One way of classifying animals is according to the constancy of their body temperature. Animals that maintain a constant body temperature are called homeotherms, and animals that experience a fluctuating body temperature are called poikilotherms. Another classification is based on the source of heat that predominantly determines body temperature. Endotherms such as birds and mammals have the ability to vary their metabolic heat production to compensate for the loss of heat to the environment. Ectotherms such as invertebrates, fishes, amphibians, and non-avian reptiles are largely dependent on environmental sources of heat. We use the terminology of ectotherm and endotherm in this book, but no classification scheme is perfect. Many ectotherms can generate considerable internal heat to raise their body temperature, and endotherms that hibernate can periodically appear to give up the ability to generate internal heat. To accommodate these departures from a strict dichotomy, we add the term heterotherms—organisms that act like ectotherms some of the time and like endotherms at other times.