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Imagine human like creatures with glands inside their forearms that can quickly produce a liquid that when exposed to air turns to a solid has hard as bone.

The liquid can be used to create and throw projectiles out of the forearm or to create a blade attached to the underside of the wrist.

How could this evolve and why?

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    $\begingroup$ I don't think shapeshifters applies here. I replaced it with natural-weapons, which seems more appropriate given the question. Other tags may also apply. $\endgroup$
    – user
    Oct 22, 2018 at 12:45
  • $\begingroup$ :-) Ditto from my last posted comment. We recommend closing your question by accepting an answer after at least 24 hours. $\endgroup$
    – JBH
    Oct 22, 2018 at 16:54
  • $\begingroup$ @JBH "Closing" a question has a very specific meaning on Stack Exchange. Please don't use the term to refer to answer acceptance. Thank you. $\endgroup$
    – user
    Oct 29, 2018 at 19:12

3 Answers 3

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Tarantulas and some lepidopteran species have what is called "urticating hair". Some plants have too. It is hair that can be shot off as a defense mechanism.

Now of course it is a long stretch from those hair to bone- bullets or forearm blades. But it shows that there is something "similar" to it in nature. A forearm blade of course could have evolved from a claw. It might grow back when lost in battle.

Don't know how important it is to you that it is made from a liquid that becomes solid when exposed to air. Maybe you could take a look at how spider silk evolved and get some ideas from there?

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  • $\begingroup$ Posted my comment then saw yours, yeah spider silk seems like the closest thing to me. $\endgroup$
    – Erin B
    Oct 22, 2018 at 14:15
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It couldn't.

Anything that hardens that fast will have a soft interior because the inside has not been exposed to air long enough to harden. You need either slow extrusion, with each thin layer forming on top of the earlier ones, like coats of paint; or thin fibres that bind together to form a single structure, i.e., a horn. Neither one is an instantaneous process, so Wolverine claws are right out.

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  • $\begingroup$ It could be hardened by oxygen/temperature combination, combined with high heat-conductivity. That way the inner heat can come out quicker too, it doesnt have to get that thick of a layer to be effective $\endgroup$
    – Martijn
    Oct 22, 2018 at 13:33
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    $\begingroup$ @Martijn: In living tissue? Besides, from the OP's question, ...glands inside their forearms that can quickly produce a liquid that *when exposed to air turns to a solid* has hard as bone. [emphasis added] $\endgroup$
    – nzaman
    Oct 22, 2018 at 14:28
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    $\begingroup$ As a side step to avoid the requirement to cure, a super saturated crystalline solution might be an interesting starting point... $\endgroup$
    – Quaternion
    Oct 22, 2018 at 22:05
  • $\begingroup$ @Quaternion: see Renan's answer below $\endgroup$
    – nzaman
    Oct 23, 2018 at 3:19
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    $\begingroup$ A crystal structure can have a lower density than its liquid form water being a prime example (although it does appear to be more the exception than the rule) and are prime examples of material hardness. $\endgroup$
    – Quaternion
    Oct 23, 2018 at 3:30
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It won't.

We do have liquid suspensions that solidify when they get out of the bloodstream or our glands: platelets become blood clots, gunk is left over when tears dry etc. But those are not very dense.

For a claw or projectile to be made, it would be as dense as the liquid it came from if it kept the same volume. In other words, for the volume of a 9mm bullet (a little less than 0.18cm3), you'd spend that much fluid to make a bullet as dense as the fluid itself. For the record, human blood is marginally more dense than water.

Every time you double the amount of liquid for the dame volume (compressing the liquid), you double density To achieve the density of human bone ([1.75g/cm3), you'd need to spend just a little more blood per bullet.

How effective would that be? Get a t-bone bone (I know it sounds silly, but I can't find another name for it), break it into bullet sized pieces and toss at a friend. For science. Now imagine a creature bleeding in order to do that. In the very least natural selection would penalize creatured who used this "weapon" to hunt, or to fend off predators. If you are willing to stretch the bow past the point where it breaks, such bone bullets could be used in sexual courtship, but that's it.

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    $\begingroup$ He said "a liquid" not necessarily blood. $\endgroup$
    – JMac
    Oct 22, 2018 at 13:28

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