Camels can travel long distances over several days with little or nothing to eat. They manage to do this by storing fat in their humps. The stored fat can be broken down in the body to produce energy, carbon dioxide and water.
At one time it was thought that the camel's hump acted as a chemical water reserve. However, in order to burn fat an animal needs oxygen, and to get more oxygen, it has to breathe faster. Breathing results in water loss, and rapid breathing means more water loss than usual, so in fact more water is lost than is gained. The camel's hump, therefore, is not a water barrel but a food reserve. A camel that has not had enough to eat has no hump at all, but one that is well fed carried a large hump. Thus a camel weighing 1100 lb (500 kg) can store up to 440 lb (200 kg) of fat in its hump, enough to keep it going for up to six months if it is not working hard.
Body fat is the most efficient way for an animal to store food because it contains up to 200 times more energy than other body tissue. One problem with fat, however, is its bulk - a fat animal cannot move as easily as a lean one. For a desert animal, another problem is heat retention. Most animals store fat all over their bodies, but a layer of fat under the skin prevents cooling. So in a hot desert the best place to store as a hump. A camel's hump allows the animal to store food for lean times and yet keep cool by losing heat freely from the rest of its body.