Thermal Physiology, Phenology, and Distribution of Tree Frogs

Abstract

Seasonal and latitudinal gradients in the thermal sensitivity of jumping performance have been examined in 10 species of hylids representing five different seasonal and distributional categories. Among temperate species, low-temperature performance is best in early-breeding northern species, intermediate in later-breeding northern species, and worst in southern species. Subtropical and tropical species have progressively higher critical thermal minima, but their thermal performance curves are not much different from those of southern temperate species in other respects. Critical upper thermal limits on jumping performance are much less variable than are lower limits, and the critical thermal minimum (CTmin) is not correlated with the critical thermal maximum (CTmax) in our sample. Effectively, CTmax is relatively inflexible, and consequently an improvement in low-temperature performance (reduction in L80, the lowest temperature at which frogs can attain 80% of maximal performance) results in a widening of the thermal breadth of performance at or above some arbitrarily selected standard (e.g., B80, 80% of maximal performance). The strong negative correlation between L80 and B80 in our sample of hylids is not found in lizards. Differences in thermal sensitivity of jumping performance are strongly correlated with naturally experienced minimum body temperatures. The thermal sensitivity of locomotion may itself be (or may be correlated with) a mechanism underlying distributional differences and temporal partitioning of habitat among frogs. We suggest that some aspects of the community organization of hylids, such as breeding phenology and the depauperate faunas of high latitudes, are constrained at least in part by thermal physiology. Ecological consequences of this constraint are currently being investigated.