Hot-Blooded or Cold-Blooded? Chemical Clues Solve One of the Oldest Mysteries in Paleontology

Dinosaur Metabolism

Schematic drawing of a subset of the animals that were investigated as part of the research. 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 debating for decades whether dinosaurs were warm-blooded, like modern mammals and birds, or cold-blooded, like modern reptiles. Knowing whether dinosaurs were warm- or cold-blooded could give us clues about how active they were and what their everyday lives were like, but previous methods to determine their warm- or cold-bloodedness — how quickly their metabolisms could turn oxygen into energy — were inconclusive. However, in a new paper published in the journal Nature, scientists are unveiling a novel method for studying dinosaurs’ metabolic rates, using clues in their bones that indicated how much the individual animals breathed in their last hour of life.

“This is 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,” says Jasmina Wiemann, the paper’s lead author and a postdoctoral researcher at the California Institute of Technology (Caltech).

“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. We also found different metabolic rates characterizing different groups, which was previously suggested based on other methods, but never directly tested,” says Matteo Fabbri, a postdoctoral researcher at the Field Museum in Chicago and one of the study’s authors.

People often talk about metabolism in terms of how easy it is for someone to stay in shape, but at its core, “metabolism is how effectively we convert the oxygen that we breathe into chemical energy that fuels our body,” says Wiemann, who is affiliated with Yale University and the Natural History Museum of Los Angeles County.

Allosaurus Bone, Blood Vessels, Cells, and Matrix

Microscopic view of extracted soft tissues from the bones of one of the dinosaur specimens (Allosaurus) that were investigated for metabolic signals (metabolic crosslinks) in the fossilization products of the proteinaceous bone matrix. Fossilization introduces additional crosslinks that, in combination with metabolic crosslinks, generate the characteristic brown color of the fossil extracellular matrix which holds bone cells (dark, ramifying structures) and blood vessels (tube-like structure in the center) in place. Credit: © J. Wiemann

Animals with a high metabolic rate are endothermic, or warm-blooded; warm-blooded animals like birds and mammals take in lots of oxygen and have to burn a lot of calories in order to maintain their body temperature and stay active. Cold-blooded, or ectothermic, animals like reptiles breathe less and eat less. Their lifestyle is less energetically expensive than a hot-blooded animal’s, but it comes at a price: cold-blooded animals are reliant 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 creatures.

With birds being warm-blooded and reptiles being cold-blooded, dinosaurs were caught in the middle of a debate. Birds are the only dinosaurs that survived the mass extinction at the end of the Cretaceous, but dinosaurs (and by extension, birds) are technically reptiles — outside of birds, their closest living relatives are crocodiles and alligators. So would that make dinosaurs warm-blooded, or cold-blooded?

“This is 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.” — Jasmina Wiemann

Scientists have tried to glean dinosaurs’ metabolic rates from chemical and osteohistological analyses of their bones. “In the past, people have looked at dinosaur bones with isotope geochemistry that basically works like a paleo-thermometer,” says Wiemann — researchers examine the minerals in a fossil and determine what temperatures those minerals would form in. “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. But we’ve realized that we don’t really understand yet how fossilization processes change the isotope signals that we pick up, so it is hard to unambiguously compare the data from fossils to modern animals.”

Another method for studying metabolism is the growth rate. “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, Fabbri, and their colleagues doesn’t look at the minerals present in bone or how quickly the dinosaur grew. Instead, they look at one of the most basic hallmarks of metabolism: oxygen use. When animals breathe, side products form that react with proteins, sugars, and lipids, leaving behind molecular “waste.” This waste is extremely stable and water-insoluble, so it’s preserved during the fossilization process. It leaves behind a record of how much oxygen a dinosaur was breathing in, and thus, its metabolic rate.

“We are living in the sixth mass extinction, 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.” — Jasmina Wiemann

The researchers looked for these bits of molecular waste in dark-colored fossil femurs, because those dark colors indicate that lots of organic matter are preserved. They examined the fossils using Raman and Fourier-transform infrared spectroscopy — “these methods work like laser microscopes, we can basically quantify the abundance of these molecular markers that tell us about the metabolic rate,” says Wiemann. “It is a particularly attractive method to paleontologists, because it is non-destructive.”

The team analyzed the femurs of 55 different groups 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 breathing-related molecular byproducts with the known metabolic rates of the living animals and used those data to infer the metabolic rates of the extinct ones.

The team found that dinosaurs’ metabolic rates were generally high. There are two big groups of dinosaurs, the saurischians and the ornithischians — lizard hips and bird hips. The bird-hipped dinosaurs, like Triceratops and Stegosaurus, had low metabolic rates comparable to those of cold-blooded modern animals. The lizard-hipped dinosaurs, including theropods and the sauropods — the two-legged, more bird-like predatory dinosaurs like Velociraptor and T. rex and the giant, long-necked herbivores like Brachiosaurus — were warm- or even hot-blooded. The researchers were surprised to find that some of these dinosaurs weren’t just warm-blooded — they had metabolic rates comparable to modern birds, much higher than mammals. These results complement previous independent observations that hinted at such trends but could not provide direct evidence, because of the lack of a direct proxy to infer metabolism.

