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In my RPG campaign setting the orcs are tougher than most other races. In hard times they can eat wood (small bushes and fresh pieces of trees). In really hard times they can eat healthy soil and survive enough to continue their march/siege, but they don't like it.

How do I begin to explain this process?

While I could hand-wave it away with the word "magic," I'd like to have at least a believable starting point

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    $\begingroup$ Directly processing the trace elements -- or eating micro-flora/fauna that do the same. $\endgroup$ – Lyndon White Aug 8 '17 at 2:06
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    $\begingroup$ Have you read about Orks? Seriously have a look at the Warhammer 40,000 Orks they're half fungus and have some of the characteristics you seem to be after. $\endgroup$ – Ash Aug 8 '17 at 16:19
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    $\begingroup$ Earthworms eat soil (sort of). Are your orcs biologically related to earthowrms perhaps? $\endgroup$ – Pharap Aug 9 '17 at 19:37
  • $\begingroup$ Why not just have it be the same way that humans do? (Oh, just realized there's an answer already touching on this. I'll leave the link in case it's useful.) $\endgroup$ – Joshua Taylor Aug 9 '17 at 20:14
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Starting from the ground up: how much caloric energy even exists in the things orcs eat? Energy metabolism for aerobes involves combining oxidizable with oxygen - in essence burning them. One can burn things in a bomb calorimeter to see what the caloric value is. I looked up caloric values for fats, ethanol, protein and conventional carbohydrates (e.g. wheat flour); then wood and leaves, then various soils.

from https://en.wikipedia.org/wiki/Food_energy

Fats and ethanol have the greatest amount of food energy per gram, 37 and 29 kJ/g (8.8 and 6.9 kcal/g), respectively. Proteins and most carbohydrates have about 17 kJ/g (4 kcal/g).

from source enter image description here

from Caloric values of organic matter in woodland, swamp, and lake soils

Caloric values of organic matter in woodland, swamp, and aquatic soils (± standard deviation of the mean) $$ \begin{array}{c|cc} \text{Soil Type} & \text{Kcal/g ash-free dry wt} & \text{Ignition loss% dry wt} \\ \hline \text{11 woodland humus layers} & 5.04±0.04 & 57 .5±5.7\\ \text{20 swamp soils} & 4.87±0.04 & 77 .3±1.5\\ \text{9 lake and pond muds} & 5.24±0.05 & 59.3 ±3.6 \\ \end{array} $$

So: for a fire, carbs, proteins, wood, leaves and soils all have comparable energy value - 4 to 5 kcal/g. I was surprised to read it for soil. Why can't you make a fire from dirt, then? Maybe my efforts have used dirt with too much sand or clay; the authors of the soil article specify that this is topsoil rich in organic matter, not deeper mineral soils.

Then why can humans eat corn but not wood? The calories in wood, tree leaves and soil are mostly as cellulose: the evolutionary masterwork of the plants. It is made of sugar and very much a carbohydrate, as energy dense as starch, but very difficult to digest.

To digest it one must have commensal bacterial to do the job. These live in the specialized gut organ called the rumen in ruminants (like cows), also in the less specialized digestive tracts of pandas, and in the cecum (part of the colon) of rabbits.

The problem rabbits have is that the cecum comes after the small intestine, and the small intestine is where nutrients are absorbed from broken down food. What good is it for your bacteria to break down cellulose into sugar if you can't soak up some of the sugar because you are about to poop it out?

The rabbit solution: refection. Eat the poop. That poop is full of microbes breaking down the cellulose. Give them a while and that poop is full of nutritious sugars. Back in it goes to absorbed the second time around.

from http://www.columbia.edu/itc/cerc/danoff-burg/invasion_bio/inv_spp_summ/Oryctolagus_cuniculus.htm

European rabbits are ravenous eaters and indulge in a diverse diet of grasses, roots, tree bark, leaves, grains, fruit, seeds, and buds. Since this diet is low in nutritional value and high in difficult-to-digest materials, they are known to reingest their feces to obtain extra nutritional value from the food the second time around.

Your orcs are set up the same way. They can extract energy value from twigs, leaves and soils by letting their colonic commensal bacteria break it down, then re-eating the product.

