Final answer:
Ectotherms regulate their body temperature using external sources, whereas endotherms maintain a constant internal body temperature through metabolic means, which enables higher metabolic activity in varying temperatures.
Step-by-step explanation:
The key difference between ectotherms (such as lizards) and endotherms (like birds and mammals) is that ectotherms use external thermal energy to help regulate and maintain body temperatures, whereas endotherms rely on metabolically generated heat to maintain relatively constant body temperature in varying environmental temperatures. Ectotherms, such as most fish, amphibians, and reptiles, manage their body temperature through behavior, like basking in the sun or seeking shade. In contrast, endotherms maintain a stable body temperature and high metabolic activity which allows them to be active even in cooler temperatures, a trait that requires higher energy intake.