Maintaining an appropriate body temperature is critical for all aspects of biochemical and physiological function, and animals expend a great deal of energy controlling their body temperature. There are two main strategies by which animals regulate their body temperatures. Ectotherms, which include invertebrates, fish, and reptiles, obtain most of their body heat directly from the environment, and their metabolic rates fluctuate with ambient temperature. Endotherms, such as mammals and birds, produce their own heat through metabolic processes and are able to maintain constant body temperatures. Almost all heat produced is ultimately lost, making endothermy an energetically expensive process. Consequently, endotherms need to eat high-energy foods or feed very often.