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I'm trying to build background information for a tabletop game and one of the things I wanted to have be a factor was upkeep such as keeping creatures fed. I am intending for players to be able to purchase/tame creatures and ride into battle or fight alongside them. Upkeep seems an ideal way to limit how many/what they can take into battle.

Is there a method for coming up with a rough approximation of how much food various mythical creatures would need based on factors such as body mass and abilities? I'm looking to determine a relationship between calories and mass/abilities of the creature that I can use as an approximation which I can adjust for game balance or adjust the creature's abilities based on its upkeep.

Dragons, for example, would need something their bodies can process into fuel for their fire breath attacks (strictly non-magical in this setting.) I believe they can process most living matter into a fuel for their breath attack. There are plenty of questions on the fire breath example, so I'm not too worried about this specific point.

Another example, as mentioned in the title, is the griffin. Since it has a half-lion body, would going off the dietary needs of a real world lion be fitting? Or would its flight equipped body need more/less food due to the combination of extra muscle work for flight and a lighter body needed for flight?

For dragons, wyverns and other larger creatures, would going off estimates of caloric intake of similarly sized dinosaurs work?

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    $\begingroup$ Bear in mind that without magic, creatures such as Griffins and Dragons tend to not be possible. This CAN be safely ignored, but that means that you can similarly get a little less exact with your food requirements. $\endgroup$ – Andon Jun 26 '17 at 0:44
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    $\begingroup$ You would, at a minimum, have to determine their approximate sizes and how they work within the square-cube law; after all, a dragon would obviously need more calories than a griffin, but just how many would probably be determined mathematically. $\endgroup$ – Snowshard Jun 26 '17 at 0:59
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    $\begingroup$ I am aware that such creatures could not exist in reality, at least not be capable of flight. I'm intending to have the abilities of the creatures tweaked to be a bit more realistic or having some other form of explanation as to why it can perform in such a way. Dragons, for example, don't fly. They soar on warm currents and cannot takeoff from a standstill. $\endgroup$ – Arvex Jun 26 '17 at 1:43
  • $\begingroup$ Related: worldbuilding.stackexchange.com/a/25387/809 $\endgroup$ – Mołot Jun 26 '17 at 5:53
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    $\begingroup$ Possible duplicate of What is a Giant's Diet? - starts with different creature but you both ultimately asked for generic method, and there it was provided. $\endgroup$ – Mołot Jun 26 '17 at 5:54
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Take a look at Kleiber's Law, which shows how many calories an animal needs per day based upon its weight. This is a well-fitted and followed empirical law; there can be deviations of course, but you will get pretty close using this, and deviate if you need to, in order to deal with game requirements. For example, you might want Dragons to be especially hungry just so they require more time and money for players to keep as pets.

Kleiber's law, says Kilocalories is proportional to $Mass_{kg}^{\frac{3}{4}}$. Convert lbs to kg by multiplying lbs by 0.4536. So for example, using Kleiber's Law, we'd expect a 154 lb person (that is 69.85kg) to require calories proportional to the number $69.85^{0.75}=24.16$. How does that help us? Proportional means we must multiply that answer by a constant, and the constant we choose is the USDA requirements for an average male: Which is 154 lbs. They say, for a middle aged 154 lb male, 2600 calories per day. Thus our proportionality multiplier is $\frac{2600}{24.16}=107.6.$

So compute your caloric requirements for the animal as:

$calories = 107.6\times M_{kg}^{0.75}$

That is all with normal life sustaining activity for an animal, so includes activities of finding food, water, shelter, evading predators, etc. Humans (and perhaps other highly intelligent magical creatures) might require fewer calories by not requiring all these activities: I don't have to hunt for food or water, and my environmental temperature seldom varies more than about ten degrees from 75F, at least for any extended amount of time, so I typically burn no calories keeping myself warm. Many of us humans are quite sedentary and expend almost no energy doing any muscular work, relative to what wild animals must do every day to survive.

Kleiber's law can work in reverse, obviously: If you want a specific number of calories K, you can compute $M_{kg} = (\frac{K}{107.6})^{\frac{4}{3}}$. So if K=2000 (calories a day), compute $M_{kg}=(\frac{2000}{107.6})^{\frac{4}{3}}=49.23_{kg}$, or 108.5 lbs.

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  • $\begingroup$ Keep in mind that Kleiber's Law was measured based on an animal's metabolic rate while feeding, i.e. when in a state of reasonably high, but not extra-high, activity. The actual food an animal requires over long periods is much more variable, as it depends on how active the animal is overall. Warm-blooded animals require significantly more food relative to their size than cold-blooded ones (reptiles can often go for months without eating), and flying birds require even more. This does work well for game balance; active animals will be more valuable to a player and will need more food. $\endgroup$ – IndigoFenix Jun 28 '17 at 6:44
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Since it has a half-lion body, would going off the dietary needs of a real world lion be fitting?

Yes, that makes sense. Use the closest real-world beast or interpolate them, as these are things you can look up.

You can add to that (size and type of ration) some unique special requirements. Beasts that use magic in their metabolism or for special abilities like flying will need to consume mana as well. So it may need to eat or deplete the magic from magical objects to remain healthy and fully-powered.

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This question really piqued my interest. In the process of world building, this is one conundrum I had yet to even consider. For myself, my mythical creatures are based off similarities to their Earthly counterparts. I have chosen a few extinct species to play around with also.

Firstly, my dragons are small, only reaching at maximum the size of a great Dane or small pony and while the are omnivores, they are usually fed by their human handlers. As such their food requirements would be much less. The Woolly Mammoths I'm also using as a beast of burden by the native inhabitants are only about the size of the pygmy mammoth which tops 5'5" max. Therefore I decided that they would only need about the same amount of food as a cow or oxen might. However, I am having difficulty deciding what the food requirements would be for my Pegasus/flying unicorns. I would think their metabolism would be greater than that of a normal horse and/or unicorn. Thereby they would require at least twice as much food, if not more, than a typical horse/unicorn.

You need to decide on the size of your griffins, what do they eat? if only the size of a lion, they may only require as much food. if they're as small as an eagle, they likewise would only require as much food. However, if you want your griffins to be ridden by say two or more people then you would have to multiply their food intake by the size you intend for them to be. My griffins are only about the size of an eagle or small house cat and like my dragons are used primarily for hunting by their human handlers, though that's not to say there aren't wild ones of either species. Similar to how our ancestors used falcons and other birds of prey.

Most animals typically eat twice their body weight in food a day. Although some can go for prolonged lengths of time without eating anything, but their digestive systems are fairly slow to accommodate this. So what they've already eaten lasts them a while. Just food for thought, and thanks for bringing this matter to my attention. I'll have to think on this a bit myself.

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