J. Zool., Lond. (1996) 240, 537-549
Temperature and the tropical tortoise Kinixys spekii:
tests of thermoregulationA. HAILEY
Department of Biological Sciences, University of Zimbabwe, PO Box MP 167, Mount Pleasant,
Harare, Zimbabwe
AND
I. M. COULSONSengwa Wildlife Research Institute, Department of National Parks and Wild Life Management,
(Accepted 17 October 1995)
(With 3 figures in the text)
The mean body temperature (Tb) of active Kinixys spekii did not vary with sex or type of activity, or between hot days (maximum ambient temperature Ta > 29 oC) and cool days. On hot days, Tb increased rapidly in the morning, and was constant during the late afternoon after midday inactivity. On cool days, Tb increased more slowly in the morning, and decreased during the afternoon. The slopes of Tb on Ta 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, Tbs were compared with temperatures of null models (Tm). Tortoises were clearly thermoregulators compared to 'active-all-day' models. 'Activity-time' models had Tm more similar to Tb. 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 Tb reached a high level. These results are discussed in relation to E, a measure of the effectiveness of thermoregulation based on comparison of Tb and Tm with the set point range (Tset) selected in a thermal gradient. A set of three indices, which separate the variability of Tb, the difference between Tb and Tm and between Tb and Tset, is suggested as a more generally applicable summary of thermoregulation in ectotherms.