Isotope clumping occurs because heavier carbon‑13 prefers to bond with heavier oxygen‑18 at lower temperatures. By comparing the clumping level in modern animals of known body temperature, scientists can infer the temperature of extinct species.

Tripati’s group first refined the method with abundant megalodon shark teeth and earlier dinosaur‑eggshell studies. Those experiments proved the technique works on very small samples before it was applied to the fragile T. rex enamel.

Because tyrannosaur teeth are thin, the museum allowed the researchers to shave only a tiny “pinch of salt” from two teeth and use a fragment from a third specimen. This provided enough material for the isotope analysis without damaging the fossils.

The analysis gave a body‑temperature estimate of 33.8 °C to 38.8 °C for the T. rex, a range that matches large mammals such as elephants and large, possibly flightless birds like ostriches, according to Tripati.

Paleontologists such as Anthony Fiorillo and François Therrien note that a warm‑blooded T. rex could have been active day and night, lived in high‑latitude regions, and moved across continents—behaviour more like modern birds and mammals than a sluggish reptile.