Every animal is a living radiator--heat formed in its cells is given off through its skin. Warm blooded animals maintain a steady temperature by constantly replacing lost surface heat smaller animals, which have more skin for every ounce of body weight, must produce heat faster than bigger ones. Because smaller animals burn fuel faster, scientists say they live faster. The speed at which an animal lives is determined by measuring the rate at which it uses oxy gen. A chicken, for example, uses one-half cubit centimeter of oxygen every hour for each gram it weights. The tiny shrew uses four cubit centimeters of oxygen every hour for each gram it weights. Because it uses oxygen eight times as fast, it is said that the mouse-like shrew is living eight times as fast as the chicken. The smallest of the warm-blooded creatures, tile humming-bird, lives a hundred times as fast as an elephant. There is a limit to how small a warm-blooded animal can be. A mammal or bird that weighted only two and a half grams would starve to death, h would bum up its food too rapidly and would not be able to eat fast enough to supply more fuel. The selection says that every animal is a living radiator because it ______.