Scientists have effectively taken the temperature of a Tyrannosaurus rex, and the reading is surprisingly familiar. Analysis of a 66-million-year-old tooth shows that the fearsome predator ran at about 97.3 degrees Fahrenheit, or 36.3 degrees Celsius, remarkably close to the human body temperature.

The finding, published in the journal Science Advances, adds hard evidence to the long debate over whether dinosaurs were cold-blooded like modern reptiles or warm-blooded like birds and mammals.

How they took a dinosaur’s temperature

Researchers at the University of California, Los Angeles, developed a technique that reads body temperature from the chemistry of tooth enamel. Rare isotopes of carbon and oxygen form bonds in the enamel, and the number of these bonds depends on temperature: fewer bonds mean a warmer animal.

Tooth enamel was chosen because its large crystal structures resist chemical change over millions of years, unlike bone, which is constantly remodelled during life. The team needed only about five milligrams of enamel powder per tooth, less than a pinch of salt, using a minimally destructive drilling technique refined over a decade.

What the numbers mean

The three T. rex teeth, unearthed in Montana’s Carter County and held by the Natural History Museum of Los Angeles County, came from a well-preserved young adult nicknamed Thomas and one other individual. Their average reading of 36.3 degrees Celsius, plus or minus 2.5 degrees, sits close to the elephant’s 97 degrees Fahrenheit and not far from the human average of 98.6.

For comparison, modern cold-blooded reptiles run at about 82 to 86 degrees Fahrenheit, while birds, the dinosaurs’ living descendants, often run much hotter at 104 to 109 degrees. Crocodile teeth from the same rock formation averaged just 87.6 degrees, confirming that the method can tell warm from cold.

A new picture of the predator

A warm-blooded T. rex would have been an active, fast-metabolised hunter rather than a sluggish animal waiting for the sun to warm it up. The study’s senior author, UCLA geoscientist Aradhna Tripati, noted that such an animal is thermally independent of its surroundings, closer in its heat regulation to an elephant or an ostrich than to a crocodile.

The finding also helps explain how tyrannosaurs thrived as far north as present-day Canada, enduring freezing conditions that would have stopped a cold-blooded reptile. It paints the most famous dinosaur of all as a far more mobile and adaptable animal than the old image suggests.