An endotherm is an organism that can generate heat to maintain its body temperature, usually above the temperature of its surroundings. Because of this constant core temperature, endotherms are able to optimize enzyme activity. Some examples are mammals and birds. Most of the internally generated heat is incidental because it is already a product of routine metabolic reactions. Endotherms have had to adapt to avoid overheating; they can do this by panting or sweating.