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Dragons. We know them. We love them. They breathe fire and wreck our towns and plunder our gold. But all that destruction has to get fueled somehow. So naturally, I would assume dragons have to eat (or at the very least have some method of caloric intake.)

Let's assume our dragon is a traditional Smaug-style dragon. Big (20 meters long, 5 meters tall lying down), flies, breathes fire (assume the firebreathing is a chemical reaction that does not draw substantially from their caloric requirements), has periods of dormancy in which they will require fewer calories.

So let's assume there are four modes for our dragon:

  • Hibernation - spending weeks or months at a time slumbering on massive piles of gold.
  • General activity - organizing treasure, holding meetings, holding court, studying/learning magic
  • Scouting - long periods of sustain flight and gliding, perhaps swooping down occasionally to pick up prey
  • Combat - short periods of intensive and aggressive flying, swinging, and firebreathing

In terms of calories per hour, what kind of caloric intakes are necessary for these different categories of activity? If we can't answer them, what pieces of information are we missing? Feel free to be liberal with your assumptions (for example, when estimating mass), as long as you clearly state where they come from. Also, if any of my givens above seem inappropriate, feel free to change them. I want realistic dragons, here!

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    $\begingroup$ Do you want that in Calories or princesses? $\endgroup$ – user243 Feb 11 '15 at 21:01
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    $\begingroup$ I'd prefer calories, since no respectable dragon would eat a princess, given their typical ransom price. I would assume a princess could be ransomed for a large number of cattle or beasts of burden. But depending on the princess, I suppose... $\endgroup$ – corsiKa Feb 11 '15 at 21:16
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    $\begingroup$ "(assume the firebreathing is a chemical reaction that does not draw substantially from their caloric requirements)" Two points here, 1) chemical reactions do have caloric requirements in a creatures body. We know this because 2) all activity in an animals body are in fact merely chemical reactions, and they all have caloric requirements. So unless you want to fiat it as Magic, you've got to take it into account, but, then you could invoke Magic for all of these requirements, thus making any answer possible, $\endgroup$ – RBarryYoung Feb 12 '15 at 4:21
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    $\begingroup$ but what produces the chemicals for the reaction? $\endgroup$ – Mr.Mindor Feb 12 '15 at 16:51
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    $\begingroup$ No, you still aren't getting it. Food has caloric energy. So does Gasoline. You cannot create energy out of nothing, it has to come from somewhere. To make gasoline from food, you need to ingest as many (more actually) calories as you produce. There's no free lunch here. $\endgroup$ – RBarryYoung Feb 13 '15 at 18:01
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In general, when calculating caloric requirements for animals, we can apply what's known as Kleiber's law. Kleiber's law states that the food requirements of various animals scale by the mass of the animal to the $\frac{3}{4}$ power. This value is then multiplied by a constant that's dependent on the type of animal.

To compute a dragon the size of Smaug's caloric intake, then, we can start by computing his weight. According to this source, Smaug was around 60 meters long. A tyrannosaurus rex, by comparison, is a little over 12 meters long. Smaug had a long tail, though. (About 2/3rds of his length is tail), while t-rexes look to only be about half tail. If we compute weight based on body length, we'd then have Smaug's 20 meter body compared to about 6 meters for our t-rex. If we assume weight scales with the cube of body length, we can then figure Smaug's weight to be around $\frac{20}{6}^3 = 37$ times the weight of a t-rex. The more bird-like assumptions about dinosaurs put the weight of a t-rex at about 4.5 tonnes. Since Smaug flies, we can assume that his weight is probably similarly proportioned to his mass, perhaps a bit less. Since 37*9 is 166.5, we'll round down to a respectable 150 tonnes for Smaug. This is comparable to the weight of a blue whale (190 tonnes), which, while only about half as long as our dragon, is significantly more densely built.

Now let's apply Kleiber's law. First, let's get some base data on how much animals eat

  • A 290 gram barn owl needs the eat about 72.9g of small mammals per day. (About 120 kCal)
  • A 20kg crocodile needs to eat about 2kg of meat per week. (About 470 kCal/day)

These numbers give us estimates for about how much reptiles that are fairly stationary (our dragons sleep a lot) and flying birds (dragons fly around to catch food) need to eat. We can scale these numbers up to get some estimates on how much our dragons need to eat per day. The data I found is specified per day (or per week) rather than per hour, but we can extrapolate based on the activities of our sample animals what sort of activities these things correspond to.

Bird-like dragons

Our dragons weigh as much as around 500,000 owls. Based on Kleiber's law, they should then eat about 20,000 times as much food, for a total of 2.3 million Calories/day. Smaug ate ponies in the lord of the rings, so we can say that this is about 8 ponies worth of meat, for comparison. This activity level corresponds to a bird that hunts quite a bit, so this is probably a fair estimate for the activity level of a dragon while scouting or hunting.

