Were dinosaurs cold-blooded lumbering creatures or warm-blooded and agile? A new study has come up with a surprising answer.

Dinosaurs were once portrayed as giant, cold-blooded, lumbering lizards. But recent evidence of their bird-like features – including feathers covering their bodies in many cases – has given them an image makeover. In recent decades, many scientists have argued that they were likely warm-blooded and agile like birds and mammals.

Still the debate continued, so evolutionary biologist John Grady and his colleagues aimed to settle it by comparing the growth rates – which are five to 10 times higher in warm-blooded animals compared to cold-blooded animals – of 381 species of animals, including 21 dinosaurs.

What they seem to have found is that dinosaurs weren't cold-blooded, but they weren't warm-blooded either – they were somewhere in between.

That was a surprise, said Grady, a graduate student in the lab of evolutionary biologist Felisa Smith at the University of New Mexico.

"I thought, like most people, it's either or," he said. "I wasn't expecting to see this kind of third way of regulating your body temperature."

The team published their results Thursday in the journal Science.

Warm-blooded advantages

Most modern fish, amphibians and reptiles are cold-blooded or ectothermic, which means their body temperature varies with that of their surroundings. Birds and mammals, on the other hand, burn energy to produce heat and keep their body temperature steady, regardless of whether it's cold outside.

"It has a big impact on where you can live," Grady said.

Being warm-blooded or endothermic has allowed mammals to do well in cold environments such as the Arctic, where animals such as crocodiles and frogs would spend much of the year at risk of literally freezing to death. Even when the temperature is cool but above freezing, cold-blooded animals are at a competitive disadvantage, Grady said.

"You just get really slow and sluggish and that makes you vulnerable to predators that are warm-blooded."

The down side is that it takes a lot of energy to maintain a high body temperature, so warm-blooded animals are less efficient and need to eat more for their size.

When dinosaurs were first discovered a couple of centuries ago, scientists thought they were most similar to reptiles and therefore cold-blooded.

"It kind of fit into the narrative of the time," Grady said, noting that people then believed that evolution produced progressive changes, "so that if these dinosaurs are extinct, maybe they were sluggish and slow and stupid and mammals beat them out."

Bird-like features

Then, starting in the 1960s, paleontologists began finding bird-like dinosaurs that looked built for speed, such as velociraptors, kicking off a decades-long debate about whether they might, in fact, be warm-blooded. The debate was fuelled by the discovery of dinosaurs that had feathers and other features, such as lung structures, that reminded people of birds, Grady added.

While it's impossible to go back in time and take a dinosaur's temperature, whether an animal is warm or cold-blooded has a big influence on its metabolic rate. That, in turn, influences its growth rate – something that can be measured from fossil evidence.

Grady gathered the growth rates and metabolic rates of hundreds of animals ranging from fish to mammals, and showed that they were closely related. He then compared the growth rates of living animals to those of dinosaurs.

"Dinosaurs really fell in the middle between reptiles and mammals," Grady said.

All dinosaurs were in that middle, including dinosaurs with feathers. For example, Grady said, the flying, feathered dinosaur archeopteryx grew to hawk size in two years. Hawks do that in just six weeks.

"That was most surprising," Grady said. "I would have expected feathered dinosaurs to be more like a warm-blooded bird."

Energy use in dinosaurs and other vertebrates. (John Grady/University of New Mexico)

Dinosaur growth rates were most like those of animals called mesotherms that include mako sharks, tuna and leatherback turtles. Such animals can burn energy to generate heat, but don't maintain their bodies at a set temperature.

Grady said that the fact that there are so few mesotherms today and that they live mainly in the ocean, where there are few warm-blooded animals, may say something about evolution and competition.

"If you look today, mammals and birds are by far the dominant animals on land," he said.

If mesothermic animals like dinosaurs lived among them, they would have to compete.

"And I think," Grady said, "they would probably lose."