For more than a century, since its fossil discovery around the early 1900s, scientists have come to understand and reconstruct everything we know about the Tyrannosaurus rex.
Through the discovery of its ancient bones and teeth, we learned how enormous it could become, how powerful its bite and jaws are, what its diet may have been, and even its total estimated population.
However, one basic fundamental question has remained difficult to answer: What is the body temperature of a living, breathing T. rex?
Being able to answer this question will not only give us an idea of how hot it actually was, but also tell us more about the T. rex way of living, range of habitat, and survival problems it may have encountered.
Now, a new discovery has been made after scientists examined the chemical signatures preserved inside fossilized T. rex teeth. Researchers estimated that the giant predator was able to maintain a body temperature of approximately 36.3 Celsius or 97.3 Fahrenheit, which is remarkably close to our own.
A discovery that would help us create a more complete picture of the life of one of the most well-known predators to have ever lived on Earth, the Tyrant Lizard King, Tyrannosaurus rex.
Scientists Found a 66-Million-Year-Old Thermometer Inside T. rex Teeth
We obviously can’t place a thermometer on an extinct animal that lived over 66 million years ago. So, to discover its actual body temperatures, scientists have to be creative, and the answer they were looking for turns out to be hidden inside the dinosaur’s teeth.
Researchers analyzed three fossilized T. rex teeth that were recovered from the late Cretaceous Hell Creek Formation in Montana. Two of those teeth belonged to a T. rex named Thomas, while the other was from a different individual.
Thomas was estimated to weigh more than 3,000 kilograms if it were alive today. But now, its fossils are preserved in the Natural History Museum of Los Angeles County.
Instead of just looking at the shape and structure of the teeth, the scientists examined their chemistry. They used a technique known as clumped isotope thermometry.
They researched the animal’s tooth enamel formation. During this process, different isotopes of carbon and oxygen become bonded together, and the frequency at which heavy isotopes bond with one another can depend partly on temperature.
Scientists found that lower temperatures generally encourage more of the heavy isotope bondings, while higher temperatures lead to fewer of these particular combinations.
Then once the enamel has formed, those same chemical patterns will be stored inside the teeth. In other words, the process essentially created a tiny chemical record of the temperature at which the tooth was developed.
Scientists can therefore analyze those isotope combinations millions and millions of years later, which allows them to estimate the body temperature of the animal when its teeth were forming.
The researchers also tried to look for any signs of tampering or chemical alteration after the burial and fossilization of the teeth, but evidence points to the idea that the original temperature was most likely preserved.
This is also not the first time the scientific technique was used, as it was also applied to other extinct animals. However, researchers have yet to obtain such a direct temperature measurement of a T. rex until today.
Then, when they were finally able to get it, the results were surprising.
T. rex Was Running at Around 97 Degrees Fahrenheit
The three examined T. rex teeth did not produce the exact same results, but they were all not very far from each other.
The two teeth that belonged to Thomas produced estimates of approximately 37.3°C and 35.9°C, while the second T. rex specimen’s teeth produced an estimate of approximately 34.7°C.
Then scientists averaged them and got to approximately 97.3°F or 36.3°C.
These estimates are relatively close to that of modern human beings. A normal body temperature for us would be around 37°C or 98.6°F, which is not far away at all.
For context, modern birds tend to run hotter. An example would be an ostrich, which has a 38.1°C to 39.7°C body temperature, with many smaller flying birds reaching up to upwards of 40°C.
In other words, T. rex was not running quite as hot as many modern birds, but it is actually still significantly warmer than our old estimates from what we would expect from many modern reptiles.
This discovery matters because for much of the history of Tyrannosaurus, these dinosaurs were essentially imagined as larger reptiles whose body temperatures depended heavily on their environments.
But the study showed us a very different picture, implying that the T. rex appears to have maintained a high internal body temperature, similar to warm-blooded animals of the current world.
If the Tyrannosaurus rex actually had this body temperature when it was alive, it would tell us much more about the places it could have lived and its way of life there.
But before that, scientists wanted another way to test this theory, to know that it isn’t just because these specific T. rex specimens lived in a warm environment.
So, they compared the T. rex with the other reptile species that lived alongside it.
Crocodilians Living Beside T. rex Were Significantly Colder
The Hell Creek Formation in Montana wasn’t only inhabited by dinosaurs tens of millions of years ago. Ancient crocodilians also lived in the same environment. This phenomenon gives researchers a very useful comparison.
Because if the T. rex body temperature, as recorded from the specimens used in the study, was just hot because of its environment, then other large reptiles living in the same place might be expected to produce similar results, a temperature close to 97.3°F or 36.3°C.
To test this idea, scientists analyzed five fossilized crocodilian teeth from the Hell Creek Formation using the very same technique.
However, the result of the study showed that they had an average temperature of approximately 30.9°C, which is considerably lower than the 36.3°C estimated for the T. rex.
