Metabolic water is the water produced inside a living organism through metabolism, by oxidizing energy-containing substances in food and adipose tissue. Animal metabolism produces about 107–110 grams of water per 100 grams of fat, 41–42 grams of water per 100 g of protein, and 60 grams of water per 100 g of carbohydrate. Some organisms, especially xerocoles (animals living in the desert), rely exclusively on metabolic water. Migratory birds must rely exclusively on metabolic water production while making non-stop flights, facilitated by the high metabolic rate during such flights. Humans, by contrast, obtain only about 8–10% of their water needs through metabolic water production. In mammals, the water produced from metabolism of protein roughly equals the amount needed to excrete the urea which is a byproduct of the metabolism of protein. Birds, however, excrete uric acid and can have a net gain of water from the metabolism of protein. Metabolic water is essential for the survival of some animals that reside in dry lands or in habitats where water sources are not available. For instance, the chief source of water for kangaroo rats (thriving in desert) is metabolic water.