Comparative Field Study of Thermal Preferenda in Garter Snakes (Thamnophis)

Abstract

Field body temperature records from seven species of Thamnophis were used to investigate intrageneric variation of body temperature selection under field conditions. There was no evidence for geographic or interspecific variation of thermal preferenda in Thamnophis. Major factors imposing con- straints or influencing body temperature were found to be season, weather, macrohabitat, time of day, reproductive status, and activity type. After controlling for these factors, field-selected body temperatures converged on an apparent intragenerically stable preferendum near 30 C, with means ranging from 28.04- 30.53 C for the study populations. Observations on thermoregulatory behavior suggest that behavioral adjustments compensate for thermal differences among habitats and across latitude. Review of the literature revealed that intrageneric variation of PBT in snakes has yet to be demonstrated unequivocally. The potential significance of, and possible reasons for, the conservatism of PBT in snakes are discussed in terms of stabilizing selection and plasticity of activity timing in the diel cycle. ABSTRACT. - Field body temperature records from seven species of Thamnophis were used to investigate intrageneric variation of body temperature selection under field conditions. There was no evidence for geographic or interspecific variation of thermal preferenda in Thamnophis. Major factors imposing con- straints or influencing body temperature were found to be season, weather, macrohabitat, time of day, reproductive status, and activity type. After controlling for these factors, field-selected body temperatures converged on an apparent intragenerically stable preferendum near 30 C, with means ranging from 28.04- 30.53 C for the study populations. Observations on thermoregulatory behavior suggest that behavioral adjustments compensate for thermal differences among habitats and across latitude. Review of the literature revealed that intrageneric variation of PBT in snakes has yet to be demonstrated unequivocally. The potential significance of, and possible reasons for, the conservatism of PBT in snakes are discussed in terms of stabilizing selection and plasticity of activity timing in the diel cycle.