These findings, the researchers say, can give us fundamentally new insights into what dinosaurs’ lives were like.

“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 ‘behavioral’ 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.”

On the other hand, she says, the 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. 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.” Meanwhile, the theropod dinosaurs — the group that contains birds — developed high metabolisms even before some of their members evolved flight.

“Reconstructing the biology and physiology of extinct animals is one of the hardest things to do in paleontology. 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. We can now infer body temperature through isotopes, growth strategies through osteohistology, and metabolic rates through chemical proxies,” says Fabbri.

In addition to giving us insights into what dinosaurs were like, this study also helps us better understand the world around us today. Dinosaurs, with the exception of birds, died out in a mass extinction 65 million years ago when an asteroid struck the Earth. “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 afterward,” says Wiemann — some scientists have proposed that birds survived while the non-avian dinosaurs died because of the birds’ increased metabolic capacity. But this study, Wiemann says, helps to show that this isn’t true: many dinosaurs with bird-like, exceptional metabolic capacities went extinct.

“We are living in the sixth mass extinction,” says Wiemann, “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.”

Reference: “Fossil biomolecules reveal an avian metabolism in the ancestral dinosaur” by Jasmina Wiemann, Iris Menéndez, Jason M. Crawford, Matteo Fabbri, Jacques A. Gauthier, Pincelli M. Hull, Mark A. Norell and Derek E. G. Briggs, 25 May 2022, Nature.
DOI: 10.1038/s41586-022-04770-6

5 Comments on "Hot-Blooded or Cold-Blooded? Chemical Clues Solve One of the Oldest Mysteries in Paleontology"

  1. Very Interesting

    Well Done Ms Jasmina Wiemann.

    1. First step towards developing a comprehensive theory of extinction of species and linking it with the modified Charles Darwin “Origin of Species”. What causes Radiation of “New Species”!!

    2. Even among identified Species there are different rates of metabolism. I suspect metabolic rates will be unique to a individual. Some humans can put away a great deal of food at the dinner table whereas others are satisfied with a couple of bites. Maybe the Warm Blooded and Cold Blooded nature is like a fingerprint or eye print and unique to the individual.

    3.We are at the early stages in the understanding of the microbiome. The Brain in the Stomach.

    4. Understanding the signaling system ( Hunger Pangs) which drives the creatures to forage for food ( matter to be converted to energy to feed the cells requirements for energy), to quell hunger pangs, is a common trait of all living creatures, from Mosquitoes to Men!

    5. Providing Food to the various Micro-Biome Creature eco-systems, aligned with the individual metabolic systems of the individual creature requirements, will result in the ideal solution to the hunger problem, aligned with food availability and its optimum use on the planet. We need to learn from the warm and cold blooded fossils who have become extinct……

    Views expressed are personal and not binding on anyone.

