Dinosaurs Took A Middle Road Between Warm- And Cold-Blooded
June 13, 2014

Dinosaurs Took A Middle Road Between Warm- And Cold-Blooded

Lawrence LeBlond for redOrbit.com - Your Universe Online

There has been a long-standing debate over dinosaurs: were they cold-blooded like modern day reptiles or warm-blooded like mammals?

In the early days of science, and in Hollywood, these prehistoric beasts were depicted as slow, lumbering giants as they were believed to be cold-blooded. But over the past few decades, these animals have been portrayed as swift-moving lizards, more reminiscent of warm-blooded behaviors.

New research, led by John Grady, a graduate student at the University of New Mexico, has possibly solved the mystery of dinosaur metabolism, stating dinosaurs were neither cold-blooded (ectothermic) nor warm-blooded (endothermic). Instead, he and his team paint a picture of something more in between.

“Most dinosaurs were probably mesothermic," said Grady in a statement. “A thermally intermediate strategy that only a few species ­– such as egg laying echidnas or great white sharks – use today.”


Publishing a paper in the journal Science, the team demonstrate the first evidence to quantitatively explore the relationship between growth rate and metabolic rates in animals and extend that to those which have been long extinct – namely the dinosaurs.

Grady and his colleagues, which included fellow graduate students Eva Dettweiler-Robinson and Natalie Wright, along with supervision from Prof Felisa Smith of UNM and Prof Brian Enquist of the University of Arizona, used an extensive database of animal growth and energy use that was developed by Grady himself to demonstrate that animals which grow fast not only require more energy, but also have higher body temperatures. Using growth estimates of extinct dinosaurs, the team then calculated dinosaur metabolic rates, unexpectedly finding that dinosaurs followed a more intermediate road between mammals and reptiles.

"I think we were all surprised by this," Smith said. "The idea certainly took some getting used to. But, the patterns were so robust."

"Our results showed that dinosaurs had growth and metabolic rates that were actually not characteristic of warm-blooded or even cold-blooded organisms. They did not act like mammals or birds nor did they act like reptiles or fish," Enquist told Reuters’ Will Dunham. "Instead, they had growth rates and metabolisms intermediate to warm-blooded and cold-blooded organisms of today. In short, they had physiologies that are not common in today's world."

The data analysis was performed by Grady, Dettweiler-Robinson and Wright.

"John spent years compiling this information… collecting more than 30,000 rows of data was quite a feat,” noted Dettweiler-Robinson.

Delving deeper, the team found feathered dinosaurs and primitive birds grew much slower than their descendants, the modern day birds. “Archaeopteryx, the first bird… took two years to reach maturity,” explained Grady. “But, a red-tailed hawk, which is about the same size, only takes 6 weeks.”

While dinosaurs didn’t grow nearly as fast as their modern day descendants, they did grow significantly faster than modern reptiles.

“This higher energy use probably increased speed and performance,” Grady said. “Mesothermic dinsoaurs were likely faster predators or better able to flee from danger than the large reptiles found earlier in during the Mesozoic.”

“It feels like almost everyone working in dino palaeontology has weighed in on [metabolism] at some point in their career,” John Hutchinson, a professor of evolutionary biomechanics at the Royal Veterinary College at University of London, said to National Geographic’s Ed Yong.

Scientists have long tried to address the issue of metabolism by looking at bone structure, leg shape, lung anatomy and presence of feathers, as well as predator-prey ratio. Others have even tried to determine how quickly these prehistoric beasts grew.

When it comes to their bones, dinosaurs are like trees, having trunks with age rings, each representing a year of growth. By studying specimens at different ages, researchers can determine how quickly these animals grew and how much energy they burned in doing so. With bone rings you get growth rates, which in turn gives you metabolic rates.

For the study, Grady and his team amassed a wealth of data on the growth rates of 381 animal species, both living and extinct. These animals include 21 dinosaurs, six extinct crocodiles and a prehistoric shark. Besides dinosaurs, like Tyrannosaurus and Apatosaurus, the team also looked at blue whales and deer mice, hammerhead sharks and Komodo dragons, as reported by NatGeo. The team estimated each animal’s mass, growth rate and metabolic rates, refining or reworking mathematical models from earlier studies in the process.

The analysis of the data clearly revealed to the team that dinosaurs sit somewhere in the middle of the road between endotherms and ectotherms. In essence, these prehistoric beasts were unable to control their body temperature as well as mammals, but weren’t as dependent on their environment as reptiles are.


“The data pointed to dinosaurs not being quite like a reptile or a mammal, but to weird things like great white sharks, leatherback turtles, and tuna,” Grady told NatGeo.

Great whites and tuna are mostly cold-blooded but their hard-working muscles naturally heat their blood. In most fish, the warm blood would lose its heat as it travels to the gills for a dose of oxygen. But in these fish, the vessels are arranged so that the warm blood from the muscles travels past cold blood from the gills, and heats it up. They create body heat, and keep that heat in their bodies—a trick that can keep certain body parts up to 14 degrees Celsius hotter than the surrounding water,” wrote NatGeo’s Ed Yong.

