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Wait — dinosaurs weren’t cold-blooded?

A boy looks inside the skull a Tyrannosaurus Rex replica at the Egidio Feruglio Museum in the Argentina’s Patagonian city of Trelew, May 18, 2014. (Maxi Jonas/Reuters)

Sometime last century, cigarettes were good for you, the Red Sox lost and dinosaurs were cold-blooded.

Now, a new study published in the journal Science may have upended the last of these quaint notions.

Evidence of mesothermy in dinosaurs,” a paper written by scientists led by John M. Grady of the University of New Mexico’s biology department, said dinosaurs weren’t quite warm-blooded or cold-blooded, but followed a middle path.

“By examining animal growth and rates of energy use, we were able to reconstruct a metabolic continuum, and place dinosaurs along that continuum,” Grady said, according to Reuters. “Somewhat surprisingly, dinosaurs fell right in the middle.”

In case it’s been awhile since you took biology: Warm-blooded creatures such as humans and other mammals regulate their own body temperature, but cold-blooded creatures such as lizards must get in the sun to keep warm.

The cold-blooded life is an idle one. Cold-blooded animals, or ectotherms, sit around catching rays so they don’t freeze to death. They move slowly. They don’t have to eat much because their metabolism is slow. They lie on rocks. And they can get really big.

This is the traditional view of dinosaurs: Giant lizards that act like Homer Simpson.

Meanwhile, warm-blooded creatures, or endotherms, live fast. They go outside without coats on and have snowball fights. They eat a lot and burn calories fast — so fast that they are relatively small.

As Grady told Reuters: “It is doubtful that a lion the size of T. rex could eat enough to survive.”

Grady and his team estimated the metabolic rate of 21 dinosaur species by estimating their body masses. The theory: the faster the metabolism, the “warmer” the blood.

How do you guess the body mass of a creature that’s been dead for about 65 million years? Look at the size of their thigh bones — and growth rings on fossils much like those found on trees.

After some science, the conclusion: T. rex wasn’t just a cold fish.

“Our results showed that dinosaurs had growth and metabolic rates that were actually not characteristic of warm-blooded or even cold-blooded organisms,” said University of Arizona evolutionary biologist and ecologist Brian Enquist, a co-author of the study. “They did not act like mammals or birds nor did they act like reptiles or fish.”

In fact, scientists may need to forget about categorizing species as warm-blooded or cold-blooded.

“Our results suggest that the modern dichotomy of endothermic versus ectothermic is overly simplistic,” the study says.

At press time, there was still no evidence that dinosaurs had WiFi.

Justin Wm. Moyer is a reporter for The Washington Post's Morning Mix. Follow him on Twitter: @justinwmmoyer.



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