Thermoregulation by a nocturnal elapid snake (Hoplocephalus bungaroides) in southeastern Australia.

Abstract

Studies of reptilian thermoregulation have tended to focus on diurnal heliothermic taxa that display overt thermoregulatory behavior, with nocturnal reptiles attracting less attention. We studied thermoregulation by the broad-headed snake (Hoplocephalus bungaroides), a small (mean snout-vent length = 57 cm) nocturnal elapid that spends long periods sequestered in diurnal retreat sites. The snakes selected body temperatures of 28.1 degrees-31.1 degrees C in laboratory thermal gradients. Prey-capture ability (strike speed and accuracy) increased at higher body temperatures over the range 20 degrees-30 degrees C. Using temperature-sensitive radio transmitters, we obtained 7,801 body-temperature measurements of 19 free-ranging snakes. Information on operative environmental temperatures was obtained at the same time. From these data, we quantified the degree to which the snakes exploit the environmental thermal heterogeneity available to them (i.e., the time they spent within their set-point range, relative to the total time that these body temperatures were available to them). Mean body temperatures (both diurnally and nocturnally) differed among seasons but not among different types of retreat sites. Inclement weather prevented snakes from attaining "preferred" body temperatures on 30% of days. However, even when preferred temperatures were available, the snakes exploited this opportunity for only 26% of the time: they remained within retreat sites and rarely emerged to bask. Nonetheless, judicious retreat-site selection resulted in snakes being within their set-point range for 60% of the time at the most crucial time of day (i.e., the 2-h period around dusk, when the opportunity to capture prey is highest). Basking may be rare not only because of its high potential costs (e.g., risk of avian predation) but also because high body temperatures enhance snake fitness for only a short time each day and can be attained over that short period without the "expense" of heliothermy. Our results suggest that precise thermoregulation may not be widespread among snakes, particularly small nocturnal species that spend long periods sequestered in retreat sites.