The pulse of T. Rex: were dinosaurs warm-blooded? Does it even matter?

Science News, May 14, 1994 by Richard Monastersky

Were dinosaurs warm-blooded? Does it even matter?

There's a moment in the movie Jurasic Park, a brief, unremarkable image, that bothers physiologist John Ruben. He doesn't mind the look of Spielberg's dinosaurs or even the way they act. Let them run, jump, stalk Jeff Goldblum, or even dance a jig if they want. What John Ruben doesn't like is their breath.

More precisely, he objects to the sight of vapor coming out of a dinosaur's snout - a small point that reveals a fundamental assumption the movie makes about the metabolism of these great beasts.

"The problem with Jurassic Park is that they portrayed dinosaurs as endothermic to underline the notion that they were interesting, active, and dangerous. How do I know that? Because they showed one of these dinosaurs lying there with steam coming out of its nose. The only way an animal can do that is if it's very tightly regulating its body temperature and is a lot warmer than its surroundings. That's why you never see reptile breath," says Ruben, a researcher at Oregon State University in Corvallis.

Welcome to the blood fued over dinosaur physiology - a debate about endothermy versus ectothermy, about warmbloodedness versus cold-bloodedness. Paleontologists launched this war 25 years ago, and it has since spread from museums into magazines, best-selling books, and even public television shows.

The zealousness of some combatants and the derogatory comments that have flown back and forth led paleontologist James O. Farlow to upbraid a few colleagues when he addressed the question of physiology in The Dinosauria (1990, University of California Press).

A dinosaur researcher from Indiana University-Purdue University at Fort Wayne, Farlow wrote: "Unfortunately, the strongest impression gained from reading the literature of the dinosaur physiology controversy is that some of the participants have behaved more like politicians or attorneys than scientists, passionately coming to dogmatic conclusions via arguments and/or data subject to other interpretations."

The debate still simmers, in large part because researchers have failed to find any means of resolving the issue. Lacking conclusive evidence one way or the other, paleontologists have had the freedom to argue ad nauseam, driven by nothing more substantial than faith in their own theories.

New discoveries, however, are providing solid information that offers the hope of ending this long-standing controversy. "We are on the edge of finding out what the metabolic physiology of dinosaurs was really like," says Ruben.

When early paleontologists looked at the oversized femur of a Triceratops or a Tyrannosaurus rex, most of them envisioned a large reptile with a physiology to match. Reptiles typically take their temperature cues from the outside environment.

In the cold, their metabolism slows and they grow lethargic. When the air warms or the sun comes out, reptiles arise from their torpor and resume an active life. In contrast, birds and mammals keep their body temperatures continuously elevated, which requires them to consume more food than a reptile of similar size.

The idea of cold-blooded, slow-moving dinosaurs came under fire in the 1960s, after John H. Ostrom of Yale University discovered a sickle-clawed terror called Deinonychus. With its slashing weaponry and flexible skeleton, this dinosaur had the look of an active, agile predator - an image inconsistent with the concept most paleontologists had of typical reptiles. Ostrom suggested that dinosaurs may have had physiologies more like mammals and birds.

Since then, other researchers have argued that the sheer bulk of the larger dinosaurs would have kept them from cooling down during the night. By dint of their dimensions, these giants could have maintained a constant body temperature.

Paleontologist Robert T. Bakker pushed the physiology argument further than other scientists in his popular book THE DINOSAUR HERESIES (1986, William Morrow). Bakker claimed that all dinosaurs were "automatic" endotherms - animals with fast, stable metabolisms that supply enough internal heat to keep their body temperatures constant. Ectotherms are just the opposite, having body temperatures that fluctuate with outside conditions.

Among his different points, Bakker contended that the structure of dinosaur bone closely resembles that of mammals and birds and does not match that of modern reptiles. He concluded that the internal structure of dinosaur bone proved such animals grew quickly, as modern endothermic mammals do.

New work on dinosaur bones, however, paints a more complex picture, suggesting that these once ruling reptiles do not fit neatly into any physiological category.

Although researchers have examined slices of dinosaur bone for decades, Anusuya Chinsamy of the University of Pennsylvania in Philadelphia advanced this work recently by comparing the bones of young and old animals from a single species. Charting the variations among the samples enabled her to reconstruct how dinosaurs grew.