Thermoregulation in leopard tortoises in the Nama-Karoo: The importance of behaviour and core body temperatures

Despite being ectotherms, reptiles have an ability to thermoregulate which is enhanced by adopting a variety of behavioural mechanisms. Different behavioural postures, the use of retreat sites and selection of microhabitats enable reptiles to maintain their core body temperatures (Tb) above that of ambient temperatures (Ta) in winter or below Ta maximum in summer. This study describes the daily activity patterns of leopard tortoises (Stigmochelys pardalis) in relation to Tb and Ta, and the extent to which leopard tortoises can manipulate their Tb in response to seasonal changes in Ta. Ten and nine leopard tortoises were radio-tracked in 2002 and 2003, respectively and cloacal Tb and behaviours observed. Core Tbs were measured using Thermocron iButtons™ surgically implanted into the body cavities of 4 and 5 adult telemetered tortoises for summer and winter 2003, respectively. There were seasonal differences in the extent to which certain behaviours were practiced and the time of day that these occurred. Leopard tortoises generally had unimodal activity patterns in winter and bimodal ones in summer. In winter tortoises were active at lower Tbs, and at lower Ta, than in summer. Tortoises maintained their core Tb well below Ta minimum profiles in summer and well above these in winter. Core Tb closely followed the increase in Ta minimum profiles in the mornings, however tortoises exhibited an extended thermal lag when Ta minimum profiles cooled overnight. By using different behavioural mechanisms in summer and winter, leopard tortoises maintained their core Tb at different levels compared with Ta minimum and maximum profiles. Consequently although they are ectotherms, they maintained their core Tb independent of Ta. ► Although ectotherms, reptiles thermoregulate using behaviour. ► Body temperature and activity patterns of Stigmochelys pardalis were investigated. ► Leopard tortoises used different behavioural mechanisms in summer and winter. ► They maintained core body temperature independent of ambient temperature.