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I've created a species that, instead of eating multiple times a day like us, eats one tremendous meal and survives off of the fat stores for long periods of time. My question is about feasibility.

First, how chemically efficient is the human digestive system? As far as digestion goes, we can process and absorb a tremendous variety of foods, indicating high efficiency. But, how much of the chemical energy stored in that food do we actually absorb?

My second question: how efficient is fat storage, and could it feasibly be more biologically efficient?

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    $\begingroup$ This seems an awful lot like hibernation. $\endgroup$ – zzz Dec 4 '16 at 1:13
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    $\begingroup$ Fat storage and efficiency of burning fat can be explained pretty easily - if they can work for hibernation, as @Zxu says, they'll work for you. The important question is if a species can lug around one tremendous meal without needing to hibernate. If it doesn't need to hibernate, you might ask if it can still power an intelligent brain with a slow metabolism, etc. $\endgroup$ – Zxyrra Dec 4 '16 at 3:25
  • $\begingroup$ I forgot to add that they DO go through a small hibernation period after eating to allow the food energy to be broken down and stored as what (I hope is feasible) extremely energy efficient fat $\endgroup$ – Gaius The Observer Dec 5 '16 at 1:37
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This seems feasible.

However, human digestion is pretty efficient - the heat of combustion of starch is ~ 4.2 Calories (kilocalories) per gram, which compares pretty well to the usually quoted 4 Calories per gram of carbohydrates for dietary purposes. (Fats are usually quoted at 9 Calories per gram, so fat storage is also pretty efficient. Not quite as efficient as, say, propane, but really quite good.)

Where you can get significant improvements isn't so much in the extraction of energy as in the use of that energy. The way humans (and other mammals) do endothermy is very energy-hungry.

Also, humans eat several times a day, but the amounts are rather small. Plenty of animals are capable of eating huge quantities relative to their own body weight. According to the USDA chicken meat is on the order of 200 calories per 3 ounces: http://www.fsis.usda.gov/shared/PDF/Chicken_Turkey_Nutrition_Facts.pdf

That's about 1000 calories per pound. So if your creature can eat 20 pounds of meat at a meal, that's 20,000 calories - which is the equivalent of 8-10 days of activity even at 2000-2500 calories/day for a human.

The combination of these two is why some large snakes can go months between large meals. You probably can't go quite as low metabolically as a snake for an intelligent species (brains are very energy-hungry), but assuming they evolved in a very stable thermal environment they might need much less energy for maintaining body temperature.

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    $\begingroup$ I came here to write about the snakes, only to see you already did. +1. But also crocodiles are pretty good at not eating for long, aren't they? $\endgroup$ – Mołot Dec 4 '16 at 2:16
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    $\begingroup$ Relevant xkcd. $\endgroup$ – Papayaman1000 Dec 4 '16 at 3:41
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    $\begingroup$ @Molot: I think so, for similar reasons - low (ectothermic / "cold-blooded" metabolism) and ability to eat large prey all at once. $\endgroup$ – cometaryorbit Dec 4 '16 at 20:43
  • $\begingroup$ Many thanks! May I ask for a little more info on how we're inefficient as far as endothermy? I forgot to add that they DO go through a small hibernation period after eating to allow the food energy to be broken down and stored as what (I hope is feasible) extremely energy efficient fat. $\endgroup$ – Gaius The Observer Dec 5 '16 at 1:36
  • $\begingroup$ @GaiusTheObserver: Well, inefficent might not have been exactly the right word - usually turning energy into heat is pretty easy ;) I just meant that we use a lot of energy maintaining our body temperature, whereas a large reptile like a boa or crocodile doesn't. But that has limits of its own, both because it limits their activity (have to sun themselves, etc.) and because they have to accept a much wider range of body temperatures than we do (which is why I'm skeptical if a truly 'cold blooded' -- poikilothermic ectotherm -- creature could have a human-level large and delicate brain.) $\endgroup$ – cometaryorbit Dec 5 '16 at 5:37
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I think the problem is that if you eat a tremendous meal compared to your body size, you have to "carry" that meal around as you move. This would severely impact your movement ability and moving around would consume a lot of energy.

The closest analogy I can think of are snakes. They eat animals that are bigger than them (a snake can eat a sheep). Then they just rest and digest. You can sometimes literraly see the shape of the animal they ate in their body.

So yes in theory you could have a gigantic flexible stomach and eat gigantic amounts of food but then you won't be able to move while you digest, leaving you vulnerable.

Most "Big" animals have a different strategy, they constantly eat.

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