Schematic drawing of a subset of the animals that were investigated as part of the study. Metabolic rates and resulting thermophysiological strategies are color-coded, orange hues characterize high metabolic rates coinciding with warm-bloodedness, and blue hues characterize low-metabolic rates coinciding with cold-bloodedness. From left to right: Plesiosaurus, Stegosaurus, Diplodocus, Allosaurus, Calypte (modern hummingbird). Credit: J. Wiemann
Paleontologists have been arguing for decades about whether dinosaurs had warm blood, like mammals and birds today, or cold blood, like reptiles today. If we knew whether dinosaurs were warm-blooded or cold-blooded, we might be able to figure out how active they were and what their daily lives were like. However, the methods used to figure out whether they were warm-blooded or cold-blooded—like how fast their metabolisms turned oxygen into energy—were not conclusive. But in a new paper published in Nature, scientists show a new way to figure out how fast dinosaurs’ bodies worked by looking at how much air each animal breathed in its last hour of life.
“This is really exciting for palaeontologists. The question of whether dinosaurs were warm-blooded or cold-blooded is one of the oldest in palaeontology, and now we think we have an answer: most dinosaurs were warm-blooded,” says Jasmina Wiemann, the lead author of the paper and a postdoctoral researcher at the California Institute of Technology.
“With Jasmina Wiemann’s new proxy, we can directly infer the metabolism of organisms that have died out, which is something we could only dream about a few years ago. We also found that different groups had different metabolic rates. This had been thought before based on other methods, but it had never been tested directly “says Matteo Fabbri, one of the study’s authors and a postdoctoral researcher at the Field Museum in Chicago.
People sometimes talk about metabolism in terms of how easy it is to stay in shape, but Wiemann, who works at Yale University and the Natural History Museum of Los Angeles County, says that at its core, metabolism is how well we turn the oxygen we breathe into chemical energy that powers our bodies.
Endothermic, or warm-blooded, animals have a high metabolic rate. Warm-blooded animals, like birds and mammals, need to take in a lot of oxygen and burn a lot of calories to keep their body temperature up and keep moving. Reptiles and other animals that are cold-blooded, or ectothermic, breathe less and eat less. Their way of life uses less energy than that of a warm-blooded animal, but it comes at a cost: cold-blooded animals depend on the outside world to keep their bodies at the right temperature to function (like a lizard basking in the sun), and they tend to be less active than warm-blooded animals.
Dinosaurs were in the middle of a debate because birds have warm blood and reptiles have cold blood. Only birds survived the mass extinction at the end of the Cretaceous. However, dinosaurs and, by extension, birds are reptiles, and crocodiles and alligators are their closest living relatives besides birds. So, does that mean that dinosaurs had warm or cold blood?
Scientists have used chemical and osteohistological tests on dinosaur bones to try to figure out how fast their bodies burned food. Wiemann says, “In the past, people have looked at dinosaur bones using isotope geochemistry, which basically works like a paleo-thermometer.” Researchers look at the minerals in a fossil to figure out what temperatures those minerals would have formed in. “It’s a really cool method that changed a lot when it was first used, and it continues to give very interesting information about how extinct animals worked. But we’ve realised that we don’t really understand how fossilisation processes change the isotope signals that we pick up, so it’s hard to compare the data from fossils to modern animals in a clear way.”
Growth rate is another way to look at metabolism. “If you cut a dinosaur bone in half and look at a cross-section, you can see a series of lines, like tree rings, that show how old the bone was,” says Fabbri. “You can figure out how fast the dinosaur grew by counting the lines of growth and the space between them. The limit depends on how growth rate estimates are turned into metabolism. Whether an animal grows faster or slower can have more to do with its age than its metabolism, just like we grow faster when we’re young and slower as we get older.”
