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I'm designing a creature that is supposed to be able to eat "basically anything", within the realm of plausibility for what is essentially an organic creature. Its digestive system can be as complicated as necessary, and evolutionary history is not an obstacle, the creature could be either alien or engineered but one way or another it is omnivorous to the extreme.

At the very least, it should be able to break down any organic, energy-storing molecule, ranging from plants and meat to fossil fuels. Any other naturally-occurring chemical with a high energy content should be edible as well, if possible.

Notably though, it is an active, mobile animal so it should only be interested in eating things that have a relatively high energy content. It is possible to get energy by oxidizing metal but probably not enough to sustain a high-energy lifestyle.

How would the creature's digestive system work, in order to give it the ability to digest as many different things as possible?

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    $\begingroup$ Digestion can be fairly simple and straightforward. The real challenge is not being poisoned by all sorts of foreign proteins or isomers. This thing needs to break everything down and synthesize its own aminos, for instance. But in the meantime it needs to make sure none of the foreign ones come in contact with its own cells... probably some mineral-based lining to its digestive organ (like teeth). $\endgroup$
    – John O
    Commented Jan 28, 2020 at 15:49

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At the very least, it should be able to break down any organic, energy-storing molecule, ranging from plants and meat to fossil fuels.

The closest thing we got on Earth are roaches. They will eat anything that they can chew.

Digesting fossil fuels is complicated because of a few factors:

  • Breaking them down within your body could lead to the formation of carbon monoxide, which is terribly poisonous;
  • They are rare enough to find in natura that most creatures had no evolutionary pressure to develop the means to break them.

That does not mean animals do not consume them. Many herbivores, ranging from elephants to deer, and also some omnivore apes will eat charcoal. They consume it not as an energy source, but as a natural antidote for toxins in their diets (the article cites monkeys only, but there is evidence for other animals).

So let's start from our best bet. You could have an evolutionary story where roaches have a symbiosis with these two bacteria:

These bacteria could live in the gut of the roach. In return for giving the roach part of their energy output, they have an environment where they can thrive without competition. Also the roach will provide them with the raw materials they need to live, so they don't need to bother about getting food. A similar process happens with termites and bacteria in their gut - termites don't break down the cellulose they eat, the bacteria in their gut do.

So there you have it. Imagine a world in which roaches could eat plastic and oil. If you had a roach infestation in your house, they could drink your car's gas and then eat the fuel tank as a dessert.

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    $\begingroup$ Humans do this too; our intestines can only absorb the most basic of nutrients, but they are inhabited by a variety of bacteria that digest complex foods into what we need. In return for this valuable function, we provide them a steady food supply and a nice, safe home. Making more foods edible is just a matter of finding (or engineering) new bacteria that can digest more things for us. $\endgroup$
    – StephenS
    Commented Jan 29, 2020 at 18:59
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I would suggest giving it multiple stomachs, like cows, however here each stomach has a different function: one for degrading meats, one for fibrous plants, one for fruits, etc. This would probably require a first stomach with some kind of filtering method, or at least something to break down tough foods, so that each subsequent stomach can fulfill its role properly.

It could also be interesting to give this animal a complex internal microbiota. A lot of micro-organisms are much more effective than we are at breaking down energy-storing molecules, and having the appropriate bacteria and fungi in each stomach pouch would give your animal an incredibly wide selection of foods to live off of.

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    $\begingroup$ A stomach full of symbiotic chemoautotrophs for the more stable fuel sources... How wonderfully combustible. $\endgroup$
    – Joe Bloggs
    Commented Jan 28, 2020 at 13:22
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    $\begingroup$ @JoeBloggs Tremendous methane outputs! You'd think twice before hunting this beast with a rifle ;-) $\endgroup$
    – Whitehot
    Commented Jan 28, 2020 at 13:24
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    $\begingroup$ @Whitehot Maybe a fish goes to the same place. But slurping a pool of oil might not. I'm suggesting that some of the more exotic substances might deserve unique orifices. $\endgroup$
    – SRM
    Commented Jan 28, 2020 at 14:04
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    $\begingroup$ @SRM: if the methane is routed from multiple stomachs to the dedicated ‘high energy gases’ stomach, then the creature can really digest efficiently. Sorry, did I say digest? I meant explode. $\endgroup$
    – Joe Bloggs
    Commented Jan 28, 2020 at 14:19
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    $\begingroup$ Ooo! Check this out: worldbuilding.stackexchange.com/questions/154285/… $\endgroup$
    – SRM
    Commented Jan 28, 2020 at 14:55
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Heat differential engine.

Your creature oxidizes molecules in its "stomach" which is more of a furnace. Anything that can be oxidized is fair game - any reduced carbon, or nitrogen, or metal salt. Oxidation produces heat. The amount of heat produced is controlled by controlling oxygen ingress to the stomach.

