The concept of thermal niche represents an important tool for understanding the link between organisms and their thermal environment over both ecological and evolutionary timescales. The scientific progress in this field has been hindered by the mismatch between multiple theories and observations. The discordance, among others, stems from the use of various indirect estimates characterizing a species’ thermal niche. In this project, an integrative approach between physiology and ecology will be applied to experimentally evaluate the current thermal niche concept in ectothermic vertebrates. This goal will be achieved (1) by examining the association between species’ thermal requirements and energy metabolism from individual to population levels, and (2) by testing propositions of the ecological niche theory about species’ coexistence in their thermal environment. Results will contribute to understanding the thermal niche from its mechanisms to within- and among- population patterns.