Coming In from the Cold

Paleontologist Bob Bakker speaks with a zealot's fervor about his crusade for warm-blooded dinosaurs. Thirty years ago, his was a lone voice in the wilderness. Today, frisky, bird-like dinosaurs are the new gospel, preached from Hollywood to paleontology textbooks. How did this happen?

Bakker's Yale adviser John Ostrom began what Bakker calls the "dinosaur renaissance" in the mid-1960s with his discovery of Deinonychus, the agile Cretaceous predator that slashed its prey with an oversized middle toe. The more Ostrom thought about Deinonychus' locomotion and habits, the less plausible it seemed that such a creature could have been cold-blooded.

During the 1970s and '80s, Bakker fired a barrage of arguments for dinosaur endothermy, or warm-bloodedness, at the fortress of dino orthodoxy. These included calculations showing that dinosaur predator-prey ratios were more like those of modern mammals than of today's reptiles, and microscopic observations of dinosaur bones that show rapid mammal-style growth. Most recently, Bakker trumpets discoveries of a wide range of dinosaurs living in northern climates where large cold-blooded animals don't survive today, as well as the recently found feathered dinosaurs in China, as conclusive proof of dinosaur endothermy.

There is still no consensus. Further studies of bone growth show that many dinosaurs did, in fact, grow so fast after hatching that they must have had high metabolisms as youngsters. One of Jack Horner's students, Jill Peterson, found that young Maiasaura grew almost three meters a year. But adult bones of the little predator Troödon show seasonal growth spurts associated with today's cold-bloods.

Like all the debates over the nature of the dinosaurs during the past century and a half, the next conclusion will probably be even more surprising than current positions. Since dinosaurs were both bird-like, but not actual birds, and reptilian, but not true reptiles, it is not outlandish that perhaps their metabolism was not one or the other, either. Ultimately, the dinosaurs were just that, dinosaurs--uniquely, tantalizingly themselves.