Reptile-like dragons

If their caloric requirements are closer to those of crocodiles, they'll need a bit less. A dragon weighs 7500 times more than a 20kg croc, and so should need about 806 times the food as one. This comes out to a mere 380,000 Calories per day, or a bit more than one pony per day. This is more like a dragon that's sitting around snoozing on some gold or schmoozing with other dragons and the odd invisible hobbit.

Final estimate

In summary, if our dragons eat about as much as owls (relative to weight) while they hunt, but only hunt once a week and snooze like crocs the rest of the time, they'll need an average of around 655000 Calories per day. That's about two ponies per day, so once a week a dragon will need to go out and eat a small herd of them before going back to snooze on its pile of gold.

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  • $\begingroup$ Great answer! My estimates of the size of smaug were obviously grossly under... $\endgroup$ – bowlturner Feb 11 '15 at 20:28
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    $\begingroup$ I have once read a book that described the theorie that dragons are basically Hydrogen balloons, which reduces weight and also explains the fire breathing nicely ... should reduce the necessary calorie intake $\endgroup$ – Jens Schauder Feb 12 '15 at 8:24
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    $\begingroup$ It is interesting that the number of ponies seems realistic and feasible. $\endgroup$ – Antony D'Andrea Feb 12 '15 at 16:22
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    $\begingroup$ That would be either 655000 Calories, 655000 kcal, or 655000000 calories. 1000 calories is usually meant to equate 1 Calorie, by convention. $\endgroup$ – Mårten Feb 13 '15 at 10:26
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    $\begingroup$ What I love about this is that it gives you a cost to the hunt. You make a simple assumption that the dragon needs a pony (or pony like animal) a day, but that he needs an additional pony for every, let's say, hour of combat or aerial hunting. Every time I revisit this answer I love it even more. $\endgroup$ – corsiKa Jul 3 '15 at 20:24
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Well first I'll point you to this, where xkcd has figured out it would take a T-Rex about half an adult human every day ~40,000 calories.

Taking that and assuming Smaug is about 4 times the mass of a T-Rex, I would start with General Activity (GA) being about 160,000 calories a day or about 2 adult humans, or four 10 year olds, or half a horse or cow.

If it stuffed itself like a snake it could likely hibernate for months maybe years after gorging on a small heard of cattle. I was unable to get a good quote on how long an anaconda can go after a large meal, I have heard of 6 months.

Scouting I would double or maybe 2.5 times as much as GA, so a whole horse.

Combat I would guess 3+ times GA. Part of the issue is even if the firebreathing is a 'chemical reaction' the dragon still needs to produce the chemicals so it would still use calories to produce or reproduce. Depending on how, this could be a LOT of calories. Unless the dragon collects the chemicals and just stores them, which just doesn't sound like a real great dragon. "Wait I can't find any sulfur! How will I belch fire now!"

EDT: I was assuming that since dragons are reptilian, that they would have a similar metabolism as the large carnivorous dinosaurs, just larger.

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  • $\begingroup$ Going by birds, flying seems to increase required calories by approximately 2/3rds (see: world-builders.org/lessons/less/biomes/annutrita.html). So that might impact your scouting/combat requirements. The difference could be more due to size though, since the flying birds they reference are much smaller than the non-flying. $\endgroup$ – Dan Smolinske Feb 11 '15 at 18:11
  • $\begingroup$ @DanSmolinske good to know, but I'm going to stick with my numbers, because first dragons are huge beasts to keep airborn, and also because at GA they are using reptilian metabolism which is pretty low, Flying they have to work more like a bird so basically would have to crank things up a notch or two (IMO ;) ) $\endgroup$ – bowlturner Feb 11 '15 at 18:15
  • $\begingroup$ Re: sulfur, it's not uncommon in tissues (it's an essential element for all forms of life, probably #6 after hydrogen, oxygen, carbon, nitrogen, and phosphorous, the so-called "CHNOPS"), even less uncommon in rocks (sulfide/sulfate minerals). $\endgroup$ – Nick T Feb 11 '15 at 20:56
  • $\begingroup$ @NickT so 'producing' it would make more sense than 'finding' it right? $\endgroup$ – bowlturner Feb 11 '15 at 20:58
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    $\begingroup$ You actually have an exact number for what Smaug liked to eat, when he thanks Bilbo for bringing the ponies to his front door "Let me tell you I ate six ponies last night and I shall catch and eat all the others before long" $\endgroup$ – Mikey Mouse Feb 12 '15 at 16:06

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