Inside the mind of a dinosaur © Magic torch

Were the dinosaurs cold-blooded?

Whether dinosaurs were cold- or warm-blooded has been debated by experts for a long time. However there is a new hypothesis...

If you read many older dinosaur books, you will see T. rex and Brontosaurus portrayed as ponderous beasts that resemble overgrown lizards or crocodiles. It was once thought that dinosaurs had the same cold-blooded (ectothermic) metabolism as reptiles. In other words, they could not control their body temperatures internally, and so relied on their environment to heat themselves up. And because of this, they grew and moved slowly.

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However, in the late 1960s and 1970s, palaeontologists started to doubt this stereotype. The discovery of feisty, svelte bird-like species such as Deinonychus revealed that some dinosaurs were faster, smarter, more agile, more energetic and faster-growing than once assumed.

Leading experts of this generation, such as John Ostrom and Robert Bakker, argued that dinosaurs were warm-blooded (endothermic), with the same physiology as today’s birds and mammals. Dinosaurs, they posited, could finely control their body temperatures, and keep them constant and high, regardless of their environment.

Over the past four decades, palaeontologists have continued to debate this question. It is now widely accepted that dinosaurs were indeed more energetic, with elevated metabolisms and growth rates, compared to reptiles. But did they achieve this through proper warm-bloodedness, or through another mechanism? There remains no definitive answer.

We must remember that nature is not black and white. Animals are not always strictly cold-blooded or warm-blooded. There are intermediates, and one new hypothesis is that dinosaurs were ‘mesotherms’: they had some control of their body temperatures, but not precise control, and many species were able to passively keep themselves warm through their enormous bulk.

With that said, we know today’s birds are warm-blooded, and birds evolved from dinosaurs like Deinonychus, so at some point during the dinosaur-bird evolutionary transition, a dinosaur must have become warm-blooded. But when, and how? These are the big mysteries that need solving.

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Asked by: Holly Cross, Cardiff

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