ENERGETICS AND BIOMECHANICS OF LOCOMOTION BY RED KANGAROOS (MACROPUS-RUFUS)

As red kangaroos hop faster over level ground, their rate of oxygen co nsumption (indicating metabolic energy consumption) remains nearly the same. This phenomenon has been attributed to exceptional elastic ener gy storage and recovery via long compliant tendons in the legs. Altern atively, red kangaroos may have exceptionally efficient muscles. To es timate efficiency, we measured the metabolic cost of uphill hopping, w here muscle fibers must perform mechanical work against gravity. We fo und that uphill hopping was much more expensive than level hopping. Th e maximal rate of oxygen consumption measured (3 ml O-2 kg(-1) s(-1)) exceeds all but a few vertebrate species. However, efficiency values w ere normal, similar to 30%. At faster level hopping speeds the effecti ve mechanical advantage of the extensor muscles of the ankle joint rem ained the same. Thus, kangaroos generate the same muscular force at al l speeds but do so more rapidly at faster hopping speeds. This contrad icts a recent hypothesis for what sets the cost of locomotion. The cos t of transport (J kg(-1) m(-1)) decreases at faster hopping speeds, ye t red kangaroos prefer to use relatively slow speeds that avoid high l evels of tendon stress. (C) 1998 Elsevier Science Inc. All rights rese rved.