Wiemann, Fabbri, and their colleagues have come up with a new way to figure out how long dinosaurs lived. This method doesn’t look at the minerals in the bones or how fast the dinosaurs grew. Instead, they look at how oxygen is used, which is one of the most important parts of metabolism. When animals breathe, by-products are made that react with proteins, sugars, and lipids, leaving behind molecular “waste.” This waste is very stable and doesn’t dissolve in water, so it stays in place when it fossilises. It shows how much oxygen a dinosaur was taking in and, by extension, how fast it was burning calories.
The scientists looked for these bits of molecular trash in fossil femurs that were dark in colour. Dark colours mean that a lot of organic matter is still there. They used Raman and Fourier-transform infrared spectroscopy to look at the fossils. Wiemann says, “These methods work like laser microscopes; we can basically count the number of these molecular markers that tell us about the metabolic rate.” “Paleontologists really like this method because it doesn’t hurt the fossils.”
The team looked at the femurs of 55 different kinds of animals, including dinosaurs, their flying cousins the pterosaurs, their more distant marine relatives the plesiosaurs, and modern birds, mammals, and lizards. They compared the amount of molecular waste from breathing to the known metabolic rates of living animals and used that information to figure out the metabolic rates of animals that are no longer alive.
The team found that the metabolic rates of most dinosaurs were high. There are two main types of dinosaurs: those with lizard hips and those with bird hips. The dinosaurs with lizard-like hips, like Triceratops and Stegosaurus, had slow metabolisms, just like cold-blooded animals today. The theropods and sauropods, such as the two-legged, bird-like predators Velociraptor and T. rex and the giant, long-necked herbivores Brachiosaurus, were warm-blooded or even hot-blooded. Researchers were surprised to find that some of these dinosaurs were not just warm-blooded, but also had metabolic rates similar to modern birds and much higher than mammals. These results add to previous observations that hinted at similar trends but couldn’t prove them directly because there wasn’t a direct way to measure metabolism.
Researchers say that these results can tell us a lot about how dinosaurs lived that we didn’t know before.
Wiemann says that dinosaurs with slower metabolisms would have been affected by the outside temperature in some way. “Lizards and turtles like to sit in the sun and warm up. Ornithischians with very slow metabolisms may also use “behavioural” thermoregulation in the same way. Cold-blooded dinosaurs may have had to move to warmer climates when it got cold, and the weather may have limited where some of these dinosaurs could live.”
She says that the dinosaurs with hot blood would have been more active and would have had to eat a lot. “The giant sauropods with high body temperatures were herbivores, and they would have needed a lot of plant matter to fuel their metabolism. Their digestive systems worked very well, and because they were so big, it was probably harder for them to cool down than to heat up.” The theropod dinosaurs, which are the group that includes birds, had high metabolisms even before some of them learned to fly.
“One of the hardest things to do in palaeontology is to figure out how animals lived and worked before they died. This new study adds an important piece to the puzzle of how physiology has changed over long periods of time. It also adds to the proxies that have already been used to look into these questions. We can now tell from isotopes, osteohistology, and chemical proxies what a body’s temperature is, how it grows, and how fast it burns calories “says Fabbri.
This study not only tells us more about what dinosaurs were like, but it also helps us learn more about the world we live in now. Except for birds, all dinosaurs died out at the same time 65 million years ago when an asteroid hit the Earth. Wiemann says that most scientists think that having a high metabolic rate is one of the best ways to survive mass extinctions and thrive afterward. For example, some scientists think that birds survived while non-avian dinosaurs died because birds had a higher metabolic capacity. Wiemann says that this study helps to show that this isn’t true: many dinosaurs with high metabolic rates like birds went extinct.
Wiemann says, “We are living through the sixth mass extinction, so it is important for us to understand how modern and extinct animals’ bodies reacted to climate change and other environmental changes in the past.” This will help us protect biodiversity now and in the future.
Further information: Jasmina Wiemann, Fossil biomolecules reveal an avian-like metabolism in ancestral dinosaurs, Nature (2022). DOI: 10.1038/s41586-022-04770-6. www.nature.com/articles/s41586-022-04770-6
Journal information: Nature
Source: Field Museum