Answer: Reptiles are cold-blooded (poikilothermous) animals; their body temperature changes with the temperature of the surroundings.
Reptiles do not have the mechanism to produce and regulate heat inside the body like birds and mammals. Their body temperature rises when they sit in the sun and falls when the surroundings become cold. That is why they bask in the sun in the morning and become inactive in winter. Hence their body temperature is not constant.