During the 1990’s there was a lot of discussion (yelling?) over the question of whether or not Dinosaurs were endothermic, that is, warm blooded. In the regular media there is still a pretty solid leaning toward the idea that they were.
I’m inclined to say they weren’t. Here are two reasons why:
Turbinate Bones
Endothermic (warm blooded) animals have a problem: water loss. In an effort to maintain a constant body temperature (in contrast to cold blooded animals who “go with the flow” of the ambient temperature outside), their metabolisms (the sum total of all chemical reactions in an organism) must run at a breakneck speed nearly all the time–at least in contrast to cold blooded animals (elephants have a rather low metabolism for a mammal, but still higher than a croc). One biproduct of this is water, H2O. Why?
The above equation is a basic cellular respiration equation (metabolism). If you put in six Carbohydrates and six Oxygens you get out six water molecules plus six molecules of Carbondioxyde plus energy (usually in the form of heat and ATP, the energy molecule).
The CO2 loss isn’t a big deal, you just breath it out. Much of the water you lose is through your urine, but you lose some water through breathing out also. This is how you can fog up your windows in the winter.
A Turbinate bone is used to reduce this water loss through your nose. How does it work? Well, in short, it is covered in specialized tissues that are responsible for humidifying, filtering, and heating the air you breath. (it does much more, but we’ll leave it at that for now).
[As a side note, if you sleep with your mouth open, thereby bypassing your turbinate bone in your nose for breathing, you’ll dehydrate more during sleep than if you sleep with your mouth closed–of course, there is the snoring problem].
Mammals and Birds both have these bones. All warmblooded species alive today infact, that have been studied for such things, have a turbinate bone to prevent excessive water loss.
Did dinosaurs have a turbinate bone? No. None of the fossils that have been collected have had them. This is strong evidence against the likelyhood of endothermy.
Climate
The second piece of evidence is in the climate of the era during which dinosaurs lived and evolved. Particularly two things: Oxygen levels and temperature. The late Triassic and Jurassic periods were both warmer (by a lot) than today and had much lower oxygen levels. We’ll cover the temperature problem first since it’s the key.
Being an endotherm in a hot environment is not easy. It’s even harder for a large creature like a big dinosaur because of a quirk of physics called the volume to surface-area ratio.
If you’re small, like a mouse, you have a large surface area relative to your overall volume. This makes heat transfer to and from the environment easy. Too easy sometimes! Rats and mice have blazing fast metabolisms because they need massive amounts of heat to be generated to maintain body temperature (remember that in the above equation, heat was a byproduct).
A lizard doesn’t have to worry about such things. He’s an ectoderm (coldblooded), so he doesn’t need to maintain the same temperature all the time. Instead he just lets himself cool down or heat up with the outside temperature. Once it’s warm enough, he can get going and catch some food. (The correspondingly low metabolism also helps explain why reptiles can go so long between feedings. A rat, in contrast, has to eat like an elephant to stay alive!)
On the other side of the scale, an elephant, a very large warmblooded mammal, has a much much larger volume than it does surface area. since heat transfer happens only at the surface (the skin), this means the elephant can maintain its temperature rather easily and doesn’t need to worry too much about losing heat. This is great if you’re a mastodon in the snow. However, cooling off in a hot temperature environment becomes a huge pain when you’re huge. As a point of interest, most desert animals are lanky to decrease their volume and increase their surface area. In contrast, cold weather creatures are often more “rounded out” to decrease surface area and increase volume, thereby decreasing heat-loss.
If we think of it that way, a warm-blooded, huge bodied dinosaur in a super hot environment makes little sense. From much of the morphological evidence, many dinosaurs appeared to be quite active. We know they were big. But, if they were big and active, that is they would be increasing their metabolic rate during activity (thereby heat) even higher than baseline we have a problem for an endoderm! Especially if they didn’t have a turbinate bone to decrease water loss!
It is more likely that dinosaurs were cold-blooded (or at least not warmblooded; there is some speculation about gray areas in these matters). A cold blooded animal in a hot environment can be quite active. And they don’t run the serious risk of overheating the way that warmblooded creatures do. Tuna are a good example.
The last thing is a side issue about oxygen levels. Oxygen levels were quite low during the period of time that the dinosaurs rose to power. But, there is mounting evidence that dinosaurs (at least the saurischians) had a specialized kind of air-sac lung system, the same kind that modern birds have (who likely evolved from this subset of dinosaur). This type of lung is FAR more efficient than the one hanging out in your chest (or any chest of a mammal). It’s why birds can fly at altitute and still be so active (they’re flying for heaven sakes!), and we need oxygen masks.
A coldblooded dinosaur with an air-sac lung could have thrived in an environment that was hot and oxygen poor.
FOLLOW UP:
The debate on this topic is far from over. I tend to side with the idea that Saurischian Dinosaurs were ectotherms, but there are plenty of arguments to throw around.
For more on the topic, here’s a site at Berkeley about it.
And the top five hypothesis:
- Dinosaurs were complete endotherms, just like birds, their descendants.
- Some or all dinosaurs had some intermediate type of physiology between endothermy and ectothermy.
- We know too little about dinosaurs to hazard a guess at what their physiology was like.
- Dinosaurs were mostly inertial homeotherms; they were ectothermic but maintained a constant body temperature by growing large. Small dinosaurs were typical ectotherms, maybe with a slightly elevated metabolic rate.
- All dinosaurs were simple ectotherms, enjoying the warm Mesozoic climate. But that’s okay; many ectotherms are quite active, so dinosaurs could be active, too.
I’m diggin’ on number 4, with a nod to number 5.
Oh, who am I kidding! I eat upwards of 1 pound of beef a day. And the amount of fat that gets rendered out via the ever amazing George Foreman Grill could power a small city. 
Sapien Games 