This also explains why they don't like it.

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    $\begingroup$ +1 for the good answer. But even if I didn't like the answer: +1 for 'This also explains why they don't like it' $\endgroup$ – Crisfole Aug 8 '17 at 13:03
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    $\begingroup$ Still having a hard time reconciling "rabbits/bunnies" and "orcs" in a single chain of thought that does not include violence or one eating the other. $\endgroup$ – LiveMynd Aug 9 '17 at 0:01
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    $\begingroup$ @LiveMynd : "Look - that rabbit's got a vicious streak a mile wide! It's a killer!" $\endgroup$ – Willk Aug 9 '17 at 1:29
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    $\begingroup$ Wait, can rabbits tell the difference between poop that they've digested once and poop that they've digested twice? $\endgroup$ – Erty Seidohl Aug 9 '17 at 3:05
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    $\begingroup$ @Erty Seidohl yes, the 2 substances are different enough to have different names. IT's called cecotropes and is coated by a special mucus $\endgroup$ – Andrey Aug 9 '17 at 16:52
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It is possible, because it is real.

See documentary video "Haitians eat dirt cookies to survive" https://youtu.be/s3337cj4sJQ

Also, in Vietnam (use google translate to read): http://news.zing.vn/nuong-dat-de-an-o-vinh-phuc-post396355.html

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    $\begingroup$ Real world examples. Cool! $\endgroup$ – JBH Aug 8 '17 at 3:28
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    $\begingroup$ Not only Haititians. Peasants in my own country did this during famine (1907). You can imagine uprisings followed shortly after. $\endgroup$ – user9981 Aug 8 '17 at 17:22
  • $\begingroup$ @Magicsowon what is your country ? $\endgroup$ – Haha TTpro Aug 9 '17 at 4:50
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    $\begingroup$ @HahaTTpro looks like it might be china - wikipedia lists a famine with 25 million (holy carp!) dead there in 1907, and some more googling tells me that there were uprisings after that (understandably) $\endgroup$ – Syndic Aug 9 '17 at 13:02
  • $\begingroup$ I was about to answer along these lines. Although I don't have any reliable reference, I've heard, as a kid, that some indigenous peoples here in Brazil have (or had, in the past) a sort of dish made of "cooked earth" as part of their diet. $\endgroup$ – dnep Aug 9 '17 at 18:14
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Loam (consists of sand clay, silt and any organic and water mixed in) contains vegetation that is breaking down. Often forest floors have layers of leaves lost each year. The carbohydrates that make a leave structure will give on energy when oxidized. Same as other food. They can also eat live roots, shoots, grubs, worms, bacteria, slugs. Fungi are usually present as single cells even if they do not show a cap above ground.

It is likely that sand and clay cause constipation.

Compost piles can have many of the possible nutrients that are in loam. Smashing everything and manufacturing a large pile of rot seams appropriate behavior for orcs.

You could have a preferred fungus that the orc bands carry with them. The fungus will have the enzymes that convert chewed beta cellulose (wood) into simple sugars.

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    $\begingroup$ Compost piles - painful condition caused by eating too much dirt. $\endgroup$ – Whelkaholism Aug 9 '17 at 15:11
  • $\begingroup$ About sand and clay causing constipation, I believe it is more up to mechanical properties of the digestive tract. Hens ingest small stones all the time, and these actually play an important part in their digestion. Also, I've seen dogs eating large amounts of sand and having no apparent trouble in getting rid of it later. $\endgroup$ – dnep Aug 9 '17 at 18:26
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You could take a slightly different approach, if your Orcs are green you could make the argument that they are able to perform limited photosynthesis in their skin, eating the dirt gives them the nutrients they need to perform photosynthesis which they don't usually get from their usual diet of meat and more meat.

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    $\begingroup$ photosynthesis on the skin gives orders of magnitudes less energy than what's necessary to move a humanoid body. That's why plants can't make significant movements for a prolonged time (except for building up tension for a long while and releasing it at once to disperse seeds, etc.) $\endgroup$ – vsz Aug 8 '17 at 13:28

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