Which is a characteristic of poikilotherms?
Poikilotherms experience substantial fluctuations in body temperature, often because their heat exchange is strongly coupled to ambient conditions. Most are ectothermic and obtain heat from sunlight, warm surfaces, air, or water rather than sustaining high internal heat production. Their metabolic, locomotor, digestive, and developmental rates consequently change with temperature. Many compensate behaviourally by basking, entering shade, burrowing, or shifting activity time, so a fluctuating temperature does not imply absence of all regulation. Energy efficiency can also characterize ectotherms because less food is spent on thermogenesis, but it is not universal and is not the defining meaning of poikilothermy. A constant metabolic rate is unlikely when biochemical reaction rates are temperature sensitive. Environmental niche breadth varies: some poikilotherms tolerate wide ranges, whereas stenothermal species tolerate narrow ones. The distinction from homeothermy concerns temperature variability, while the distinction between ectothermy and endothermy concerns the main heat source. These categories can cross: an ectotherm in stable water may remain nearly homeothermic, and an endotherm in torpor may become temporarily heterothermic. Nevertheless, fluctuating body temperature is the characteristic captured by the term.
Ref: Evolutionary Analysis, Herron & Freeman, 5th Ed., Ch. 10