Hot and Cold Running Dinosaurs

An Ultraraptor attack: Did it kill in cold blood? (Copyright Lee Krystek 1996)

The velociraptor sneaks up to the window and looks through. His hot breath fogs the glass. Through the mist he draws a bead on his next victims...

Was this scene from the movie Jurassic Park accurate? Were dinosaurs warm-blooded? Did they have the kind of hot breath that would have fogged up a window?

The question of whether dinosaurs were warm-blooded, like mammals and birds, or cold-blooded, like reptiles and fish, has been debated since dinosaur fossils were first discovered in 1825.

Cold-blooded, or ectothermic, animal's bodies stay about the same temperature as the environment around them. This means cold-blooded animals tend to be sluggish in cool weather. In the morning a lizard must sun itself on a rock to warm up before it can take on any vigorous activity.

Warm-blooded, or endothermic, animals generate their own heat and maintain a nearly constant temperature no matter what conditions are like outside the body. This means that an animal, like a dog, can be active in even freezing weather.

Richard Owen, who named the dinosaur suborder in 1841, speculated early on dinosaurs were a version of reptiles that were very active and perhaps warm-blooded. Despite this scientists that followed Owen classified dinosaur behavior as lizard-like: Generally sluggish animals that dragged their tails around because they couldn't lift them. Some scientists even speculated that large sauropods, like the Apatosaurus were so weak they could only stand by half floating in lakes.

This picture began to change in 1964 when John Ostrom, a paleontologist at Yale, found a long, thin, scythe-like claw in the rock next to the skeleton of a Deinonychus. "Startling. I couldn't believe what I saw, " recalls Ostrom.

Close examination showed that the claw, similar to ones found on the animal's hands, had been attached to the foot. Ostrom wondered of what use a knife-like claw on the toe would be. His conclusion was that the animal must have attacked by jumping and slashing with its feet and hands while it counter-balanced with its stiff tail. Very unreptile-like behavior.

Ostrom, along with paleontologist Robert Bakker, began to argue that dinosaurs were much more active than had been thought. The animals they pictured walked, even ran, with their feet squarely under their shoulders. Ostrom, Bakker and others also maintained that dinosaurs were perhaps more closely related to birds than reptiles. Since birds are warm-blooded, why not dinosaurs too?

Examination of dinosaur fossils seemed to suggest they were indeed warm-blooded. The remains of microscopic holes in the bone seemed to indicate tiny blood vessels had been trapped by the rapid growing bone tissue. Any animal growing that fast must be warm-blooded, scientists reasoned.

Museums soon started to mount their dinosaur exhibits to reflect this new, active, warm-blooded view of the dinosaurs. Skeletons on display were changed to show the animals in leaping and running positions. Now, new evidence has been found that suggests dinosaurs may not have been warm-blooded after all.

Tomasz Owerkowicz, a researcher at Harvard, has worked with living animals, both lizard and mammal, and shown that tiny blood vessels trapped in bone doesn't mean that an animal is warm-blooded, but that the animal is very active. He trained half his lab animals on tread mills everyday for months, while the other half sat around in their cages. Upon examining their bones he couldn't tell the cold-blooded ones apart from the warm-blooded ones. He could only tell the active ones from the couch potatoes.

This alone doesn't show that dinosaurs were cold-blooded, just that the tiny-vessels trapped in their fossilized bones are a sign that they were very active, not necessarily warm-blooded. The new evidence that dinosaurs were cold-blooded comes from John Ruben, a physiologist with Oregon State University. He noticed that all warm-blooded animals alive now, both birds and mammals (including humans), have tiny, thin, curled structures behind their noses called turbinates. Cold-blooded animals don't have them. The purpose of these structures are to cool the warm air as it leaves the body during exhalation.

Since cool air holds less moisture than warm air this causes water to condense on the turbinates (just like droplets condense on the outside of a glass of ice water standing in the hot summer air). An inhalation of dry air causes the droplets on the turbinates to be reabsorbed and go back into the body. This saves the animal from losing precious water. This is especially important for warm-blooded animals because they breathe much more, and much warmer, air in and out than do cold-blooded animals.

Ruben has shown fossilized dinosaur skulls lack the boney ridges that turbinates would have attached to if dinosaurs would had had them. He can find them in fossilized mammals of the same period. This suggests dinosaurs weren't warm-blooded, or at least not warm-blooded like animals today. Ruben notes that this does not mean dinosaurs were sluggish as thought earlier in this century. The evidence still favors very active, but ectothermic, dinosaurs.

Could this be the final word on dinosaur metabolism? Probably not. We are constantly learning more about prehistoric creatures as scientists develop new ways to probe the past. These new discoveries also open new questions. How could dinosaurs be so active and not warm-blooded? Did they have some special way of warming themselves up to get ready to run and fight? We don't know. All we can say for sure is that dinosaurs are even stranger than we had thought.

In April of 2000 scientists who had examined the fossil of a dinosaur found in 1993 announced a startling discovery. A CT (computerized tomogrpahy) of the chest area of a Tescelosaurus dinosaur revealed something that looked like a four chambered heart with a single arched aorta. Normally soft tissue is not preserved in fossils, but the tescelosaurus, found in South Dakota and nicknamed "Willo," was in exceptionally good condition. What is particularly significant about this find is that most reptiles alive today have a three chambered heart. Of those few that do have four chambered hearts, like crocodiles, they also have a double set of arteries. Only warm blooded creatures, like birds, are known to have four chambered hearts with single aortas.

Does this mean that dinosaurs, or at least some dinosaurs, were warm blooded? Not all scientists are sure that the CT really shows a heart. Others believe that dinosaurs may have developed a wide variety of hearts and some species may have been warm-blooded while others remained cold-blooded or somewhere in between. What is certain is that the controversy is not over yet.

Next Stop on Dinosaur Safari

Copyright Lee Krystek 1996. All Rights Reserved.