One of the oldest controversies in dinosaur research centers around the animal’s metabolism. One side of the argument holds that they were cold blooded, like modern reptiles. The other side of the believes that they were warm blooded, like modern mammals and birds.

A third school of thought has now been introduced into the debate by scientists at the University of New Mexico. In the paper, “Evidence for Mesothermy in Dinosaurs,” published in the journal Science, the researchers argue that the dinosaurs were neither warm-blooded (“endotherms”) or cold-blooded (“ectotherms”). Instead they argue that the dinosaurs were in-between, or mesothermic-like egg laying echidnas or great white sharks.

To reach this conclusion, the researchers used a database of energy use as it relates to animal growth developed by University of New Mexico graduate student and research leader John Grady. The research demonstrated that faster growing animals require more energy and have higher body temperatures. Then, using paleontologists’ estimates for the growth rate of extinct dinosaurs, Grady attempted to estimate the metabolic rates of the animals. The researchers discovered that the estimates put the dinosaurs in between the metabolic rates of reptiles and mammals.

“I think we were all surprised by this. The idea certainly took some getting used to. But, the patterns were so robust,” said Professor and PiBBs Director Felisa Smith in a statement.

The researchers also found that feathered dinosaurs and feathered birds grew at a slower rate than their modern avian descendants.

“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,” said Grady.

The research demonstrates that, although dinosaurs did not grow as fast as modern birds, they did grow faster than reptiles.

“This higher energy use probably increased speed and performance. Mesothermic dinsoaurs were likely faster predators or better able to flee from danger than the large reptiles found earlier in during the Mesozoic,” said Grady.

While the additional speed and power were a genetic advantage, the researchers explain that warm blooded dinosaurs would have faced fuel shortages.

“A lion the size of a T-Rex, while a frightening thought, would quickly starve to death because it would be so hard to find enough food,” said Smith.

The mesothermic approach would have provided distinct advantages over reptiles but without the high overhead energy costs in terms of food faced by mammals and birds.