The Caloric Contents of Foods Knowing the general caloric content of foods will help you to figure out what your animals will need.
 Caution:   This information has been gathered from many web sites. Often the information on the web sites is contradictory, and I have picked numbers somewhere in the middle of the ranges. This information is only generally accurate, but I hope that it will work for world builders as we learn to understand the concepts.  Your comments and suggestions are welcome! Caloric Values of Proteins, Carbohydrates, and Fats      I am assuming that carbon life forms on other planets will have metabolisms similar to animals here. This may not be true, but our life forms work! Certainly all life forms need energy in order to live and reproduce. The experts here seem to agree that the following values hold:      1 gram of protein = 4 Kilocalories (food calories)      1 gram of carbohydrate = 4 Kilocalories      1 gram of fat = 9 Kilocalories One ounce = 28.3 grams. Let's multiply and get an estimate that is easy to work with.   Translating the food values to ounces we get; 1 ounce of protein = (28.3 * 4 = 113.2)  about 100 Kilocalories 1 ounce of carbohydrate = (28.3 * 4 = 113.2)  about 100 Kilocalories 1 ounce of fat = (28.3 * 9 = 254.7)  about 250 Kilocalories In general, meat is protein, plants are carbohydrates.  Meat (protein) builds animal bodies: carbohydrates provide energy for moving, breathing, digestion, etc. If an animal eats only meat, it will burn some of the protein for energy. Foods are usually made up of a mixture of components: carbohydrates, proteins, fat, roughage (indigestible things like stalks, hair, feathers, etc) minerals, vitamins, and so on. For this project we will assume that the minerals and vitamins work out all right in the foods that our aliens eat. Scientists use the metric system, so I have given you information in that format as well as in pounds and ounces. Begin by using measurements that make sense to you  so that you can think about what you are doing. 1000 grams = 1 Kilogram one ounce = 28.3 grams (I have been using ounces times 30 = grams to get approximate values) The stated caloric values of foods vary, so for this project I am picking the following values. These are from pages for human dieters. I have rounded off the numbers to simplify the math. Notice that some of the plants have a very low caloric content per ounce. This is because plant tissues may contain up to 90% water.
 Calories in Ounces or Grams for Different Types of Foods Food KiloCalories per Gram of Food KiloCalories per Ounce of Food Comments Fruit 0.5 15 contain a sugar, fructose leafy vegetables, leaves 0.13 4 carbohydrates meat from mammals 3 100 mostly protein meat from birds 2 50 mostly protein fish and sea food 1 30 mostly protein nuts 5 140 shelled nuts are 180 Kcal per ounce barley (and other grains?) 3 100 mostly carbohydrates legumes: peas, beans 1 30 contain some protein roots: tubers 0.3 10 mostly carbohydrates

I found the following information from the manufacturers of animal food. I also got some information from zoos. These foods keep the animals healthy and reproducing. These are specially prepared feeds, and probably ideal for the animals involved. However, in the wild the food might be lower in calories per gram because it might also contain indigestible items like feathers, hair, tough stalks, twigs, and bones.

 Calories in Ounces or Grams of Commercial Animal Food Food is for KiloCalories per Gram of Food KiloCalories per Ounce of Food Comments Rodents: mice, hamsters 3 90 grain, seeds Rabbit 2 60 larger rodent Grazing Herbivores 1.3 39 need roughage - poor pasture? Large Herbivore: elephant 2.6 78 Carnivores: cat, dog 3.5 105 Omnivore 3.2 96 Carnivore: meat and bones 1.8 54 meat and bones Fish Eaters 1 30 Reptiles

Note that large grazers, such as cows, deer, elk, and antelope, eat food that is largely low calorie food.  They have adapted to low calorie leaves and stems by developing long digestive tracts that can deal with a lot of roughage.  Now they need the roughage in order for their digestions to work properly.

Meat eating animals, on the other hand, eat food that is high calorie.  Their intestines are relatively short, as this concentrated food does not require a long period of digestion.  High calorie food is also less bulky than low calorie food.

Now go on to Caloric Requirements for the Animals inYour Eco Systems to calculate how many calories your animal will need each day.