It depends largely on whether they were cold- or warm-blooded.
When dinosaurs were first discovered in the mid-19th century, paleontologists thought they were plodding beasts that relied on their environment to keep warm, like modern-day reptiles.
But research during the last few decades suggests that they were faster creatures, nimble like the velociraptors or T. rex depicted in the movie Jurassic Park, requiring warmer, regulated body temperatures.
Now, researchers, led by Robert Eagle of the California Institute of Technology, have developed a new way of determining the body temperatures of dinosaurs for the first time, providing new insights into whether dinosaurs were cold- or warm-blooded. The method could shed new light on the evolution of some of Earth's most enigmatic lifeforms.
"Eagle and colleagues have applied the newest and most innovative techniques to answering the question of whether dinosaurs were warm- or cold-blooded," says Lisa Boush, program director in the National Science Foundation's (NSF) Division of Earth Sciences, which funded the research.
"The team has made important strides in discovering that the body temperature of dinosaurs was close to that of mammals, and that the dinosaurs' physiology allowed them to regulate that temperature. The result has implications for our understanding of dinosaurs' ecology--and demise."
By analyzing the teeth of sauropods--long-tailed, long-necked dinosaurs that were the biggest land animals ever to have lived--the scientists found that these dinosaurs were about as warm as most modern mammals.
"The consensus was that no one would ever measure dinosaur body temperatures, that it's impossible to do," says John Eiler, a co-author and geochemist at Caltech. But using a technique pioneered in Eiler's lab, the team did just that.
The researchers analyzed 11 teeth, unearthed up in Tanzania, Wyoming and Oklahoma, that belonged to the dinosaurs Brachiosaurus and Camarasaurus.
They found that Brachiosaurus had a temperature of about 38.2 degrees Celsius (100.8 degrees Fahrenheit) and Camarasaurus had one of about 35.7 degrees Celsius (96.3 degrees Fahrenheit), warmer than modern and extinct crocodiles and alligators, but cooler than birds.
The measurements are accurate to within one or two degrees Celsius.
"Nobody has used this approach to look at dinosaur body temperatures before, so our study provides a completely different angle on the long-standing debate about dinosaur physiology," Eagle says.
The fact that the temperatures were similar to those of most modern mammals might seem to imply that dinosaurs had a warm-blooded metabolism.
But, the researchers say, the issue is more complex. Because sauropod dinosaurs were so huge, they could retain their body heat much more efficiently than smaller mammals like humans.
The measured temperatures are lower than what's predicted by some models of dinosaur body temperatures, suggesting there is something missing in scientists' understanding of dinosaur physiology.
These models imply that dinosaurs were so-called gigantotherms, that they maintained warm temperatures by their sheer size.
To explain the lower temperatures, the researchers suggest that dinosaurs could have had physiological or behavioral adaptations that allowed them to avoid getting too hot.
The dinosaurs could have had lower metabolic rates to reduce the amount of internal heat. They could also have had something like an air-sac system to dissipate heat.
Previously, researchers have only been able to use indirect ways to gauge dinosaur metabolism or body temperatures.
For example, they inferred dinosaur behavior and physiology by figuring out how fast dinosaurs ran based on the spacing of dinosaur tracks, studying the ratio of predators to prey in the fossil record, or measuring the growth rates of bone.
But these lines of evidence were often in conflict.
"For any position you take, you can easily find counter-examples," Eiler says. "How an organism budgets the energy supply it gets from food, and creates and stores the energy in its muscles--there are no fossil remains for that."
Eagle, Eiler and colleagues developed what's known as a clumped-isotope technique that shows that it's possible to determine accurate body temperatures of dinosaurs.
"We're getting at body temperature through a line of reasoning that I think is relatively bullet-proof, provided you can find well-preserved samples," Eiler says.
In this method, the researchers measured the concentrations of the rare isotopes carbon-13 and oxygen-18 in bioapatite, a mineral found in teeth and bone.
How often these isotopes bond with each other--or "clump"--depends on temperature.
"What we're doing is special in that it's thermodynamically-based," Eiler says. "Thermodynamics, like the laws of gravity, is independent of setting, time and context."
Because thermodynamics worked the same way 150 million years ago as it does today, measuring isotope clumping is a reliable technique, says Eiler.
Identifying the most well-preserved samples of dinosaur teeth was one of the major challenges of the analysis.
The scientists used several ways of finding the best samples. For example, they compared the isotopic compositions of resistant parts of teeth--the enamel--with easily altered materials like the fossil bones of related animals.
The next step, the researchers say, is to determine the temperatures of more dinosaur samples, and extend the study to other species of extinct vertebrates.
In particular, discovering the temperatures of unusually small and young dinosaurs would help test whether dinosaurs were indeed gigantotherms.