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.