Abstract
The ability to increase metabolic rate during locomotion has been important in the structural evolution and evolutionary success of both birds and mammals. Greater endurance capabilities are conferred directly by greater maximal metabolic rates, which vary between species. These maximal rates are known for many mammals1 but have not been determined for birds. We have measured oxygen consumption in a large flightless bird, the rhea, Rhea americana , while it was running on an inclined treadmill, and find an upper limit to aerobic metabolism that is 36 times greater than the minimum resting rate, a factorial increase exceeding that reported for nearly all mammals.
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