When the first dinosaur fossils were recognized in the mid-19th century, scientists envisioned that the creatures were basically giant, lumbering lizards. They also presumed that dinosaurs were like present-day, cold-blooded lizards, meaning that their body temperature depended on the surrounding environment. However, this notion was later fiercely debated.  

“The general picture that we have of dinosaur physiology has changed quite a bit through the last [several] decades,” says Jasmina Wiemann, a molecular paleobiologist at the California Institute of Technology. “Our understanding of what dinosaurs looked like and lived like is directly related to the question of whether they were cold-blooded, warm-blooded, or somewhere in between.” 

A new analysis published by Wiemann and her collaborators on May 25 in Nature indicates that the ancestors of dinosaurs were warm-blooded, or capable of maintaining a constant internal temperature. The researchers used a new technique to estimate the metabolic rates of modern and extinct animals based on the molecular composition of their bones. They concluded that many iconic dinosaurs such as Tyrannosaurus rex and the giant sauropods were warm-blooded, but cold-bloodedness later emerged in some dinosaurs such as Stegosaurus

Enrico Rezende, an evolutionary biologist at Pontifical Catholic University of Chile who has studied the evolution of warm-bloodedness, or endothermy, calls the findings “quite impressive.”

The results are “not entirely surprising, but it’s definitely good to have some estimate of metabolic levels,” he says, explaining that it breaks away from rigidly categorizing dinosaurs as warm-blooded or cold-blooded. “Essentially what this shows is that we have this whole gradient of metabolic levels.” 

Modern lizards or crocodiles must bask in the sun to raise their body temperature, while warm-blooded animals such as birds and mammals don’t need to do this. Being endothermic could have allowed dinosaurs to be more active and range over larger areas, Rezende says. They would also be less vulnerable to chilly temperatures, which means they could be more active at night and would fare better on elevated terrain or at high latitudes. On the other hand, warm-blooded dinosaurs would require a lot of energy to fuel their high metabolisms, which means they would need to spend a lot of time feeding.

“Understanding the metabolic levels would tell us quite a lot about how they could interact and how these ecosystems could be built,” Rezende says.

[Related: Spinosaurus bones hint that the spiny dinosaurs enjoyed water sports]

Researchers have used various procedures to explore the extent to which dinosaurs were able to generate their own heat, says Lucas Legendre, a paleontologist at the University of Texas at Austin. One line of evidence comes from body temperature estimates based on temperature-sensitive minerals preserved in fossils. Other researchers study the growth rings in dinosaur thighbones to gauge how fast the animals grew. Legendre and his colleagues have also used blood vessel and bone cell size to infer that carnivorous dinosaurs had high metabolic rates close to those of today’s birds. 

The Nature paper indicates that, in terms of physiology, dinosaurs typically had more in common with their closest living relatives—birds—than with lizards, Legendre says. “This is a new piece of evidence that confirms what a lot of researchers have been saying for the past decade,” he says. 

For the new work, the researchers took a more direct approach than earlier investigations, says Matteo Fabbri, a paleontologist at the Field Museum of Natural History in Chicago and coauthor of the study. The team examined byproducts of metabolism—the process by which animals convert nutrients and oxygen into energy—preserved in newly-formed as well as fossilized thighbones. 

“It is the metabolism that determines whether a lot of excess heat is generated as part of the breathing process and whether an animal is cold-blooded or warm-blooded,” Wiemann says.

During this process, chemicals called reactive oxygen species form and generate molecules called advanced lipoxidation end-products. These leftovers build up and “leave a fingerprint in pretty much every tissue,” Rezende says. An animal with a high metabolic rate uses more oxygen than one with a low metabolic rate, so it should have higher levels of these compounds in its body. 

Wiemann and her team scanned the bones of 30 fossilized animals and 25 modern birds, mammals, and reptiles using techniques called Raman and Fourier-transform infrared spectroscopy. This allowed them to measure the accumulated amounts of advanced lipoxidation end-products. 

“We basically use these data to infer the evolution of metabolism,” Wiemann says. “What we figured out is that dinosaurs were ancestrally warm-blooded.”

[Related: The fiery end of the dinosaurs kicked off the golden age of mammals]

The findings indicate that endothermy independently evolved in the group encompassing dinosaurs and the flying reptiles known as pterosaurs, in mammals, and in marine reptiles known as plesiosaurs. The researchers calculated particularly high metabolic rates for a long-necked diplodocid, Allosaurus, and birds, while T-rex had a somewhat lower metabolic rate than other carnivorous theropod dinosaurs. Strikingly, several of their more distant relatives had metabolic rates on par with modern lizards, indicating they were cold-blooded, or ectothermic. These included Stegosaurus, Triceratops, and a duck-billed hadrosaur.

“That is quite fascinating because it means the range of metabolisms realized in dinosaurs is a lot broader than originally thought,” Wiemann says. “That brings up interesting questions as to what triggers the evolutionary increase or decrease in the metabolic rate, and what does this mean for the lifestyles of the animals?”

Researchers have previously suggested that warm-bloodedness helped prehistoric birds and mammals adapt during the mass extinction that killed off the rest of the dinosaurs about 66 million years ago. However, the evidence that many Late Cretaceous dinosaurs had high metabolic rates hints that other traits such as body size were probably key to the survivors’ success, Wiemann says.  

The findings will need to be verified with further analyses that include more extinct animals, Legendre says. Still, the metabolic byproducts Wiemann and her team probed offer a source of data that researchers can compare with other traits.

“The fact that they used this new method adds one additional piece of the puzzle,” Legendre says. “Hopefully we’ll be able in the next few years to come up with a more precise picture of how dinosaurs and their close relatives were able to produce metabolic heat.”

Updated (May 26, 2022): The headline of this story has been updated to better reflect the research study’s question and the debate about dinosaur endothermy.