Latitudinal Patterns of the Thermal Sensitivity of Sprint Speed in Lizards

Abstract

A classical evolutionary hypothesis predicts that tropical ectotherms should be adapted physiologically to a narrower range of temperatures than are temperate-zone ectotherms. This hypothesis is partially supported by data on the thermal sensitivity of sprint speed in lizards. The tolerance ranges (total ranges of temperatures over which lizards can locomote) of tropical lizards are narrower than those of temperate-zone lizards, apparently because tropical lizards experience a narrower total range of body temperatures than do temperate-zone lizards, and tolerance range is correlated with total body-temperature range. In contrast, the performance breadths (ranges of temperatures over which sprint speed is high) of tropical lizards are not consistently narrower than those of temperate-zone lizards. This may reflect many lizards' use of behavior for thermoregulation when they are active in the field, with the result that their body-temperature variabilities are independent of latitudinal patterns in ambient temperatures. Even so, performance breadths are usually correlated with the variability in body temperatures of lizards that are active in the field (when high sprint speeds should be important). Consequently, the classical latitudinal hypothesis is supported for tolerance range but not for performance breadth. The classical hypothesis has been proposed as a possible explanation for latitudinal patterns of high species diversity and narrow altitudinal ranges in the tropics. However, because the results of this study only partially support the classical hypothesis, the contribution of latitudinal patterns in the thermal sensitivity of physiology toward explaining these biogeographic patterns remains unclear.