Thermoregulatory Response to Heat in the Waterproof Frogs Phyllomedusa and Chiromantis

Abstract

The thermal relations of waterproof frogs of two genera (Phyllomedusa and Chiromantis) were studied in an outdoor enclosure and, in the laboratory, in a thermal gradient, in a heated wind tunnel, and under an imposed radiant heat load. When allowed to move freely in a thermal gradient, no frogs showed a distinct preferred temperature, although Chiromantis spp. consistently avoided the cool end of the gradient. Both Chiromantis spp. and Phyllomedusa sauvagei voluntarily tolerated high body temperatures of 38 and 40 C, respectively. When subjected to a convective heat load, either outdoors or in the laboratory wind tunnel, both C. xerampelina and P. sauvagei allowed body temperature (Tb) to track air temperature (Ta) until Tb reached 38-39 C. Further increases in Ta resulted in little or no increase in Tb, whereas evaporative water loss (EWL) increased in direct proportion to the temperature difference (Ta − Tb) and with wind speed to the power of about 0.4. Phyllomedusa azurae increased water loss at a lower Tb (ca. 35-36 C) and did not regulate as precisely. A similar pattern was seen when frogs were subjected to rapid radiant heating. A sudden increase in EWL was observed when Chiromantis spp. reached a body temperature of 39 C and when P. azurae reached 35 C. Glands in the skin begin secretory activity when EWL increases, and the mechanism for thermoregulation in these frogs is apparently analogous to sweating.