BEHAVIORAL THERMOREGULATION BY BUFO-AMERICANUS - THE IMPORTANCE OF THE HYDRIC ENVIRONMENT

Tadpoles and adults of American toads, Bufo americanus, were forced to sprint at different body temperatures. Tadpoles sprint at 80%, or bet ter, of their maximal sprint rate between 17 and 36 C, whereas adults achieve the same performance level over a narrower range of 21-34 C. I n a laboratory thermal gradient with a wet sandy substratum, presumabl y well-hydrated juvenile toads preferentially select body temperatures that tend to optimize sprint performance. In a thermal gradient with a dry substratum, presumably dehydrated juvenile toads select body tem peratures that minimize loss of water. In a thermal gradient with both wet and dry substrata, juvenile toads generally remain on wet substra ta and select temperatures available in those wetter environments at w hich they can best approach maximal sprint performance. They typically avoid dry substrata that would result in dehydration, even when those environments off er temperatures conducive to better sprint performan ce. juvenile toads, owing to their diurnal habit and small body size, are more able than adults to regulate body temperature through behavio ral selection of thermal microsites. Indeed, adult American toads in n ature likely rarely achieve body temperatures necessary for optimal lo comotor performance. We hypothesize that the thermal optimum for locom otion in B. americanus evolved in response to selective pressures on j uveniles.