Warm-blooded dinosaurs

For a long time, dinosaurs were thought to be just big reptiles, cold-blooded like modern crocodiles, snakes, lizards etc. Recently people began to wonder whether dinosaurs were more mammal- or bird-like, whether they could control their body temperatures.

Generally, fish and reptiles have the same body temperature like the air or water that surrounds them. Mammals and birds control their body temperature from inside. They always keep their bodies equally warm, no matter what the outside temperature is. They do this by burning extra food - no less than ninety percent of what mammals and birds eat is used to keep their body temperature. In other words, a herd of antelopes would feed (for a certain amount of time) one lion, or ten crocodiles of the same weight.

In any fauna the proportion of the meat-eaters to plant-eaters varies according to whether the meat-eater is warm- or cold-blooded. A warm-blooded lion eats ten times more than a cold-blooded crocodile of the same weight. This tells you whether the meat-eater is warm-blooded or not, but it tells you nothing about the plant-eaters.

So, we can make an analogy in the fossil faunas. If we find ten meat-eating dinosaurs for every hundred of plant-eating dinosaurs, we might say that the meat-eaters were cold-blooded. If we find only one meat-eating dinosaur for every hundred of plant-eating dinosaurs, we can say that the meat-eater was warm-blooded. As it happens, we find very low numbers of meat-eaters in fossil faunas, and this is what brought us to the idea of the warm-blooded dinosaurs.

But, this isn't completely proving the theory. We find very big meat-eaters eat similar amounts of food no matter they are cold- or warm-blooded. There are also some plant-eaters, which are protected by their size. Elephants are hardly ever attacked by lions because they are just too big. Apatosaurus was probably also too big for Allosaurus to attack. So we should not include Apatosaurus in this calculation. But Allosaurus may have fed on the dead bodies - so we cannot tell how much it really ate.

Bone structure

The other main evidence of dinosaurs being warm-blooded is the bone structure. Most living reptiles have slow-growing bones. These bones are solid, with noticeable rings. Mammals have fast-growing bones with lots of canals for blood vessels. The dinosaur bones look exactly like mammal bones.

Once again, we cannot prove it. Only large mammals have lots of canals in their bones. Small mammals and birds have rather solid bones, just like crocodiles, and sea turtles have bones with canals. So, lots of canals in the bones seem to indicate large size of the animal. And we don't have to explore dinosaurs' bone structure to know that they were big.

So, we cannot prove that the dinosaurs were warm-blooded in the way that mammals are today. But, they were different from modern reptiles. Most scientists have agreed that it is likely that the small and medium two-legged dinosaurs were warm-blooded with some inside control of body temperature. This is not proven, and scientists need to study fossil bones in more detail. The whole story is surely way more complicated than we think.

Scientists who study alligators and crocodiles have noticed that large animals have far more constant body temperature than the small ones, just because of their large size. This is easy to explain: larger animals have better insulation because they have more thick layers of fat and flesh under the skin, which keeps them warm. Now, imagine how well insulated would a big dinosaur be!

So, the big dinosaurs were warm-blooded, but not in the way that we and other mammals and birds are - this was a by-product of their large size. Also, they did not use ninety percent of their food to keep them warm. For instance, just look at the pictures of Apatosaurus or Diplodocus and examine the size if their mouths. Think how difficult would it for Diplodocus be to eat ten times its ordinary amounts of food (phew!)