Ectotherms, also known as cold-blooded animals, warm their bodies by absorbing heat from their surroundings. These animals have large variations in normal body temperature due to their changing environment, with the most common ectotherms being invertebrates, fish, reptiles, and amphibians. The body temperature of endotherms, also known as warm-blooded animals, depends on the heat produced by the animal’s metabolism. Mammals, birds, some fish, and some insects are endotherms. Their normal body temperature is fairly constant, even when vast temperature differences in their environment occur.