Were dinosaurs cold-blooded, tail-dragging monsters or fast, energy-guzzling predators like we've seen them in Jurassic Park? Science might just have found the answer, but it might not be the answer we've been expecting to find. They were neither.

The question began when scientists unearthed the giant thigh bones that could only belong to huge, sluggish dinosaurs that had to lumber around ancient Earth with their massive weight. Plus, they are classified as reptiles, so it only made sense to conclude that dinosaurs, like lizards, turtles, snakes and other reptiles, were ectotherms. They had to rely on their environment to maintain their temperature. However, when researchers discovered that dinosaurs may have actually lived active lifestyles in pursuit of prey, the scientific pendulum swung the other way to the extreme opposite in support of the theory that dinosaurs were endotherms, or warm-blooded animals that can control their body temperatures.

"Most dinosaurs were probably mesothermic," says John Grady, a University of New Mexico graduate student and theoretical ecologist. "A thermally intermediate strategy that only a few species - such as egg laying echidnas or great white sharks - use today."

Grady and his team used a method developed by paleontologist Greg Erickson of Florida State University to determine the growth rate of a variety of dinosaurs and other animals including both extinct and existing sharks and birds by examining the growth rings in their bones. Animal bones are similar to tree trunks. They have rings in them that signify a period of growth. By looking at their bone rings, the researchers were not only able to identify the growth rate of dinosaurs and their metabolisms.

The researchers found that warm-blooded animals grow on average 10 times faster than cold-blooded ones. They also saw that warm-blooded animals have metabolism rates 10 times as fast as that of cold-blooded animals. Dinosaurs, however, did not grow as rapidly as endotherms. Their growth did not linger like ectotherms either. And they were neither warm-blooded nor cold. They were somewhere in the middle.

This could possibly explain why dinosaurs ruled the Earth for millions of years. As mesotherms, dinosaurs were able to move and reproduce much faster than cold-blooded reptiles, allowing them to elude predators while hunting for prey and ensuring their continuity into the future. It could also be the reason why most dinosaurs were larger than most warm-blooded mammals, since they did not have to eat so much to sustain their growth.

"For instance, it is doubtful that a lion the size of a T.rex would be able to eat enough wildebeests or elephants without starving to death. With their lower food demands, however, a real T.rex was able to get by just fine," explains Grady.

The researchers, however, are not quick to conclude that all dinosaurs were mesotherms. Feathered dinosaurs, in particular, are quite the mystery not many scientists can figure out. Grady cites the first bird, the archaeopteryx, which was more dinosaur than bird. The archaeopteryx grew to full maturity in two years, while the similar-sized red-tailed hawk can grow to the same size in six weeks.

"Despite feathers and the ability to take flight, the first bird was not the active, hot-blooded fliers their descendants came to be," Grady says.