Temperature selection in wood turtles (Glyptemys insculpta) and its implications for energetics

Abstract

For turtles in northern climates, the primary function of body temperature (Tb) regulation should be to maximize energy gain. The increase in energy gain with Tb is explained primarily by the increase in food consumption and passage rate, both of which have thermal reaction norms similar to that of metabolic rate. We measured Tb continuously for fed and food- deprived wood turtles (Glyptemys insculpta) in a dry thermal gradient and in an enclosure with access to water containers and infra-red radiation during the photophase. We also measured the increase in metabolic rate with Tb to estimate the increase in energy gain with increasing Tb. In the thermal gradient, fed juveniles maintained higher Tb. In the enclosure, feeding had no significant effect on Tb selection when infra-red radiation was available 10 h-d−1, but when infra-red radiation was available only 5 h-d−1 fed juveniles maintained higher Tb. Metabolic rate increased exponentially with Tb with a Q10 of 1.96 ± 0.10 (SD). We argue that, for turtles, the 95th percentile of selected Tb (Tupper) better approximates the optimal Tb for energy gain than the preferred Tb range (25th to 75th percentiles of selected Tb) commonly used in other studies. Tupper remained at 30 °C in all treatments, although Tb became increasingly skewed towards Tupper for fed turtles and when infra-red radiation was limited. We conclude that Tupper approximates the optimal Tb that fed turtles try to maintain.