From:
http://www.newyorker.com/fact/content/?020715fa_fact1
Saris compared the metabolic rates of professional cyclists while they were riding with those of a variety of animal species, and he created a kind of energy index—dividing daily expenditure of energy by resting metabolic rate. This figure turned out to range from one to seven. An active male rates about two on Saris's index and an average professional cyclist four and a half. Almost no species can survive with a number that is greater than five. For example, the effort made by birds foraging for food sometimes kills them, and they scored a little more than five. In fact, only four species are known to have higher rates on Saris's energy index than the professional cyclists in his study: a small Australian possum, a macaroni penguin, a large seabird called a gannet, and one species of marsupial mouse.