Were dinosaurs cold-blooded, warm-blooded or perhaps something else?

Scott Sutherland
July 23, 2013

A new study released earlier this month may have finally provided the evidence that lays to rest the idea that dinosaurs were cold-blooded.

When Jurassic Park hit the big screen in 1993, it introduced an idea that many in the general public hadn't heard about — that dinosaurs may have been warm-blooded. Although there had been some speculation about dinosaurs being warm blooded as far back as the 1850s, the long-standing idea about these huge creatures was that they were cold-blooded, like lizards. It was John Ostrom and his student Robert Bakker (who was a consultant on the Jurassic Park movies), who approached the idea again in the 1960s, while they worked on linking the evolution of dinosaurs and birds.

Bakker argued that dinosaurs were active animals, based on their size and body structure, with similarities between their bodies and the bodies of birds and mammals, and differences between them and lizards. For example, brain size. Dinosaurs had brain-sizes more like mammals, who have fairly large brains compared to their body size. Lizards have fairly small brains for their size. Another example is their ability to walk upright, like birds do, which would have required bigger muscles, stronger bones and more energy. Lizards, who walking on all four legs, would have smaller muscles, smaller bones and they definitely use less energy.

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This 'active' view of the dinosaur was definitely at-odds with the idea of the lumbering, sluggish lizard-like dinosaurs, and there's been a lot of debate about this idea since then, with evidence on both sides. It's even been suggested that dinosaurs were large enough that the body heat they gathered during the day wouldn't all dissipate during the night. This is called 'inertial homeothermy' and it would allow them to keep a fairly constant body temperature, and thus they could remain more active while still having a cold-blooded metabolism. However, even if they could keep their body temperatures up, there's still the question of muscle strength.

A new study has tested this, though. Taking the modern saltwater crocodile as an example of what a cold-blooded dinosaur would be like, since they're large enough to retain a good portion of their body heat overnight, Professor Roger Seymour, of the University of Adelaide, in South Australia, examined blood and muscle samples from crocodiles to see how much energy they produced during activity, and he compared this to similar samples from mammals.

Even at smaller sizes, the crocodile was already at a disadvantage. A 1 kilogram crocodile can produce about 57% as much energy as a 1 kilogram mammal. As size goes up, this gets worse for the crocodile, and a 200 kg crocodile only produces as much as 14% of the energy that a 200 kg mammal can produce. Comparing this to a Tyrannosaurus rex, which could weigh up to several tons, and it seems more and more likely that such a creature would have to be warm blooded in order to be active.

"The results further show that cold-blooded crocodiles lack not only the absolute power for exercise, but also the endurance, that are evident in warm-blooded mammals," Professor Seymour said in a University of Adelaide news release. "So, despite the impression that saltwater crocodiles are extremely powerful animals, a crocodile-like dinosaur could not compete well against a mammal-like dinosaur of the same size."

"Dinosaurs dominated over mammals in terrestrial ecosystems throughout the Mesozoic," Seymour added. "To do that they must have had more muscular power and greater endurance than a crocodile-like physiology would have allowed."

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Does this settle the question once and for all? Not yet. Even though this and other strong evidence exists for dinosaurs being warm-blooded, there's still plenty of evidence around that they were cold-blooded (otherwise, there wouldn't be a debate).

It's possible that some species of dinosaur were warm-blooded while others were cold-blooded. It's also possible that dinosaurs were something completely different: something in-between cold- and warm-blooded, with characteristics of both. That would make them a stepping stone on the path of evolution, as the species on this planet developed from the more simple cold-blooded metabolism to the more complex warm-blooded metabolism.

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