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It's tempting to think that anything with the outer shape of a human would simply collapse in a heap if its skeleton were not there. That is surely true if the internal structure was the same as ours but with the bones simply non-existent.

However the human tongue, the elephant's trunk and the octopus's whole body work perfectly well without bones to support them. An octopus can squeeze through remarkably small spaces https://en.wikipedia.org/wiki/Octopus

I'd like my land-living approximately human-shaped aliens to be able to walk upright on land and disguise themselves as humans but also have the ability to get where humans can't by squeezing themselves through small apertures.

The question is - could a creature without a skeleton actually hold and balance itself upright on land for any appreciable length of time? What difficulties might it experience?


Video of octopus walking on land https://youtu.be/TFzpC_e44Tg?t=43

EDIT

A hydrostatic skeleton, or hydroskeleton, is a flexible skeleton supported by fluid pressure. https://en.wikipedia.org/wiki/Hydrostatic_skeleton

A muscular hydrostat is a biological structure found in animals. It is used to manipulate items (including food) or to move its host about and consists mainly of muscles with no skeletal support. https://en.wikipedia.org/wiki/Muscular_hydrostat

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  • $\begingroup$ Maybe they have some kind of cartilage skeleton instead of bones. $\endgroup$ – takintoolong Dec 25 '18 at 0:28
  • $\begingroup$ @takintoolong - a non-bone skeleton won't allow for the "able to squeeze through remarkably small spaces" part of OP's question. $\endgroup$ – G0BLiN Dec 25 '18 at 10:46
  • $\begingroup$ Cartilage can be remarkably flexible. It does not need to be extensive, just at important structural places. Octopuses have cartilage: "The only hard parts of their bodies are a parrot-like beak and a nub of cartilage around their brain: nationalgeographic.com/magazine/2016/11/… $\endgroup$ – takintoolong Dec 25 '18 at 15:52
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    $\begingroup$ Was this inspired by The Future Is Wild? IIRC, they depicted giant squids that could stand upright, evolved to fill the niche of elephants. $\endgroup$ – Tristan Klassen Dec 27 '18 at 19:17
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As you can see in the video, the squid isn't really "walking", it's dragging its body across the ground and can't keep it's shape up.

The human skeleton is incredibly important. It attaches the muscles we have, and if you look at the trunk of an elephant or your tongue you can see it's attached to bone at its beginning and only really needs to support its own weight, and can "rest" by being limp most of the time. Simply removing them and replacing them with flesh would be a bad option. Replacing bones with more muscle could work. Up to a point.

Muscle is pretty heavy, and a solid tense muscle is very strong and applied well could partially replace bone. In fact your muscles right now are well capable of pulling itself out of its attachments with your bone, but smart design in your brain prevents your muscles from doing this unless in extreme situations, where you might actually damage yourself in order to survive imminent death. This is also where the stories about parents who single-handedly lift a car from their child come from. The big problem here is that it would make you heavier than you already were and cost more energy just to stand upright. And energy requires oxygen for most animals, and your human impersonator would be panting as if he was jogging when just idly standing by.

