Temperature-dependent performance scales with maximum heat tolerance across ectotherms

Temperature fundamentally shapes ectotherm biology, from continuous changes in sublethal performance to acute functional failure at upper thermal limits. Sublethal performance is commonly characterized using thermal performance curves (TPCs), which describe continuous changes in performance across temperatures, whereas acute heat tolerance is often quantified by the critical thermal maximum (CTmax), the temperature marking organism-level functional failure. Whether these dimensions of thermal physiology vary independently or covary as components of a coordinated warm-end thermal phenotype remains unclear. Synthesizing data from more than 100 ectotherm species, we tested how CTmax scales with two key warm-end TPC traits: the thermal optimum (Topt) and the upper limit of performance (TPCmax). Across diverse taxa, both Topt and TPCmax scaled approximately 1:1 with CTmax, and these relationships were broadly consistent across the biological, experimental, and environmental contexts represented in our dataset. Although both slopes were individually consistent with 1:1 scaling, when Topt and TPCmax were compared directly, TPCmax increased more steeply with CTmax than did Topt, such that the declining portion of the TPC widened as CTmax increased. Species with greater acute heat tolerance therefore tended to retain positive performance across a broader range of supra-optimal temperatures before reaching the upper performance limit. Together, these relationships show that sublethal warm-end performance and acute heat tolerance covary systematically across species, supporting their interpretation as broadly coordinated components of the thermal phenotype rather than independent axes of thermal biology.