Larger dinosaurs likely had more body heat

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Research suggests the largest of the prehistoric dinosaurs had body temperatures even higher than the heat wave levels now wilting Washington.

A new study says the seven-ton, 40-foot-long, meat-eating Tyrannosaurus rex had a cruising temperature of just above 91 degrees, and the Sauroposeidon -- a 90-foot-long vegetarian that weighed up to 70 tons -- probably averaged a body temperature of 118 degrees.

That is hotter than most living creatures can get before proteins in the body begin to break down, according to researchers from the University of Florida; the National Center for Ecological Analysis and Synthesis in Santa Barbara, Calif; and the University of New Mexico, who provided the dinosaur body heat data.

"These results provide what is perhaps the first direct evidence that dinosaurs were reptiles whose body temperatures increased systematically with body size," said the authors, whose report appears in the online journal PLoS Biology.

"These findings suggest that maximum dinosaur size may have ultimately been limited by body temperature," said lead author James F. Gillooly of the University of Florida's zoology department.

The body temperature of dinosaurs has long been a subject of debate in biology.

For many years, scientists thought dinosaurs were coldblooded -- or ectotherms -- with a slow metabolism rate that required the sun's heat to regulate their temperature.

But starting in the late 1960s, some began promoting the idea that the beasts may have been endotherms -- warmblooded creatures, much like mammals and birds -- with relatively constant high body temperatures that were internally regulated.

Still others argued that dinosaurs were reptilelike in their metabolism, but that large dinosaurs maintained higher, more constant body temperatures than smaller reptiles because of a phenomenon known as thermal inertia.

Under the so-called "inertial homeothermy" hypothesis, the researchers explained, dinosaur temperatures were primarily determined by the interaction between environmental temperature and the production and dissipation of heat.

The goal of the research by Mr. Gillooly and his colleagues was to settle that debate and to come up with firm estimates of the body temperatures of specific dinosaurs.

Toward that end, the researchers combined their understanding of relationships among factors such as body size, temperature and growth rates with newly available fossil data on the growth rates of eight dinosaur species.

They concluded that dinosaurs were coldblooded, but that thermal inertia was at work in setting the body temperatures of the largest dinosaurs. They determined the largest ones dissipated heat more slowly and so maintained higher, more constant body temperatures, similar to those of today's mammals and birds.

"We're quite confident of our findings," Mr. Gillooly said yesterday in a telephone interview. "We provided strong support [for the idea] that the higher body temperatures of large dinosaurs were due to their heat capacity. They were much like a large lake, which cools and heats more slowly than a small lake."