A reluctant heliotherm: thermal ecology of the arboreal snake Hoplocephalus stephensii (Elapidae) in dense forest

Dense forests exhibit little thermal heterogeneity, and hence pose substantial difficulties for behavioral regulation of body temperatures by ectotherms. The problems are exacerbated for nocturnally active secretive taxa that spend most of their time hidden deep within shaded retreat-sites, such as our study organism Hoplocephalus stephensii in wet sclerophyll forests and rainforests of near-coastal eastern Australia. In laboratory studies these elapid snakes selected temperatures around 28°C if these levels were available within retreat-sites, but rarely basked. Body temperatures of radio-tracked snakes in the field were well below these "preferred" levels, and generally around 24°C during the active season. Nonetheless, the radio-tracked snakes consistently maintained body temperatures slightly higher than shaded air temperatures, primarily by microhabitat selection (elevated tree hollows) and occasional covert basking. The snakes exploited higher radiation levels available in the tree canopy; a snake's height in the tree was positively correlated with its body temperature. Calculated thermoregulatory indices (Am. Nat. 142 (1993) 796) confirm the low thermal quality of the habitat, and the trend for snake body temperatures to fall midway between shaded air temperatures and the animals' set-point range. Thus, H. stephensii will bask covertly when conditions permit, but rarely do so in the field. Our study shows the potential danger of extrapolating laboratory observations of thermal preferenda to the field, and clarifies the thermal constraints influencing ectotherms in forest habitats.