When dinosaurs were first recognized by European naturalists during the early 19th century, they were interpreted as being immense, lumbering reptiles similar to iguanas and crocodiles. Since that time our understanding of dinosaurs has changed substantially; early paleontologists such as Gideon Mantell, William Buckland, and Richard Owen would not recognize dinosaurs as we know them today. The once revolutionary idea that dinosaurs were dynamic creatures is now the standard view, yet the details of dinosaur physiology are still not completely known. A new study published in the journal PLoS One adds to the ongoing debate about dinosaur biology, and it suggests that dinosaurs might have actually inherited the physiology necessary to lead very active lives.
Most of the debate has centered on whether dinosaurs were endothermic like birds (i.e. internally regulated their body temperature through their metabolism) or ectothermic like living reptiles (i.e. had body temperatures that fluctuated more widely according to their surrounding environment). As some scientists have pointed out, it is not necessary to think that dinosaurs were precisely like living birds or reptiles—they could have had a unique physiology all their own—but the broad questions of whether dinosaurs were more like endotherms or ectotherms has remained.
Given that all the non-avian dinosaurs are extinct, though, we can't simply stick a thermometer into a dinosaur and take their temperature. (Nor would such an activity be necessarily advisable, at least without wearing a protective suit of armor.) The questions that remain must be approached more indirectly, and in the new study scientists Herman Pontzer, Vivian Allen, and John Hutchinson looked at how much energy it would take for dinosaurs to walk and run. If they could figure out the cost of moving around, they reasoned, they could determine whether an ectothermic or endothermic metabolism would be able to provide the amount of energy the dinosaur required.
The team estimated the leg length of the bipedal dinosaurs, as this measurement has been used to estimate the cost of walking and running in living animals. They also estimated the volume of the muscles that would have attached to the leg bones based upon the size of muscles required to move the legs of the dinosaurs. These estimates could then be compared to what has been observed in living animals, providing an indirect way to see whether dinosaurs were more like ectotherms or endotherms.
What the scientists found was that the largest dinosaurs in the study (Plateosaurus, Dilophosaurus, Allosaurus, Gorgosaurus, and Tyrannosaurus) would have required an endothermic metabolism to move around, while the smaller dinosaurs, such as Archaeopteryx, fell more within the range expected for ectotherms. This created something of a paradox as the small, feathered dinosaurs are the ones thought to be most bird-like in terms of physiology.
Size might have made all the difference. While the study produced clear results for the larger dinosaurs the results for the smaller dinosaurs were ambiguous. Even though the smaller dinosaurs in the study (such as Archaeopteryx, Compsognathus, Velociraptor, and Microraptor) had anatomical traits suggestive of endothermy, the study placed them into the ectotherm range. What this probably means, the authors argue, is that energy expenditure in these smaller animals might have been different than in the large dinosaurs, but the technique they used could not successfully distinguish between the two metabolic ranges in the smaller dinosaurs.
More certain were the results of the larger dinosaurs. It had been proposed that large dinosaurs could afford to be ectothermic as their large body size would have allowed them to retain heat, thus living a "warm-blooded" lifestyle without actually being endothermic. If the new analysis is correct, however, then it is more likely that the largest dinosaurs would have to have been endotherms. And since they evolved from small ancestors, that makes it possible that the smaller dinosaurs were also endotherms. The fact that pterosaurs, close relatives of dinosaurs (which were not included in the present study), also have traits that seem to indicate more bird-like metabolic rates suggests that endothermy either evolved multiple times or that it is an ancestral trait for the common ancestor of both pterosaurs and dinosaurs. Determining which scenario is the case, however, will require further study in combination with other lines of evidence from the fossil record.