Were dinosaurs hot or not?

Dinosaur Mysteries digs into the secretive side of the “terrible lizards” and all the questions that keep paleontologists up at night.

WE’RE MORE THAN 66 million years too late to take a dinosaur’s temperature the old-fashioned way. Paleontologists can’t put a Tyrannosaurus rex under anesthesia and leave a wax-sealed thermometer inside the dinosaur’s body, as some zoologists did to track crocodile temperatures. While experts think that nonavian dinosaurs were active, fast-growing creatures with body temperatures that were higher than their surroundings, Dinosaur Physiology 101 is still a subject with more questions than answers. Those answers can shed light on everything from how much dinosaurs needed to eat each day to how they withstood environments ranging from Arctic forests to searing deserts.

Paleontologists have gone back and forth over the years on whether dinosaurs were warm-blooded or cold-blooded. During the late 19^th century, some scholars aptly guessed that dinosaurs were agile and on the move most of their lives. Their deductions were based in part on the limb proportions of prehistoric reptiles like the shovel-beaked Hadrosaurus foulkii and the sharp-toothed tyrannosaur Dryptosaurus aquilunguis, which indicated that these were busy animals rather than giant meandering lizards. But as the field of paleontology shifted its focus to evolutionary questions, researchers became more interested in the abundant, familiar fossil mammals rather than the strange and sometimes inscrutable dinosaurs. It took until the late 1960s—and the discovery of Deinonychus antirrhopus, the inspiration for Jurassic Park’s velociraptor—for paleontologists to revisit the idea that dinosaurs ran hot. The debate is still raging today.

Of course, it isn’t a simple question in the first place. The idea of being either warm-blooded or cold-blooded—a binary choice between one physiological profile and another—is a pretty flawed concept, limited to an organism’s body temperature rather than how its metabolism and physiology regulate that temperature. Consider a horned lizard scurrying around in the desert as the sun is coming up. Because it is “cold-blooded,” or what zoologists call ectothermic, the lizard’s body temperature shifts with its surrounding environment. This means the reptile will probably act sluggish in the cool of the morning but get much more active as the sun warms its spiky little body. “Warm-blooded” or endothermic creatures like humans, which maintain a near-constant and elevated body temperature, don’t have to thaw out in the sun, but also have to be careful not to overheat or exceed the narrow temperature range they can properly function in.

Then there’s the in-between. Some animals, including great white sharks, keep their bodies warmer than the surrounding seawater but still experience significant shifts in temperature as they move between warm and cold waters. Then there are marsupials that undergo daily torpor, a period when they are fully awake but physically inactive to conserve energy. Studies of a small, shrewlike animal called a planigale, for example, found that its body temperature dropped at least 6°F for up to 12 hours a day.

Even with all the wishy-washiness, there are some set biological factors that play into managing heat and related body functions. Mass seems to be a big one. In 1946, paleontologist Edwin Colbert and colleagues conducted a set of cruel experiments on alligators, exposing them to direct sunlight in apparatus that positioned the reptiles like tail-dragging dinosaurs. The larger alligators changed temperature more slowly, owing to the relationship between volume and surface area, but the smaller study animals had a harder time regulating their body temperatures. A few even perished from getting too hot in the unrelenting sun. But the study, and many others that came after it, showed that more mass means body temperatures change more slowly, requiring bigger animals to adopt strategies to gain or dump heat so they can stay comfortable.

Dinosaurs, like modern beasts, probably didn’t share one standard physiological profile. After all, the word dinosaur encompasses a huge, still-living group of animals of different shapes and sizes that have been around for more than 200 million years. But to investigate the internal thermostats of long-dead species like Triceratops and Allosaurus, paleontologists had to get inventive.

Some researchers have attempted to take dinosaur temperatures directly from fossilized bones. Organisms take in isotopes of oxygen when they drink water; as the element is incorporated into bones and teeth, its geochemical signature is modified by the body’s temperature. After applying the technique to T. rex, a 1994 study concluded that the dinosaur’s oxygen isotopes looked like those of a homeotherm—an animal that maintains a near-constant body temperature, usually as a result of generating internal body heat. But other experts questioned the results, and later analysis proposed that while T. rex generated its heat internally, it may have had a fluctuating body temperature like great white sharks today.

Interpreting the microscopic details of dinosaur skeletons has also been challenging because experts are still learning how physiology affects bone growth in living animals. Many ancient reptilian fossils show lines of arrested growth, or LAGs, created when the organisms periodically stopped maturing. These bone rings were once taken as a sign that nonavian dinosaurs periodically halted their growth, like many modern reptiles, and therefore might have been cold-blooded. But a 2012 study found that modern mammals create LAGs in their bones during cold seasons when food is scarce and going through a growth spurt isn’t the best option. Still, various studies of dinosaur growth rates have generally found that the  “terrible lizards” developed faster than living reptiles and even at rates comparable to those of living mammals. But the correlation is vaguer than the detailed information scientists have been seeking from fossils.

Paleontologists are still scraping together clues using various techniques as they try to come up with the big picture on body temperatures. One study from 2022 looked at biological markers of metabolic stress preserved in dinosaur bones that can act as clues to an animal’s metabolism. Big names like Tyrannosaurus and Brachiosaurus more closely matched endothermic animals studied today, while those like Stegosaurus and Triceratops seemed to have evolved from an endothermic ancestor but switched to be more ectothermic. Then again, a different analysis that looked at where long-necked dinosaurs like Brachiosaurus lived indicated that these dinosaurs never inhabited cold, polar environments, hinting that they had variable body temperatures and couldn’t withstand the cold like carnivorous theropods. In short, the available evidence doesn’t offer a straight answer just yet.

The good news is that we’re not teetering on the edge of a total dinosaur rethink. Experts have uncovered plenty of proof that our Mesozoic favorites were behaviorally complex animals that locked horns, bit each other on the face, dug after prey hiding in their burrows, and sometimes moved together in social groups. Dinosaurs generally grew up fast and, one way or another, maintained high body temperatures that required a lot of food to fuel. It’s the specific details that remain unclear, and they may be as different between any two dinosaur species as they are between any two mammal or bird species alive today.

We hope you enjoyed Riley Black’s column, Dinosaur Mysteries. Check back on PopSci+ in July for the next article.