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If an alien species ate minerals (aka a lithovore species), how might their bodies differ from ours? How would they eat/digest the materials? Do those answsers change if they eat substances like diamond versus metals like gold?

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    $\begingroup$ Do you mean biological or non-biological aliens? Things like you describe are a little hard to implement if you like to stick with biological ones. They can eat things like fertilizer because it can be used to build tissues and cells, but metals are another story. $\endgroup$ – Alexander Jan 31 '17 at 19:01
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    $\begingroup$ Possible duplicate of Can anything survive by eating rocks and other ground minerals? $\endgroup$ – Secespitus Jan 31 '17 at 19:05
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    $\begingroup$ Have you put any thought into their biology at all? The question may answer itself if you figure that out. A creature made of cells needs to have things to make more cells. If it's made of rock, then ways to get more rocks... $\endgroup$ – AndyD273 Jan 31 '17 at 19:06
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    $\begingroup$ I added the term "lithovore" to your question because that's a term commonly used to describe creatures like this in sci-fi and fantasy... I'm unaware of any true lithovores in real-world biology, although lithotrophs are close. Here's a link that describes some of what you're looking for (I'd turn it into an answer but it isn't quite on topic): alt-sites.tripod.com/lithovore.htm $\endgroup$ – SRM Jan 31 '17 at 19:13
  • $\begingroup$ More similar questions that might help OP: Dragons that eat gems - digestion and Could eating rocks provide sufficient calories for survival? $\endgroup$ – Secespitus Jan 31 '17 at 19:18
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Animals like us eat for two reasons: to get carbon that we burn for energy, and also carbon and nitrogen (protein) that we turn into our tissues. Bacteria have those same two needs but sometimes have the two functions separate. They might get energy via oxidizing or reducing inorganic chemicals / minerals - even gold. They need carbon just like us but might be able to use oxidized carbon (CO2) or reduced carbons we cannot use (for example alkane petrochemicals).

Eating 101. 1: Break it up so you can take it in and work with it. This can be mechanical (teeth, gizzards) or enzymatic or both. Enzymes can work inside the body (as we do) or outside (as fungi do - they squirt the stuff around and then sop up the breakdown products). Also, breaking it up increases surface area which facilitates step 2: chemistry.

2: Work chemistry on it. We oxidize the food molecules we make into energy, carefully, via enzymes which is a narrower term for a catalyst. Any kind of chemistry that is energetically favorable in your circumstance can be harnessed to produce energy for the organism. If you want to use materials you ingest to make your own tissues, then you work that kind of chemistry to make them into you.

If we had commensal organisms able to oxidize more exotic carbon sources then we could use those carbon sources for food. That works for cows and termites. If we had commensal organisms capable of metabolizing sulfur into energy then we could chew up sulfur-containing compounds and present them to our commensals for processing. That works for some types of clam and worm.

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The same way it works for plants: picture the gut uses the processes used by plant roots to extract needed types of atoms from minerals.

The animal still needs energy from someplace! If it uses sunlight or chemical energy like some bacteria, it will have a slow metabolism and be more like coral than what you normally think of as animals.

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