Dinosaurs fit in between warm-blooded mammals and cold-blooded reptiles, according to a study reported in the journal Science. The study is the first to explore the relationship between growth rate and metabolic rates in animals and extend that to long extinct animals such as dinosaurs.

Dinosaurs dominated the landscape for more than 100 million years, but all that remains today are bones. This has made it difficult to solve a puzzle: were they cold-blooded or warm-blooded creatures?

To answer this question, a team of scientists led by Prof Felisa Smith from the University of New Mexico developed and used an extensive database of animal growth and energy use.

The researchers demonstrated that animals which grow faster, not only require more energy, but have higher body temperatures.

Then, using growth estimates made by paleontologists for extinct dinosaurs, they calculated dinosaur metabolic rates.

The result was unexpected: dinosaurs were clearly intermediate between modern mammals and reptiles.

“Most dinosaurs were probably mesothermic, a thermally intermediate strategy that only a few species ­– such as egg laying echidnas or great white sharks – use today,” said study first author John Grady of the University of New Mexico.

The team also found that feathered dinosaurs and primitive birds grew distinctly slower than their descendants, modern birds.

“Archaeopteryx, the first bird took two years to reach maturity. But, a red-tailed hawk, which is about the same size, only takes 6 weeks. While dinosaurs didn’t grow as fast as modern birds or mammals, they did grow significantly faster than modern reptiles,” Mr Grady explained.

“This higher energy use probably increased speed and performance. Mesothermic dinosaurs were likely faster predators or better able to flee from danger than the large reptiles found earlier in during the Mesozoic. Dinosaurs quickly became the new ecological incumbents.”

Mesothermy in dinosaurs may have helped them become ecologically dominant and probably also helped them become enormous.

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John M. Grady et al. 2014. Evidence for mesothermy in dinosaurs. Science, vol. 344, no. 6189, pp. 1268-1272; doi: 10.1126/science.1253143