The debate among paleontologists over whether dinosaurs were warm- or cold-blooded may have an unusual answer -- the famous extinct animals may have been both.
In our modern world, most animals are either warm-blooded endotherms, like humans, dogs and birds, or cold-blooded ectotherms, such as snakes and turtles. Cold-blooded animals such as amphibians and reptiles must constantly move in and out of sunlight, trying to maintain their body temperature. Warm-blooded creatures, like birds and mammals, are able to keep a near-constant level of heat.
Dinosaurs roamed the Earth for 135 million years, dominating the land. The creatures were classified as reptiles, and modern species in this class of animal are cold-blooded. This led scientists to believe they were also without a source of internal heating.
One of the challenges to the idea of dinosaurs as a cold-blooded species are birds. With their warm bodies and active lifestyles, biologists questioned how the animals could be descendants of ancient species related to dinosaurs.
Dinosaurs may have been mesotherms, animals possessing characteristics of both warm- and cold-blooded species. There are not many of these animals alive today. The best-known of these are white sharks, often called great white sharks, and tuna -- their muscles work hard as they swim and naturally heat their blood.
Lesser-known species include leatherback turtles and the echidna or spiny anteater, an egg-laying mammal from Australia.
Researchers found a new way to measure the metabolic rate of extinct species by looking at the rates the animals grew through their lives. Warm-blooded animals grow faster, on average, than members of cold-blooded species. Mammals that grow five times faster than an equivalent reptile also have metabolic rates that are five times faster.
"Most dinosaurs were probably mesothermic," said University of New Mexico lead researcher John Grady. "A thermally intermediate strategy that only a few species -- such as egg-laying echidnas or great white sharks -- use today."
Greg Erickson, a paleontologist at Florida State University, developed a new technique to read those growth rates, similar to the way one might read tree rings. The process involved comparing dinosaur bones with bones of modern mammals to judge how quickly the animals grew. From that data, the team was able to deduce information about the metabolic rates of dinosaur species.
"[W]hen the effects of size and temperature are considered, dinosaur metabolic rates were intermediate to those of endotherms and ectotherms and closest to those of extant mesotherms," Grady and his team wrote.
A mesotherm body system could have allowed dinosaurs to move quickly, while not requiring the animals to constantly eat to maintain body temperature. The largest animals can be classified as gigantotherms -- animals large enough to sustain body temperature after a period of heating.
"Our results suggest that the modern dichotomy of endothermic versus ectothermic is overly simplistic," investigators wrote in a journal article detailing the results of their study.
Research into the possible existence of mesothermy in dinosaurs was published in the journal Science.
