Altitudinal variation in the ecophysiological limits of a carabid beetle depends on temperature and aridity

Abstract Global warming is expected to strongly impact insect populations, particularly in montane environments where elevational gradients impose steep climatic variation. Insects in these systems are thought to be especially vulnerable because upward range shifts reduce available habitat. However, few studies have considered the combined influence of temperature and aridity on thermal tolerance at the intraspecific level. Here, we examined physiological and morphological variation in populations of the carabid beetle (Coleoptera) Baripus nevado along an altitudinal gradient (2380, 2644 and 2959 m) in Cerro Nevado, Argentina, an arid mountain system. We measured upper thermal limits (CTmax and upper lethal temperature [ULT]), body size and water loss. Our results revealed significant intraspecific variation: CTmax, ULT, body size and water loss/body size all declined with elevation. Low‐elevation populations exhibited the highest thermal limits and body mass, consistent with adaptations to endure greater aridity and desiccation stress. These patterns contrast with previous studies reporting conserved upper thermal limits within species across elevations, highlighting the role of water availability as an additional selective pressure. Our findings suggest that physiological and morphological diversity within species can buffer climate change impacts, but also indicate that increasing aridity may pose critical challenges even for heat‐tolerant populations. Overall, we demonstrate that intraspecific variation in thermal physiology is shaped by the interaction between temperature and aridity, and that this variation could be key for the persistence of montane insects under climate change.