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Thermophysiological traits, particularly thermal tolerances and sensitivity, are key to understanding how organisms are affected by environmental conditions. In the face of ongoing climate change, determining how physiological traits structure species' ranges is especially important in tropical montane systems. In this study, we ask whether thermal sensitivity in physiological performance restricts montane lizards to high elevations and excludes them from the warmer environments reported at low elevations. For three montane lizard species in the Brazilian Atlantic Forest, we collect thermophysiological data from lizards in the highest elevation site of each species' distribution, and ask how well the individuals exhibiting those traits would perform across the Atlantic Forest. We use microclimatic and organism-specific models to directly relate environmental conditions to an organism's body temperature and physiological traits, and estimate measures of thermophysiological performance. Our findings demonstrate that thermophysiological constraints do not restrict montane lizards to high elevations in this system, and thus likely do not determine the warm boundaries of these montane species' distributions. Results also suggest that competition may be important in limiting the warm boundaries of the species' ranges for two of the focal species. These experimental results suggest that caution should be used when claiming that physiology drives patterns of diversity and endemism within montane environments. They also highlight the importance of interdisciplinary experimental studies that bridge the fields of evolution and ecology to improve predictions of biological responses to future environmental shifts.