Abstract
Range expansion into cooler climates frequently drives phenotypic changes within a species to increase thermal tolerance. The oxygen and capacity-limited thermal tolerance (OCLTT) hypothesis maintains that thermal tolerance changes when animals move from one climate to a cooler or warmer one based on rates of oxygen demand and supply in the new environments. We tested central tenets of the OCLTT hypothesis across the north-south distribution of the green anole lizard (Anolis carolinensis) in North America, predicting that northern populations in cooler climates would have higher cold tolerance due to decreased standard metabolic rates (SMR; decreased oxygen demand), greater heart ventricle mass or hemoglobin concentrations (increased oxygen supply), or both. We further predicted that the adaptations leading to greater cold tolerance would also increase endurance capacity, with a corresponding decrease in sprint speed and exertion. Our results provided mixed support for the OCLTT hypothesis. Heart ventricle mass of the northern populations was greater, but only for females. There was no difference in SMR across populations, and hemoglobin concentrations did not follow a latitudinal pattern. However, performance traits generally matched our predictions, as endurance increased with latitude and sprint speed decreased, likely due to physiological tradeoffs associated with increased aerobic capacity. There are clearly selection pressures other than temperature acting on these populations to cause patterns differing from those expected under the OCLTT hypothesis.