Tyrannosaurus Rex has traditionally been portrayed as a cold-blooded killer.
But now scientists have found the world’s most infamous predator was actually warm to hot-blooded, and it would have made the species far more menacing.
A study of fossilised leg bones shows that the metabolism of the T-Rex was fast, which would have made the dinosaur more active, and given it a voracious appetite.
In contrast, dinosaurs like Triceratops and Stegosaurus had low metabolic rates comparable to those of cold-blooded modern animals like lizards.
Cold-blooded animals, such as reptiles, breathe less and eat less.
“Dinosaurs with lower metabolic rates would have been, to some extent, dependent on external temperatures,” said lead author Dr Jasmina Wiemann at the California Institute of Technology.
“Lizards and turtles sit in the sun and bask, and we may have to consider similar ‘behavioural’ thermoregulation in (dinosaurs) with exceptionally low metabolic rates.
“Cold-blooded dinosaurs also might have had to migrate to warmer climates during the cold season, and climate may have been a selective factor for where some of these dinosaurs could live.”
Palaeontologists have always been unsure whether dinosaurs were cold-blooded like their reptile ancestors, or warmer-blooded, like their avian descendants.
‘Most dinosaurs were warm-blooded’
To find out, researchers looked at how they used oxygen. Warm-blooded animals, like birds and mammals, take in lots of oxygen and have to burn a lot of calories in order to maintain their body temperature and stay active.
When animals breathe, side products form that react with proteins, sugars, and lipids, leaving behind molecular ‘waste’.
The waste was preserved during fossilisation, leaving behind a record of how much oxygen a dinosaur was breathing in, and therefore its metabolic rate.
The team analysed the femurs of 55 different groups of animals, including dinosaurs, flying pterosaurs, marine plesiosaurs, as well as modern birds, mammals, and lizards.
The researchers were surprised to find that bird-hipped dinosaurs, including theropods and the sauropods - the two-legged, more bird-like predatory dinosaurs like Velociraptor and T-Rex and the giant, long-necked herbivores like Brachiosaurus - were warm or even hot-blooded.
“This is really exciting for us as palaeontologists - the question of whether dinosaurs were warm or cold-blooded is one of the oldest questions in palaeontology, and now we think we have a consensus, that most dinosaurs were warm-blooded,” added Dr Wiemann.
Some scientists have proposed that birds survived the asteroid strike 65 million years ago because they were warm-blooded, while cold-blooded dinosaurs struggled with the cooling temperatures when the sun was obscured by debris.
However, the new study suggests that explanation cannot be true.
The research was published in the journal Nature.