Thermal biology of the Australian forest dragon, Hypsilurus spinipes (Agamidae)

Unlike all other Australian agamid lizards, forest dragons (Hypsilurus spp.) are restricted to cool moist forest habitats where there is little light penetration and, thus, little opportunity for heliothermy. In keeping with this habitat preference, our field and laboratory studies of thermal availability and temperature selection reveal that Hypsilurus spinipes show little overt thermoregulatory behavior. Body temperatures of five radio-tracked lizards were low and variable (11-26 C), highly correlated (r > 0.96) with operative temperatures measured at the same time, and lower in the field than in a laboratory thermal gradient, where the lizards consistently avoided temperatures < 16 C. Quantitative analysis of perch-site characteristics, compared to availability, revealed minor but significant habitat selection (for high, vertical perches, of intermediate diameters, in areas with relatively open canopy) that may have elevated the lizards' body temperatures. Although other Australian agamid lizards differ from H. spinipes in their heliothermic behavior and preference for open habitats, they share a correlated suite of traits (e.g., ambush predation, reliance on camouflage to evade predators) that centers on prolonged immobility. These features are retained by H. spinipes in its densely forested habitat, where the high "costs" of heliothermy have favored a shift to extreme thermoconformity.