"We found that growth rate is a good indicator of energy use in living animals. Warm-blooded (endothermic) mammals grow 10 times faster than cold-blooded (ectothermic) reptiles, and metabolise 10 times faster; in general doubling one's metabolic rate leads to a doubling in growth rate," Grady explains.
However, when they examined the growth rates of dinosaurs, although there was some variation in the rate they grew, they had neither the high metabolic rate of mammals and birds, nor the low metabolic rate of reptiles.
"Surprisingly we found that, instead, they occupied the middle energetic ground."
Today, mesothermic animals are uncommon, but living species come from across the evolutionary spectrum, and include leatherback turtles, tuna, great white sharks and the echidna.
These animals at times rely on internally-generated metabolic heat to maintain body temperatures, while being subject to external temperatures in others.
"They generate enough heat to warm their blood above ambient temperature, but don't do anything to maintain it, such as shivering which humans do when they are cold," says Grady.