These differences are statistically significant. But researchers compared another factor to their estimates, which is the environmental temperatures of the Late Cretaceous, the era from which the teeth in the studies were formed.
Once again, the T. rex appeared to be warmer than its natural surroundings.
These two comparisons were necessary because a huge animal can simply retain heat because of its massive size. A phenomenon often observed in cases of gigantism. Basically, it suggests that the bigger the animal, the longer it can take for its body to gain or lose heat.
So, the new evidence showed that the T. rex’s temperature was both elevated above other crocodilian species living alongside it and above its natural surroundings.
In simpler words, the study’s results support the idea that the Tyrannosaurus rex actively maintained a warm internal body temperature. This meant that it is not just a cold-blooded reptile that experts have thought of it before, but instead an animal possessing the physiology to generate and regulate its own body heat.
A discovery that may help explain a scientific mystery found in the far north that has been puzzling scientists since its inception.
Tyrannosaurs Were Living in Places That Dropped Below Freezing
Tyrannosaurs aren’t only restricted to warm and tropical environments. Fossils belonging to young Tyrannosaurids have been found in northern Alaska. A location far closer to the pole than the Hell Creek Formation in Montana, where the T. rex teeth used in the new study were found.
Those environments weren’t tropical paradises, but instead, their temperature could fall below freezing point at certain times of the year.
The puzzling and important part is the discovery of the young dinosaurs in these northern locations.
Because if a group of dinosaurs was merely migrating during the summer seasons to the north, then returning to the south once the winter came close, then scientists wouldn’t have expected to find evidence that they were reproducing there.
In other words, the presence of young Tyrannosaurids in the area suggests the possibility that some populations weren’t only passing by, but had actually remained and survived at these high latitudes throughout different seasons of the year.
This meant that they survived long seasons of freezing cold temperatures and darkness.
However, for a large reptile whose body temperature is completely dependent on its surroundings having warmth and sunlight, those conditions could pose some serious problems.
But an animal capable of generating and maintaining its own body heat at around 97.3°F or 36.3°C would have had a major advantage that could allow it to thrive in these same environments.
Because even if the air around it becomes cold and the winter seasons make it near or pass the freezing point, a warm-blooded animal could be able to continue to keep its muscles, organs, and nervous system operating at relatively high temperatures.
These findings don’t particularly mean that the T. rex individuals in the study were walking in the Alaskan snow. Their teeth were recovered in Montana.
However, the discovery of young dinosaur fossils in northern latitudes suggests that their measured physiology provides another piece of evidence for understanding how tyrannosaurs as a group could tolerate living in cold environments.
And when scientists incorporate these findings together with the estimated body temperatures, the potential range of T. rex’s habitat becomes enormous.
A Warm Body May Have Allowed T. rex to Roam Across Much of North America
After the estimation of the T. rex body temperature, the researchers went up another level. They combined the physiological information with simulation of the Late Cretaceous climate to try to find out where else the T. rex could have potentially survived.
The results suggested that the suitable thermal environments could have extended across most of the North American paleocontinent, which includes very cold regions at higher latitudes.
These findings meant that T. rex had an important advantage that cold-blooded animals didn’t.
Being a cold-blooded species means that you are heavily dependent on your environment for heat. When temperatures outside naturally fall, their metabolism and activity slow down dramatically.
However, an endothermic animal, or warm-blooded animal, has more independence and flexibility.
It can generate heat internally, which allows it to be capable of living in a much broader range of environmental temperatures.
It does not mean that the animal is immune to extreme levels of cold. But generating and maintaining a temperature around 36°C could have still opened up habitats for T. rex that were unavailable to reptiles with more environmentally dependent body temperatures.
Additionally, its enormous size could have helped as well.
A full-grown adult T. rex, which could weigh about 6 to 8 tons or 12,000 to 16,000 pounds, would lose heat much more slowly than a smaller animal. This is because it possessed a relatively smaller surface area compared to its enormous body volume.
Then, combined with internally generated heat, inferring that a T. rex is a formidable cold -tolerant predator becomes completely plausible.
The result is an animal capable of remaining active across diverse environments ranging from comparatively hot regions to places experiencing really cold seasons.
However, warm blood comes with a high cost.
Because keeping a multi-ton predator heated continuously requires a whole lot of energy.
Being Warm-Blooded May Have Made T. rex a Hungrier Predator
To put it simply, generating your own body heat is metabolically expensive.
Modern humans constantly burn energy simply to remain alive and maintain our bodily functions, as well as internal temperature. Mammals and birds need regular access to food partly because their metabolic processes in the body require a continuous supply of energy.
A giant T. rex, if it really is warm-blooded, would have faced the same fundamental problem but on a much bigger scale.
If the dinosaur maintained an elevated temperature through endothermy, one that is above its natural environmental temperature, it would have required much more energy than an equivalently sized cold-blooded animal.
In other words, T. rex needed food, a lot of it.
