Factorial scopes of cardio-metabolic variables remain constant with changes in body temperature in the varanid lizard,Varanus rosenbergi

Abstract

The majority of information concerning the cardio-metabolic performance of varanids during exercise is limited to a few species at their preferred body temperature (Tb) even though, being ectotherms, varanids naturally experience rather large changes in Tb. Although it is well established that absolute aerobic scope declines with decreasing Tb, it is not known whether changes in cardiac output (V̇b) and/or tissue oxygen extraction, (CaO2− Cv�[Formula: see text]), are in proportion to the rate of oxygen consumption (V̇o2). To test this, we studied six Rosenberg's goannas ( Varanus rosenbergi) while at rest and while maximally exercising on a treadmill both at 25 and 36°C. During maximum exercise both at 25 and 36°C, mass-specific rate of oxygen consumption (V̇o2kg) increased with an absolute scope of 8.5 ml min−1kg−1and 15.7 ml min−1kg−1, respectively. Interestingly, the factorial aerobic scope was temperature-independent and remained at 7.0 which, at each Tb, was primarily the result of an increase in V̇bkg, governed by approximate twofold increases both in heart rate ( fH) and cardiac stroke volume (VSkg). Both at 25°C and 36°C, the increase in V̇bkgalone was not sufficient to provide all of the additional oxygen required to attain maximal V̇o2kg, as indicated by a decrease in the blood convection requirement V̇bkg/V̇o2kg; hence, there was a compensatory twofold increase in (CaO2− [Formula: see text]). Although associated with an increase in hemoglobin-oxygen affinity, a decrease in Tbdid not impair unloading of oxygen at the tissues and act to reduce (CaO2− Cv�[Formula: see text]); both CaO2and Cv�[Formula: see text] were maintained across Tb. The change in V̇o2kgwith Tb, therefore, is solely reliant on the thermal dependence of V̇bkg. Maintaining a high factorial aerobic scope across a range of Tbconfers an advantage in that cooler animals can achieve higher absolute aerobic scopes and presumably improved aerobic performance than would otherwise be achievable.