Difficulties encountered by teachers in giving a definition of the term "energy", and by students in grasping its actual meaning, reflect the lengthy process through which the concept eventually came to maturity around 1850. Tracing the history of this process illuminates the different aspects covered by the term and shows the important role played by advancements in animal physiology in the concept's elaboration. A unique example of cross-fertilization between historically separate fields, the history of the studies on animal heat, is recounted here. The recount starts from the early experiments by Boyle and Hooke on the effect of void on living beings and from Lavoisier's revolutionary interpretation of respiration as a "slow combustion" process, touching on the contributions by Spallanzani, von Humboldt and Liebig. It ends with the first enunciation of an energy conservation law by two German physicians, Meyer and Helmholtz, in advance of the elaboration of a coherent thermodynamic framework by Kelvin.