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.
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!)