Dinosaurs, the rulers of Earth for over 100 million years, dominated the planet through strategic energy use based on latest study. They were neither cold-blooded, like reptiles, nor warm-blooded, like modern birds and mammals. They were just right.

Mesothermy is an intermediate strategy that combines less required energy consumption than endothermy (warm-blooded), but leads to higher energy use abilities than ectothermy (cold-blooded).

The matter of which energy use strategy dinosaurs evolved plagued paleontologists for quite some time. John Grady, a graduate student at the University of New Mexico, led the study that suggested mesothermy in Earth's early beasts, and published his findings in Science. Along with graduate students Eva Dettweiler-Robinson and Natalie Wright, Grady studied the relationship between growth rates and metabolic rates in dinosaurs. His research group first analyzed them in existing animals and then extended the data to quantitatively study the relationships in extinct organisms.

The animal growth rate and energy use data was used to show that animals with faster growth rates require more energy and have higher body temperatures. After showing this relationship, Grady and his team used data collected by paleontologists who predicted dinosaur growth rates to calculate metabolic rates.

"I think we were all surprised by [the results]," says Grady's supervisor Professor Felisa Smith. "The idea certainly took some getting used to. But, the patterns were so robust." And the patterns showed this: dinosaurs grew significantly faster than modern reptiles, but not as fast as modern birds and mammals, so they had, according to Grady, "a higher energy use that probably increased speed and performance."

To get these results researchers had to use a formula that looked at body mass from the dinosaurs' thighbones, and growth rates from the growth rings in fossil bones (like growth rings on trees). 21 species of dinosaurs were evaluated and compared to data from mammals, birds, bony fish, sharks, lizards, snakes and crocodiles, resulting in over 30,000 rows of data to analyze.

Existing animals today that also function mesothermically include the great white shark, the leatherback sea turtle and tuna. Researchers suggest that mesothermy in dinosaurs is what allowed them to grow so large.

"It allows a performance advantage over the ectothermic reptiles, but without the high overhead costs of modern birds and mammals. In any case, it was a successful formula for a long reign in the Mesozoic," says Grady.

While a study released in February also discussed the probability that dinosaurs were neither cold-blooded nor warm-blooded, this is the first time anyone has presented calculated metabolic rates in a discussion of extinct animals.