

The simulacrum seems just animate matter. The hollow structure can then be used to our advantage. Inside the simulacrum will be air chamber(s). Something will move inside the air chamber, moving air out one side and in the other. This thing can then assimilate into the edge while a new one will be created on the other side, repeating the process. This way you can suck air into the creature and out the other side. (Alternatively you just have propellor blades.) This provides the pressure needed to keep the creature on the ground and look appropriately heavy. Multiple air chambers and entry/exit holes can be along the creature for pressure in the right direction. The holes can open and close at will and are hidden in the hairs. As the hairs are controlled by the simulacrum, it can ignore the wind of the creature moving the hairs. The larger the creature, the more air is available, making a sort of scaling along with the size.


  • The wind rushing through the holes can make a lot of sound.
  • It requires a lot of moving inside the simulacrum with possibly very thin structures. Although the thinness might not be a problem with the way you described the creatures.
  • Produces more wind than normal which can be detected.
  • Might not be able to get equal simulation of gravity in some situations.
  • Despite the wind simulating gravity, it isn't actually heavy. This can be a problem when pushing or pulling objects in some cases (although for most pretty accurate).

Second option is magnetic fields, but it's even more outlandish. Have it produce immense magnetic fields to use the earth magnetic fields to pull the creature down. However, if you've ever been near an MRI machine you know the weird feelings it creates from just being inside the magnetic fields outside the machine. In addition, the magnetic fields of the earth are so weak you would need an insanely powerful magnetic field to actually push one way or another.

  • $\begingroup$ I think the main problem with generating immense magnetic fields is that MRI machines are massive. I don't think a shifter with the mass of a border collie and unconnected to a hefty electricity supply would be able to generate these fields even if they simulated an MRI machine. A horse with the mass of an MRI wouldn't even be able to walk! $\endgroup$ Commented Sep 5, 2020 at 9:15
  • $\begingroup$ @chasly-reinstateMonica certainly true. And the point is, even if they could, it would still be woefully short of being able to increase your mass significantly. The power output alone would be so immense you would know something is going on by the way the iron in your blood would stick to one side of the veins. $\endgroup$
    – Trioxidane
    Commented Sep 5, 2020 at 10:15

Unfortunately probably not.

Human are hyper aware of how things of middling mass and force should move, that is why CGI movement often just looks wrong even if the lighting is perfect. Your horses will never move properly because they don't have enough mass.

A real horse has about 25 times the mass of a Border Collie(1000/40lbs) and is roughly the same shape. Since both have the same density you can extrapolate the density of your "horse". Your "horse" is about 25 times less dense than it should be. That means the Horse is less than half the density of styrofoam. It is not light enough to float in air but only just. Unless it never goes outdoors it will be hard to hide.

  • $\begingroup$ We are really good at spotting badly animated people, but an animator has to be pretty far off for an animal's animation to catch our eye unless it is an animal you are personally very familiar with. $\endgroup$
    – Nosajimiki
    Commented Sep 3, 2020 at 21:11
  • 1
    $\begingroup$ @Nosajimiki by far off you would include moving as if it weighs less than a tenth of what is should. Artists will tell you after horses horses are the hardest thing to get right. $\endgroup$
    – John
    Commented Sep 3, 2020 at 21:23
  • $\begingroup$ Weight does not affect how things move nearly as much as we think they do. What animators often get wrong is Gravity and Cause-effect stuff. This horse will still fall at 9.8m/s^2 as long as you don't make it so light that it can't overcome air resistance very well. $\endgroup$
    – Nosajimiki
    Commented Sep 4, 2020 at 21:34
  • $\begingroup$ @Nosajimiki the horse will have approximately the same density as styrofoam. $\endgroup$
    – John
    Commented Sep 5, 2020 at 3:41
  • $\begingroup$ Yes, and it does not matter. A solid horse made of Styrofoam behaves very differently in air than packing peanuts the same way that a 2x4 behaves differently than sawdust. The larger the object, the more easily it overcomes air resistance for its density. Once you get to horse sized objects, the difference in of air resistance between Animal vs Styrofoam is even smaller than a bowling ball vs a basketball youtube.com/watch?v=ujY045qFktM. $\endgroup$
    – Nosajimiki
    Commented Sep 5, 2020 at 5:54

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .