Cardiovascular responses to graded activity in the lizards Varanus and Iguana

Abstract

Respiratory gas exchange and blood O2 content were measured in 2 lizard species at rest and during treadmill activity at low speeds to evaluate the changes in cardiac output (.ovrhdot.Q) and O2 extraction from the blood during activity. The high aerobic scope (1.07 ml O2/g per h) of Varanus allows a wider range of running speeds that can be sustained aerobically than can Iguana with its low aerobic scope (0.65 ml O2/g per h). Varanus and Iguana increase their (.ovrhdot.Q) and O2 extraction in response to activity. As these lizards increase their running speeds to the maximum speed which is sustained for .gtoreq. 20 min (0.5 km/h, Iguana; 1.2 km/h, Varanus), heart rate and stroke volume change in a manner which affords Varanus a greater factorial scope in cardiac output than Iguana (3 .times. vs. 1.7 .times.) despite equal maximum rates of .ovrhdot.Q (20.5 ml/g per h). The greater O2 content of arterial blood in Varanus permits a greater O2 extraction from the blood than Iguana (6.1 .+-. 0.4 vol% vs. 3.8 .+-. 0.5 vol%) while running at speeds that elicit maximum O2 consumption. The cardiovascular responses to activity in these lizards are similar to each other and to other vertebrates. The larger aerobic scope of Varanus appears due to a quantitative increase in the O2 content of arterial blood relative to Iguana and in the contribution of stroke volume to increasing the factorial scope of cardiac output in Varanus.