The economy of locomotion (gas mileage – gal/mile) is remarkably similar for animals that differ in leg architecture, leg number, skeletal type, and body temperature when the effects of size are removed. A 4-legged mouse, 4-legged lizard, and 8-legged crab and a 42-legged centipede of the same size have similar economies. Size alone can explain much of the variation we see. Ideas that attempt to explain the similarity in metabolic cost which are restricted to mammals or humans will most likely be inadequate because we see this general trend. Equally impressive is the variation in metabolic cost observed at a given body mass. Differences in metabolic cost can exceed 6-fold, but the reasons for such variation remain largely unknown.