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What variables (in a hypotetical alien planet) can lead an underground alien species to have red skin pigmentation? I was thinking of widespread presence of pervasive ferrous compounds in the environment. Would that be scientifically plausible?

On a general note, which planetary features have an impact on the colour of the skin?

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  • $\begingroup$ Why are true moles (Talpidae) black? Why are marsupial moles (Notoryctidae) creamy yellow? Why does the tricolor hognose snake have its striking color pattern, with, yes, lots of red? All those animals are fossorial, if that is what is meant by "underground". $\endgroup$
    – AlexP
    Feb 12, 2023 at 23:51
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    $\begingroup$ You're overthinking it. For an alien world with an alien biology, the chemistry of that place can be sufficiently different to allow any color. Even bizarrely spectacular stuff via "structural coloration". On Earth, such things are relegated only to the taxons that (at some point in the past) had need to develop it (so mammals won't get bright metallic blue fur any time soon), but on your alien world anything's plausible. The only thing you might stay away from is photosynthesizing organisms, as their color does have something to do with the color of light they can absorb. $\endgroup$
    – John O
    Feb 13, 2023 at 6:08
  • $\begingroup$ @JohnO color of blood also is a result of which chemicals are responsible $\endgroup$
    – Trish
    Feb 13, 2023 at 8:46
  • $\begingroup$ @Trish Though related, color of the blood isn't the question here. In an alien physiology, it's not even clear that the color of blood would impact the color of their integument, or that they even have blood at all. $\endgroup$
    – John O
    Feb 13, 2023 at 14:40

2 Answers 2

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Blood

The skin of the species is actually transparent, as subterranean species don't benefit at all from any coloration of the skin. If they never go to the surface, making something like melamine to shield internal organs from sunlight is a waste of energy. As a result, the skin is translucent. Due to the lack of natural light, the species also will be most likely blind, eyeless, or just detect light and try to stay away from it. I suggest reading a paper on cave adaption.

What you actually see as the color of the aliens' skin is the thin layer of blood vessels, which makes the skin appear red. It's the same that gives human skin a red-to-pink tint. That is, the skin looks red without actually being red once removed from the body and dried.

The red color in this case is Haemoglobin, an iron complex. Would it bind in a different molecule, the blood (and skin) would have a different color:

  • Red is Hemoglobin when it binds Oxygen to its iron.
  • Purple-blue is Hemoglobin when it binds Carbon-monoxide - usually lethal
  • Blue is Hemocyanin and binds copper instead of iron.
  • Green is Chlorocruorin and binds its iron differently to Hemoglobin.
  • Purple is Haemerythrin and binds two iron per molecule.
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I mean - you've got so many options:

  • Diet
  • Predator/Prey relationship
  • Radiation
  • Environment
  • Blood
  • Sexual display
  • DA RED ONES GO FASTER! WAAAAAAAAAAAAAAGH!
  • Interspecies Communication

I could elaborate on each - but the question should be: "From a story perspective, which one is more interesting or more consequential?"

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