Thermoregulation of the box turtles Terrapene carolina and Terrapene ornata.

Abstract

Terrapene ornata and T. carolina are closely related box turtles that live in different habitats: grasslands and desert edges, and forested areas, respectively. Considering these species' habitat selection, I predicted that T. ornata would select for higher body temperatures (Tb) and would be a more precise thermoregulator than T. carolina. I recorded time series of cloacal T b's in thigmothermal linear gradients from acclimatised (LD 12:12; 10, 20° C) box turtles. I used three analytical methods to evaluate and characterise turtles' activity: a ratio-dependent index that measured activity as an indirect function of Tb changes, a comparison of hourly mean variance of Tb (ratio-independent), and autocorrelation. I tested the thermoregulatory differences of active T. carolina and T. ornata with a factorial ANOVA, and characterised the turtles' thermoregulatory cycles with correlograms. Overall, T. ornata had significantly higher mean Tb's than T. carolina. Both species had similar diel thermoregulatory cycles with a period of approximately 24-hr. 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 preferences that may help explain the different habitat choices.