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In a couple of games, you encounter a creature or two that regrows limbs immediately after losing them, not necessarily the same kind or aesthetic of limb. You shoot the creature, it loses an arm, it grows a horrible mutant arm, you shoot it again, it dies.

TLDR: how do you make a creature, or evolve one, to generate new limbs, but automatically and instantaneously, and not necessarily the same type?

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Regrowing anything resembling an arm "instantly" is outside all known biological theories. There is no creature on earth which suggests, however remotely, that this is even a possibility within the evolutionary structures found on earth. As a reality check, growing a limb is impossible. It's simply too hard to do it fast. The timeline is off by 4 or 5 orders of magnitude at least.

As a creature design, what if the creature already had the limb pre-grown inside of it, and it simply erupts when it is needed. This is exactly how the human body develops permanent teeth as a child while it continues to use the baby teeth. The teeth are developed inside the jaw, and they only erupt when needed. Also, this satisfies the need for the limbs to be of different types -- it would be easy to have different morphologies for the two limbs Teeth in a a jaw, ready to be pushed out.

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  • $\begingroup$ Exactly the kind of answer I searched for. $\endgroup$ – Caleb Woodman Feb 22 '16 at 20:20
  • $\begingroup$ Last time I checked, adult teeth don't pop out instantly once the baby tooth is removed. Look at the design of shark's teeth for a better model of rapid tooth replacement. About one day as the replacement teeth are pre-loaded on what amounts to a conveyor belt. $\endgroup$ – Gary Walker Feb 23 '16 at 12:27
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Fiction

Did you see the recent spider-man movie? He fights an evil scientist-turned-lizard and he has the ability to regrow limbs at an astonishing rate. However, he suffers no consequences for doing so.

This is, of course, preposterous.

Facts

Consider that when you regrow a limb the mass of said limb has to come from somewhere. What are you sacrificing in order to regenerate that tissue? Muscle mass? Fat deposits? Secondary organs?

For a limb to grow that quickly your body would probably go into some kind of shock, as it is being deprived of all of its energy deposits in only a few seconds. It would be incredibly traumatic.

Implications

Even growing it over time would take a toll on your system. You'd probably be very tired, eat a lot more, not be able to maintain/gain, or even lose, muscle mass, etc.

Furthermore, how powerful would that limb be? Would it have the musculature of your existing arms/leg? Probably not. It would be incredibly weak, and vulnerable, and in need of therapy to use as your uninjured one.

Other Aspects

if your metabolism is capable of accomplishing this feat, how else might your organism be impacted?

How would your immune system react to viruses, or bacteria? Would you ever catch a cold, or get an infection?

Would you need to build up massive deposits of fat "just in case" you suffer an injury? Could your body trying to grow a lost limb potentially exhaust you to the point of entering a coma, or even killing you, if you don't have the resources in your body to provide the "building blocks" for said limb?

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  • $\begingroup$ Maybe this creature does have a massive fat deposits that it builds up, just for this reason, and as the OP described, dies the second time having depleted its "new limb reserves". $\endgroup$ – Whitcomb L. Judson Jan 8 '16 at 3:49
  • $\begingroup$ @WhitcombL.Judson - a human being would go into shock and die. Also, "and as the OP described, dies the second time having depleted its "new limb reserves" <- the OP never said anything like that, only I did, to point out how flawed the idea is. Cells regenerating that fast just doesn't occur on Earth. $\endgroup$ – AndreiROM Jan 8 '16 at 13:28
  • $\begingroup$ "You shoot the creature, it loses an arm, it grows a horrible mutant arm, you shoot it again, it dies." -Riley Santos. IDK, I was just thinking out loud as to how this could work (since it is tagged "creature-design), I didn't mean to challenge what you said. $\endgroup$ – Whitcomb L. Judson Jan 8 '16 at 21:14
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    $\begingroup$ You ask a lot of new questions that I find fascinating. But I'm looking not for "can it" but rather, "what would work?" $\endgroup$ – Caleb Woodman Jan 27 '16 at 16:56
  • $\begingroup$ Evolution is on the answerer's side, if he will use it. Pop-out limbs, Jelly-squirt limbs, concrete limbs, these things are ALL viable. Don't be a naysayer, especially keeping in mind that the only condition on this creature is "multi-celled" $\endgroup$ – Caleb Woodman Jan 27 '16 at 17:02
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Look at the problem from the other direction, how fast can cells grow and divide? Optimizing for the fastest growing eukaryote cells under artificial conditions, the research obtained a cell cycle rate or 52 minutes much below that reported to date by any other eukaryote. I.e., to manage a single cell division, you need about 52 minutes or more. To regrow a limb you would need at a minimum, many thousands of cycles of cell divisions.

There can be no biological method to regrow a limb in a very short time period.

Is there any possible non-magical alternative? Only 1 I could envision is that if the replacement limb were already fully grown and kept in reserve ready to "spring into place" as a replacement. Note that such a reserve limb could not have any bones. There is still a problem though, to what internal structures could such a limb be attached? It could not reuse the structure of the first arm as it would take time to attach to the old stump, it would in fact have to have its own blood vessels, etc. i.e., a complete duplicate limb that would emerge nearly in the same location as the old limb.

Needless to say, this is both impractical as well as inconceivable that this would not be an advantage to a creature unless the creature's limb detachment itself is designed to be an advantage (such as the gecko's tail to aid escape). Instant regrowth is not a necessary part of the escape strategy

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  • $\begingroup$ So you need to find a way to make the divisions not sequential, but able to work in parallel. This means filling out the volume in a fractal manner, rather than growing forward at a solid stump. $\endgroup$ – JDługosz Jan 11 '16 at 9:36
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    $\begingroup$ @JDługosz - Supposing you could somehow regrow a limb fractally, instead of having growing from supportable structures at the solid stump. I don't see how this would eliminate the need for hundreds of generations of cell divisions that makes near instant limb regrowth impossible. $\endgroup$ – Gary Walker Jan 11 '16 at 12:21
  • $\begingroup$ What's 2 to the "few hundreds" power? That's way way too many cells! Take the number of cells to be generated, log base 2. Natural log is pretty close. 30 divisions will allow one initial cell to produce over a billion. I recall that the difference between a person and a whale is around 40 divisions. $\endgroup$ – JDługosz Jan 11 '16 at 19:50
  • $\begingroup$ I find it funny that everyone assumes that these limbs are human limbs with bone and highly-developed structure. Popping a new limb out from a deliberate hole in the old one in the case of amputation would be totally viable, I agree. Don't burden yourself with assumptions of what the "perfect limb" is like! $\endgroup$ – Caleb Woodman Jan 27 '16 at 16:59
  • $\begingroup$ A trillion cells per kg is best approximation I found. You need about 5kg for a minimal limb. It means 42 divisions from single cell. My result seems similar to @JDługosz. And you don't have to start from a single cell. Still, few hours seems to be the best. But you overestimated division count, heavily. $\endgroup$ – Mołot Feb 23 '16 at 11:47

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