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I'm trying to create a new race of humanoids that are in appearance almost identical to humans with one exception: They have an extension of their spine protruding from their back, much like a tail. It would however be very long and be used similarly to a scorpion's tail, minus the poison. The way I envisioned it is: it would appear as if it's only bone, similiar in appearance to a human spine, but longer and meant as a weapon, tool for movement (like a spring for long distance jumps) etc.

My concern for this idea is the following: Would the humanoid have any control over the 'limb', considering there is no muscle around the bone? Do I specifically need muscles to be able to control it or is there another solution?

if there are any additional details necessary, I'd be glad to provide.

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You could control the tail with something inside the bones. Easy answer would be to think of a regular tail just encased in bone, but if you want something thinner, you could imagine discs between each section of bone that swell in specific ways to move the tail. This swelling could be through blood engorging soft tissue, or if you want something more imaginative, a piezoelectric material that swells in reaction to a voltage.

It seems like these methods would be slow and clumsy, but evolution can pull off some neat tricks. I'm pretty much just a meaty water-balloon, but I can move well enough to write posts!

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  • $\begingroup$ This sounds the closest to what I wanted to imagine, thanks a lot. $\endgroup$ – Chooba Dec 30 '17 at 11:33
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(...) it would appear as if it's only bone, similiar in appearance to a human spine, but longer and meant as a weapon, tool for movement (like a spring for long distance jumps) etc.

My concern for this idea is the following: Would the humanoid have any control over the 'limb', considering there is no muscle around the bone?

It may look like bones, but that's just appearance.

Think of a platypus bill. It looks like a proper duck bill. But did you know that it's not bony? The platypus's bill is made of leather!

So you may have a multi-articulated tail that looks like a spine, and feels like a spine to the touch, but it's actually hardened, pale human skin. The muscles and real bones are all inside.

As for how agile it is, that depends on a lot of factors - but if it is thin as an arm or thinner, there is no reason why it couldn't be at least as agile as a monkey's tail.

The real issue here is what survival advantage that would provide that would cause humans to evolve such a limb. Probably your race evolved from simians that did have prehensile tails, unlike us which have evolved from tailless apes.

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Let's take inspiration from the claws of mantis shrimp. Their club- or spear-like claws are deployed by flexing and releasing bands of chitin. In the same way, your creature's tail would need such a structure, and there would need to be a strong group of muscles at the base. Also, just as the shrimp's claws can pretty much do just this one thing (i.e. club or spear at high speed) your creature's tail would not have fine control, just coil and strike once every couple of seconds.

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  • $\begingroup$ Thanks, this looks very helpful and informative so far. I want the race to have fine control over the 'tails' so it seems a re-design is inevitable so far. $\endgroup$ – Chooba Dec 29 '17 at 12:46
  • $\begingroup$ You don't need much muscle mass for fine control. You do need mass for heavy lifting, though, in which case bipedal walk might be difficult. $\endgroup$ – John Dvorak Dec 29 '17 at 23:51
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Any tail to be used as a prehensile tail would need significant muscles to accommodate the fact of it's use.

https://animalsake.com/animals-with-prehensile-tails

The pictures of the monkeys and other animals in the link above show large amounts of muscle near the base of the tail, tapering out at the end. Visually comparing the tail to the animals arms and legs, it's about the same size, so it's basically another limb for them.

https://en.wikipedia.org/wiki/Prehensile_tail

That link might be a good start for you. Looking at the references might take you to the anatomy of a tail. My Google searches can't seem to find that in 5 minutes, so hopefully you have more luck there.

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Can you flex your index finger? In case you can (hopefully!), tell me where the muscle is.

So, in one word, yes this is possible, and nature has done it. You can, in principle, place muscles somewhere close to the center of mass, and move stuff around with tendons that are bound by tendon sheaths and go around accessory bones. Not the most energy-efficient thing to do, not the least stressful thing for tendons, but certainly doable (and done).

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