Dinosaurs were warm-blooded runners, skeletons hint

New research has added strength to the theory that dinosaurs were warm-blooded like mammals and birds, not cold-blooded like reptiles.

Herman Pontzer of Washington University in St. Louis used data from fossil dinosaur skeletons and the energy needs of modern-day animals to conclude that dinosaurs were probably warm-blooded.

Pontzer's previous work found that an animal's energy cost for walking and running was closely associated with leg length for a wide variety of land animals.

'I'm sure the jury is still out and I don't expect that any one study is going to put the nail in the coffin.' —Herman Pontzer, Washington University, St. Louis

He found the distance from an animal's hip joint to the ground could predict with 98 per cent accuracy the energy cost of the animal's movements.

Pontzer and his team measured leg bone lengths from 14 dinosaur species, both those that walked on all fours and bipedal dinosaurs like Archaeopteryx and Tyrannosaurus rex. The researchers also performed more complex calculations to estimate the leg muscle volume the dinosaurs would have needed to move around.

Both sets of calculations suggested that dinosaurs were warm-blooded and, therefore, active and acrobatic animals.

"It's always possible that something that lived a long time ago … had a wildly different physiology than anything we've seen today," said Pontzer. "But we find that pretty unlikely and really I think the only assumption we're making is that physics worked the same way in the time of the dinosaurs as it does today."

If dinosaurs were warm-blooded, it suggests that they could have lived in colder climates than modern-day reptiles can, such as mountain ranges and polar climates.

Being warm-blooded, or endothermic, comes at a cost, though. Endothermic animals need to eat a lot more than cold-blooded, or ectothermic, animals do to keep their metabolisms going.

Evolution pushed back millions of years

The research, published this week in the journal Public Library of Science ONE, isn't the final word on the thermodynamics of dinosaurs, but it does add fuel to the debate.

"I'm sure the jury is still out and I don't expect that any one study is going to put the nail in the coffin," said Pontzer.

In fact, when the researchers plotted endothermy on a dinosaur family tree, they found that it's possible that warm-bloodedness is the common ancestral trait for all dinosaurs, pushing back the evolution of endothermy by millions or years.

"It suggests that warm-blooded metabolisms were probably prominent for a lot of species of dinosaurs," said Pontzer.