Unless you’ve been fossilized in a cave for the last few decades, you’ve probably heard about the debate over whether dinosaurs were cold-blooded or warm-blooded. Researchers have attacked this question using computer modeling to determine things like body mass and heat loss rates, or compared locomotion and energy use. They’ve studied bone structure and they’ve even used oxygen isotopes in those bones to help determine body temperatures. High growth rates observed in bones have suggested high metabolic rates (i.e., warm-blooded or endothermic), but modeling has shown that very large dinosaurs, if endothermic, would probably have had problems with overheating because they wouldn’t shed heat quickly enough. Nothing has settled the debate.

A new method for determining temperature using isotopes is now being applied to dinosaurs, and promises to provide hard data that could advance the debate. This new method, called "clumped isotope thermometry," is based on the tendency of a heavier isotope of carbon (13C) to preferentially bond with a heavier isotope of oxygen (18O) during calcite formation in teeth and bones. The lower the temperature, the more those heavy carbons and oxygens tend to "clump" together instead of being scattered about randomly.

This analysis is much more accurate than traditional oxygen isotope thermometry, which requires assumptions about the oxygen isotopic content of the water involved in calcite formation, assumptions that increase the uncertainty in the calculation. The clumped isotope method, which requires no knowledge of initial conditions, has been demonstrated on modern animal bones to be accurate within about 1°C.

The researchers analyzed teeth from two types of sauropod, Brachiosaurus and Camarasaurus, and calculated a body temperature of 36-38°C (96.8-100.4°F). So that settles it: they were warm-blooded, right? Not really—remember, it's not a question of body temperature, it's a question of how they maintain that body temperature. Do they generate heat with a high metabolism (endothermic), or do they keep a low metabolism and absorb heat from outside the body (ectothermic)? Unfortunately, merely knowing the temperature range doesn't answer these questions.

So, what can this information tell us? The researchers compare the calculated body temperature to several groups of modern animals. It’s 5-12°C higher than temperatures maintained by crocodiles and alligators, but lower than most birds, which tend to stay above 40°C. As you may have noticed, it’s right on the dot for mammals like us (and even large ones like elephants and whales).

Although it won't settle the debate, this new information can provide constraints for evaluating models, constraints that were previously much too fuzzy to be definitive. For example, take the best estimate of body temperature for an endothermic creature that size, and figure out whether plausible metabolic and heat dissipating adaptations could get body temperature down to 38°C. If not, would an ecothermic metabolism make the numbers work? With enough work, we could puzzle out which scenario is better supported by the evidence. Data gathered via this clumped isotope analysis may allow researchers to turn the corner and finally get some real answers about the physiology of these ancient beasts.

Science, 2011. DOI: 10.1126/science.1206196 (About DOIs).

Listing image by Photo by Flickr user davidd