So I had an idea for a vaguely humanoid creature called Krah (which I will post more questions about in time), and one of its many horrifying bodily functions is an ability I call “loathsome limbs”. Basically, it has something in its musculoskeletal system that creates or shrinks calcium compounds in bones/muscles, making them softer/harder as a result. This power lets it become short and stocky (lots of bones into muscles) or tall and gangly (lots of muscles into bones). Is this biologically feasible? Could any creature do this? If so, how?

  • $\begingroup$ Could you explain a bit better how you'd like this to work? I feel like this would involve much more than just converting muscle into bone and vice versa, and be much less practical than it might seem. For starters: calcifying a muscle doesn't make it longer, just hard and bony. There's actually a human condition called fibrodysplasia ossificans progressiva (FOP) where tissues in your body (including muscles) essentially get turned into bone, but that doesn't really change the length of the pre-existing bones, just adds more bone into the equation. $\endgroup$ Nov 14, 2022 at 19:39
  • $\begingroup$ @ProjectApex I messed up in writing that. I meant something more along the lines of converting fleshy or soft organic matter to hard, bony organic matter, but the cells would bond to preexisting ones of the same type. This would make the creature thinner, taller, and much scarier. $\endgroup$ Nov 14, 2022 at 19:47
  • $\begingroup$ I'd say you're still with a problem: assuming it has an endoskeleton, for your creature to work as you want it to, it'd need the ability to rearrange its internal anatomy drastically, as it'd need not only to just calcify part of its tissues, but rearrange them as well, potentially involving breaking and healing the bones in its limbs and stretching of the skin, something that would naturally leave it very vulnerable for quite some time whenever it transformed, otherwise it'd just look the same, but with internal tissue turned into bone. $\endgroup$ Nov 15, 2022 at 2:29
  • $\begingroup$ How does changing bone density change shape? Or are you considering squishy substrate that is can't be used for structural support until filled with mineralized solid material. Would perhaps hydrostats be a better option? $\endgroup$ Nov 15, 2022 at 2:36

1 Answer 1


this is probably not good for anything that wants to use its limbs fully in the next couple hours at least.

Like I said in my comment, there are naturally cases of soft tissue converting into something more bone-like, a process usually known as ossification. Diseases like fibrodysplasia ossificans progressiva (FOP), are a great example of this. However, we don't just want for some tissues to turn into bone. From what I get, we also want this to happen in both ways, and in less than a day. Even in FOP, it can take years for something nearly as big as what we want to happen.

There's also another problem: the creature doesn't only need to turn some of its tissues into bone, it needs its limbs to be rearranged in terms of length. The best example of this in humans is us going through puberty, which, as you'd expect, still requires several years for the bones to grow and isn't exactly something we can reverse on a whim. We would ideally want something similar, but much more complex, and we'd want it to happen in seconds.

Essentially, what you want is borderline impossible to happen feasibly. Your creature would need the tissues in its limbs to essentially break down and rearrange themselves (possibly involving the necessity for the thing to break its own bones,which would definitely take way more than a couple minutes to be ready). Essentially, unless this monster also has the power to magically recover from severe fractures in seconds without cooking itself alive (cell division produces a ton of heat). It'd probably jut stick to one form for good if it plans to hunt anything in the next month.

But we still want a creature that can appear both tall and stocky at will, and to be ideally able to change between both "forms" as quickly as possible, so what do we do? Simple: loose skin and maybe some extra joints.

To better demonstrate what I mean: let's look at this green heron:

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From seemingly Small and Stocky neck and body to long and thin neck in a second. How does it do it? Simple: the neck was long to begin with and kept folded, the folded nature hidden by its skin and feathers.

Instead of going through the sci-fi extra complicated route of bome-to-tissue-to-bone rearrangement, it's much simpler to simply add in somewhat loose skin and more joints. Once it's spotted prey, the creature usually walks around in a crouched, compact way, with its long limbs folded up closer to its body, ready to spring out for a pounce. Thanks to the loose skin and/or covering of feathers/fur, to someone who's not paying attention or does not know it, the thing may seem to be smaller and bulkier than it actually is. For another example of how this is far from outrageous: did you know owls have pretty long legs?

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Once again the feather covering and the anatomy of the owl means we usually only see a relatively small part of their legs, and so we commonly assume they're shorter than they actually are. If anything, this is good for most predators, because it's prey is underestimating it's range.

If your creature keeps its limbs folded in a way that makes them appear like stockier, shorter versions of themselves (something potentially helped by the loose skin appearing to cover the hands when they're actually hidden in the folds somewhere else), they can still benefit from having long limbs without having to deal with any bone rearrangement, which then means it's always ready to stretch out said limbs when it gets within range of unsuspecting prey. A long neck with more vertebrate, if owls and other birds are anything to go by, also enables your creature to have a greater range of neck articulation (potentially up to 270 degrees).

For a crude concept demonstration:

You and your friend walk through the forest, it feels like a stupid thing to do, but you both did loose a bet, and you're no cowards nor a pair that'd go back on their word. After you've stayed for the time you needed to, you start making the path back, when you begin to feel like you're being watched. It comes from beind a tree, and whatever it is, it's about the size and shape of a person, but the skin looks loose, almost soggy, like it is halfway melted. If it even has hands and feet, they're probably hidden under the folds. It approaches slowly, almost clumsily, like a grown man walking while crouched down, and you both take a few steps back to keep some distance in response. Whatever it is, it doesn't look too athletic, and you might be able to outrun it. You start uttering a question to your friend as it pounces, spine springing straight as its limbs stretch out, each almost as long as a person. It grabs onto your friend with large talons originally hidden among the folds of skin, crushing his ribs like a harpy eagle that just caught a monkey. It stops for a bit, appearing to admire its catch before looking at you, it's long neck stretching out as it turns in an unnatural way, folding its arms up again like snakes curling up for a strike. You begin to run away, but the pursuer's long limbs give it an unnaturally long stride, and it catches up with little effort. Hopefully no one comes looking for you, or if they do, hopefully they'll see through what it pretends to be in time.

Naturally there's some exaggeration here, but I hope this helps show a more plausible alternative solution. It's pretty much biologically impossible for an animal to casually reform its limbs from a shorter to a longer version (especially from long to short), and even then, that's a process that should normally take at least a couple months to happen properly, and could probably render the limb useless or at least partially limited in function in the meantime. However, we can have a similar concept that's still relatively plausible by making the scary, elongated form the default and simply granting it a more flexible body, longer limbs capable of folding tightly, and a covering of feathers, loose skin, or something else that helps to hide the fact that the thing's limbs, neck, etc are actually curled up and longer than they appear at first.

  • $\begingroup$ That is brilliant. I am most definitely going with that system instead. $\endgroup$ Nov 15, 2022 at 12:28

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