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How to take a dino's temperature

Carin L. Cain / AAAS / Science

An artist's conception shows a typical sauropod with a long neck, long tail and massive body.


"What we're basically doing is sticking a thermometer up a dinosaur's butt," study co-author John Eiler, a geochemist at Caltech, told me jokingly.


Past experiments have shown that the heavier isotopes — carbon-13 and oxygen-18 — are more likely to clump together when the carbonates are formed at lower temperatures. At higher temperatures, the bonds are more randomly distributed, and you don't see as many of the heavy isotopes clumping together. The precise proportion of the clumped isotopes can tell you the average body temperature of a toothy organism.

"This is the basis of the 'thermometer,' but it's a thermometer where all the information that allows you to rigorously calculate temperature is preserved in a single phase," Eiler said.


The analysis was done on 11 teeth from Brachiosaurus and Camarasaurus dinosaurs from Tanzania, Oklahoma and Utah. Other samples were judged unsuitable for the sensitive chemical tests. "Did we do it perfectly?" Eiler said. "We believe that we found a result that we're confident in, but it's not easy."


Warm blood ... but warm-blooded?
It turns out that the temperature Eiler and his colleagues came up with — 36 to 38 degrees Celsius, or 97 to 100 degrees Fahrenheit — is consistent with previous estimates produced by other methods. But Eiler said those methods, including one that involves comparing the different oxygen isotopes in dinosaur teeth, resulted in less direct measurements.


The key point behind the research isn't necessarily that dinosaurs had blood as warm as ours. "The deeper question is, how did they get that way?" Eiler said. "What allowed them to get to that temperature? Was it through regulation of their metabolism, like it is for us? Was it simply their size? This is the next step."

Some scientists hold to the view that dinosaurs as a group might not have been strictly warm-blooded (like today's birds and mammals) or cold-blooded (like most reptiles). They suggest that some dinosaurs were big enough to keep their blood warm due to thermal inertia — as alligators and Galapagos tortoises do.

Eiler said the newly published temperature estimates are not consistent with that hypothesis. "They're not as hot as they were supposed to be," he said, "and that tells us there's something about dinosaur physiology that we don't understand."


"If sauropods were like mammals, you would expect the small animals to have the same body temperature as the large ones," Caltech biologist Robert Eagle, the Science study's lead author, told me. But if the temperature readings vary significantly, that would tip the scales back toward the thermal-inertia hypothesis — or it might force scientists to come up with a whole new hypothesis.

Caltech's John Eiler and Robert Eagle discuss the chemical "thermometer."

Beyond dinosaurs

If it turns out that dinosaurs were truly warm-blooded, scientists could work their way backward toward the common ancestor of dinosaurs and crocodiles, to study how body temperature regulation might have evolved during the Permian geologic period, more than 250 million years ago. "The only limitation is whether there's enough well-preserved fossil material," Eagle said. "That doesn't necessarily depend on age. ... It's definitely theoretically possible that we can go all the way back to the Permian."  

Eagle said the clumped-isotope thermometer has already been used on rocks and the shells of marine organisms to study ancient climate change, and now it's being applied to a meteorite from Mars as well. In that experiment, researchers want to find out "whether this meteorite experienced high temperatures on the journey from Mars to Earth," he said. If there's no evidence of high temperatures, that might support claims that organisms could make their way between the planets inside such meteorites — an idea that set off a storm of controversy earlier this year.

Eiler said we'll probably be hearing much more about this temperature-taking technique in the years to come: "It's still recent enough and exotic enough that you wouldn't quite want to call it mature, but it's established. ... Applying it to teeth, I think our lab remains unique in doing that, although I'm sure other labs will be doing it soon."

Update for 11:45 p.m. ET: I followed up with a phone call to Luis Chiappe, the dinosaur expert at the L.A. museum, and he said there may well be other applications for this technique. "I would imagine that the way it happens is, people realize there's a new tool on the table and they will grab it and use it in various ways." For example, Chiappe is an expert on dinosaur eggs, and it's not unthinkable that you could measure egg temperatures to investigate how dinosaur moms hatched their young. Modern-day crocodiles use temperature to determine the sex of their hatchings. Did dinos do the same?

Chiappe agrees that the next task is to test other sizes of dinosaurs.

"If we were to find comparable temperatures in hatchlings of sauropods and juveniles of sauropods, that would definitely support the idea that these animals were endotherms [warm-blooded]. So that needs to be done," he said. "The problem is that the technique is a destructive technique. Therefore, not too many museum curators will be happy to donate their specimens to be destroyed."

More about dinosaur physiology:

"Dinosaur Body Temperatures Determined from Isotopic (C13-O18) Ordering in Fossil Biominerals" was published today on the journal Science's website and will appear in a future issue of the print version. In addition to Eagle and Eiler, the authors include Thomas Tütken, Taylor S. Martin, Aradhna K. Tripati, Henry C. Fricke, Melissa Connely and Richard L. Cifelli.
