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The assumption that dinosaurs were ectothermic -- cold-blooded -- was originally based on a simple argument. Reptiles are ectothermic -- they can't regulate their body heat. If they get too hot, they die. If they get too cold, they get sluggish. Dinosaurs were closely related to reptiles. End of argument.
As early as the 1950s, though, some researchers claimed that the rich blood supplies within dinosaurs' bones, as evidenced by the channels left behind in fossils, were more like those of fast-growing (and warm-blooded, or endothermic) birds and mammals than like those of reptiles. Maybe dinosaurs were warm-blooded after all.
There are no maybes about it as far as Robert Bakker is concerned. Long- haired, bearded and strongly opinionated, free-lance paleontologist Bakker has been the bad boy of the field for years, and does not suffer fools gladly. "There are still a few of my colleagues who think, 'If it walks like a duck, breathes like a duck and grows like a duck, it must be a turtle.' "
The fact that dinosaurs were warm-blooded should be especially obvious, says Bakker, because they were known to have had chest cavities large enough to hold huge hearts, like birds. Additional evidence is found in their migratory patterns. "There's no question that dinosaurs got as far north and as far south as there was land," says Bakker. "What should have been the tip-off is that the ones you find in the far north are the same ones you find in the south, so they could live in a wide range of climates. Also, I don't see any way dinosaurs could have survived up there unless they migrated, and migration takes energy. They would have to have been warm-blooded."
Scientists now recognize that there are, in fact, five or six different kinds of warm- and cold-bloodedness, and they are sometimes hard to distinguish, even in living animals. Moreover, making generalizations about the relationship between an animal's activity level and its metabolism can be misleading. "We tend to think that cold-blooded animals are sluggish, but that's not very accurate," says Yale paleontologist John Ostrom. "Some snakes, lizards and crocodiles can move faster than humans can. At the same time, we tend to think that warm-blooded animals are fast and very active, but the average house cat spends a lot of time snoozing."
The current consensus is that dinosaurs were not strictly ectothermic but fell short of full-fledged endothermy. "The problem," notes Michael Brett- Surman of the Smithsonian Institution, "is that there is no such thing as 'the dinosaur.' There were seven groups living 150 million years ago that started out as one thing and perhaps evolved into something else." Although Deinonychus, Velociraptor and other small, meat-eating bipeds may have been warm-blooded, Brett-Surman believes large predators like Tyrannosaurus rex, which went through three vastly different growth stages, may have been equipped with a variable metabolism.