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The oceanic whitetip shark and the Pacific halibut are ectothermic (cold-blooded) fish, whereas the shortfin mako shark and the yellowfin tuna are regional endotherms-they retain metabolic heat resulting in body temperatures above the ambient water temperature. Modeling the effect of falling ambient temperatures on ectotherms indicated to researchers Haley R. Dolton and colleagues that ectotherms' body temperatures inexorably decrease toward the ambient temperature. Data from wild basking sharks show that even though their body temperature consistently remains 1.0 to above ambient, it also declines as the ambient temperature falls. This suggests that the body temperatures of the