Poikilothermic animals:
Poikilothermic animals have body temperatures that vary appreciably over time, often approaching the temperature of their surroundings. Most obtain much of their heat externally and therefore are also ectothermic. A lizard may warm in sunlight and cool in shade; a fish’s body temperature generally follows the surrounding water. This dependence reduces the energetic cost of producing metabolic heat, so food requirements are often lower than in similarly sized endotherms. It does not mean poikilotherms passively accept every temperature. Behavioural thermoregulation—basking, burrowing, changing
Ref: Evolutionary Analysis, Herron & Freeman, 5th Ed., Ch. 10