  2. Thanks for the review. You reserved the lizard and bird hips species.

  3. Is this a joke were are all the evidence . I see no scientist take this story as truth in the scientific world only tabloid .first bird are not dinosaur they are not thecodont. velociraptor is a bird not a thecodont .dinosaur is a crocodilian animal it’s a thecodont .thecodont mean crocodilian teeth when they first found dinosaur they wanted to know which it is close too they found they share unique teeth with modern crocodilian but dinosaur was much more primitive than modern crocodilian that why they name all them thecodont .true crocodilian is mesoeucrocodylia animal like spinosauridae and the gator they have death roll system they have feature for that.and dinosaur are not dead modern crocodilian is last spinosauridae and last tyrannosaur they are king of dinosaur they have better gator cold blooded and need the sun have to bask the answer is no it’s clear by the photo are you blind it has cat eyes most modern crocodilian hunt at night were it’s cold no sun and gator live in cold places and can live in extreme cold like New York a feral gator live many winters in the Penn state .dinosaur time was not like New York it was subtropical .allso one dwarf caiman live in the high mountain it’s known to hunt in 40 degree of snow run off water and hate warm water .evidence show gator does not need warm of day to hunt it’s in science report were you find on internet easy and YouTube video eating food at night allso open it’s mouth like cold blooded lizard at the day but crocodilian do that at night too clearly these scientist know nothing about crocodilian the last dinosaur that are alive.crocodilian are nothing like other reptiles they can do this because they are dinosaur.any won call modern crocodilian primitive know nothing about dinosaur it has pseudorotunda like birds and other good hearing things from pseudorotunda report .only find pseudorotunda in fake dinosaur like velociraptor line and gallimimus and animal they claim is not a dinosaur but are a dinosaur advance protosuchus .will they answer why the gator heart is more advance than birds and mammal and why the gator kidney is more advance than mammal a clear sign the animal is warm blooded. reptile have a primitive kidney because they do not use a lot of energy they do not need to fight the high acid count like mammal and gator .mammal and gator have advance kidney the gator is better.gator have a advance spleen like the very high tech horse that need a lot of oxygen because it’s a very fast animal .the answer is why do the gator need so much oxygen it allso very fast animal with very high tech monster centra vertebrae joint simular to great predator cats.gator is a reptile.gator lungs are not advance like mammals but it has flow throw lungs like high tech birds mammal lack flow throw lungs gator allso has big lungs like very good flying birds.what with rows armor back on the gator and dinosaur they are giant in dinosaur a very heavy primitive feature is it really primitive i here other story on YouTube it could be advantage when animal is very fast like the gator they do not need them whale mesoeucrocodylia have smooth skin but in water life is not that stressful like land .easy to know if dinosaur is warm blooded by looking at modern crocodilian skull the nose by looking crocodile skull very high tech nose it’s not fuse like mammal most mammal skull I saw it is not fuse include humans because they need a lot of oxygen the bone is in the way most reptile it is the more advance American gator and cold place living Chinese gator it is fuse because of strong bite force in the primitive gatorcaiman it is not fuse like it’s ancestor Nile crocodile and Nile crocodile ancestor .in dwarf crocodile it is fuse they can get away with this because the hip move to get more oxygen there system probaly take too much oxygen but it’s a reptile .predator dinosaur hip are fix does not move in bird hip dinosaur the advance won it move too so it’s not only modern crocodilian have these advance feature .the Nile crocodile is first modern crocodilian .lack of fuse nose mean warm blooded .bird system is very high tech so it can be fuse .with air sac like the very advance monitor lizard.there is nothing primitive about modern crocodilian .dinosaur yes that why they died in a death roll.dinosaur could not hear like the gator there bite force was weaker gator can smell bettter fossil land mesoeucrocodylia can smell better than modern type .gator can sprawl and can be bipedal dinosaur could not sprawl they can not climb trees there ankle is fuse.there is nothing primitive about the gator it has fully secondary bones palate it has full palate like mammal 6 bone fuse to braincase like mammal it’s eusuchian crocodilian .if you use that it’s going to make people understand it’s a dinosaur only reptile with a full palate is t.rex and the gator the top dinosaur .t.rex secondary bony palate is short that why it’s not a spinosauridae .t.rex has no chance against spinosauridae just like early gator type mesoeucrocodylia has no chance against eusuchian gator .spinosauridae is true mesoeucrocodylia the top dinosaur line .they say land mesoeucrocodylia look like mammal with the snout chewing and teeth and having advance feature because of full palate only gator mammal they talk about .the full palate animal can eat breathe at the same time in dinosaur spinosauridae t.rex only dinosaur could do that the first dinosaur did not have any palate at all they are wons are primitive not the gator .palate how they test bite force the palate is roof of the skull in the mouth were the teeth at human have 2 palate like the gator the different is in human one is a bone the other soft flesh it can turn into bone in a million years in future .gator have 2 bone palate the bigger palate stronger the skull gator have massive skull not as big as spinosauridae gharial baryonyx these 3 have stronger skull than the gator or Nile crocodile these are longostrine skull .predator dinosaur have small skull snout that why they not around any more .clearly modern crocodilian have back up in there skull that why they are so successful and are advance so they can battle lion or tiger.spinosauridae lack back up there skull has to be long all time modern crocodilian fix the problem they can have strong short skull.modern crocodilian is extremely dangerous animal no dinosaur can match them.dinosaur is the primitive ancestor of the gator but was warm blooded like the gator even with a fuse nose .they can tell because dinosaur was fast they have furcula the arm was on a violent force quadrupedal so they develope furcula so arm will not break like birds .today we have fast quadrupedal mammal without furcula or monster centra vertebrae joints or well develope pectoral girdle were you will find in birds and gator a flight feature.allso in dinosaurs compsognathus tibia is bigger than the femur like a horse and bird.compsognathus was fast the same thing can be found in fossil land mesoeucrocodylia macelognathus .the scipionyx fossil has unique hip like the gator so heart spleen lungs were the same it has same system

  4. Gator blood have more oxygen than the mammal human .gator is a technology marvel .modern crocodilian is the greatest tyrannosaur ever and mite be king of the beast

  5. “We are living in the sixth mass extinction,” says Wiemann,
    No we’re not.
    “But whatever species number we use—2.1 million, 8.7 million, even 30 million—by no stretch of the imagination does a plausible extinction rate in our time compare to the mass die-off of 65 million years ago nor the four mass extinctions before that.”

    It’s a shame that a reputable site like scitechdaily allows itself to be used for “anthropogenic climate change” propaganda.

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