Why the Hypothesis That All Dinosaurs Were Cold Blooded Could Be Completely Wrong

A new study has called into question the long-held belief that dinosaurs were cold-blooded. What does this mean for our understanding of these ancient creatures?

When the remains of the first dinosaur were discovered in the mid-nineteenth century, paleontologists assumed the beasts were nothing more than huge lumbering lizards. For decades various paleontologists couldn’t come to a consensus on their metabolism.

Some paleontologists believe that all dinosaurs were “warm-blooded,” like birds and mammals. Some believe that no dinosaur could have such a rapid metabolism. Like some sea turtles today, some scientists believe that huge dinosaurs may have had warm bodies due to their large body size. Scientists also assumed that dinosaurs were similar to modern-day cold-blooded reptiles in that their body temperature was affected by their surroundings. The issue is that it’s difficult to find proof that revealed exactly what dinosaur metabolisms were like. This theory, however, was later hotly contested.

768px 047 bernarro gr
Exceptionally preserved articulated sauropod dinosaurhttps://commons.wikimedia.org/wiki/File:047-bernarro_gr.jpg by Titan2155 under CC BY-SA 4.0

Jasmina Wiemann, a molecular paleobiologist (paleobiologist examine fossilized remains of dead biological life) at the California Institute of Technology, says, “The broad picture of dinosaur metabolism has evolved quite a bit in the past decades.” “Our knowledge of what dinosaurs looked like and how they lived is strongly tied to whether they were cold-blooded, warm-blooded, or somewhere in between,” she stated. 


A recent paper published on dinosaur physiology

Wiemann and her team published a paper on dinosaur physiology in Nature on May 25 that proves something we “knew” wasn’t true. The study casts doubt on the idea that dinosaurs were cold-blooded reptiles, implying that dinosaurs’ ancestors were warm-blooded or capable of keeping a consistent internal temperature.

Cold blooded reptile
Cold-blooded reptile by S N Barid. Licensed under CC BY-SA 4.0

The metabolic rate generates the “warmth” of warm-blooded animals. Being warm-blooded is a collection of characteristics. The fact that these qualities occur together in living animals doesn’t mean they have to be linked. Perhaps some of these characteristics are used to give benefits unconnected to “warm-bloodedness.”

The new research shows that we can’t presume that all dinosaurs, or even any, were warm-blooded. After all, it’s debatable if all mammals are warm-blooded in the same way. Echidnas, for instance, appear to have fluctuating body temperatures, while others, such as humans, do not.


The issue is that dinosaurs are no longer alive, making it difficult to assess their temperatures and metabolic rates. However, the metabolic rate of extinct creatures, such as dinosaurs, must be calculated based on other factors.

Enrico Rezende: The findings were Impressive

The findings are “very impressive,” according to Enrico Rezende, an evolutionary biologist at the Pontifical Catholic University of Chile who has studied the evolution of warm-bloodedness, or endothermy. The findings are “not wholly surprising,” he says, but “it’s beneficial to have some approximation of metabolic levels,” as it avoids labeling dinosaurs as warm-blooded or cold-blooded. “This illustrates that we have a wide range of metabolic levels,” says the researcher.

Warm-blooded species like birds and mammals do not need to bask in the sun to elevate their body temperature, while crocodiles and modern lizards need to. “Dinosaurs could have been more active and ranged over bigger territories as endothermic,” explains Rezende. They’d also be less susceptible to cold temperatures, allowing them to be more active at night and thrive on higher ground or higher latitudes.


Warm-blooded dinosaurs, however, would need a lot of energy to power their high metabolisms, which means they’d have to spend a lot of time feeding.” “Having a clear knowledge of metabolic levels will tell more about how these ecosystems relate and develop,” Rezende says. 

It is a new line of evidence – Lucas Legendre.

According to Lucas Legendre, a paleontologist at the University of Texas at Austin, “researchers have utilized various approaches to study how dinosaurs could generate their heat.”

Estimates of body temperature based on temperature-sensitive minerals preserved in fossils are one line of evidence. Other scientists examine the growth rings in dinosaur thigh bones to determine how quickly the animals grew. Similarly, Legendre and his colleagues have used the size of blood vessels and bone cells to conclude that carnivorous dinosaurs had high metabolic rates similar to those of modern birds.


According to Legendre, the Nature research shows that dinosaurs had more in common with birds than lizards in terms of physiology. “This is new proof that backs up what many scientists have been stating for decades.”

Matteo Fabbri: This is a new direct approach

According to Matteo Fabbri, a paleontologist at the Field Museum of Natural History in Chicago and coauthor of the paper, the researchers employed a more direct approach than previous studies. The researchers looked at metabolic byproducts retained in newly-formed and fossilized thighbones. Metabolism is the process through which animals transform oxygen and nutrients into energy.

The study

“Metabolism defines whether an animal is warm-blooded or cold-blooded and if excess heat is produced during the breathing process,” Wiemann explains.


Using Raman and Fourier-transform infrared spectroscopy, Wiemann and her colleagues scanned the bones of 30 prehistoric creatures and 25 modern reptiles, mammals, and birds to estimate the total amount of advanced lipoxidation end-products.

The researchers applied the new method to assess modern and extinct animals’ metabolic rates based on the molecular composition of their bones. Reactive oxygen species occur during this process, resulting in compounds known as advanced lipoxidation end-products. According to Rezende, these residues accumulate and “leave an imprint in pretty much every tissue,” according to Rezende.

Research results

According to the findings, many iconic dinosaurs, such as Tyrannosaurus rex and the giant sauropods, were warm-blooded. Still, cold-bloodedness later emerged in some dinosaurs, including Stegosaurus, Triceratops, and a duck-billed hadrosaur.

1024px Hadrasaurus upper leg bone
Hadrasaurus upper leg bone by Hectonichus under CC BY-SA 3.0

The researchers found that a long-necked diplodocid, Allosaurus, and birds had particularly high metabolic rates, while T-rex had a lower metabolic rate than other carnivorous theropod dinosaurs. Because an animal with a high metabolic rate consumes more oxygen than one with a low metabolic rate, the levels of these compounds in its body should be higher.

“It’s quite interesting since it implies that the range of metabolisms observed in dinosaurs is far broader than previously imagined,” Wiemann says. He explained further that “this raises intriguing concerns about what causes the evolutionary increased or decreased metabolic rate, and what this means for animal lifestyles.”

Researchers have previously suggested that warm-bloodedness may have helped prehistoric birds and mammals adapt during the great extinction that wiped off the dinosaurs around 66 million years ago. However, the fact that many Late Cretaceous dinosaurs had high metabolic rates suggests that other characteristics like body size were likely important in the survivors’ survival, according to Wiemann.


Future studies

Legendre thinks the findings will need to be confirmed with other research that involves more extinct animals. Nonetheless, the metabolic byproducts investigated by Wiemann and her team provide data that scientists can compare to other features.

Wiemann and her team have demonstrated that both proponents and opponents of warm-blooded dinosaurs have attempted to simplify this concept more than possible. We should, however, expect more of these kinds of protests in the future.

Legendre explains, “The fact that Wiemann and her team used this innovative method adds one more piece to the puzzle.” “Hopefully, we’ll achieve a clearer picture of how dinosaurs and their close relatives generated metabolic heat in the next few years.”


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