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Dinosaurs: cold-blooded or warm?

ALBUQUERQUE, N.M. — Science magazine, considered by many to be the nation’s top scientific journal, published this month two articles written in large part by University of New Mexico researchers.

One of the articles in the prestigious journal is about the discovery of a huge amount of non-liquid, non-ice, non-vapor water deep below the Earth’s surface under North America in the Earth’s rock mantle.

The second, unrelated article concerns dinosaurs, at least their metabolic and growth rates. In conducting their research, lead author John Grady, a Ph.D. student with UNM’s Department of Biology, and his team believe they managed to answer an age-old question: Was the ever-popular but long-extinct dinosaur warm- or cold-blooded?

Grady has been pondering animal size and metabolic rates and studying the evidence for years, beginning when he was an undergraduate at the University of Arizona. Only recently did he extrapolate the data to dinosaurs.

The surprising answer, he and the team found, is that most dinosaurs were probably neither cold- nor warm-blooded, but something in between.

Scientists call warm-blooded animals – mammals and birds – endotherms, while cold-blooded reptiles and most fish are ectotherms. But there is also a middle category, mesotherms, which includes great white sharks and most large tuna – and now, perhaps dinosaurs.

“Mesotherms are not quite reptilian and not quite mammalian, but right in the middle,” Grady said. “Most dinosaurs were probably mesothermic.”

That means dinosaurs’ metabolic rates generated enough heat to make them warmer than their external environments, just as great whites are warmer than the surrounding ocean temperature.

The study, “Evidence for Mesothermy in Dinosaurs,” is the first to quantitatively examine the relationship between growth rate and metabolic rates in animals and extend that to dinosaurs.

Grady was joined by fellow UNM graduate students Eva Dettweiler-Robinson and Natalie Wright. Dettweiler-Robinson did much of the programming, and Wright added the evolutionary analysis.

The research was supervised by UNM Professor Felisa Smith and Professor Brian Enquist of the University of Arizona. It was supported by grants from the National Institutes of Health.

Grady collected a database of more than 30,000 lines pertaining to animal growth and energy consumption, which the research team used to show that faster-growing animals use more energy and have higher body temperatures.

The study found feathered dinosaurs and primitive birds grew much slower than modern birds. “Archaeopteryx, the first bird, took two years to reach maturity,” Grady said. “But, a red-tailed hawk, which is about the same size, only takes six weeks.”

But even if they didn’t grow as fast as modern birds or mammals, dinosaurs did grow significantly faster than modern reptiles. The higher energy use probably increased speed and performance, Grady said. “Mesothermic dinosaurs were likely faster predators or better able to flee from danger than the large reptiles found earlier during the Mesozoic,” he said.

The Mesozoic period, which lasted from about 252 million to 66 million years ago, is also known as the Age of Reptiles. Mesothermy in dinosaurs may have helped them become dominant and probably also helped some of them grow quite large, in ways few mammals could ever achieve.

By adopting a medium-powered strategy for energy, it seems, dinosaurs had found the perfect solution for survival.

“A lion the size of a T-Rex,” said Smith, “while a frightening thought, would quickly starve to death, because it would be so hard to find enough food.”

Mesothermy would also allow for a performance advantage over cold-blooded reptiles, but without the high overhead costs of birds and mammals, Grady said. “In any case, it was a successful formula for a long reign in the Mesozoic.”