Hot and Cold: Dinosaur Blood Couldn't Make Up its Mind

It seems that dinosaurs couldn't make up their minds. Neither were they cold-blooded like modern reptiles, nor warm-blooded like mammals and birds, but rather an in-between rarely seen nowadays, researchers say.

Dinosaurs are currently classified as reptiles, and so scientists assumed that in all their 135 million years roaming the planet, their body temperatures depended on the climate. However, birds are considered modern day dinosaurs and they are warm-blooded, meaning they control their own body temperature, raising the question of whether these prehistoric beasts were hot or cold.

John Grady, an ecologist at the University of New Mexico and lead author of the study, now offers a third option. "What I'm suggesting is neither," he explained to NPR. "Rather, they took a middle way - kind of like Goldilocks. And it seemed to work out very well for them."

To get to the bottom of this mystery, scientists developed a new method for analyzing growth and metabolism of extinct species. Much like cutting into a tree and looking at its ring thickness to determine its growth rate, the research team examined the layers of bone deposits in fossils, which revealed how quickly or slowly that animal might have grown, according to Live Science.

The study, published in the journal Science, analyzed 21 species of dinosaur bones, including super predators Tyrannosaurus and Allosaurus, long-necked Apatosaurus, duckbilled Tenontosaurus and bird-like Troodon, as well as a swath of mammals, birds, fish, sharks, lizards, snakes and crocodiles.

"Our results showed that dinosaurs had growth and metabolic rates that were actually not characteristic of warm-blooded or even cold-blooded organisms. They did not act like mammals or birds nor did they act like reptiles or fish," University of Arizona evolutionary biologist and ecologist Brian Enquist told Reuters.

Researchers dubbed these indecisive creatures as mesotherms, opposed to ectotherms (cold-blooded animals) or endotherms (warm-blooded).

Dinosaurs may have to thank their intermediate metabolisms for their enormous, fear-inducing size - which was larger than any other mammal around. Warm-blooded animals use heat from metabolic reactions to maintain a normal body temperature, and so need to eat frequently.

"It is doubtful that a lion the size of T. rex could eat enough to survive," Grady pointed out.

Grady believes that classifying animals as either cold- or warm-blooded is too black and white. Like dinosaurs, some animals alive today like the great white shark, leatherback sea turtle and tuna do not fit easily into either category, he added.