A new tooth test suggests T. rex was not cold-blooded like modern reptiles; the giant predator’s body temperature was estimated at a human-like 97°F

For years, scientists have debated one basic question: was T. rex warm-blooded, as we are, or cold-blooded, as lizards are? A new study adds direct physical evidence to that debate. The study, ‘The body temperature of Tyrannosaurus rex,’ published in Science Advances on September 16, 2026, found that T. rex had a body temperature of about 36°C (97°F), roughly the same as a human being. The research, conducted by geobiologists at UCLA, provides this figure from direct evidence in T. rex tooth fossils rather than from guesswork or growth models. This appears to be the first direct temperature estimate from a T. rex tooth, according to the study.

How scientists read a temperature from a tooth

Getting this reading has been no easy feat. T. rex fossils are rare, and museums are usually unwilling to allow researchers to dissect such specimens. This is why the UCLA team spent years developing a method that used 90% less material than earlier approaches. This innovation made the Natural History Museum of Los Angeles County agree to provide some tissue samples from the teeth of “Thomas the T. rex,” one of the most complete tyrannosaur skeletons on display there.


The science behind it comes down to chemistry. Tooth enamel contains rare isotopes of both carbon and oxygen. The atoms are more likely to bond when tooth enamel forms in a cooler body than in a warmer one. Using a mass spectrometer to count these atoms, it's possible to determine body temperature. The study notes this method builds on older work: a 2011 study published in Science titled 'Dinosaur Body Temperatures Determined from Isotopic (13C-18O) Ordering in Fossil Biominerals' in which the researchers analyzed fossilized teeth from Jurassic sauropods, including Brachiosaurus brancai and Camarasaurus, using the same isotope analysis technique. They found that these animals had body temperatures around 97°F and 100°F, similar to most modern mammals. It's worth noting that in sauropods that size, elevated body temperature is often attributed at least partly to gigantothermy, sheer body mass retaining heat passively, rather than the same kind of active, mammal-like metabolism now inferred for the much smaller T. rex. That earlier paper helped show the method could work on animals that died tens of millions of years ago.

A temperature between reptile and bird

So where does 97°F fit in? Cold-blooded reptiles typically run between 82 and 86 degrees Fahrenheit today and slow down in cold weather. Birds, the living relatives of T. rex, run much hotter, often 104°F to 109°F. The study says T. rex’s temperature fell between typical reptile and bird ranges. To prove that this difference is not due to fossilization, researchers also tested ancient crocodile teeth found at the same site in Hell Creek, Montana. These teeth registered temperatures closer to 86°F. If fossilization were driving the readings, both creatures would be expected to show more similar results. This gap supports the T. rex result.

Why a warmer body makes sense

The 97°F body temperature suggests that the dinosaur would need to burn a lot of energy. This indicates that the animal could be one that was able to run fast and for a long time, rather than one that just waited for its prey. This supports the researchers' interpretation that T. rex was an active predator or at least an energetic scavenger, rather than a sluggish reptile-like animal. This theory might also help explain a riddle that has puzzled paleontologists for decades. Fossils of T. rex have been found as far north as Alaska, but there is no evidence that lizards, turtles, or crocodiles lived there during that era. In the Cretaceous era, even if the days were warm, the winters in Alaska were too harsh for any cold-blooded animal to survive. But for a warm-blooded T. rex, that might not have been a problem.

This is not the first sign that ancient Alaska had tougher predators than people used to think. A 2014 study in PLOS ONE described Nanuqsaurus hoglundi as a smaller cousin of T. rex that lived on the North Slope of Alaska about 70 million years ago. Its presence suggested that tyrannosaurs could live in cold, high-latitude conditions that were difficult for many reptiles. The new temperature data provides a physical reason for why that may have been possible.

What this does, and does not prove

It helps to be careful here. One reading taken from one dinosaur doesn't explain the whole story of its life. The measurement is a snapshot of what the tooth enamel captured while it formed, not a lifelong record of T. rex's temperature. The researchers stop short of claiming that T. rex was warm-blooded exactly like mammals. The data show that its body was significantly warmer than a reptile and more in line with what you'd expect from an active, energetic animal. Even so, this is a real number, calculated from measurements rather than estimation based on the rate of bone growth or muscle mass estimations. For an animal extinct for 66 million years, this was a notable result. Scientists now have a temperature estimate for the King of the Tyrant Lizards, and it suggests T. rex was about as warm as humans.