Final answer:
Vertebrates with a nearly constant body temperature are endotherms, which include mammals and birds. They can internally regulate their temperature to remain active in varying environmental conditions. Ectotherms depend on external temperatures, while poikilotherms have varying body temperatures.
Step-by-step explanation:
Vertebrates that maintain a nearly constant body temperature, despite environmental changes, are known as endotherms. These animals, which include mammals and birds, regulate their body temperature internally through metabolic or other physical changes. Endothermy allows these animals to maintain high levels of activity and a stable metabolic rate, regardless of external temperatures.
They have adaptations such as fur, feathers, blubber, or behavioral strategies like basking in the sun or seeking shade to manage their body temperatures. Conversely, ectotherms are animals that rely on external environmental temperatures to regulate their body temperature. Examples of ectotherms are fish, amphibians, and reptiles. The term poikilotherms refers to animals with body temperatures that vary considerably. It is important to note that the correct answer to the question, 'Which type of animal maintains a constant internal body temperature?' would be 'endotherm', not 'poikilotherm'.