A Model of the Dynamic Exchanges of Water and Energy between a Terrestrial Amphibian and Its Environment

Mechanistic models of the H 2 O and energy dynamic interrelationships between a terrestrial amphibian and its environments were developed and applied in a study of the temperature and water relations of the northern leopard frog Rana pipiens Schreber. The models illustrate the relative importance of air temperature, relative humidity, wind speed, absorbed solar and thermal radiation, substrate temperature, and soil H 2 O potential on the core temperatures, hydration levels, and rates of hydration level change during desiccation and rehydration for leopard frogs of different size. Leopard frogs' core temperatures are influenced most by ambient air and substrate temperatures, whereas absorbed radiation has a subordinant effect. Evaporative H 2 O loss rates are strongly influenced by absorbed radiation and virtually not influenced at all by substrate temperature (relative humidity, air temperature, and wind speed were also shown to be strongly influential on evaporative H 2 O loss). Water uptake from wet soils (ψ s o i l > —150 mb) is determined primarily by the properties of the frog, whereas H 2 O uptake from relatively drier soils (ψ s o i l < —300 mb) is determined by the conductive properties of the soil. Results of computer simulations suggest that leopard frogs in southern Wisconsin environments should not be found at great distances from standing H 2 O or soils with H 2 O potentials < —150 mb. Data taken from the literature and viewed in context of the theory of H 2 O exchange between an amphibian and soil, suggests that leopard frogs may rarely venture from saturated soils (ψ s o i l = 0). Results of simulations also imply that leopard frogs in most natural environmental circumstances will not generally have core temperatures greatly different from ambient air temperatures; thus, implying that any behavioral thermoregulatory ability would likely be crude. The modeling approach provides an extremely practical technique to study mechanistic interactions of the many environmental variables involved in the temperature and water relations of terrestrial amphibians.