Physiological Responses to Temperature in the Horned Lizards, Phrynosoma cornutum and Phrynosoma douglassii

Abstract

In the laboratory, Phrynosoma cornutum and Phrynosoma douglassii from central New Mexico had mean preferred body temperatures of 38.5? C and 35? C respectively. The mean critical thermal minimum established in the laboratory for P. douglassii was 2.75? C and 9.46? C in P. cornutum. The mean critical thermal maximum in P. douglassii was 43.5? C and 47.9? C in P. cornutum. The range of thermal tolerance in P. douglassii was 2.30 greater than that of P. cornutum. Oxygen consumption and heart rate increased with temperature. There were no significant differences between the heart rates of P. douglassii and P. cornutum at various temperatures. Oxygen consumption in P. douglassii at 5? C and 15? C was almost double that in P. cornutum at those temperatures, but their rates of oxygen consumption were not significantly different at 25, 35, and 40? C. Differences in the oxygen pulse values of these species at high and low temperature extremes indicate differences in the cardiac output and efficiency. It is suggested that P. douglassii may have a more efficient transport of oxygen by the respiratory pigments at low temperatures than P. cornutum.