Were dinosaurs warm- or cold-blooded?

A decades-long debate among palaeontologists – were dinosaurs cold- or warm-blooded? – has been resolved, with new research published in Nature providing a new technique for analysing extinct animal metabolisms.

“[M]etabolism is how effectively we convert the oxygen that we breathe into chemical energy that fuels our body,” says the paper’s lead author, Jasmina Wiemann, affiliated with Yale University and the Natural History Museum of Los Angeles County.

The team’s findings, she says, are “really exciting for us as paleontologists – the question of whether dinosaurs were warm- or cold-blooded is one of the oldest questions in paleontology, and now we think we have a consensus – that most dinosaurs were warm-blooded.”

“The new proxy developed by Jasmina Wiemann allows us to directly infer metabolism in extinct organisms, something that we were only dreaming about just a few years ago,” says co-author Matteo Fabbri, a postdoctoral researcher at the Field Museum in Chicago. “We also found different metabolic rates characterising different groups, which was previously suggested based on other methods, but never directly tested.”

With all that’s left of dinosaurs being their fossilised remains, determining whether they were endothermic (warm-blooded) or ectothermic (cold-blooded) hasn’t been easy.

Palaeontologists have had to use chemical analyses of fossils and comparison of fossilised bone structure to today’s animals to try to gauge extinct animals’ metabolisms.

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“In the past, people have looked at dinosaur bones with isotope geochemistry that basically works like a paleo-thermometer,” says Wiemann.

Minerals often form at certain temperatures, and minerals in a fossil can give clues to the warmness inside an extinct animal’s bones. “It’s a really cool approach and it was really revolutionary when it came out, and it continues to provide very exciting insights into the physiology of extinct animals,” Wiemann says. “But we’ve realised that we don’t really understand yet how fossilisation processes change the isotope signals that we pick up, so it is hard to unambiguously compare the data from fossils to modern animals.”

Alternatively, scientists have tried to assess dinosaur metabolism by analysing the growth rate of different species.

“If you look at a cross section of dinosaur bone tissue, you can see a series of lines, like tree rings, that correspond to years of growth,” says Fabbri. “You can count the lines of growth and the space between them to see how fast the dinosaur grew. The limit relies on how you transform growth rate estimates into metabolism: growing faster or slower can have more to do with the animal’s stage in life than with its metabolism, like how we grow faster when we’re young and slower when we’re older.”

The new method proposed by Wiemann’s team focuses not on minerals inside fossils or dinosaur growth rates, but on oxygen use. Breathing produces proteins, sugars and fats in animals. When these organic compounds are made, there is an associated production of molecular “waste”, which is preserved in the fossilisation process. So, the researchers were able to measure how much oxygen different dinosaur species were breathing.

Looking at fossil femurs darkened by the presence of high amounts of organic matter, the team used non-destructive infrared spectroscopy to measure the organic “waste” compounds in the fossils.

Analysing 55 species, including dinosaurs and their also-extinct flying cousins, pterosaurs, and slightly more distant marine cousins, plesiosaurs, the researchers compared the content to the femurs of living animals such as birds to infer the metabolic rates of the extinct ones.

Dinosaur metabolisms are generally high, the team found.

The two big groups of dinosaurs are saurischians (lizard-hipped) and ornithischians (bird-hipped), and lizard-hipped dinosaurs such as Triceratops and Stegosaurus were found to have low metabolic rates, like today’s cold-blooded animals.

Bird-hipped dinosaurs, including sauropods (massive, long-necked dinosaurs such as Brachiosaurus) and theropods (generally meat-eating, two-legged dinosaurs such as Tyrannosaurs and Velociraptor) were warm- and even hot-blooded.

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The team’s findings complement earlier work that hinted at, but could not prove, such trends. The results also provide new insights into the daily lives and activities of dinosaurs.

“Dinosaurs with lower metabolic rates would have been, to some extent, dependent on external temperatures,” says Wiemann. “Lizards and turtles sit in the sun and bask, and we may have to consider similar ‘behavioural’ thermoregulation in ornithischians with exceptionally low metabolic rates. Cold-blooded dinosaurs also might have had to migrate to warmer climates during the cold season, and climate may have been a selective factor for where some of these dinosaurs could live.”

Conversely, hot-blooded dinosaurs would have been more active and would have needed to eat a lot. “The hot-blooded giant sauropods were herbivores, and it would take a lot of plant matter to feed this metabolic system,” Wiemann says. “They had very efficient digestive systems, and since they were so big, it probably was more of a problem for them to cool down than to heat up.”

“Reconstructing the biology and physiology of extinct animals is one of the hardest things to do in palaeontology,” Fabbri says. “This new study adds a fundamental piece of the puzzle in understanding the evolution of physiology in deep time and complements previous proxies used to investigate these questions.”

While giving us a picture of animals in the past, Wiemann also believes it can help us understand animals today.

“Having a high metabolic rate has generally been suggested as one of the key advantages when it comes to surviving mass extinctions and successfully radiating afterwards,” she says. Some scientists postulate that only avian dinosaurs survived the extinction event 65 million years ago because of the birds’ higher metabolic rate. Wiemann, however, believes the new study helps disprove this theory as many dinosaurs with high metabolisms also went extinct. “We are living in the sixth mass extinction,” she says, “so it is important for us to understand how modern and extinct animals physiologically responded to previous climate change and environmental perturbations, so that the past can inform biodiversity conservation in the present and inform our future actions.”