Tyrannosaurus rex ran at about 97 degrees Fahrenheit, close to a human's normal body temperature, according to the first direct estimate of the dinosaur's body temperature. The study, published Sept. 16 in Science Advances, analyzed chemical bonds preserved in the enamel of three T. rex teeth from Montana's Hell Creek Formation and arrived at 36.3 degrees Celsius, with an uncertainty of plus or minus 2.5 degrees (one standard error).

That result places T. rex well above modern cold-blooded reptiles and below today's birds, its living relatives. It supports the picture of an active animal that kept its body warmer than its surroundings, and, combined with climate modeling, it offers physical evidence for how tyrannosaurs could live as far north as Cretaceous Alaska, where winters would have been too cold for crocodiles and turtles.

The finding also carries a broader lesson. Senior author Aradhna Tripati, a UCLA isotope geochemist and climate scientist, has noted that an animal's thermal physiology shapes its behavior, its range and how it responds to a changing climate, a question that applies to species today as much as to dinosaurs.

Reading Temperature from Ancient Enamel

The method is called clumped isotope thermometry. Tooth enamel contains carbonate built from carbon and oxygen atoms, including rare heavy forms known as carbon-13 and oxygen-18. When enamel forms, those heavy isotopes bond to each other more often at cooler temperatures and less often at warmer ones. Measuring how common those bonds are reveals the temperature at which the tooth grew inside the living animal.

Enamel is the best place to look because it resists chemical change far better than bone, which the body constantly remodels. The team drilled small amounts of enamel, dissolved the powder in phosphoric acid to release carbon dioxide, and measured the gas with a mass spectrometer. "The enamel chemistry is truly recording the temperature at which it formed," Tripati told Scientific American.

The researchers also used infrared analysis to check the teeth for signs of chemical alteration during burial. They reported that the Hell Creek samples looked more like modern alligator teeth than like older Jurassic dinosaur teeth or a known chemically altered reference sample, and that their carbonate content matched modern reptile enamel.

Why the Museum Agreed to Drill Thomas

Two of the teeth came from Thomas, the most complete of the T. rex skeletons in the dinosaur hall at the Natural History Museum of Los Angeles County, and the third was an isolated tooth from a second animal. A decade ago, the UCLA method needed more material than any museum would give up for a T. rex. Over that decade, the team cut the amount required by roughly 90%, down to a few milligrams per measurement.

That change made the trade-off acceptable. According to UCLA's announcement of the study, Luis Chiappe, curator of the museum's Dinosaur Institute, said the museum routinely weighs damage to irreplaceable specimens against the knowledge gained, and that sacrificing small portions of two teeth not on display was worth it to learn T. rex's body temperature.

The team built in a check against false results. As Smithsonian magazine reported, the researchers compared the dinosaur with animals from the same rocks. Five crocodilian teeth from Hell Creek averaged about 30.9 degrees Celsius (about 88 degrees Fahrenheit), and fossil clams, which record environmental temperatures, averaged about 25.9 degrees Celsius. If burial chemistry had reset the fossils, the crocodilians and T. rex would likely have produced similar readings. Instead, they differed by more than 5 degrees Celsius.

Warmer Than a Crocodile, Cooler Than a Bird

Modern cold-blooded reptiles typically run about 28 to 30 degrees Celsius (82 to 86 degrees Fahrenheit), while most birds run 40 to 43 degrees Celsius (104 to 109 degrees Fahrenheit). T. rex landed in between, near mammals such as humans and elephants. "We found T. rex was 36 degrees Celsius (97 degrees Fahrenheit), about the same as humans," said study co-author Robert Eagle, a UCLA geobiologist.

A regulated, internally generated body temperature supports the view that tyrannosaurs were active animals that chased or scavenged food to fuel a fast metabolism, the UCLA team said. The study measures physiology, not hunting behavior, so it does not settle debates about exactly how T. rex obtained its meals. It does argue strongly against the old image of a sluggish, sun-basking reptile.

Independent paleontologists welcomed the number while placing it in context. Thomas Holtz Jr., a vertebrate paleontologist at the University of Maryland who was not involved in the study, told CNN that scientists have wondered about dinosaur warm-bloodedness "literally since the first time dinosaurs got their name." He said comparing T. rex with crocodiles and clams from the same time and place gives high confidence that the result reflects a genuinely high body temperature. Jasmina Wiemann, a paleobiologist at Johns Hopkins University who also was not involved, praised the result's precision but told the Associated Press, "I almost would have expected a higher body temperature," based on her own earlier work.

A Range Stretching from Mexico to Alaska

The Cretaceous was one of the warmest periods in Earth's history, roughly 11 to 25 degrees Fahrenheit warmer than today. Even so, winters in what is now Alaska would have been too frigid for a cold-blooded animal, said co-author Alessandro Chiarenza, a paleontologist at University College London. He noted that paleontologists have found no fossils of lizards, turtles or crocodiles from Cretaceous Alaska, but they have found Tyrannosaurus fossils there.

Using paleoclimate models, the team mapped where an animal with a 97-degree body could have survived 66 million years ago. The suitable range stretched across North America from Mexico to Alaska.

The evidence has limits worth stating plainly. This is a peer-reviewed measurement study combined with climate modeling, but it rests on three teeth from a single rock formation, two of them from the same animal. The stated uncertainty of one standard error spans roughly 93 to 102 degrees Fahrenheit. The authors say they cannot completely rule out seasonal bias, although samples from different parts of Thomas's teeth produced statistically indistinguishable temperatures. The result applies to T. rex, not all dinosaurs, and it records the temperature at which enamel formed rather than showing how the animal regulated heat day to day. The research was supported in part by the National Science Foundation.

The researchers hope to apply the method to other dinosaurs, and Holtz suggested that groups sometimes considered cooler-blooded, such as Triceratops, Stegosaurus and Brachiosaurus, would be telling next targets. For readers in Southern California, Thomas remains on display at the Natural History Museum of Los Angeles County, now with a measured body temperature to go with its bones.

What Readers Want to Know

What was T. rex's body temperature?

The study estimated 36.3 degrees Celsius, or about 97 degrees Fahrenheit, with an uncertainty of plus or minus 2.5 degrees Celsius. That is close to a typical human body temperature.

How can scientists measure the temperature of an extinct animal?

They analyze how often rare heavy carbon and oxygen isotopes bonded together in tooth enamel as it formed. Those bonds are more common at cooler temperatures, so their abundance works like a thermometer.

Does this prove T. rex was warm-blooded?

It is strong evidence that T. rex kept its body warmer than its surroundings and warmer than the crocodilians living alongside it. The sample is small: three teeth from two animals, and scientists would like more specimens to confirm it.

Why did the museum allow scientists to drill a famous fossil?

UCLA refined the method so it needs only a few milligrams of enamel per measurement. The Natural History Museum of Los Angeles County decided small portions of two undisplayed teeth were a worthwhile trade for the knowledge gained.

Did T. rex really live in Alaska?

Tyrannosaurus fossils have been found in Cretaceous-age rocks in Alaska, according to study co-author Alessandro Chiarenza. A warm body temperature helps explain how it could survive winters too cold for reptiles.

Where can I see the T. rex that was tested?

Thomas the T. rex is displayed in the dinosaur hall at the Natural History Museum of Los Angeles County.