Thermoregulation in the box turtles Terrapene carolina and Terrapene ornata

Abstract

Terrapene ornata and Terrapene carolina are box turtles that live in different habitats, the former in grasslands and desert edges and the latter in forested areas. Considering these species' habitat selection, we predicted that T. ornata would select a higher body temperature (Tb) and would be a more precise thermoregulator than T. carolina. We recorded time series of cloacal Tb's in thigmothermal linear gradients from acclimatized (12 h light : 12 h dark; 10 or 20°C) box turtles. We used three analytical methods to evaluate and characterize turtles' activity: a ratio-dependent index that measured activity as an indirect function of changes in Tb, a comparison of hourly mean variances of Tb (ratio-independent), and autocorrelation. We tested the thermoregulatory differences between active T. carolina and T. ornata with a factorial ANOVA and characterized the turtles' thermoregulatory cycles with correlograms. Overall, T. ornata had significantly higher mean Tb's than T. carolina. The two species had similar diel thermoregulatory cycles with a period of approximately 24 h. No clear differences in absolute thermoregulatory precision of Tb's were detected. These species' thermal behaviours were consistent with those reported from field studies, suggesting that there are intrinsically determined differences in thermal preference that may help explain the different habitat choices.