Under hot conditions, certain metalloproteins change configuration. The circulation brings these proteins into hot conditions near the stomach and they capture heat energy with the configuration change. When the hot-configured metalloprotein circulates out to the cold exterior (possibly radiator plates or fins) it shifts back to the cold configuration. This conformational shift is linked to an ATPase and generates ATP for the creature to use for its muscles and metabolic processes, as Earth life does.

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  • $\begingroup$ It sounds like you would need very fine tuning of the heat of your furnace for this to work. Do you know of any animals that can tune their oxygen intake this finely? $\endgroup$
    – Whitehot
    Commented Jan 28, 2020 at 14:39
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    $\begingroup$ @Whitehot: I am such an animal and unless you are a robot you are too. Oxygen capture in the lungs and subsequent delivery to tissues is tightly regulated according to metabolic need of those tissues. Oxygen is toxic stuff and you only want the amounts you can use floating around your delicate biology. If you are a robot I guess your secret it not safe with me since I just outed you; sorry. $\endgroup$
    – Willk
    Commented Jan 28, 2020 at 14:46
  • $\begingroup$ I do understand the transmission of oxygen throughout a metazoa- I mean, my body, but this involves a whole number of very finely tuned enzymes circulating in blood. In my mind the furnace that you propose function with gaseous exchanges, but I may have misunderstood how it functions exactly. Could you describe your stomach-furnace in a little more detail maybe? :) $\endgroup$
    – Whitehot
    Commented Jan 28, 2020 at 15:21
  • $\begingroup$ I was thinking of using heat, but how efficient is using heat compared to utilizing chemical energy directly? As far as I know, no living organism uses heat differentials as its primary power source, and given how easy it is to produce heat by breaking down organic molecules you'd think that something would make use of it, yet heat production seems to only be used for homeostasis in living things. $\endgroup$
    – IndigoFenix
    Commented Jan 28, 2020 at 15:54
  • $\begingroup$ @Whitehot: Air control might be like a damper in a steam engine. The firebox of a steam engine has fuel intake and air intake, and you can reduce the air intake using the damper, which reduces the heat. I envision the "firebox" in this creature to work along the lines of the heating of a wet haystack. todayifoundout.com/index.php/2014/09/… $\endgroup$
    – Willk
    Commented Jan 28, 2020 at 15:55
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You got humans!

No, seriously - think about this.

We can eat basically anything on this big, weird world of us with proper preparation. What we can't eat right away, we can find ways to cook and make it edible. We already use stuff like coal, petroleum, and even wood. It is just a matter of time before we start making food directly out of carbon via some sort of weird 3D Food-Printer.

We already got the printers, and we aren't that far away from using carbon directly either.

Sure, it isn't directly our delicate stomachs that do this. Instead, we use our brainpower and societal resources to enable ourselves to eat whatever we think might be remotely tasty - emphasis on "remotely".

We usually think of our "digestive system" starting on our mouths, but for some foodstuffs this process happens way, way before it even hit our plates. From taming special bacteria to make cheese to using almost ritualistic processes to prepare meat, we devised thousands upon thousands of ways to make all sorts of things edible - sometimes for far longer than they should be, sometimes even if it tries really hard to kill us.

In that sense, we aren't that much different from the termite that stores special bacteria in its gut to be able to digest cellulose or from the animals that literally spit acid on top of their food to them slurp up the resulting juices. The major difference is that for them, this is the only way they have to eat.

For us, it is different. We're crafty bastards.

We don't eat with our mouths.

We eat with our brains.

So, in the end, what your creature needs isn't a super digestive system that can eat anything - instead, give them the ways to make whatever they want to eat, edible. Don't make them monster eaters. Make them monster cooks.

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    $\begingroup$ This is an oddly insightful answer. We consume all available resources -- not internally, but all in the service of our energy needs. Neat. $\endgroup$
    – SRM
    Commented Feb 3, 2020 at 1:51
  • $\begingroup$ @SRM Thank you for the kind words, and thank you for the bounty! >.<" $\endgroup$
    – Mermaker
    Commented Feb 3, 2020 at 13:32
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How about having your creature be in fact a multi-combo symbiote? Maybe it's part-way through the evolutionary process of merging into a single complex being (similar to the mix of prokaryotes into eukaryotes, sort of), or maybe it's like the combo-animals of "A Fire In The Deep". Either way, its component creatures 'share' all food sources as appropriate.

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Its name is "Carl"...with the iron stomach.

The mouth registers what type of "food" is being eaten. This triggers a complex neural message to glands that support the digestive process. A series of acid emitters are engaged to create the right acid mix for what is landing in the "iron gullet" this also triggers any needed mucus increases to protect the stomachs based on the acid strength. For complex or harder materials, there is a holding stomach that is designed for the full power acid and only really bizarre foodstuffs are diverted there where they get softened up before moving down to the regular stomach. For the complex gases associated with certain digestive processes, there is the ability to back-feed "burp" gases from either stomach, and the bowels "farts".

The question is for the really tough meals, is it the creature who suffers the most or those near the creature who get the brunt of the process.

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