Most animals cannot sustain speeds above that at which the rate of oxygen consumption reaches a maximum (VOO2max). Fowler's toad (Bufo woodhousii fowleri), by contrast, has a maximum aerobic speed (MAS, the speed at VOO2max) of 0.27 km h−1 but can sustain speeds as high as 0.45 km h−1 without increasing the VOO2max above the VOO2max. The present study investigates the discrepancy between MAS and the maximum sustainable speed (MSS). Toads switched from walking to hopping as their speed increased. The cost of a hop (4.1×10−4 O2g−1hop−1) was greater than the cost of a walking stride (2.5 × 10−4 ml O2 g−1 stride−1) and was independent of speed for both hopping and walking. However, individual hops were much longer than walking strides, which more than offset the greater cost of a hop. The calculated cost to traverse a given distance was approximately 1.9 times as much for walking as for hopping. During natural locomotion animals used combined walking and hopping. Individual toads that favored walking had higher locomotor costs than those that favored hopping. The estimated cost of exclusive hopping was less than the cost of natural locomotion at all but the highest speeds. This discrepancy may reflect the fact that the natural gait is a combination of both the less economical walking gait and the more economical hopping gait. To achieve speeds above the MAS toads walked less and used the more economical hopping gait more, and thus did not increase energy cost above that of VOO2max. The speed at which the estimated cost of exclusive hopping exceeded the cost of a natural gait and approached the VOO2max was close to the MSS. Creatine phosphate and lactate concentrations in the muscles of the thigh and calf did not change from resting levels at sustainable speeds greater than the MAS.
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