Thermal biology of the common garter snake Thamnophis sirtalis (L.) : I. Temporal variation, environmental effects and sex differences.

This paper describes the thermal biology of the common garter snake, Thamnophis sirtalis, a diurnal, viviparous colubrid. The body temperatures of snakes caught in the field are cooler and more variable early and late in the day, and early and late in the active season. On sunny days in midsummer, females average about 1 C° warmer than males. The frequency distribution of body tempratures for females is skewed to the left, whereas that for males is nearly symmetrical. Under cloudy skies, body temperatures are lower and more variable than under sunny skies, and the difference between males and females disappears. This and related considerations suggest to us that males are less determined, or less precise, thermoregulators than are females. Air and ground-surface temperatures, and snout-vent length, are poor predictors of body temperature. In contrast with other studies, we found no indication that the body temperatures of gravid and nongravid females differ either in mean or variance. We discuss our conclusions in light of previous studies and identify in the latter, analytical shortcomings which we believe hinder interpretation and synthesis.