For decades, paleontologists have debated whether dinosaurs were cold-blooded, like reptiles, or warm-blooded, like mammals or birds. Since cold- and warm-bloodedness are closely related to animals’ metabolic rates – how quickly their metabolism could transform oxygen into energy – clarifying this issue could shed more light on how active dinosaurs were and what their daily life looked like.
A research team that included scientists from the Field Museum in Chicago has now developed a new method for studying dinosaurs’ metabolic rates by using clues in their bones which indicated how much oxygen they breathed. By using this groundbreaking technique, the experts found that most of the dinosaurs, such as the theropods or sauropods (the so-called ornithischians, or bird-hipped dinosaurs), were warm-blooded, while saurischian (lizard-hipped) species, such as the Triceratops and Stegosaurus, were cold-blooded.
“This is really exciting for us as paleontologists – the question of whether dinosaurs were warm- or cold-blooded is one of the oldest questions in paleontology, and now we think we have a consensus, that most dinosaurs were warm-blooded,” said study lead author Jasmina Wiemann, a postdoctoral fellow at the California Institute of Technology.
When animals breathe, side products from this activity react with proteins, sugars, and lipids, leaving behind molecular “waste” which is extremely stable and water-insoluble. This waste is preserved during the fossilization process, leaving behind a reliable record of how much oxygen a dinosaur was breathing in, and thus, its metabolic rate.
The researchers analyzed the femurs of 55 different groups of animals, including dinosaurs and some of their relatives such as the pterosaurs or plesiosaurs, as well as modern birds, mammals, and lizards. By comparing the amount of breathing-related molecular byproducts with the known metabolic rates of the living animals, they found that dinosaurs’ metabolic rates were generally high, suggesting that they were warm- or even hot-blooded. For instance, bird-hipped dinosaurs – including predators such as Tyrannosaurus Rex or Velociraptor, as well as giant, long-necked herbivores such as Brachiosaurus – had extremely high metabolic rates, comparable to those of modern birds, and much higher than mammals. However, most of the lizard-hipped dinosaurs, such as Stegosaurus or Triceratops, had low metabolic rates similar to those of modern reptiles.
These findings could over fundamentally new insights on how various dinosaurs lived. “Dinosaurs with lower metabolic rates would have been, to some extent, dependent on external temperatures,” said Wiemann. “Lizards and turtles sit in the sun and bask, and we may have to consider similar ‘behavioral’ thermoregulation in ornithischians with exceptionally low metabolic rates. Cold-blooded dinosaurs also might have had to migrate to warmer climates during the cold season, and climate may have been a selective factor for where some of these dinosaurs could live.”
On the other hand, hot-blooded creatures were likely more active and needed to eat a lot. “The hot-blooded giant sauropods were herbivores, and it would take a lot of plant matter to feed this metabolic system. They had very efficient digestive systems, and since they were so big, it probably was more of a problem for them to cool down than to heat up.”
“This new study adds a fundamental piece of the puzzle in understanding the evolution of physiology in deep time and complements previous proxies used to investigate these questions,” concluded study co-author Matteo Fabbri, a postdoctoral fellow at the Field Museum. “We can now infer body temperature through isotopes, growth strategies through osteohistology, and metabolic rates through chemical proxies.”
The study is published in the journal Nature.