One of the oldest questions in paleontology has finally been answered.

For decades scientists debated a pressing question: were dinosaurs warm-blooded, like birds, or cold-blooded, like reptiles? Since dinosaurs were related to both, paleontologists couldn’t stop talking about that question. A new study now seems to have put an end to this long-standing mystery. Because researchers are revealing compelling evidence that, among other things, the terrifying T. rex was more like us than we thought.

What does warm-blooded and cold-blooded mean?
Animals with a high metabolism are warm-blooded. Warm-blooded animals – think of birds and mammals – take in a lot of oxygen and have to burn a lot of calories to maintain their body temperature. Cold-blooded animals – such as reptiles – breathe and eat less. Their lifestyle is energetically ‘cheaper’ than that of a warm-blooded animal, but it comes with a price: cold-blooded animals depend on the temperature of their environment for their body temperature. For example, when a lizard wants to raise its body temperature, it takes a seat in the sun to warm up. Cold-blooded animals tend to be less active than their warm-blooded counterparts.

According to researchers, it’s very important to find out whether dinosaurs were warm-blooded or cold-blooded — and thus how quickly their metabolism could convert oxygen into energy. This also provides more insight into how active these animals were and what their daily lives looked like. However, it has not been so easy to unravel this mystery. Existing methods to determine warm- or cold-bloodedness of long-extinct animals were not conclusive. But in a new study, researchers took a different tack.

New method

To study the metabolism of dinosaurs, the research team did not look – like previous studies – at minerals present in bone or how fast a dinosaur grew, but at one of the most fundamental features of metabolism: oxygen use. When animals breathe, certain substances are produced that react with proteins, sugars and lipids, leaving behind molecular ‘waste’. This waste is extremely stable and insoluble in water, meaning it is preserved during fossilization. And that’s interesting. Because this waste can therefore provide insight into how much oxygen a dinosaur breathed. And then that again reveals secrets about his metabolism.


The researchers looked for bits of molecular debris in dark-colored, fossilized femurs of 55 different groups of animals, including dinosaurs, their flying cousins ​​the pterosaurs, their even more distant marine relatives the plesiosaurs and modern-day birds, mammals and lizards. They compared the amount of respiratory-related molecular byproducts to the known metabolic rates of the living animals and used that data to derive the metabolic rates of the extinct animals.


The study solves the long-standing mystery about whether dinosaurs were warm- or cold-blooded. There are in fact two major groups of dinosaurs, the Saurischia (the group of dinosaurs whose pelvises usually resemble that of reptiles) and the ornithischia (the group of dinosaurs whose pelvises usually resemble that of birds). The findings now show that dinosaurs with “reptile jaws” — think the horned Triceratops and spiny Stegosaurus — had low metabolisms, comparable to those of cold-blooded, modern-day animals. But beaked dinosaurs, including theropods and sauropods—think bipedal and predatory dinosaurs like the Velociraptor and the famed T. rex, as well as giant longnecks like the Brachiosaurus—were warm-blooded. And that only means one thing: most dinosaurs were probably warm-blooded, just like humans. “This is really exciting,” said researcher Jasmina Wiemann. “Whether dinosaurs were warm-blooded or cold-blooded is one of the oldest questions in paleontology. And now we think we know that most dinosaurs were warm-blooded.”

Surprisingly, some warm-blooded dinosaurs also had much higher metabolisms than mammals. In fact, their metabolic rate was comparable to that of modern-day birds. And that puts previous theories that pointed to such trends more firmly in the shoes.

New insights

The results provide fundamentally new insight into dinosaur life, the researchers argue. “Dinosaurs with lower metabolic rates would have been dependent to some extent on external temperatures,” Wiemann explains. “Lizards and turtles often lie in the sun. And maybe we should have similar ‘behavioural’ thermoregulation in dinosaurs that belong to the ornithischia belong, consider. In addition, cold-blooded dinosaurs probably had to migrate to warmer climates during the cold season. So the climate may have been a selective factor for where some of these dinosaurs could live.” On the other hand, the warm-blooded dinosaurs would have been more active, but also had to eat more. “The warm-blooded giant sauropods were herbivores and may have fed on many plants,” Wiemann continues. “They had a very efficient digestive system. And because they were so big, it was probably a bigger problem to cool down than to warm up.”

The new study can be considered groundbreaking. “The new method allows us to directly infer the metabolism of extinct organisms, something we only dreamed of a few years ago,” said researcher Matteo Fabbri. It also provides an interesting new insight. Some scientists have suggested that unlike dinosaurs, birds survived the disastrous mass extinction of 65 million years ago because of their increased metabolism. But the new study banishes that theory to the realm of myth. “Many dinosaurs with exceptionally high metabolisms also became extinct,” Wiemann says. And that also gives us a glimpse into the mass extinction that we are currently heading for. It shows that having a high metabolism does not always guarantee success.