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    $\begingroup$ Muscle is not stronger than bone, not by mass, volume, cross section or any other comparison I know of. the muscle that moves a bone may be stronger than that bone but only by being far far larger. Using that to say muscle is stronger is like saying wood is stronger than steel because you can use a log to crush a soup can. $\endgroup$ – John Dec 25 '18 at 5:57
  • $\begingroup$ Muscle isn't stronger than bone. Bone supports more weight, including that of the muscle. Muscle moves weight, but with the support of MULTIPLE muscles working in unison. If bones lacked joints, it's safe to say most muscles wouldn't work at all. Simplest proof: if a person is severely morbidly obese and dies, it's safe to assume it's because their heart, not their bones, gave out. $\endgroup$ – Sora Tamashii Dec 25 '18 at 17:07
  • $\begingroup$ @John could you show a few of your comparisons I'm interested. The thing is ofcourse that a muscle has a maximum cross-section where it creates most of the force and the minimum cross-section where it changes into the tendon where it also carries all those forces build up in the thicker cross-section. I learned muscle was stronger for my study and looking on the internet I just found several dozen different strengths in a wide variety of units for each. I'll check my old textbooks if they have a more clear answer. $\endgroup$ – Demigan Dec 25 '18 at 17:14
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    $\begingroup$ @Sorahtamashii thats a wrong comparison. If your bones "give out" they break, but unless they break and destroy arteries or organs they wont kill you. So you cant say "oh the heart gave out and killed him so its muscles that are weaker". Especially since muscles have far more space and adaptability for increased weight but the heart has a much more limited space to grow and keep pumping. Besides that obese people often avoid stressing their bones by not moving much while their heart will always need to be active. And muscles that lie next to eachother can share forces and unburden the skeleton $\endgroup$ – Demigan Dec 25 '18 at 17:22
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    $\begingroup$ @Demigan the femur comes to mind, it is one of the strongest bones and the muscles that move it are gigantic in comparison, with a cross sectional area many times as large. The spine is an even better example since the mucle there is actually providing direct support, and each single muscle is the same cross sectional size as the vertebra or larger. nlm.nih.gov/research/visible/photos.html $\endgroup$ – John Dec 25 '18 at 23:54
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In short: No.

I think you misunderstand what a skeleton is, though. A skeleton isn't a structure of bones exclusively. That's what we typically think of because that's what OUR skeleton is, but there are two main types of skeletons: endoskeletons and exoskeletons. An endoskeleton is like ours where its skeleton is on the inside. An exoskeleton is like an insect's where the skeleton is on the outside.

A skeleton is merely a support framework. In the case of most larger creatures, it's bones. In insects, it's an external protective shell. The skeleton doesn't have to be composed of the same rigid materials as ours are. You can substitute our calcium-based skeletal system for something more malleable like a naturally developed rubber of sorts (think something like Monkey D. Luffy) or a skeleton that is a series of non-Newtonian fluid-filled sacs that are activated and deactivated at will or by certain stimuli allowing them to shift between stiff and "fluid".

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    $\begingroup$ According to Wikipedia there is a difference between a hydrostatic skeleton and a hydrostat. I'll add something about this to my question. $\endgroup$ – chasly from UK Dec 25 '18 at 17:20
  • $\begingroup$ Can you explain how your comment and edit conflict with my answer, please? $\endgroup$ – Sora Tamashii Dec 25 '18 at 17:40
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    $\begingroup$ I suppose I'm saying that a skeleton may be all or any of the things you mention but that none of them is actually needed if you have a muscular hydrostat as defined by Wikipedia. $\endgroup$ – chasly from UK Dec 25 '18 at 18:02
  • $\begingroup$ A muscular hydrostat would not work as a full-body skeletal replacement, though. The problem with substituting a muscular hydrostat for the full skeletal sustem is that there is no support to prevent unnatural twisting or bending. You could point to an elephant's trunk and say "that lacks support and doesn't twist or bend unnaturally", but if you were to compare that to how an arm moves, it does. skeletons provide rigidity. A muscular hydrostat cannot provide quite the same rigidity in that way, though it can still stiffen. Besides, using your very own source: $\endgroup$ – Sora Tamashii Dec 25 '18 at 19:06
  • $\begingroup$ "In a muscular hydrostat, the musculature itself both creates movement and provides skeletal support for that movement." A muscular hydrostat works if you're trying to create a skeleton for an organism... but its method of function is significantly different than that of a human's skeleton that one wrong movement or being bumped into would show the difference. $\endgroup$ – Sora Tamashii Dec 25 '18 at 19:07
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If you wish you can go to a different direction and form your "alien" on a "swarm" cell system, if the cells comunicate to each other and can specialize themselves very fast, they would not build a skeleton by the actual definition of the word, more like a temporal skeleton I guess. This would make sense if they travel from planet to planet and have to adapt themselves in order to survive. They could also gain interesting abilities like fast evolving, claws, etc. These would be great feats for a horror book alien, if this is what you are building.

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