We’re slowly getting used to the idea that some dinosaurs may have looked more like birds than reptiles. But there’s been a long-running debate about what dinosaurs were like on the inside – and specifically on how they kept warm. Were they warm-blooded like modern birds and mammals, or cold-blooded like modern lizards and crocodiles?

Now new evidence has emerged for the case that they were somewhere in between, neither just regulating their body temperature using internal energy (endothermy) nor relying solely on the warmth of their environment (ectothermy). Working out how dinosaurs’ bodies worked is very difficult because we usually only have fossils of their bones. Almost all of the soft tissue and organs have usually rotted away. But the new evidence in this instance has come from a surprising place: fossilised eggshells.

Dinosaurs have always elicited wonder because of their great size and their dominance over the land for 170m years. Admittedly, many dinosaurs were turkey-sized, but most of our attention has focused on the giants. Some, like the sauropods Brachiosaurus or Brontosaurus, may have weighed as much 50 tonnes – ten times the mass of the largest living elephant.

Dinosaur physiologists wonder how such vast creatures could have operated. How did they find enough food each day? How did they pump blood up their long necks? How did they manage their body temperature so they neither overheated in summer nor froze in winter?

Gerald Grellet-Tinner

The thermophysiology debate flared in the 1970s, when a daring young researcher named Bob Bakker proposed that all dinosaurs were full endotherms (warm-blooded), just like their descendants the birds. Initially the debate was hugely polarised, with Bakker’s opponents claiming dinosaurs must have shared more with their other living relatives, the crocodiles.

Soon, the physiologists pointed out that much of the argument was unnecessary. Modern vertebrates can vary in many subtle ways between full-scale endothermy and ectothermy. So it’s reasonable to believe that dinosaurs could have fallen somewhere in between as well.

Too big to cool

Another theory, known as mass homeothermy, suggests some dinosaurs were so big that their bodies would have maintained a relatively stable temperature. Organisms gain and lose heat in proportion to their cross-sectional area or mass. A small animal like a mouse has to eat far more in proportion to its body mass than a large animal such as an elephant because the mouse radiates heat faster.

A big warm-blooded animal has greater thermal inertia – meaning it cools more slowly at night and warms up more slowly during the day – than a smaller animal of similar shape. When the alternative is eating huge amounts of food to keep the body’s furnaces firing, this thermal stabilisation by mass makes sense.

The discovery that some theropod dinosaurs had avian-style feathers helped confirm that the smaller dinosaurs, like birds, probably used some kind of endothermy. The theory was further strengthened when evidence was found for feathers even in early, scaly dinosaurs. Animals that possessed feathers covering their bodies, even simple bristle-like structures, must have been insulated and so must have had some form of endothermy.

Gerald Grellet-Tinner

The new research, published in Nature Communications, relies on a different kind of evidence from the structure of fossilised dinosaur eggs shells. The researchers, led by Robert Eagle of the University of California, Los Angeles, used a new geochemical technique called clumped isotope analysis. They looked at the kind of oxygen and carbon atoms the shells were formed from and worked out the corresponding temperature at which they were created. They argue the egg shell is formed deep within the mother’s oviducts and so records her core body temperature.

By analysing exceptionally preserved eggshells from different types of dinosaur in Mongolia and Argentina, they found that giant sauropods such as Titanosaurus maintained body temperatures around 38℃, similar to modern large mammals. Meanwhile, small to medium-sized, bipedal and lanky oviraptorid theropods such as _Oviraptor _had temperatures of around 32℃, lower than most modern endotherms but about 6℃ higher than the typical environmental temperature of the time. If these animals maintained their body temperatures above the background environmental temperature, they were by definition endotherms, however they achieved it. But it is most likely that it was via their internal mechanisms.

The clumped isotope method is controversial – and critics may reject the exact values obtained in this study. But the researchers are at pains to justify their values, rejecting cases in which the eggshell has been altered by later physical processes and comparing measurements with those from surrounding carbonate rocks. They have also calibrated these fossil eggshell measurements with measurements from the eggshells of modern reptiles and birds.

What’s particularly surprising about the results is that they suggest oviraptorid dinosaurs, which had feathers and so were probably more similar to modern birds, had lower body temperatures than the more reptilian sauropods. This means they probably used an intermediate, less-developed mode of endothermy to keep warm, whereas modern birds tend to be fully endothermic. The sauropods probably kept their temperature stable using their large mass, but why they were hotter is still a mystery.