Which term describes animals that cannot generate internal heat and have body temperatures that vary with the environment?

The term that describes animals that cannot generate internal heat and have body temperatures that fluctuate with their environment is indeed ectothermic. Ectothermic animals rely on external sources of heat to regulate their body temperature, such as sunlight or warm surfaces, which causes their body temperature to vary with ambient conditions. This adaptive strategy is seen in many reptiles, amphibians, and fish, which means they often become less active in cooler environments and seek warmth when needed.

In contrast, endothermic animals are capable of generating their own heat through metabolic processes that maintain a relatively constant internal body temperature, regardless of external conditions. Warm-blooded is a colloquial term often used synonymously with endothermic, implying a similar ability to maintain stable body temperature. Homeothermic refers to organisms that maintain a constant body temperature, further emphasizing the capability to regulate temperature internally. Therefore, ectothermic distinctly describes the characteristic of varying body temperature based on environmental influences.