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 (T(b)) even though, being ectotherms, varanids naturally experience rather large changes in T(b). Although it is well established that absolute aerobic scope declines with decreasing T(b), it is not known whether changes in cardiac output (V(b)) and/or tissue oxygen extraction, (Ca(O2) - Cv(O2)), are in proportion to the rate of oxygen consumption (Vo(2)). 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 degrees C. During maximum exercise both at 25 and 36 degrees C, mass-specific rate of oxygen consumption (Vo(2kg)) increased with an absolute scope of 8.5 ml min(-1) kg(-1) and 15.7 ml min(-1) kg(-1), respectively. Interestingly, the factorial aerobic scope was temperature-independent and remained at 7.0 which, at each T(b), was primarily the result of an increase in V(bkg), governed by approximate twofold increases both in heart rate (f(H)) and cardiac stroke volume (V(Skg)). Both at 25 degrees C and 36 degrees C, the increase in V(bkg) alone was not sufficient to provide all of the additional oxygen required to attain maximal Vo(2kg), as indicated by a decrease in the blood convection requirement V(bkg)/Vo(2kg); hence, there was a compensatory twofold increase in (Ca(O2) - Cv(O2)). Although associated with an increase in hemoglobin-oxygen affinity, a decrease in T(b) did not impair unloading of oxygen at the tissues and act to reduce (Ca(O2) - Cv(O2)); both Ca(O2)) and Cv(O2)) were maintained across T(b). The change in Vo(2kg) with T(b), therefore, is solely reliant on the thermal dependence of V(bkg). Maintaining a high factorial aerobic scope across a range of T(b) confers an advantage in that cooler animals can achieve higher absolute aerobic scopes and presumably improved aerobic performance than would otherwise be achievable.

Oxygen transport is not compromised at high temperature in pythons

“…Skovgaard et al, 2009;Enok et al, 2012). The ability to increase f H during digestion, even at high temperatures, is consistent with the increased postprandial V · O 2 at these temperatures, and the postprandial metabolic and cardiac responses of the pythons therefore resemble the exercise-mediated responses at high temperatures in toads (Overgaard et al, 2012) and varanid lizards (Clark et al, 2005). It thus seems that amphibians and reptiles do not 'conform' to the OCLTT predictions, although this has been investigated explicitly in only very few species.…”

“…(OCLTT) model has, however, been questioned (see Clark et al, 2013) and for air-breathing ectothermic animals in particular, there is limited support for this model (Clark et al, 2005;Stevens et al, 2010;Seebacher and Franklin, 2011;Overgaard et al, 2012).…”

Digestive state influences the heart rate hysteresis and rates of heat exchange in the varanid lizardVaranus rosenbergi

“…The lizards were instrumented using similar methods to those described in Clark et al (2005a). Briefly, V O ∑ was obtained by lightly taping a lightweight (approximately 10·g), transparent, loose-fitting mask over the head of the animal.…”

Venous tone and cardiac function in the South American rattlesnakeCrotalus durissus: mean circulatory filling pressure during adrenergic stimulation in anaesthetised and fully recovered animals

“…Increased heart rate and stroke volume during exercise or digestion have been documented in a number of reptiles (e.g. Gleeson et al, 1980;Stinner, 1987;Wang et al, 1997Wang et al, , 2001Secor et al, 2000;Hicks et al, 2000;Farmer and Hicks, 2000;Munns et al, 2004;Clark et al, 2005). Stroke volume is the difference between enddiastolic and end-systolic volumes and is determined by myocardial contractility and cardiac filling.…”