Response of the Thermal Preferendum and Heat Resistance to Thermal Acclimation under Different Photoperiods in the Lizard Anolis carolinensis

Abstract

Freshly captured Anolis carolinensis regulated body temperatures between 28-36 C in a laboratory photo-thermal gradient, and the mean preferred temperature (MPT) was about 31 C. There was virtually no change in the MPT after 4 months acclimation to 32 C, 20 C or a thermal cycle fluctuating between these two temperatures, with photoperiods of 0, 6, or 14 hours of light daily. Heat resistance, measured as survival time at 42.5 C, was more labile than the thermal preferendum. Maintenance under long day-lengths tended to improve heat resistance at each acclimation temperature, but differences were significant only in animals kept at 20 C. Lizards acclimated to 32 C tended to be more heat resistant than those acclimated to 20 C. The thermal preferendum and heat resistance appear to be somewhat independent with regard to their capacity for modification. These results support the conclusion that the thermal preferendum is independent of previous thermal history and is a reliable character for comparing the extent of thermal adjustment among lizards. INTRODUCTION Interspecific differences in preferred body temperatures (thermal preferenda) and heat resistances among lizards have been regarded as mechanisms for ecologically isolating sympatric species (Bogert, 1949a; Inger, 1959; Licht et al., 1966a), as reflecting phylogenetic relationships (Bogert, 1949a, b; Hirth, 1965; Ushakov, 1964; Licht et al., 1966a), and as indicators of species divergence in physiological adjustments to temperature (Schmidt-Nielsen and Dawson, 1964; UJshakov, 1964; Licht, 1967). Unfortunately, all of these interpretations have been hindered by a lack of information on the phenotypic stability of saurian thermal responses. Some seasonal shifts may occur in the body temperatures at which lizards are active in nature (Mayhew, 1963; Warburg, 1965; McGinnis, 1966). However, these observations may reflect the harshness of climatic conditions during certain seasons rather than a change in the inherent thermal preferendum of the species, since lizards may tolerate temperatures outside of their preferred range when climatic conditions are unfavorable for thermoregulation (Licht et al., 1966a; DeWitt, 1967). Species generally continue to regulate at the same levels when provided with wide choices of temperatures in thermal gradient chambers during different seasons even though they show significant variations in the body temperatures at which they are active in nature (DeWitt, 1967; McGinnis, 1966; Licht et al., 1966a). On the 1 This work was supported in part by a grant (GB-2885) from the National Science Foundation to the author.