An Altitudinal Cline in Critical Thermal Maxima of Chorus Frogs (Pseudacris triseriata)

Abstract

Adult male chorus frogs (P. triseriata) were collected from 14 breeding congregations situated along an altitudinal gradient running from approximately 1500 m in the piedmont of northcentral Colorado [USA] to approximately 3000 m in the Front Range of the Rocky Mountains. Critical thermal maxima (CTM), determined following acclimation of frogs at constant temperatures of 5 and 20.degree. C, were found to decline clinally with increasing elevation of the collecting sites. Since the decline in heat resistance with increasing elevation corresponds with a similar elevational gradient in ambient temperature, it appears that interpopulation variation in CTM reflects physiological adaptation of chorus frogs to the different temperature conditions encountered at different localities.