Data from: Physiology at near-critical temperatures, but not critical limits, varies between two lizard species that partition the thermal environment

The mechanisms that mediate the interaction between the thermal
environment and species’ ranges are generally uncertain. Thermal
environments may directly restrict species when environments exceed
tolerance limits (i.e. the fundamental niche). However, thermal
environments might also differentially affect relative performance among
species prior to fundamental tolerances being met (i.e. the realized
niche). We examined stress physiology (plasma glucose and corticosterone),
mitochondrial performance, and the muscle metabolome of congeneric lizards
that naturally partition the thermal niche, Elgaria multicarinata
(southern alligator lizards; SAL) and E. coerulea (northern alligator
lizards; NAL), in response to a thermal challenge to quantify variation in
physiological performance and tolerance. Both NAL and SAL displayed
physiological stress in response to high temperature, but neither showed
signs of irreversible damage. NAL displayed a higher baseline
mitochondrial respiration rate than SAL. Moreover, NAL substantially
adjusted their physiology in response to thermal challenge whereas SAL did
not. For example, the metabolite profile of NAL shifted with changes in
key energetic molecules, whereas these were unaffected in SAL. Our results
indicate that near-critical high temperatures should incur greater
energetic cost in NAL than SAL via an elevated metabolic rate and changes
to the metabolome. Thus, SAL displace NAL in warm environments that are
within NAL's fundamental thermal niche, but relatively costly. Our
results suggest that sub-critical thermal events can contribute to
biogeographic patterns via physiological differences that alter the
relative costs of living in warm or cool environments.