Interrelationships between water economy and thermo-regulation in the Canyon tree-frog Hyla arenicolor

Canyon tree-frogs, Hyla arenicolor, are found along permanent waterways, but during daylight hours they bask on vertical rock faces in open sun, about a metre above water line. We examined the thermoregulatory activity of H. arenicolor by measuring body temperatures (Tb) and rates of evaporative water loss (EWL) under field conditions, and EWL under controlled laboratory conditions. Rates of EWL for the northern leopard frog, Rana pipiens, were included for comparison. Around midday in June, average Tb, 30•6 ± 0•2°C, was independent of air temperature, water temperature, and surrounding rock surface temperature. Between dawn and dusk, the EWL of basking frogs averaged 24•8 ± 3•5% of body mass, equivalent to the bladder water reserves, 25•1 ± 1•4% of body mass, of these animals. The average resistance to water loss for H. arenicolor, 14•9 ± 0.3 s/cm, was nearly 7-fold greater than the skin of a typical frog, R. pipiens, 2•2 ± 0.3 s/cm, but only about one-fifteenth the average skin resistance of ʹwaterproofʹ frogs. The relatively high skin resistance to EWL is adaptive for the thermoregulatory activity of H. arenicolor; preventing excess heat loss through EWL when ambient temperature (Ta) is less than Tb, but permitting excess heat loss through EWL when Ta exceeds Tb , thereby stabilizing Tb during basking