Growing big has its advantages — like escaping those nasty predators — but it also brings new problems. For giant dinosaurs, dealing with overheating was a massive issue, but they managed to overcome it by developing specialized cooling systems.
Dinosaurs were an extremely varied group, and size is one regard in which their variability went off the charts. Some dinosaurs were as small as hummingbirds, weighing only a few grams, while others could weight as much as 100 tons.
Sauropods were the largest and heavier dinosaurs — larger than everything else in their ecosystem, and an order of magnitude more massive than anything that ever walked the Earth until them.
There are several advantages to growing this much. For starters, you can simply outgrow predators, and become too big for predation. Secondly, you can also use energy more efficiently, and also gain significant longevity. But there are a few issues to overcome too. Thermal inertia is such an issue.
“Small dinosaurs could have just run into the shade to cool off,” said study co-author Professor Lawrence Witmer, “but for those giant dinosaurs, the potential for overheating was literally inescapable. They must have had special mechanisms to control brain temperature, but what were they?”
If you want to cool down, one of the best things you can do is use evaporation. It’s what many creatures do through sweat, and it’s also what air conditioning units do. This process is particularly important around the head, where some of the most sensitive organs (like the brain and eyes) lie. Chemical analyses of fossil sauropod teeth previously suggested that, despite their massive bodies, the animals maintained body temperatures similar to those of modern mammals, so we know that they were successful in their thermoregulation.
“The brain and sense organs like the eye are very sensitive to temperature,” said Ruger Porter, Assistant Professor of Anatomical Instruction and lead author of the study. “Animals today often have elaborate thermoregulatory strategies to protect these tissues by shuttling hot and cool blood around various networks of blood vessels. We wanted to see if dinosaurs were doing the same things.”
In order to see how this process took place, researchers looked at blood vessels — the body’s natural cooling system. Thankfully for the researchers, blood vessels basically inscribe themselves into bones and well-preserved fossils can reveal the ridges associated with blood vessels.
The team started out with carcasses of birds and reptiles, using CT scans to trace blood flow from sites of evaporative cooling to the brain. They also measured the bony canals and grooves that conveyed the blood vessels. They were then able to take what they’ve learned from this process and adapt it to information from dinosaur fossils. They found that different dinosaurs used different cooling strategies.
“The discovery that different dinosaurs cooled their brains in a variety of ways not only provides a window into the everyday life of dinosaurs, it also serves as an exemplar of how the physical constraints imposed by specific environmental conditions have shaped the evolution of this diverse and unique group,” said Sharon Swartz, a program director at the National Science Foundation, which funded the research. “Using a combination of technological innovation and biological expertise, these researchers were able to take a direct reading from the fossil record that provides new clues about how dinosaur skeletal form and function evolved.”
A key factor that decided how dinosaurs regulated their temperature was body size. Smaller dinosaurs had balanced blood flows, whereas larger dinosaurs favored blood flow to different parts of the head — the blood vessel patterns that researchers found goes way beyond what is necessary for simple nourishment. This unbalanced pattern allowed thermal strategies to be more focused, emphasizing one or more cooling regions.
For a while, the team also had a puzzling question. Most of the giant dinosaurs were herbivores, but large predators (such as the 10-ton Tyrannosaurus Rex) also existed — and the researchers’ analysis showed that they had a balanced vascular pattern, like the small dinosaurs.
“This finding had us scratching our heads until we noticed the obvious difference–theropods like Majungasaurus and T. rex had a huge air sinus in their snouts,” Witmer said. But the eureka moment came when the team found evidence that this antorbital air sinus was richly supplied with blood vessels. Witmer had previously shown that air circulated through the antorbital air sinus like a bellows pump every time the animal opened and closed its mouth.
“Boom! An actively ventilated, highly vascular sinus meant that we had another potential cooling region. Theropod dinosaurs solved the same problem…but in a different way,” concluded Witmer.
The team now wants to expand the research to other species of dinosaurs.
The study has been published in the journal Anatomical Record.