Temperature and the tropical tortoise Kinixys spekii: tests of thermoregulation

The mean body temperature (T b ) of active Kinixys spekii did not vary with sex or type of activity, or between hot days (maximum ambient temperature T a > 29°C) and cool days. On hot days, T b increased rapidly in the morning, and was constant during the late afternoon after midday inactivity. On cool days, T b increased more slowly in the morning, and decreased during the afternoon. The slopes of T b on T a overall, and on hot days, were close to one, suggesting that tortoises were thermoconformers; the slope was greater than one on cool days. In a second test of thermoregulation, T b s were compared with temperatures of null models (T m ). Tortoises were clearly thermoregulators compared to ‘active‐all‐day’ models. ‘Activity‐time’ models had T m more similar to T b. Nevertheless, detailed comparison showed that tortoises were thermoregulating in the late morning, and that this was by choice of microenvironment, rather than ceasing activity when T b reached a high level. These results are discussed in relation to E, a measure of the effectiveness of thermoregulation based on comparison of T b and T m with the set point range (T set ) selected in a thermal gradient. A set of three indices, which separate the variability of T b , the difference between T b and T m, and between T b and T set, is suggested as a more generally applicable summary of thermoregulation in ectotherms.