Dinosaur metabolism—was their metabolism cold-blooded or warm-blooded? This question has been debated since dinosaurs were first discovered. Did they produce so little body heat that they were only as warm as their surroundings, like other reptiles and frogs? Or, could they produce enough heat to warm their bodies well above ambient temperatures, like birds and mammals, and thus maintain a relatively constant and more active lifestyle?
The debate over the controversial subject was rekindled in the 1970s when palaeontologist John Ostrum suggested that the metabolism of theropod dinosaurs was less reptilian-like and more like that of mammals and birds.
Firstly, metabolism is a complicated subject. How do we define metabolism? Metabolism is the set of chemical processes that occurs within the cells of living organisms for the maintenance of life. The metabolism of modern animals is only now being understood. Understanding metabolism in extinct animals is even more complicated because palaeontologists do not have the benefit of direct evidence from studies conducted with living animals. Palaeontologists, therefore, have to rely on indirect clues about metabolism in dinosaurs.
Many avenues of evidence have been explored in relation to dinosaur metabolism, including predator-prey ratios, dinosaur feeding adaptations, speed, agility and activity levels, bone microstructure, social behaviour, body size, bird ancestry, geographic distribution, blood pressure and chemistry (oxygen isotope ratios), and absence of respiratory or nasal turbinate structures.
None of these lines of exploration is conclusive on whether dinosaurs were warm- or cold-blooded. Recently, fossils of dinosaurs belonging to several groups of carnivorous (theropod) dinosaurs were found with feathers in China. These discoveries not only support the hypothesis that birds came from dinosaurs, but they also shed a light on the issue of dinosaur physiology.
The filament-like feathers of the most primitive feathered dinosaurs certainly were not used for flying because the feathers' structure was too simple. But feathers are also good for keeping warm, as those who wear down-filled coats in winter know. With this in mind, palaeontologists think that feathered dinosaurs may have had a high metabolic rate and the feathers protected against heat loss.
If so, these dinosaurs were warm-blooded (like birds and mammals), at least to a certain degree. Other, non-feathered dinosaurs, however, were most probably cold-blooded or had metabolic rates and activity levels similar to those of living reptiles.