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 (T-b) on the aerobic capacity, metabolic cost, and endurance of sustained, terrestrial locomotion. In the ghost crab, Ocypode quadrata, T-b was a function of ambient temperature (T-a), relative humidity (RH), and the duration of acute exposure. At 15 degrees C and 24 degrees C, T-b was similar to T-a. At high T-a (30 degrees C to 35 degrees C) and low RH (40% to 50%), T-b's were 6 degrees C below T-a. When the RH was 99%-100% at a T-a of 30 degrees C, T-b stabilized at 29.7 degrees C. The depression in T-b resulted from evaporative water loss. The maximal rate of oxygen consumption (Vo(2max)), determined during treadmill exercise, decreased by nearly 75% as T-b was decreased from 24 degrees C to 15 degrees C. The minimum cost of locomotion (C-min, the slope of the steady state oxygen consumption vs. speed function) did not change at low T-b (15 degrees C). As T-b was increased from 24 degrees C to 30 degrees C, Vo(2max) decreased to half of its original value. At the lowest walking speeds, steady state oxygen consumption (Vo(2ss)) at a T-b of 30 degrees C did not differ significantly from or may have even exceeded that at a T-b of 24 degrees C (the y-intercept of the Vo(2ss) vs. speed function was elevated relative to resting rates). The minimum cost of locomotion decreased by nearly two-thirds when T-b was raised from 24 degrees C to 30 degrees C. The reduced C-min increased the range of sustainable speeds by nearly threefold over that predicted from a thermally insensitive C-min at a T-b of 30 degrees C Endurance correlated with the speed at which Vo(2max) 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.