Scientists confirm T. rex was warm-blooded, measuring dinosaur's body temperature at 97°F

For nearly six decades, paleontologists have suspected that Tyrannosaurus rex was warm-blooded. The discovery of a fossilized T. rex footprint in Alaska in 2022 strengthened that suspicion—here was evidence that these massive predators could thrive in environments where cold-blooded reptiles would struggle. But suspicion is not certainty. Now, for the first time, researchers have measured the dinosaur's actual body temperature: approximately 97 degrees Fahrenheit, or 36 degrees Celsius, matching the warmth of a living human.

The measurement comes from a study published Wednesday in Science Advances, conducted by a team led by geobiologist Robert Eagle at UCLA and climate scientist Aradhna Tripati, also at UCLA. The researchers analyzed chemical signatures preserved in tooth enamel from Thomas, a roughly 70 percent complete T. rex skeleton housed at the Natural History Museum of Los Angeles County. The work represents the first direct estimate of the dinosaur's body temperature and required more than a decade of methodological refinement. The scientists had to reduce the amount of fossil material needed by roughly 90 percent before the museum would allow them to sample two teeth that were not on public display. "Nobody can measure a T. rex tooth unless you can show them you only need a few milligrams," Tripati explained.

The technique works by measuring isotopes—different forms of carbon and oxygen—that form chemical bonds within tooth enamel at rates determined by temperature. Cooler conditions produce more bonds; warmer ones produce fewer. By extracting a few milligrams of enamel and analyzing those bonds, the researchers could determine the temperature at which the enamel formed, effectively reading a 66-million-year-old thermometer. Teeth work best for this purpose because their hard enamel resists chemical change over geological time, unlike other skeletal material.

The finding places T. rex in a distinct metabolic category. At 97 degrees Fahrenheit, the dinosaur ran warmer than modern cold-blooded reptiles, which typically maintain temperatures between 82 and 86 degrees Fahrenheit. Yet it fell short of modern birds, some of which exceed 104 degrees Fahrenheit. This intermediate position makes sense given evolutionary history: dinosaurs, though classified as reptiles, share a lineage with birds. The warm body temperature had immediate behavioral implications. A T. rex maintaining such heat could sustain activity over extended periods—not the explosive sprints of a cold-blooded crocodile, which tires quickly, but the sustained pursuit and endurance of a metabolically active predator. It also meant the dinosaur could inhabit cooler climates where sun-basking reptiles could not survive. Paleoclimate models suggest T. rex could have ranged across a broad swath of North America, from present-day Mexico to Alaska, during the late Cretaceous period 66 million years ago.

The high body temperature carried another cost: T. rex would have needed to hunt and consume more prey than cold-blooded competitors to fuel its metabolism. This reshapes how scientists understand the dinosaur's place in its ecosystem—not as an opportunistic scavenger or a sluggish ambush predator, but as an active, energetically demanding hunter.

Vertebrate paleontologist Thomas Holtz Jr. of the University of Maryland, who was not involved in the study, called the new method a significant advance. While previous evidence—bone structure, biomechanics, the Arctic footprint—had suggested warm-bloodedness, this direct measurement offers something earlier research could not: a specific number that can be compared against the body temperatures of other animals from the same time and place. Comparing T. rex's 97 degrees with the temperatures of cold-blooded crocodiles and clams from the same era "gives high confidence that the values they find for Tyrannosaurus really show a high body temperature," Holtz said.

The technique opens new avenues for paleontology. Researchers have already applied it to other extinct species, including woolly mammoths and megalodons. In the future, scientists hope to measure the body temperatures of other dinosaurs—Triceratops, Stegosaurus, Brachiosaurus—to determine whether warm-bloodedness was universal among dinosaurs or varied by species. Eagle expressed particular interest in studying the ancestors of mammals, a lineage where the timing and emergence of warm-bloodedness remains poorly understood. "That's in itself an extra challenge because then we're getting even further back in time," he said. "But that would be really exciting if that works."