Body temperature varies with environmental temperature.

• A. Ectotherm
• B. Endotherm
• C. Poikilotherm
• D. Homeotherm

Answer: (A)

Body temperature that varies with environmental temperature shows heat coming from outside rather than being produced inside. That is characteristic of ectotherms, animals that rely on external heat sources and thus their temperature tends to rise and fall with the surroundings. They often warm up by basking in the sun or cool down by seeking shade or water. Endotherms generate metabolic heat to keep a relatively constant internal temperature, so their body temperature stays more stable despite environmental changes. Homeotherms refer to maintaining a constant internal temperature, which clashes with the idea of environmental influence. Poikilotherms describe organisms whose body temperature varies with the environment as a general idea, but the direct description of relying on external heat aligns most clearly with ectotherms.