It must be hard keeping cool when you're the size of a semitrailer – imagine finding some shade or a hat big enough.
So how did sauropod dinosaurs prevent heat stroke back in the late Jurassic, and keep their brains from overheating?
New research, published today in the journal The Anatomical Record, suggests that gigantic dinosaurs, including the Tyrannosaurus rex, evolved systems to cool their blood on its way to their brains.
But, in what was a surprise to the researchers, many of these dinosaur groups came up with slightly different ways to do so.
It's getting hot in here
There's long been a debate about whether dinosaurs were cold-blooded like reptiles (ectotherms) or warm-blooded like bird and mammals (endotherms), said study co-author and palaeontologist Larry Witmer of Ohio University.
But if you're gigantic, this becomes a bit of a moot point when it comes to overheating.
"One thing that became obvious to us was that regardless of their physiology, huge dinosaurs were going to be hot, they couldn't help it," Professor Witmer said.
"They had really enormous volumes but relatively low surface areas, which means once they heated up, it would be hard for them to cool down in the absence of special mechanisms."
That posed a problem, because their brains (like ours), eyes and other sense organs were very sensitive to temperature.
If they got too hot, basic functions the brainstem controls, such as blood pressure and heart rate, would become compromised, Professor Witmer said.
A bad case of heat stroke could potentially be fatal.
The kicker was hot blood heated up in their body's core was being pumped to their brain.
For sauropods it wasn't only the warm climate in which they lived that was to blame.
"Not only were they baking in the sun, but the huge fermentation vat of their gastrointestinal tract was also generating significant heat," Professor Witmer said.
The solutions that these huge dinosaurs evolved were all based on the principle of evaporative cooling, which is the way we keep ourselves cool.
"Sweat emerges as a liquid onto the hot surface of our skin, and the heat causes the liquid to turn into a vapour (that is, the sweat evaporates)," Professor Witmer said.
The evaporating sweat takes the heat energy with it, leaving the surface cooler.
"This same process works in the nose and mouth of a person or a dog or a dinosaur – wherever there's moisture," he said.
"When the surface you're cooling happens to be richly supplied with blood, that blood will also be cooled.
"What we were able to show was that a lot of that cooled blood went back to the region of the brain."
By first looking at modern-day birds and reptiles, the researchers were able to trace patterns of blood flow from these cooling sites to the brain.
"The handy thing about blood vessels is that they basically write their presence into the bones," said palaeontologist and lead author of the study Ruger Porter of Ohio University.
"The bony canals and grooves that we see in modern-day birds and reptiles are our link to the dinosaur fossils," Dr Porter said.
By looking for similar canals and grooves in dinosaur skulls the scientists were able to reconstruct their blood flow patterns.
The idea behind the paper is a good one, said Roger Seymour, Emeritus Professor of Physiology at the University of Adelaide, who was not involved in the study.
Professor Seymour said as far as he knows, he was the first person to look at skeletal material to see if they showed anything about the circulatory system.
"For animals that have gone extinct all we have to guide us in the circulatory system are bones," he said.
"You can judge the size of the openings in the skull as they've done and make inferences about how much blood went through."
Infographic of a diplodocus head showing blood vessels in the nose and mouth that it used to cool its brain
Sauropods may have walked around with their mouths open to help keep their brains cool.
What the research team found was multiple ways the huge dinosaurs had evolved of keeping a cool head.
Ankylosaurs had increased blood flow to their nose, beyond what was needed to provide nutrition to their tissues, turning this into a cooling region.
Whereas sauropods had increased blood flow to both their nose and mouth.
"Sauropods almost certainly didn't pant like dogs," Professor Witmer said.
"But we're suggesting our evidence shows that they may have routinely walked around with their mouths open when they were overheated."
Computer generation graphic illustrating the different sinuses of therapod dinosaurs and how that contributed to cooling
Theropod like T-rex used their jaw like an old-fashioned bellows pump to cool their blood on its way to their brain.
Theropods, like T-rex, were different again.
Instead of increasing the blood flow to their nose or mouth or eyes, they increased the blood supply to an enlarged air sinus off to the side of their nasal cavity in their snouts.
We have similar air sinuses, Professor Witmer said, that get clogged when we have a cold.
"What makes theropods – including living theropods (birds) – different from us is that the air was pumped in and out of the sinus, like an old-fashioned bellows pump, by jaw movements," he said.
And that airflow set up a good evaporative cooling system.
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The research is interesting because it's finally given us quite a bit of quantitative data on how huge dinosaurs deal with overheating and the risks it poses to their brains, said palaeobiology PhD student Douglass Rovinsky who was not involved in the study.
"At a certain point you get so big that mathematically you shouldn't be alive," Mr Rovinsky of Monash University said.
"This shows that they changed the arrangement of their blood vessels to keep from cooking alive."
Mr Rovinsky said it would be interesting to see if this study prompts bird researchers to understand how they cope with heat better too.
The researchers are now keen to expand their analysis to other groups of dinosaurs, including one closer to home, Muttaburrasaurus.
