Dinosaurs' active lifestyles suggest they were warm-blooded
H. Pontzer, V. Allen, J.R. Hutchinson/PLoS ONE
Whether dinosaurs were warm-blooded or cold-blooded has been a long-standing question in paleobiology. Now, new research on how two-legged dinosaurs walked and ran adds new evidence to the argument for warm-bloodedness, and suggests that even the earliest dinosaurs may have been warm-blooded.
Warm-blooded (or endothermic) dinosaurs — able to regulate their own body temperatures — would have been more active and could have inhabited colder climates than cold-blooded (or ectothermic) dinos, which would have functioned more like modern reptiles — animals that become animated only as temperatures warm. Endothermic dinosaurs would have also required more energy to maintain their higher metabolic rates. Evidence such as rapidly growing bones, bird-like feathers and athletic builds have led most paleontologists to believe that dinosaurs were endothermic, says paleobiologist Greg Erickson of Florida State University in Tallahassee, Fla., who was not involved in the new research.
But many scientists are still averse to the idea of warm-blooded dinosaurs. For example, some researchers have suggested that larger, more massive dinosaurs may have radiated much less heat than smaller dinosaurs — and thus, they could have been cold-blooded while still able to maintain relatively high body temperatures.
In the new study, published today in PLoS ONE, biomechanist Herman Pontzer of Washington University in St. Louis, Mo., and colleagues sought to figure out whether the lower metabolism of an ectotherm would have afforded dinosaurs the energy they needed to walk and run. To test this possibility, the team looked at two factors thought to be linked with energy requirements in modern animals: hip height and the volume of muscle used to hold up and move an animal’s body forward. If the limb length and active muscle volumes of dinosaurs required more energy than an ectotherm’s metabolism would have been able to provide, Pontzer and colleagues reasoned, then the dinosaurs were likely endothermic.
The team studied 13 different two-legged dinosaur species, ranging in size from Tyrannosaurus to the tiny, bird-like Archaeopteryx, as well as one early dinosaur relative, Marasuchus. Based on hip height, the results showed that the five largest dinosaurs (including Tyrannosaurus) would have needed endothermic metabolisms just to have the energy to walk, and all of the dinosaurs would have required endothermy to run at a moderate speed. Results based on estimated active muscle volume revealed a similar pattern: The five largest dino species would have needed to be endothermic to walk or run, while smaller, very active dinosaurs such as Velociraptor, must have been endothermic to be able to run.
In addition, even the most ancient dinosaur-like relative, Marasuchus, may have been endothermic based on the data from the hip study, Pontzer says, suggesting that endothermy evolved very early on in the dinosaur lineage. Therefore, the results also suggest that all dinosaurs were endothermic, the team wrote.
“I think their study is pointing to what a lot of other studies are saying — that these animals were endothermic,” Erickson says. “It’s just, what grade of endothermy were we dealing with?” For example, modern marsupials, although endothermic, generally grow more slowly and have lower metabolic rates than other mammals, he says.
The study may not put the final "nail in the coffin" for the idea that large dinosaurs could have been ectothermic, but it does provide positive evidence for an alternative metabolic strategy, says Patrick O’Connor, a paleontologist at the Ohio University College of Osteopathic Medicine in Athens who was also not involved in the new research. "Studies like this add crucial new lines of evidence that help us refine existing hypotheses," O'Connor says.
Estimating dinosaur metabolisms based on modern animals can only go so far, according to Erickson. For example, Pontzer and colleagues focused on two-legged dinosaurs because if they had used four-legged dinosaurs, they would have also needed to estimate how the dinosaurs’ weight was distributed across all four legs.
But because all modern ectotherms, such as alligators, are four-legged, Pontzer and colleagues had to gauge the hypothetical ectothermic capacity for the two-legged dinosaurs against four-legged modern animals, Erickson notes. Moreover, even the largest modern ectotherms are much smaller than a 6-metric-ton Tyrannosaurus. “There are limitations from living organisms that make it so we may never be able to test all these ideas,” Erickson says.
Still, Erickson says he thinks scientists are “honing in on the real answer” on the question of when endothermy evolved in dinosaurs and other ancient vertebrates. Other evidence, such as rates of bone growth, suggests pterosaurs, or flying reptiles, were also endothermic. “When you have all these different lines of evidence kind of pointing towards [endothermy],” he says, “I think it’s fairly compelling collectively.”