Tyrannosaurus Rex Was a Warm-Blooded Dinosaur, With a Body Temperature Similar to That of Humans, Study Suggests

Knowing whether an animal is warm-blooded or cold-blooded can tell researchers a lot about the species’ behavior and geographic range. While warm-blooded creatures, called endotherms, such as humans, can regulate their own body temperatures, cold-blooded animals, or ectotherms, rely on the environment, limiting where they can live. Until the early 2000s, researchers thought dinosaurs resembled modern-day reptiles, which are mostly cold-blooded.

But mounting evidence over the past couple of decades has challenged that idea. Now, scientists have revealed the first precise body temperature measurement for one of the world’s most iconic dinosaurs: Tyrannosaurus rex. The fearsome predator ran at a toasty 97 degrees Fahrenheit, around the same temperature as modern-day humans, according to a study published September 16 in the journal Science Advances.

The findings further suggest that the beast was indeed warm-blooded, allowing experts to infer a host of information regarding the long-extinct creature. We had guesses about T. rex metabolism, mainly from bone and biomechanics. We did not have a temperature,” study co-author Aradhna Tripati, a biogeochemist at the University of California, Los Angeles, tells CNN’s Taylor Nicioli.

Measuring a live animal’s temperature usually requires a simple thermometer, but collecting that data from a giant that’s long gone is significantly more complicated. The team turned to three fossilized dinosaur teeth from the Natural History Museum of Los Angeles County, two of which came from a T. rex named Thomas, one of the most complete skeletons of the species ever found. Drilling tiny holes in the teeth and collecting the powder allowed the researchers to analyze the chemical bonds between certain forms of carbon and oxygen in the enamel.

The number of these bonds increases as temperature decreases, so a warm-blooded animal has fewer of these bonds than a cold-blooded animal. Had geology messed with the fossils’ molecular compositions, both species would have yielded similar temperature measurements, study co-author Robert Eagle, a geobiologist at the University of California, Los Angeles, explains in a statement. But their temperatures were different; five teeth from the crocodilian suggested the animal ran at a cooler 88 degrees.

Warm-blooded animals have faster metabolisms and higher activity levels than cold-blooded creatures, so the results bolster previous proposals that T. rex could expend lots of energy. But according to the study, the dinosaur’s body temperature sat above that of today’s reptiles, which run at about 82 to 86 degrees, and below that of their living descendants, birds, which have internal temperatures of around 104 to 109 degrees. At a stable internal temperature of 97 degrees, T. rex would have been able to inhabit regions too chilly for cold-blooded animals.