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I apologise in advance for the graphic nature of this question. I guess it just had to be asked.

I am looking for a way to make the feces of my alien vertebrates some color other than brown or green. In our Terran vertabrates, bile that is broken down in the gut along with items in the diet affect the color of fecal matter. We usually see green, rust, or the 'classic' brown unless something unusual such as a beet was eaten. My first thought when finding an answer to this question was to find an alternative to bile for breaking down fats into fatty acids. I am looking for something that would change the color almost regardless of what was eaten, as bile does. Anything that would change the color in carnivores, herbivores and everything in between would work. As long as the feces will be distinguishable from Terran feces, it should work.

Thanks for bearing with this one.

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    $\begingroup$ I feel sorry for whoever is going to go through the comments on this one . . . . $\endgroup$
    – FoxElemental
    Jun 19, 2018 at 20:08
  • $\begingroup$ @FoxElemental Me too $\endgroup$
    – Roki
    Jun 19, 2018 at 20:08
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    $\begingroup$ Are you asking if the feces will be a different color when beheld (ugh) here upon Earth? Or do we have the wavelength of an alien sun and the chemical composition of alien sky and soil to deal with? (If it makes you feel any better, this isn't the most disgusting question asked here... not by a long shot....) $\endgroup$
    – JBH
    Jun 19, 2018 at 21:11
  • $\begingroup$ @JBH Assume that the atmospheric conditions on the alien planet and the soil compositions are extremely similar to those on Earth. The sun that planet is orbiting is the same as the sun on Earth. So, if the alien were in either world, the feces would look the same. $\endgroup$
    – Roki
    Jun 19, 2018 at 21:33
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    $\begingroup$ @Aify Many bird droppings are actually very dark brown or green along with their white urate. $\endgroup$
    – Roki
    Jun 19, 2018 at 22:23

3 Answers 3

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Bone = white dung.

If an animal eats a lot of bones, much of the bone minerals is not digested. It passes through into the feces and turns them white. I have seen this with dog droppings. Hyaenas routinely eat bones; depicted are hyaena droppings.

hyaena droppings https://thomsonsafaris.com/blog/identify-animal-scat-walking-safari/

Another way to get white feces is how reptiles do it: concentrate the nitrogenous wastes that mammals pass as urine, and pass that concentrate as feces. The nitrogenous waste (mostly urea) is a white substance with the regular poop brown. Reptile feces is bicolor for this reason.

snake poop

http://www.feedpecker.com/snake-poop-look-like/

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    $\begingroup$ With pictures? Sigh. $\endgroup$
    – ShadoCat
    Jun 19, 2018 at 20:23
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    $\begingroup$ goodbye Oreo, it was nice until it lasted... $\endgroup$
    – L.Dutch ♦
    Jun 20, 2018 at 5:06
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    $\begingroup$ Dogs also used to leave white dung, but we now feed them very rich food so it's less common. It's not about the bone it's about their ability to be wasteful. $\endgroup$
    – Separatrix
    Jun 20, 2018 at 6:54
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Blue dung (or purple or red)

If you can design the metabolism of your aliens to excrete anthocyanin (an organic pigment) you could have a reason for their poop to be blue, purple or red (depending on the PH of the remaining components of the dung). So, the same way we humans have a variety of standard colors, they could have their own pantone (depending on what they have eaten).
More info about the anthocyanin in this Wikipedia link:
https://en.wikipedia.org/wiki/Anthocyanin

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Their normal gut flora includes something that releases a chemical that is whatever color you want.

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    $\begingroup$ could you add some concrete example? $\endgroup$
    – L.Dutch ♦
    Jun 20, 2018 at 5:07

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