Thermoregulation
Small warm-blooded animals have three main tactics for overcoming this problem of heat loss. First, they have a higher metabolic rate than large animals. This is universal in birds and mammals: only the details differ. A higher metabolic rate affects the whole of their lives, and is the main reason why they have shorter lives than large animals.p58/9 The smallest mammals (shrews) and the smallest hummingbirds are about the same size and weight. They have fast heartbeats, up to 1,200 per minute,p129 and they use up more calories per gram than any other endotherms. The rate of oxygen use is similarly high. As a result of the metabolism running at such a high rate, they do produce more heat per gram than any other living things.