It is hard to imagine a more pathetic dinosaur. Sprawled famished and exhausted on the ground for most of its life, barely able to bite flesh from even the most putrid carcass, Gorgosaurus, as reconstructed by Canadian paleontologist Lawrence Lambe, was easily the laughing stock of the Cretaceous Period. Although there are now other reasons to wonder whether Gorgosaurus (later renamed Albertosaurus) and the rest of its tyrannosaur kin weren't scavengers after all, Lambe was driven by the overriding assumption that the large dinosaurs were as cold-blooded as a crocodile.

Although Tyrannosaurus rex gained full predator status, Henry Fairfield Osborn and William Diller Matthew at the American Museum of Natural History still suggested that T. rex could only fight in slow motion or attack in sudden lunges like a lizard.

In the 1920s, the slow-and-dumb dogma was supplanted by a bona fide theory, called "mass homeothermy" by Yale professor Richard Swann Lull. The math was deceptively simple: the bigger your body, the more heat you retain. In 1946, Osborn's protégé Edwin Colbert tried an experiment to show how some dinosaurs might be "rather active--at least for reptiles"--and cold-blooded at the same time. With no living dinosaurs on hand, Colbert and his collaborators settled for alligators. They parked a baker's dozen of gators, ranging from a few ounces to about fifty pounds, in the Florida sun at high noon and took their temperatures every ten minutes. Sure enough, the little ones overheated faster and the big ones kept their cool longer.

Colbert concluded that mass homeothermy, along with a benign Cretaceous climate and a long lifetime of slow growth, would have allowed the giant sauropods to get along without having to eat as much as a mammal. This was fortunate, as their teeth were considered to be too weak and their jaws too small to consume enough food to fuel a hot-blooded body.