In order to understand mammals and their amazing variety, we must first learn about their shared biology. ‘The biology of mammals’ first considers how and why mammals stay warm and active. Mammals are endotherms, meaning they have a high metabolic rate, high and constant body temperature, and high maximum activity level. A high metabolic rate requires a lot of food and oxygen, but the benefits are that mammals can remain active for longer and they are more complex internally. Their ability to save water, methods of obtaining food, modes of locomotion, sense organs, and methods of reproduction are also considered alongside their brain and behaviour.