Dinosaurs weren't cold-blooded creatures like reptiles, but on the other hand they weren't warm-blooded like mammals, either, researchers say, explaining the may have evolved a "Goldilocks" just-right temperature that helped them dominate the Earth.
Whether dinosaurs that inhabited Earth for 100 million years were cold-blooded lumbering beasts or more active warm-blooded creatures has long been debated, with conclusive evidence of either difficult to determine, scientists say.
Now a University of New Mexico biologist and his colleagues say they've come up with a new way of analyzing dinosaur metabolic rates.
Biologist John Grady led a study of energy levels and growth rates in vertebrate groups both living and extinct.
They included 21 dinosaur species in that analysis, they reported.
"We found that growth rate is a good indicator of energy use in living animals," Grady says. "Warm-blooded (endothermic) mammals grow 10 times faster than cold-blooded (ectothermic) reptiles, and metabolize 10 times faster; in general doubling one's metabolic rate leads to a doubling in growth rate."
When they analyzed dinosaur growth by looking at growth rings in fossil bones, they found the ancient beasts had neither as high a metabolic rate as found in birds and mammals nor as low a rate as is common in reptiles.
"Surprisingly we found that, instead, they occupied the middle energetic ground," Grady says, noting such creatures are known as "mesotherms."
Such "middle ground" animals are not that common today, but some examples include tuna, leatherback turtles, great white sharks and the echidna, an egg-laying mammal in Australia that's also called the spiny anteater.
Being mesotherms would have given dinosaurs the ability to grow, move and reproduce at a faster rate than cold-blooded reptiles of their era, the researchers say.
That alone would have made them more dangerous as predators and much more elusive as prey.
However, if dinosaurs had been fully warm-blooded like mammals it would have limited a maximum size they could grow to because it would create food demands they couldn't keep up with, the researchers say.
The lowered food demands needed as a mesotherm would have allowed dinosaurs to reach great size while still keeping ahead of their competition.
Still, not all dinosaurs would have been mesotherms, Grady says.
"Dinosaurs were a big and diverse bunch, and some may have been endotherms (warm-blooded) or ectotherms (cold-blooded,)" he says.
The findings could yield clues to the evolution of warm-blooded animals, including humans, Grady says.
"The origins of endothermy in mammals and birds are unclear," he says. Studying the growth rates of the ancestors of birds and mammals, including dinosaurs, "will shed light on these mysterious creatures," he adds.