In a similar fashion, leatherback turtles utilize kind of a thermal inertia to keep itself warm. Leatherbacks are big animals, and with big animals, heat loss is slower than in small critters. Grady and his team believe that most dinosaurs used a similar strategy, called mesothermy. In essence, dinosaurs were “lukewarm-blooded.”

With endotherms – meaning internal heat – metabolism is used to keep body temperatures at a relatively fixed position, about 98.6 degrees F. With ectotherms – meaning external heat – body temperatures vary as animals use the environment to warm themselves up. Some big crocodiles, however, rely on their size to keep a stable body temperature once they get warm from the day’s heat, called homeothermy.

Dinosaurs followed a more mesothermic path, relying on their own metabolism to raise their body temperature. But unlike mammals, their temperatures do not keep at a fixed point. They can turn the heat on, but do not have an internal thermostat to keep it regulated.

Grady and his team’s conclusions about mesothermy in dinosaurs really isn’t that new. Many paleontologists believe there is somewhat of a middle road for dinosaur metabolism, rather than swerving too far to the right or too far to the left.

“There have been many studies arguing for intermediate metabolic rates in dinosaurs,” Hutchinson explained to Yong. “But this one stands out on its statistical treatment. It is very clear and testable, and it fits with other evidence.

“To me, a lynchpin would be how this works for polar dinosaurs,” he added.

For the record, Grady and his colleagues focused on warm climate dinosaurs, but many lived in places with cold winters. Would a baby dinosaur living in a colder climate still be mesothermic, or would it be something different? Hutchinson explained that is something Grady’s team should be looking into next.


Gregory Erickson, from Florida State University, praised the team’s attention to detail.

“This is a remarkably integrative, landmark study [that] sets a new standard for growth research on extinct animals,” Erickson, who has himself studied dinosaur growth, told NatGeo.  “Now we can more rigorously compare how dinosaurs and the earliest birds grew relative to [living] animals and infer their metabolic status.”

"Personally I don't find the result very surprising," commented Dr Paul Barrett from the Natural History Museum in London. He was impressed, however, that the team convincingly added metabolic rates to the picture.

"It's nice that they've taken real, empirical data from living animals and come up with a model," he told the BBC’s Jonathan Webb. "There's always been some discussion over whether you can use growth rate as a proxy for metabolic rate. What this shows is that maybe we can have more confidence.

"They've brought a lot of data together and make a nice case," Dr Barrett added.

Prof Roger Seymour, a reptile physiologist at the University of Adelaide in Australia, was less enthusiastic of the results.

"They could find only eight species living today that are mesotherms," he explained to BBC News. "Doesn't that suggest that there's an adaptive value in being either a good ectotherm or a good endotherm, but not in the middle?"

Prof Seymour has long argued dinosaurs were in fact warm-blooded, as is evident in large blood vessel tracks found in dinosaur bones, as well as suggestive evidence of high blood pressure and big, warm blood supply.

"It would be disadvantageous to be in the middle," he told Webb, noting he remains convinced that powerful endothermic dinosaurs would have sufficient energy to remain dominant for as long as they did.

Mieke Köhler, from the Catalan Institute of Palaeontology, is also a bit more reserved. She noted most modern mesotherms, such as tuna and leatherbacks, control their body temperatures in different ways.

“They rely on completely different metabolic machinery,” Köhler told Yong. “They’re not a discreet group, but a collection of specialists that shifted their physiological state away from the extremes to converge somewhere in the middle.”

She added that we risk suppressing the important differences in dinosaur lifestyles if we lump them all together under the same label.


Dinosaurs were, after all, a varied group. Dominating the planet for more than 180 million years, these animals were very diverse, ranging from mega titans to mini lizards. Some had feathers and others didn’t. Some lived in more tropical climates while other lived in the colder extremes. If modern fish and mammals can vary in their physiology, than dinosaurs would have likely done so too.

“There was probably variety, but I think many to most were mesotherms,” said Grady. “It makes sense of the conflicting back and forth evidence we’ve had. They’re not like modern birds or like reptiles.”

Hutchinson believes the results of Grady’s study brings about a more interesting question than “Were dinosaurs warm-blooded or cold-blooded?” Instead, he would ask: “When did the ancestors of birds evolve a high metabolic rate?”

Smith explained mesothermy may have helped dinosaurs become ecologically dominant as well as enormous creatures.

“A lion the size of a T-Rex, while a frightening thought, would quickly starve to death because it would be so hard to find enough food,” said Smith.

"If I were eating sandwiches all day... I might have to eat five," Grady explained to the BBC’s Jonathan Webb. "But a reptile [my size] can eat maybe a couple of sandwiches in a whole week."

By adopting a medium-powered energetic strategy, mesothermy may have provided the perfect solution.

"It allows a performance advantage over ectothermic reptiles," Grady added, “but without the high overhead costs of modern birds and mammals.

“In any case, it was a successful formula for a long reign in the Mesozoic," Smith concluded.


Image 2 (below): Growth rates across an evolutionary tree. Dinosaurs growth rates fall in between warm blooded mammals and birds ('endotherms') in red, and cold-blooded fish and reptiles ('ectotherms') in blue. They are closest to living mesotherms. Illustration provided by John Grady.