Were dinosaurs warm or cold-blooded? Scientists might finally have an answer

Birds, the last surviving dinosaurs, are warm-blooded. But scientists have long debated whether other dinosaurs were also warm-blooded like birds or were cold-blooded like reptiles. Now, in a new study published last week in the journal Nature, researchers find many dinosaurs such as Tyrannosaurus and Brontosaurus may have indeed been warm-blooded, as well as the flying reptiles known as pterosaurs and the Loch Ness Monster-like creatures known as plesiosaurs.

Here’s the background — Birds have the most active metabolisms among living animals. The exceptional degree to which they can burn the fuel stores in their bodies powers the flight seen in most birds, helping them spread around the globe. It also helps them generate body heat. This makes birds endothermic, or warm-blooded, meaning they are able to keep their body temperatures stable despite surrounding temperatures.

When birds evolved such high metabolisms is still unknown. Birds are the last members of the dinosaurs known as theropods, which included famous creatures such Tyrannosaurus and Velociraptor. Prior work found that some extinct theropods may have shared traits once commonly thought unique to birds, such as feathers, raising the question of when bird features such as their high metabolisms or warm-bloodedness arose in dinosaurs.

However, scientists have long argued over whether dinosaurs were warm-blooded or cold-blooded. Birds, the only surviving dinosaurs, are warm-blooded, but the other closest living relatives of dinosaurs, crocodiles, and alligators, are cold-blooded.

Previously, researchers had no clear way of deducing a species' metabolic rate from its fossils. One strategy analyzed the minerals in fossils to estimate the temperatures at which they might have formed, but scientists have found it hard to draw conclusions from this research as it's still unclear how the process of fossilization might affect this mineral data. Another method looked at growth rates, focusing on a series of lines in bones that, like tree rings, correspond to years of growth, but growth rates may have less to do with metabolism than an animal's stage in life.

However, in 2020, molecular paleobiologist Jasmina Wiemann at Yale University and her colleagues revealed that analyzing biomolecules such as proteins, fats, and sugars located within ancient fossils could yield valuable clues on their metabolisms.

"Wiemann is single-handedly taking paleontology into the future and drastically transforming our understanding of past life probably to a greater extent than any other paleontologist before her," Jingmai O'Connor, a vertebrate paleontologist at the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing who did not take part in this research, tells Inverse.

WHAT DID THE SCIENTISTS DO? — In the new study, Wiemann and her colleagues analyzed the femurs of 55 different groups of animals. These included dinosaurs, their flying cousins the pterosaurs, their more distant marine relatives the plesiosaurs, and modern birds, mammals, and lizards.

The strategy the scientists employed is based on chemicals that form when animals breathe. These react with proteins, fats, and sugars, leaving behind waste byproducts that are extremely stable, enough to survive fossilization. These can serve as a record of how much oxygen a creature was using, and thus its metabolic rate.

Using laser-based techniques known as Raman and Fourier-transform infrared spectroscopy, the researchers estimated the abundance of these breathing-related molecular byproducts in fossils without destroying these ancient bones. They compared the amounts of these compounds with the known metabolic rates of the modern animals and used those data to infer the metabolic rates of the extinct ones.

WHAT DID THEY FIND? — The scientists discovered the metabolic rates of dinosaurs were generally high. "This new information regarding metabolic rates will drastically change how we interpret the biology and behavior of many of these extinct groups," O'Connor says.

There are two big groups of dinosaurs, the "bird-hipped" ornithischians, and the "lizard-hipped" saurischians. The bird-hipped dinosaurs included Triceratops, Stegosaurus, and Ankylosaurus. The lizard-hipped dinosaurs included theropods as well as the long-necked, long-tailed giant herbivores known as sauropods, such as the Brontosaurus.

The lizard-hipped dinosaurs were warm-blooded, with some possessing metabolic rates comparable to modern birds, much higher than mammals. Pterosaurs and plesiosaurs may also have been warm-blooded. To maintain their active metabolisms, Wiemann notes all these ancient creatures would likely have had to eat a lot.

The bird-hipped dinosaurs had low metabolic rates, ones comparable to those of cold-blooded modern animals. Like modern cold-blooded reptiles such as lizards and turtles, Wiemann suggests they may have basked in the sun and may have also had to migrate to warmer climates during the cold season. (Intriguingly, their ancestors likely possessed higher metabolic rates, suggesting that bird-hipped dinosaurs reduced their metabolisms over time.)

The new study "is very far-reaching in its findings and number of species examined which makes it exciting," geobiologist Robert Eagle at the University of California, Los Angeles, who did not participate in this study, tells Inverse.

O'Connor adds that "while it is very fascinating to finally know the metabolic rates of enigmatic long-extinct groups like plesiosaurs and pterosaurs, it is the new methods that are most important. Wiemann's ability to harness fossilization byproducts to understand key aspects of extinct animal biology — in this case, directly determining metabolic rate — is truly revolutionary."

The new study did find metabolic variation within groups. For instance, while pterosaurs may often have possessed high metabolisms, the pterosaurs known as Pteranodon may have had relatively low metabolisms. "Not all warm-blooded organisms have the same metabolic rates — for example, the modern hummingbird has a higher metabolic rate than a human, and a human higher than a sloth," Eagle notes.

This discovery raises questions regarding the end of the age of dinosaurs. When it comes to why birds survived when all other dinosaurs went extinct, previous research often suggested the high metabolic rates of birds helped give them a key advantage. However, these new findings suggest that factors other than metabolism may have driven these survivals and extinctions, Wiemann says.

WHAT'S NEXT? — These findings raise a number of new and exciting questions, Wiemann says. For example, what evolutionary pressures might favor the independent development of the high metabolic rates seen in many dinosaurs, marine reptiles, and mammals? In addition, "how did dinosaurs with low metabolic rates sustain their large body size, and did they lead lifestyles more comparable to modern reptiles?" she tells Inverse.

The scientists are now seeking to gather more data on a variety of animals, such as the ancestors of mammals, to trace the evolution of metabolism and warm-bloodedness. "We can soon include amphibians and fish as well," Wiemann says.