Selected Body Temperature and Thermoregulatory Behavior in the Sit-and-Wait Foraging Lizard Pseudocordylus melanotus melanotus

Abstract

We investigated the thermoregulatory abilities and behavior of Pseudocordylus melanotus melanotus (Drakensberg crag lizard) in terms of the relationship between the operative temperature (Te), selected temperature (Tsel), set-point range (Tset) and field active body temperature (field Tsel), exposure to low temperature, body posture and activity. The Te range for P. m. melanotus was about 58 C (−3.20 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 Tset (defined as the interquartile range of Tsel, after Hertz et al., 1993) between 29.00 ± 0.36 C and 31.78 ± 0.16 C in winter and 29.61 ± 0.28 C and 32.47 ± 0.18 C in summer. The mean Tsel was 30.08 ± 0.14 C in winter and 30.99 ± 0.11 C in summer. In the field, however, lizards achieved significantly lower Tb, which suggests that the thermal environment limited the Tb that lizards were able to achieve. Lizards were active for significantly longer and selected significantly higher Tb in summer than in winter. During winter, lizards spent a significant amount of time at Tb 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 Tb maintained may be constrained by the range of Te 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.