Tyrannosaurus rex Had a Body Temperature Similar to Humans, Study Finds

New analysis of dinosaur teeth suggests that Tyrannosaurus rex had a body temperature similar to humans. The finding supports the idea that these iconic predators were active, energetic animals—not reptiles that relied on basking in the sun to warm themselves.

How scientists estimated T. rex body temperature

Tyrannosaurus rex lived approximately 68 million to 66 million years ago, during the Cretaceous Period. It was one of the largest carnivorous dinosaurs ever known.

Previous research has suggested that T. rex and its relatives were warm-blooded. Now, scientists have estimated dinosaur body temperatures by analyzing different isotopes, or forms, of the same chemical element in the enamel of three dinosaur teeth, including a T. rex tooth discovered in Montana’s Hell Creek Formation.

Rare, heavy isotopes of carbon and oxygen form chemical bonds in tooth enamel in different ways depending on temperature. More bonds form at colder temperatures, while fewer form at warmer temperatures. This gives scientists a way to estimate the body temperature of an animal while its tooth was growing.

By measuring carbon dioxide released from small enamel samples, Landon Flores, a geologist at the University of California, Los Angeles, and his colleagues were able to estimate the dinosaurs’ body temperatures.

T. rex was warmer than modern reptiles

The researchers estimated that T. rex had a body temperature of 97.3 degrees Fahrenheit (36.3 degrees Celsius)—similar to humans and elephants.

Modern cold-blooded reptiles typically have body temperatures of about 82 to 86 F (28 to 30 C), while most birds, the living descendants of dinosaurs, have temperatures between 104 and 109 F (40 and 43 C).

The findings, published Wednesday, Sept. 16, in the journal Science Advances, suggest that T. rex had already diverged significantly from cold-blooded dinosaurs such as Triceratops and Stegosaurus.

“Their body temperature is what I expected, higher than reptiles and slow-footed mammals like sloths, but lower than birds,” UCLA geobiologist Robert Eagle, a co-author of the study, said in a statement.

Could warm blood have helped T. rex live in colder regions?

If T. rex was warm-blooded, it may have been an active predator and scavenger that used food to fuel a fast metabolism rather than basking in the sun for energy, the study authors wrote.

This ability may also have helped T. rex survive across much of North America. During the Cretaceous Period—one of the warmest intervals in Earth’s history—global temperatures were about 11 to 25 F (6 to 14 C) warmer than today. Even so, T. rex may have been able to live in cooler, high-latitude environments where cold-blooded reptiles could not survive.

That could help explain why Tyrannosaurus fossils have been discovered in Alaska’s Kikak-Tegoshiek quarry, even though fossils of other reptiles, including lizards, turtles and crocodiles, have not been found there from the same period.

The ability to tolerate cool climates is also consistent with evidence that T. rex crossed the Bering land bridge between Asia and North America.

How much do you know about the king of dinosaurs? Test your knowledge with our Tyrannosaurus rex quiz.