This physiological feature actually fits the modern interpretation of T. rex, an active animal instead of a slow, big reptile imagined during the early ages of its discovery.
A warm body means muscles are ready to do an activity instead of waiting for the sun to heat the animal first. It can also support sustained movement across changing environmental conditions.
However, one caveat of the study is that it does not tell the scientists how active the T. rex really was, or how much it hunted versus scavenged.
An important detail to note, that body temperature alone is not enough to reveal every detail of an animal’s metabolism or behavior.
Still, when it is combined with other evidence, a temperature of 36°C adds another piece to the puzzle, giving us a more complete picture. That the T. rex may have perhaps wasn’t just waiting for the Cretaceous sun to heat its body.
It appears to be capable of maintaining a temperature significantly above its surroundings.
And surprisingly, scientists have been finding hints of this discovery for decades before the new study.
Scientists Suspected T. rex Was Warm-Blooded Long Before This Discovery
Early reconstructions often portrayed dinosaurs as sluggish, cold-blooded giant reptiles.
But as paleontologists discovered more about its anatomy, growth, behavior, and even their evolutionary relationships with modern birds, that simple picture of their portrayal began to change.
In 1994, researchers examined oxygen isotopes preserved in a T. rex skeleton and found, surprisingly, little temperature variation across different parts of its body. It suggests that the dinosaur was capable of maintaining its temperature relatively stable.
But there was still a missing piece: scientists do not have an estimate of what that temperature they are capable of maintaining is.
However, because of the new study, they could estimate a number and not just conclude that the dinosaur was warmer than its surroundings or that it is able to maintain a relatively stable temperature.
That number, 36.3°C, allowed scientists to have something they can compare to other ancient animals, ancient environmental temperatures, and scientific climate models.
Moreover, it demonstrates how these fossils contain much more than the eye can perceive.
A T. rex tooth doesn’t merely tell us what its mouth looked like. It also carried on historical chemistry preserved in its enamel that gives us a glimpse of what was happening inside its body tens of millions of years ago.
T. rex May Have Been Far Less Reptile-Like Than We Once Imagined
Tyrannosaurus rex disappeared approximately 66 million years ago.
Its muscles, blood, organs, and body temperature vanished together with it. However, tiny chemical bonds inside fossilized teeth have now allowed scientists to estimate something so fleeting such as heat and temperature inside a living organism.
The answer is remarkably close to ours, modern humans: around 36.3°C, or 97.3°F.
It’s a temperature close to our own bodies, and very similar to huge modern mammals such as elephants. But more importantly, it seems capable of maintaining this elevated temperature independently of its surroundings.
Ancient crocodilians living in the same environment as it is produced significantly lower temperature estimates. The climate models also suggest that the T. rex had a warmer body temperature relative to its environment around it.
Finally, Tyrannosaur relatives seemed to have managed to inhabit and survive in northern environments where seasonal temperatures could go down to freezing point.
Putting all these pieces together, we end up with a completely different picture of a Tyrannosaurus rex.
It may not have just been a giant lizard waiting for the sun to heat up its body, being sluggish and slow.
But instead, it could be an enormous predator capable of generating its own body heat and living actively across a diverse range of environmental climates.
Its skull, head, and powerful bite made the Tyrant Lizard King famous and physically terrifying. But its physiology may have been just as important to its success as a species.
For decades, scientists also already suspected that these dinosaurs ran hot. Now, after approximately 66 million years, its fossilized teeth gave us as close an answer as we could perhaps get to.
And surprisingly, T. rex may have felt almost as warm as we do.
Read next: Scientists May Have Finally Figured Out Why T. rex Had Such Tiny Arms
References & Further Reading
Flores, R. J., Eagle, R. A., Trayler, R. B., Larocca Conte, G., Kim, S. L., Chiarenza, A. A., Farnsworth, A., Valdes, P. J., Chiappe, L., & Tripati, A. (2026). The body temperature of Tyrannosaurus rex. Science Advances, 12(38), eaeb7653. DOI: 10.1126/sciadv.aeb7653. The researchers used clumped isotopes from T. rex tooth enamel to estimate a body temperature of 36.3 ± 2.5°C and modeled its potential thermal habitat. https://pubmed.ncbi.nlm.nih.gov/42748265/
UCLA. (2026). Scientists finally know the body temperature of the Tyrannosaurus rex. Explains the tooth-enamel technique, the approximately 97°F estimate, and what the result may mean for T. rex physiology and activity. https://newsroom.ucla.edu/releases/scientists-finally-know-the-body-temperature-of-the-tyrannosaurus-rex
Barrick, R. E., & Showers, W. J. (1994). Thermophysiology of Tyrannosaurus rex: Evidence from oxygen isotopes. Science, 265(5169), 222–224. Earlier isotope evidence suggested T. rex maintained a relatively stable internal temperature. https://pubmed.ncbi.nlm.nih.gov/17750663/
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