The earliest warm-blooded animals to evolve were which group?

• A. Reptiles
• B. Amphibians
• C. Birds
• D. Mammals

Answer: (C)

Endothermy, or the ability to keep body temperature regulated through metabolic heat, evolved in two separate vertebrate lineages. The mammal lineage shows evidence of endothermy quite early in the fossil record, with true mammals and their closest relatives appearing in the Late Triassic and displaying indicators of higher metabolic rates. Birds, while also endothermic, come from theropod dinosaurs and show endothermy by the time they appear in the fossil record in the Late Jurassic to Early Cretaceous, which is later than the earliest mammal endothermy. Reptiles and amphibians are ectothermic and rely largely on environmental heat sources. Therefore, among the given groups, the earliest warm-blooded animals were mammals.