Fossil evidence suggests that mammal-like reptiles and some dinosaurs could internally regulate their temperatures. Most contemporary reptiles, amphibians and most fish (except the talented tuna) cannot do this. Birds, most mammals, skunk cabbages and lotus flowers do regulate the temperatures of their bodies. Snakes in colder regions hibernate, lowering their metabolism to such a slow rate that even experts cannot tell if they are dead or alive. How did this regulation evolve?
Many mammals which can thermoregulate still hibernate — no sense hanging about if there's nothing to do in the heat o’ the sun or the furious winter’s rages. Is falling into deep unconsciousness, as we do at night, a pre-adaptation?