Fifty years ago, dinosaurs lumbered across our screens like the slow-moving, shambling oafs they were thought to be. Now, they stride and sprint. They’re portrayed as active animals, often lithe and agile.
This popular makeover was inspired by a scientific one. Scientists used to believe that dinosaurs, like modern-day reptiles, were cold-blooded ectotherms. That doesn’t mean their blood was literally cold, but that they relied on the environment to heat their bodies. Many lines of evidence challenged this view, suggesting instead that some dinosaurs were warm-blooded endotherms like mammals and birds: they generated body heat by burning energy at a much greater rate than most reptiles.
This debate about dinosaur metabolism—were they ectotherms, endotherms or something in between?—is one of the longest-running in palaeontology. (“It feels like almost everyone working in dino palaeontology has weighed in on it at some point in their career,” says John Hutchinson.) Scientists have tried to address the issue by looking at the structure of their bones, the shape of their legs, the inferred anatomy of their lungs, the presence of insulating feathers, and the ratios of predators to prey. And some have tried to work out how quickly they grew.
Dinosaur bones are like tree trunks—they have rings, and each ring represents a year of growth. By studying specimens of different ages, scientists can work out how quickly these animals grew and how must energy they must have burned to do so. Bone rings give you growth rate, which gives you metabolic rate.
It’s an attractive approach because, unlike bone structure or the presence of feathers, growth can be quantified and clearly compared. “You can put a number on it,” says John Grady from the University of New Mexico. But these numbers come in drips, through piecemeal studies of a few species at most. Grady had grander aspirations.
His team has now amassed data on the growth rates of 381 animal species, both living and extinct, including 21 dinosaurs, 6 extinct crocodiles, and one prehistoric shark. He looked Tyrannosaurus and Apatosaurus, blue whales and deer mice, hammerhead sharks and Komodo dragons. He estimated each animal’s mass, growth rate, and metabolic rates, often refining (or completely reworking) mathematical models from earlier studies.
His analysis revealed that dinosaurs sit somewhere between the endothermic and ecothermic extremes, epitomised by most mammals and reptiles. They couldn’t control their body temperature as precisely as a horse or human; equally, they weren’t as dependent on their environment as a snake or lizard. “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.”
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,
The leatherback turtle uses similar heat exchangers, and it’s also very big. Big animals lose heat more slowly than small ones, so the leatherback has a sort of thermal inertia that keeps it warm.
Grady thinks that most dinosaurs used a similar strategy, which he calls mesothermy. They were lukewarm-blooded.
This isn’t just a wishy-washy middle-man term; it has a specific meaning. Endotherms use their metabolism to keep their body temperatures at a fixed point—excepting the occasional chill or fever, you’re almost always at 37 degrees Celsius. Ectotherms have more variable body temperatures and rely on the environment to heat themselves up. Some, like big crocodiles, are homeotherms—they rely on their size to keep a stable body temperature once they bask their way to warmth.
Mesotherms are different. Unlike a basking crocodile, they rely on their own metabolism to raise their body temperature. But unlike you, they don’t keep their temperatures at a fixed point. They turn the heating on, but they have no thermostats. Great whites and leatherbacks are good examples but mammals can be mesotherms too. The echidna—a spiny, egg-laying mammal from Australia—metabolises its way to an average temperature of 31 degrees Celsius, but that can vary by 10 degrees in either direction. It has a thermostat, but a very wobbly one.
Grady’s conclusion isn’t that new. Many, if not most, palaeontologists see the warm-blooded/cold-blooded debate as too simplistic. Instead, they believe there’s a continuum between these extremes, and dinosaurs fell somewhere in the middle. “There have been many studies arguing for intermediate metabolic rates in dinosaurs,” says Hutchinson, from the Royal Veterinary College, UK. “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 adds. Grady’s team focused on species that lived in warm climates, and many dinosaurs lived in places with uncomfortable winters. Would a baby dinosaur living in a cold place still be mesothermic, or would it do something different? That’s something for the team to check next.
For now, Gregory Erickson from Florida State University, who has studied dinosaur growth, effusively 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,” he says. “Now we can more rigorously compare how dinosaurs and the earliest birds grew relative to [living] animals and infer their metabolic status.”
Mieke Köhler from the Catalan Institute of Palaeontology is a bit more reserved. She notes that echidnas, tuna, and leatherbacks are all mesotherms, but control their body temperatures in very different ways. “They rely on completely different metabolic machinery,” she says. “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.” By bundling the dinosaurs together under the same label, we risk whitewashing important differences in their lifestyles.
They were, after all, a very varied group. They dominated the planet for 185 million years, and there’s more time between Stegosaurus and Tyrannosaurus than between Tyrannosaurus and you. They diversified into forms both titanic and minute. Some had feathers and others didn’t. Some lived in the tropics and others lived in the freezing poles. If modern fish and mammals can vary in their physiology, they would have too. “There was probably variety, but I think many to most were mesotherms,” says Grady. “It makes sense of the conflicting back and forth evidence we’ve had. They’re not like modern birds or like reptiles.”
Even feathered dinosaurs like Archaeopteryx, which was either not quite a bird or just about a bird, came out as mesotherms. That surprised Grady. “This thing that was feathered like a bird wasn’t that much different to these non-feathered dinosaurs in how fast it grew,” he says. And it grew slowly! It took around 2 years for Archaeopteryx to reach adult size. A similarly sized hawk gets there in 6 weeks. “Its energy use was much lower than modern birds, but it was covered in feathers. Maybe it was an endotherm with a low metabolic rate, or something like the echidna. The jury’s still out.”
To Hutchinson, these results hint at 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?”
Grady also wonders if mesothermy could help to explain the long reign and frequent large size of dinosaurs. By raising their body temperatures, they could move their muscles faster and fire their nerves faster, becoming temporarily better at escaping or hunting. That’s why sharks and tuna do it. Swordfish and marlin can even warm up their brains and eyes to process information faster when they hunt.
But fully endothermic animals need to eat a lot to fuel their inner furnaces, which sets a limit to how big they can get. Grady wonders if mesothermy strikes a happy medium, allowing animals to stay competitive while also getting big.
Reference: Grady, Enquist, Dettweiler-Robinson, Wright & Smith. 2014. Evidence for mesothermy in dinosaurs. Science http://dx.doi.org/10.1126/science.1253143
PS: Dinosaur fans might be wondering about a recent controversy in which physicist Nathan Myhrvold challenged many published estimates of dinosaur growth rates, and argued that several papers contained serious flaws and discrepancies in their data. Grady’s paper was mid-way through the peer-review process when Myhrvold’s analysis landed, and he paid serious attention to it. But when he omitted data from the problematic papers (or even for problematic species), his results didn’t change. There are five pages of discussion on this in the supplemental materials for statisticians to pore over.