Hot and Cold Running Dinosaurs

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Earlier this month I wrote about a new scientific paper that described an ancient, dinosaur-filled habitat that existed in what is now Siberia. Commenter Naruto raised a point of confusion to many;

I think there is a mistake in this article. The mistake is at the second paragraph, on the last line. “growing understanding that they were not cold-blooded creatures.”, and I think the right one should be “growing understanding that they were cold-blooded creatures.” The “not” shouldn’t be in that line. …

In order to answer this question we have to untangle what phrases like “warm-blooded” and “cold-blooded” really mean, especially since they can be more confusing than helpful.

Let’s start with the “cold-blooded” animals like fish, amphibians, and reptiles. Their body temperatures fluctuate with that of their surrounding environment, which means they are ecothermic. This does not automatically mean that these animals are sluggish, though. If the temperature of their surrounding environment is high enough they can be very active (meaning that they are literally “warm-blooded” in those circumstances), and some of these animals even have special physiological mechanisms that help them maintain a high body temperature. Great white sharks, for instance, are able to keep their body temperature several degrees Celsius above the temperature of the cold coastal waters they inhabit.

The animals we often refer to as being “warm-blooded,” by contrast, are more aptly described as being “endothermic.” This means that they generate their own body heat and often keep it at a relatively high, constant temperature. Living mammals and birds are the main examples of this kind of physiology, but there are some species that can switch between being endothermic and ectothermic. Some small birds and bats are endothermic for part of a day or part of the year but ectothermic during other parts. They are so small and burn energy so fast that if they were not able to switch their metabolisms, they would have to constantly collect food or they would die.

So, were dinosaurs ectothermic, endothermic, or something else entirely? Read more after the jump.

It is difficult to say, but they definitely were not “cold-blooded” in the sense that they were slow, stupid, and could only survive as long as the global thermostat stayed above 65 degrees Fahrenheit. Given that dinosaurs were a very diverse group of vertebrates it is probable that different groups had different physiologies. The immense sauropods, for instance, were so big that even if they were ecothermic they could have maintained a high body temperature. The bigger an animal is, the harder it is for them to gain or lose heat, so sauropods might have been endothermic when they were young but became more ectothermic as they became larger. A high, internally-generated body temperature is energetically expensive to maintain, and the largest of dinosaurs may have undergone a physiological shift that allowed them to remain active but not have to spend their entire lives eating.

If any dinosaurs were endothermic in the way that living mammals and birds are, however, it was the small predatory dinosaurs closely related to birds. The close association of dinosaurs like Deinonychus and Dromaeosaurus with birds suggests that they might have been endothermic, and this is reinforced by the presence of this kind of dinosaur within the Arctic Circle. Even though the world was warmer in the Cretaceous than it is today, it could still get very cold, cold enough to snow, in the highest latitudes. If dinosaurs were physiologically like crocodiles or lizards they probably could not survive in such a cold place, but discoveries in Siberia and Alaska show diverse communities of dinosaurs might have lived there year round. This suggests that many dinosaurs were endothermic and could internally maintain a high body temperature, especially the small dinosaurs that would more quickly lose heat if they were ectothermic.

Unfortunately we can’t take the temperature or study the physiology of any non-avian dinosaur today, but the evidence suggests that if they were not fully endothermic like most modern birds and mammals, then dinosaurs had another physiological strategy that allowed them to maintain high body temperatures. The idea that they were “cold-blooded” animals just like living lizards has gone extinct.

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