Sauropod dinosaurs, the enormous plant-eating dinos with long tails and necks, had body temperatures ranging from 96.3 to 100.8 degrees Fahrenheit -- making them as warm as most mammals -- including people.
Because body temperature usually rises the larger an animal gets, the findings, published in the latest issue of Science, suggest huge sauropods had mechanisms for cooling themselves off.
"What we can say is that sauropods did not have body temperatures that were as cold as modern crocodiles and alligators," lead author Robert Eagle, a researcher at the California Institute of Technology, told Discovery News.
Eagle pointed out that many models had predicted that sauropods would have high body temperatures of over 104 degrees.
"This suggests that sauropods may have had cooling mechanisms to prevent very high body temperatures being reached due to their gigantic size," he said.
So-called "gigantotherms" maintain warm temperatures due to sheer size. Plant-eating dinosaurs may have then been cold-blooded, in the sense that they could have depended on their environment for heat, as opposed to generating it internally, as warm-blooded species do.
"If you're an animal that you can approximate as a sphere of meat the size of a room, you can't be cold unless you're dead," explained co-author John Eiler, a Caltech geochemist.
Eiler, Eagle and their colleagues made the determinations after studying 11 teeth belonging to Brachiosaurus brancai and Camarasaurus dinosaurs.
The scientists measured 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 depends on temperature, so the lower the temperature, the more carbon-13 and oxygen-18 clumps exist. Measuring these clumps revealed the temperature of the environment in which the mineral formed inside the dinosaur.
This geochemical "thermometer" shows Brachiosaurus had a temperature of about 100.8 degrees, while Camarasaurus had a temperature of approximately 96.3 degrees.
Eagle said that Jurassic temperatures were probably "significantly hotter than today," so even if sauropods were cold-blooded they likely were not very restricted, in terms of habitat range, due to climate. In fact, they probably spent a lot of time trying to cool down their hefty bodies.
The researchers suspect the long necks and tails of these animals might have dispelled some heat. Lower metabolic rates might have helped to reduce internal warmth.
"Sauropods could also have had other adaptations to cool themselves down, like internal air-sac systems to dissipate heat or behavioral adaptations, such as searching for shade when it was hot," Eagle said, adding that there is fossil evidence for the air sacs.
Luis Chiappe, director of The Dinosaur Institute at the Natural History Museum of Los Angeles County, told Discovery News that the new study is "very interesting." "I believe the approach based on clumped isotope thermometry is opening a new window in what has been a long and rather difficult controversy," Chiappe said.
Chiappe's colleague John Harris, who is head of vertebrate studies at the L.A. museum, echoed the praise, saying the researchers hit a scientific home run with their "very elegant piece of work that is a major contribution to our understanding of these iconic but enigmatic animals."
Eagle and his team next hope to take the temperature of juvenile and dwarf sauropods to see if age or size variations affected body heat. They are also interested in applying the tooth analysis technique to dinosaur-bird transitional species and to the early ancestors of mammals to try to determine when warm-bloodedness first arose in both the bird and mammal lineages.