Thermal Dependence of Locomotor Energetics and Endurance Capacity in the Ghost Crab,Ocypode quadrata

We tested a general model predicting the effect of body temperature (Tb) on the aerobic capacity, metabolic cost, and endurance of sustained, terrestrial locomotion. In the ghost crab, Ocypode quadrata, Tb was a function of ambient temperature (Ta), relative humidity (RH), and the duration of acute exposure. At 15°C and 24°C, Tb was similar to Ta. At high Ta (30°C to 35°C) and low RH (40% to 50%), Tb's were 6°C below Ta. When the RH was 99%-100% at a Ta of 30°C, Tb stabilized at 29.7°C. The depression in Tb resulted from evaporative water loss. The maximal rate of oxygen consumption V̇o2max, determined during treadmill exercise, decreased by nearly 75% as Tb was decreased from 24°C to 15°C. The minimum cost of locomotion (Cmin, the slope of the steady state oxygen consumption vs. speed function) did not change at low Tb (15°C). As Tb was increased from 24°C to 30°C, V̇o2max decreased to half of its original value. At the lowest walking speeds, steady state oxygen consumption (V̇o2ss at a Tb Of 30°C did not differ significantly from or may have even exceeded that at a Tb of 24°C (the yintercept of the V̇o2ss vs. speed function was elevated relative to resting rates). The minimum cost of locomotion decreased by nearly two-thirds when Tb was raised from 24°C to 30°C. The reduced Cmin, increased the range of sustainable speeds by nearly threefold over that predicted from a thermally insensitive Cmin at a Tb of 30°C. Endurance correlated with the speed at which V̇o2max was attained (maximum aerobic speed) and, therefore, was reduced significantly at both low and high temperatures. On the basis of the present study of quantified locomotion, we conclude that thermal optima of maximal oxygen consumption and endurance are caused by a true thermal effect on oxygen utilization and muscle function rather than by a submaximal effort by the animal.