In ectotherms, an increase in body temperature increases metabolic rate and may increase rates of digestive processes. We measured the thermal dependence of the apparent digestive and apparent assimilation efficiencies (ADE and AAE), gut passage time (GP) and appetite in Cordylus melanotus melanotus, a medium sized Crag Lizard, which is endemic to South Africa. Trials were conducted at 20, 22, 25, 30, 32 and 35 degrees C under controlled conditions. Trials lasted 14 days, during which, lizards were fed ca. 1 g mealworms per day. Glass beads were used as markers to determine GP at the beginning and end of trials. Faeces and urates were collected daily and oven dried at 50 degrees C. The energy content of egested matter was then measured using bomb calorimetry. ADE and AAE were not affected by temperature for either males or females. The mean+/-SE ADE and AAE were 94.4+/-0.3% and 87.2+/-0.6%, respectively. GP was not significantly different between males and females at any temperature, but decreased significantly with increasing temperature. Appetite was significantly different between the different temperatures measured. The decrease of gut passage time with increasing temperature was expected, since the digestive and assimilation efficiencies are similar over the range of temperatures tested. Lizards are thus assimilating a similar proportion of ingested energy, but at faster rates at higher temperatures. The results indicate that the digestive physiology of this species results in maximum energy gain per meal in environments where food is scarce.

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We investigated the thermoregulatory abilities and behavior of Pseudocordylus melanotus melanotus (Drakensberg crag lizard) in terms of the relationship between the operative temperature (T e ), selected temperature (T sel ), set-point range (T set ) and field active body temperature (field T sel ), exposure to low temperature, body posture and activity. The T e range for P. m. melanotus was about 58 C (23.20 C in winter to 54.94 C in summer). In a laboratory thermal gradient, in a setting that is independent of ecological costs or thermal constraints, lizards maintained T set (defined as the interquartile range of T sel , after Hertz et al., 1993) between 29.00 6 0.36 C and 31.78 6 0.16 C in winter and 29.61 6 0.28 C and 32.47 6 0.18 C in summer. The mean T sel was 30.08 6 0.14 C in winter and 30.99 6 0.11 C in summer. In the field, however, lizards achieved significantly lower T b , which suggests that the thermal environment limited the T b that lizards were able to achieve. Lizards were active for significantly longer and selected significantly higher T b in summer than in winter. During winter, lizards spent a significant amount of time at T b below their lower critical limiting temperature (defined by loss of righting). The most frequently assumed body postures in summer were those where the head or body were raised, whereas, in winter, lizards usually lay with head and body flat on the rock substrate. We suggest that these differences reflect the physiological requirements of the lizards: Head-up postures in sit-and-wait foragers are consistent with scanning for prey while head-down postures are likely motivated by thermoregulatory needs. It is clear that P. m. melanotus can thermoregulate efficiently, but the T b maintained may be constrained by the range of T e available to the lizards in their natural environment. Pseudocordylus m. melanotus currently appears to be geographically constrained by low environmental temperatures at the edge of its range. Should global warming become a reality in southern Africa, this species could inadvertently benefit by occupying new habitat that was previously unavailable because of thermal constraints.

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