Animals on Earth are naturally exposed to environmental fluctuations, including changes in temperature, food availability and food quality. Their capacity to adapt to these environmental variations may be challenged by anthropogenic stressors, including global warming, extreme weather events, and changes in land use. The capacity of individuals to produce and use energy is an essential parameter in determining animals’ ability to cope with these environmental changes. In my talk, I will discuss how environmental stressors affect energy allocation in animals, exemplifying with own work on birds and fish, with the overarching goal to identify broad synergies and divergences in response to environmental change amongst endotherms and ectotherms. I will highlight the value of studying changes in energy metabolism at various stages of life and biological levels of organisation, to understand the mechanisms by which endotherms and ectotherms may maintain fitness in a changing world.