The data suggest that the smallest dinosaurs used in the study had body temperatures very close to 77°F (25°C), about what scientists believe to be the average air temperature at the time.
The study's large dinos had considerably higher temperatures, reaching up to 108°F (42C).
"The smallest [dinosaurs] had temperatures similar to today's reptiles, but the largest had temperatures more like today's birds and mammals," Gillooly said.
That casts light on the dinosaurs' activity levels, says Gillooly's co-author, Andrew Allen of the National Center for Ecological Analysis and Synthesis in Santa Barbara, California.
"Given their large size, they were quite active, unlike today's small reptiles," he said.
"Based on the mathematics, I think we have pretty good evidence that they were quite similar to mammals." (Photos: "The Rise of Mammals" in National Geographic magazine [April 2003].)
The findings suggest that as dinosaurs grew, their body temperatures changed rather dramatically, especially for the biggest species.
This would imply that young dinosaurs, even of large species, were relatively lethargic while mature ones were much more active.
Finally, the study suggests that the very largest dinosaurs, weighing in at 60 tons (54,431 kilograms), had body temperatures of 114°F (45.5°C)—internal temperatures that would kill most of today's animals.
The scientists speculate that excessive body temperature was the factor that kept the very largest dinosaurs from being even larger.
Bone of Contention
Gillooly's study has already provoked controversy.
This is partly due to erroneous Celsius to Fahrenheit conversions in a press release, which caused early media reports to misstate the findings.
But even without that confusion, Kevin Padian, a paleontologist at the University of California, Berkeley, is suspicious of the new findings.
Padian, who has spent 15 years studying dinosaur growth rates, says his main concern is that the study is a mathematical model that is too far removed from the actual data.
"This has nothing do to with the actual biology of the animals," he said.
But he is also dubious that the growth rates for the eight dinosaur species used in the model are known well enough to allow such a calculation.
The growth-rate data that Gillooly's team relied on for its calculations, Padian says, sometimes involved as few as three juvenile animals—too few for accuracy.
Also, he says, there is considerable uncertainty as to the animals' actual weights.
"We can't put dinosaurs on a scale," he said. "There are at least four different methods to try to get estimates of dinosaur mass."
Finally, Padian is not sure that there is a great deal of biological relevance to determining the animals' body temperatures.
"If you stuck a thermometer into the butt of a dinosaur," he asked, "what would you learn? The body temperature itself is less important than the metabolic rate"—the rate at which an animal uses energy when at rest.
Study leader Gillooly disagrees. Body temperature can tell us interesting things about how dinosaurs lived, he says.
Based on modern animals, we know that rates of reproduction, food consumption, and other important factors increase exponentially with body temperature.
"It also affects life span," he said. "If you're 10 degrees [Celsius, or 18 degrees Fahrenheit] hotter, your life span tends to be two to three times shorter for any given size."
The same increase in body temperature also represents a threefold increase in metabolic rate and food consumption, he says.
The bottom line: Hot-blooded, fast-moving dinosaurs were going to have to eat a lot more than slow-moving cold-blooded ones.
In this case, big predators like the Tyrannosaurus rex would have been very, very hungry.
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