Final answer:
The ability of mammals to maintain a constant internal temperature is called homeostasis, which is a crucial biological process allowing endotherms to regulate their body temperature through various mechanisms.
Step-by-step explanation:
The ability of mammals to maintain a constant internal temperature despite external environmental changes is called homeostasis. This biological process ensures that internal conditions such as body temperature are kept within a narrow, stable range through mechanisms like thermoregulation. Animals that have this ability, such as humans, birds, and mammals, are known as endotherms. These organisms utilize various physiological processes, like shivering or changing the blood flow, to regulate their internal environment and maintain a high level of metabolic activity even in cooler temperatures.