Was T. Rex Warm-Blooded? Its Giant Teeth Offer Evidence

One of the most basic biological questions about dinosaurs is still debated among paleontologists: Were dinosaurs warm-blooded or cold-blooded?  

A century ago, many scientists assumed that all dinosaurs were sluggish, cold-blooded reptiles that lazed in the sun like lizards to regulate their body temperature. But starting in the late 1960s, a more varied picture of dinosaurs started to emerge that included active, birdlike, social creatures that were intelligent and could move quickly to hunt prey.  

It’s now established that modern birds are the surviving lineage of birdlike dinosaurs that survived the mass extinction 66 million years ago. Since birds, like all mammals, are warm-blooded or endothermic—meaning they generate body heat internally through their metabolism—what does that mean for their dinosaur ancestors? 

A 2026 study found that Tyrannosaurus rex, one of the largest carnivores to ever walk the Earth, had a body temperature of around 97.3°F (36.3°C), roughly the same as that of large land mammals like Indian and African elephants. Coupled with other evidence, the new temperature data makes it very likely that T. rex was an endotherm. But does that mean other dinosaurs were warm-blooded, too?  

The T. rex study, led by a research team from UCLA, took samples from the banana-sized teeth of three fossilized T. rex specimens. The scientists analyzed the enamel of the 70-million-year-old Tyrannosaurus teeth, looking for special “clumped” isotopes—different forms of the same chemical element—that function like a “paleothermometer.”

"The abundance of the isotope clumps reflect temperature," says UCLA geoscientist Aradhna Tripati, one of the co-authors of the study, in an email. "The enamel chemistry recorded the temperature at which the enamel formed, and 66 million years later we can read it in fossil teeth."

The UCLA team determined that the average body temperature of the three T. rex specimens was 36.3°C (97.3°F), with a margin of error of 2.5°C, roughly the same as that of large, warm-blooded mammals like elephants and slightly lower than that of flightless birds like ostriches and emus. High body temperature alone isn’t proof that the T. rex was warm-blooded, but the data is the latest in a series of clues that the massive carnivore regulated its temperature like a bird. 

“There's a lot of good evidence at this point that Tyrannosaurus and its relatives were probably endothermic,” says David Gold, a paleobiologist from UC Davis who was not involved with the T. rex study. “Feathers, for example. We don't have any direct evidence that Tyrannosaurus rex itself had feathers, but many of its close cousins had downy feathers as insulation. And animals typically have insulation if they’re regulating their body temperature.” 

Another sign that T. rex was warm-blooded was its geographical range. T. rex fossils have been found as far north as the Alaskan Arctic, where temperatures were certainly warmer 70 million years ago, but still fell below freezing in the winter. If these tyrannosaurs were truly cold-blooded (ectothermic) and relied on their environment to stay warm, they “probably could not have survived in those places,” says Gold.  

Tyrannosaurus rex and its theropod (“beast-footed”) cousins weren’t the only dinosaurs with high body temperatures. There’s evidence that long-necked, plant-eating sauropods, including giant titanosaurs that could reach 70 tons and 120 feet long, also had body temperatures close to 100°F. But does that mean those massive, slow-moving dinosaurs were also warm-blooded? Not necessarily, says Gold. 

First, there are a number of behavioral tactics that cold-blooded animals use to maintain a steady body temperature. Crocodiles, for example, bask in the sun to warm up and retreat into the water to cool down. Long-necked sauropods may have used similar methods to maintain their high internal temperatures, or they could have relied on a different trick. In truth, paleontologists know very little about dinosaur behavior.  

Another theory is that sauropods were in fact cold-blooded, but they were so big that they didn’t have to bother with temperature regulation. It’s a phenomenon called “gigantothermy” that’s found in the biggest modern reptiles like Galapagos tortoises and Komodo dragons.  

“The bigger you get, the smaller your surface area is to your volume,” says Gold. “As you get very, very big, it takes more energy both to heat you up and to cool you down. That's why African elephants have these giant ears that they're flapping. It's an easy way to increase their surface area to help them cool off.” 

Unlike T. rex, long-necked sauropods lived almost exclusively in warm climates. And even if the air temperature dipped, it would take a very long time for their massive bodies to lose heat.  

While the gigantotherm theory makes sense for truly massive dinosaurs, it doesn't hold up in the case of Tyrannosaurus rex, says UCLA geobiologist and study co-author Robert Eagle. If T. rex was an ectotherm (cold-blooded), its body size wouldn't have been large enough to reach such high internal temperatures.

Dinosaurs dominated life on the planet for 170 million years and new species are discovered every year. The sheer diversity of dinosaur ecology required each species to adapt to its environment in unique ways, including how it maintained its temperature. What's clear is that the old "cold-blooded reptile" narrative no longer holds up.

"It's thought that many dinosaurs, but not necessarily all, were warm-blooded," says Eagle in an email. "However, even amongst warm-blooded (or endothermic) organisms there are questions of the degree—they can be warm-blooded like sloths or anteaters (low level endothermy), like modern large mammals, or avian endotherms, which typically have higher metabolic rates than mammals."