Staying warm or moist? Operative temperature and thermal preferences of common frogs (Rana temporaria), and effects on locomotion

Ambient temperature largely determines the body temperature of amphibians, and thus their hydration state and physiological performance. Microhabitat conditions chosen by terrestrial amphibians may represent a trade-off between high ambient temperatures, which maximize performance but cause high rates of water loss, and low temperatures, which, in turn, slow desiccation, but potentially hinder performance. We determined the operative temperature of common frogs (Rana temporaria) by placing 3% agar models in different microhabitats and measuring their temperature and water loss. Temperature measurements derived from the models accurately matched the body temperature of live frogs placed in the same microhabitat. Operative temperatures were lower than ambient temperatures on a warm day, probably because of evaporative water loss, but they were similar to or even slightly higher than ambient temperatures on a cool day, possibly because of warmth from the substrate. Frogs in the field selected moist and cool habitats, and their body temperatures ranged from 15 to 21 °C. In a temperature gradient in the laboratory, captive frogs chose significantly higher temperatures (19.4±1.7 °C) when the gradient floor was covered entirely with wet sand than when sand was wet in the cool end, but dry in the warm end (17.6±2.5 °C). The relevance of the preferred temperature was assessed through jumping performance experiments, using frogs with different body temperatures. Jump length was lower at low body temperature (6 °C) than at higher body temperatures, and peaked at 15 °C. Our results suggest that the frogs select favourable microhabitats of intermediate temperature, which could result in reduced water loss and peak physiological and behavioural performance.