Dinosaurs were neither warm-blooded like mammals, nor cold-blooded like reptiles, instead they were somewhere in between, suggests a new study.
Exploiting the middle ground as a strategy may have helped dinosaurs rule the Earth for more than 100 million years, scientists report today in the journal Science .
The question of whether dinosaurs were lumbering cold-blooded or active warm-blooded animals has been debated for decades, but finding a definitive answer has proven difficult.
Now, biologist John Grady from the University of New Mexico and colleagues, have developed a new method for analysing dinosaurs' metabolic rates.
Building on previous work by palaeontologists and physiologists, they created a large database on growth and energy in both living and extinct groups of vertebrates including 21 species of dinosaurs.
They then used statistical analyses and energetic models to determine the relationship between growth rate and energy use.
Annual growth rings in fossils were used to determine growth rates, while metabolic rates were estimated by using changes in body size as an animal grows from birth to adult (known as ontogenetic growth).
"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.