What is the most plausible way for an organism to have (limited) shapeshifting abilities?

Question

These shapeshifters - known as Doppels - aren't the stereotypical "blob of goo that becomes whatever it wants" shapeshifters. Instead, they're humanoids that can assume the physical appearance of another human, usually to steal their identity. What is the most plausible (or the least rule-breaking) way for them to do this, biologically speaking?

Some parts of this have obvious solutions. Color changes are a no-brainer - many animals use modified chromatophores to change their skin color, and it seems reasonable that they could also be used to change eye color. Hair changes can probably be accomplished using a way to grow it extremely fast, and then simply cutting and dying it (but if there's a way to do it purely biologically, let me know). Changing their gait and voice could be done with nothing more than good control of their movements and speech. What I haven't been able to figure out is how they could change their actual body shape.

Here's the specifics. Doppels can:

• Alter their height and the length of their limbs, up to an inch or two in either direction.
• Alter their facial structure to assume a wide range of faces, as long as the face has no major deformities (such as a missing eye).
• Alter their apparent weight within a moderate range (they can't appear malnourished or extremely obese, but most humans are within their range).
• Alter their muscle definition within a similar range.

Doppels can't change their apparent sex (unless having their other abilities would already let them).

I haven't decided how long the transformation takes, mainly because it'll be based on these answers. Doppels usually assume a person's identity for months or years at a time, so it doesn't have to be a quick and easy process, but ideally they'd be able to transform within a few weeks at most.

Answer 1

Octopuses and other cephalopods change their shape and even texture using small regions in their skin known as papillae, they can also change the color of their skin with cells called chromatophores Adapting this to a humanoid with with one or more varieties of condition known as Ehlers–Danlos syndrome. You would have the base for a fairly formidable shape shifter.

The speed of the transformations in cephalopods, color and texture, is quite striking. Indeed cuttlefish can rapidly alternate their colors in which can only be described as a psychedelic maner to "hypnotise" their prey.

Answer 2

Most people don't know that muscles don't contract. They use what can be referred to as calcium ratcheting, with the muscle fibers sliding against each other instead of contracting. You could do something similar with the bones. Instead of having solid bones, they'd have interleaved bones, where they are several layers that slide against each other like intertwined fingers.

The cost of this would be that the shapeshifters would be more fragile. If you don't mind a much longer transition time, they could use osteoblasts and osteoplasts to release and freeze the bones in place.

To a certain extent, you could use natural swelling mechanisms in the soft tissue to bulk a person up, but you're going to have to get really clever if you want the shapeshifter to be able to increase its apparent mass by fifty kilos.

Head size would be a problem. The things would have to commit a lot of brainpower to shifting shape, which would increase its size. The head would unavoidably have a minimum size, and you wouldn't want the brain case to expand, but you could shift around the bones in the face.

Other issues would include hairiness and the shape of the ears. You'd also have a really tough time changing the ocular sockets, since you couldn't change the size of the eyes.

Answer 3

1 Pseudo Bones

Their bones are not actually bones, but dense muscle like structures that can change in length. Much stronger than most muscles we think of, but only able to slowly change in length as an integral part of them being so stable as to pass for bones in the first place.

2 Water sacks/sponge organs

Doppel's have skin which can be adjusted to be water absorbent in various areas, as well as a water absorbent substance within their bodies, and so allows for the growing and extruding of apparent areas of fat as local areas are allowed to become wet and swell, or the dopple can extrude all the water quicly and get rid of the apparent fat. It is worth noting that if you want them to still be possible, they still need to have body fat(assuming that they're at least adjacent to humans) and can actually get fat as a result. And so they won't be able to get rid of everything.

3 Sexual organs

Dopplers can already change their apparent sex though, if the dopplers where hemaphrodic this could be supplemented even further, with retractable penis further resulting in variability.

4 Variable length vocal cords

In continuation of the established topic, any voice changes would be supplemented if their vocal cords could tighten and relax to an extreme degree, and so would be able to mimic nearly any voice, barring ones that would result from damaged vocal cords.

Ummmmm, so, there you go.