Thermal constraints of energy assimilation on geographical ranges among lungless salamanders of North America

Aim Understanding biogeographical patterns of variation in distributional ranges of lungless salamanders in North America by examining the relationship between geography, temperature and range. We examined how these patterns fit with Rapoport's rule, that species distributional ranges increase with increasing elevation and latitude. We also examined a possible physiological mechanism behind Rapoport's rule, the climate variability hypothesis (CVH). Location Eastern North America. Methods We determined the thermal optimum and thermal performance breadth of energy assimilation for 12 species of lungless salamanders ( Plethodon ). From known collection localities, we determined species range characteristics (area, latitudinal and elevational range, and latitudinal and elevational mid‐point), and environmental characteristics (average mean temperature and temperature variability). We used an interspecific approach and phylogenetic generalized least squares (PGLS) to test assumptions of Rapoport's rule and the CVH. For Rapoport's rule we compared elevational and latitudinal mid‐points with measures of range size. For the CVH we examined the relationship between temperature variation, range size and thermal performance breadth. We also explored the relationship between thermal optima, average annual temperature and elevational and latitudinal mid‐points. Results Species range size increased respective to both their elevational and latitudinal mid‐points. Temperature variability and thermal performance breadth increased with increasing elevational and latitudinal mid‐points. Range size increased because of increases in thermal performance breadth. Average annual temperature and thermal optimum were positively correlated and decreased with increasing elevational and latitudinal mid‐points. Phylogenetic signal was strong with the relationships between physiological measures and geography. Main Conclusions Support for Rapoport's rule was found with respect to latitude and elevation among Plethodon . The thermal physiology of salamanders appears to be co‐adapted to the prevailing environmental conditions within their geographical distributions. We provided support for the predictions of the CVH and suggest it is a driving mechanism for their distributional patterns. Our results also suggest that important aspects of Plethodon physiology are evolutionarily conserved.