The first thing to say is that it may be meaningless to ask about the metabolism of dinosaurs in general, when the Dinosauria are so varied. Remember that we're talking about a group that varies in mass by a factor of forty thousand (2.5kg for Compsognathus longipes and 100 tonnes for Argentinosaurus huinculensis.)
In the same way, although we conveniently think of mammals and birds as warm-blooded and reptiles and fish as cold-blooded, the truth - as is so often the case - is more complex.
Some fish, including the tuna, are mildly warm-blooded, in as much as they maintain the body temperature consistently higher than that of the water in which the swim. Similarly, pythons may maintain a high body temperature for months at a time, when incubating their eggs, and some large turtles also maintain body temperature above that of the water in which they swim.
Conversely, some mammals can be thought of cold-blooded. For example, the smaller bats are unable to maintain their temperatures overnight while roosting, and may approach ambient temperature. Other examples include hedgehogs, bears, and indeed all mammals which hibernate - a long-term lowering of body temperature.
Some more and better examples would help here.
Before looking closely at the evidence on dinosaur metabolism, we need to define our terms.
There are several closely related issues here. Ultimately, we are interested in whether dinosaurs had active lifestyles like those of mammals (characterised by constant activity) or like those of reptiles (with long periods of rest and basking punctuated by bursts of activity.)
This lifestyle is made possible by two factors: the ability to metabolise oxygen efficiently; and a constant, high body temperature, at which the chemical reactions which drive the body can proceed rapidly.
Are these two factors related?
Hence, we are interested in:
At this point the debate tends to get a bit blurred, because (with a few exceptions) all existing animals with constant, high body temperature achieve it via the thermal strategy in which the body's temperature is actively maintained at a high level by a high metabolic rate, aided by insulation in the form of fur (for mammals) or feathers (for birds). This strategy is called endothermy - that is, ``heat from within''.
In pretty much all other existing animals, the alternative to endothermy is ectothermy, meaning ``heat from without''. Ectothermic animals are unable to warn their own bodies, but must arrange for them to be warmed by some outside agency, which is why lizards spend so much time basking on rocks, especially in the morning: they are raising the temperature of bodies which have become cold during the night, and this must be done before any energetic activity, such as hunting for food, is undertaken.
The fairly clear endotherm/ectotherm division among contemporary animals tends to blind people to the fact that there are other possible strategies for maintaining high body temperatures.
For example, one alternative body temperature maintenance strategy - unfortunately one which leaves no trace in the fossil record - is controlling the rate of fermentation of food in the gut. For animals with large guts, the amount of heat produced in this way could be significant and the fact that no animal alive today controls this rate as a means of modulating body temperature should not lead us to the conclusion that it's impossible. In our haste to think of dinosaurs by analogy with mammals, reptiles, or birds, it's sometimes easy to forget that they were entirely different animals from anything we have with us today. There's no reason they couldn't have regulated temperature in an entirely different way.
Am I right that no existing animal uses this method of thermo-regulation? Does anyone have a counter-example?
Is there a technical term for this thermo-regulatory strategy?
Another alternative to endothermy and ectothemy is for large animals to maintain a close-to-constant body temperature simply by means of their bodies' tendency to retain heat, due to their high volume-to-surface-area ratio. (In animals of the same shape, volume varies with the cube of the length, while surface area varies only with the square of the length: so an animal twice as long as another has twice as high a volume-to-surface-area ratio as its smaller cousin.)
This strategy is called inertial homeothermy, meaning roughly ``the tendency of mass to retain the same temperature''. The large, warm-blooded turtles mentioned above achieve their high body temperature by this means - smaller turtles can't do it. (Interestingly, large crocodiles, which weigh more than the largest turtles, do not appear to be inertially homeothermic - presumably because a croc's longer, thinner body has more surface area than the compact body of an equivally heavy turtle.)
Am I right about the crocodiles?
Our problem is that, with all the dinosaurs having been dead for sixty-five million years (yes, OK, except the birds, if you must), we can't make direct measurements of any of the variables that would help us to resolve the question of dinosaur body temperatures. If we could measure, for example, the rate at which dinosaurs consume oxygen when at rest, we could determine their base metabolic rate; or if we could get a thermometer into the appropriate part of the anatomy - a risky business - we could measure the internal temperature directly. In the absence of such data, we must reason from more indirect evidence.
Fortunately, there's plenty of it.
Unfortunately, most of it is equivocal.
It's been noted that the two existing classes of endothermic animals - the mammals and birds - are also the two only classes that walk erect (that is, with the legs underneath the body rather than sprawled out sideways as in reptiles and amphibians.) Crocodiles have a semi-erect, semi-sprawled posture, and chameleons have a close to erect posture, but are not quite there; both cases are very much the exception to the general rule among reptiles.
Since dinosaurs also had erect posture - their skeletons tells us this unambiguously - it doesn't seem unreasonable to assume that they too were endotherms.
But why should this be? In the 60s, John Ostrom postulated that endothermy (or at least high, constant body temperature, however achieved) is actually a pre-requisite for the erect posture, as erect animals must perpetually support their own weight, whereas sprawlers spend most of the time resting their bodies on the ground between their feet. The best part of forty years on, this connection has yet to be proved, but it is at least suggestive.
We can go further by looking at dinosaur skeletal features beyond the simple erect posture. Many groups of dinosaurs, including the ornithomimids, dromaeosaurs, troodontids and even tyrannosaurs are clearly built as cursorial animals (that is, with special adaptations for running.) Skeletal features are shared with fast-running flightless birds and even fast quadroped mammals such as horses, cheetahs and antelopes: these include the enlongation of the tibia relative to the femur, the enlongation of the toes, etc.
What are the other cursorial adaptations?
It seems nonsensical that these adaptations should have arisen in animals that lacked the metabolism to make us of them for sustained running, so this seems to give us more evidence that at least these groups of dinosaurs had high metabolism.
What exactly is the relation between metabolism and body temperature?
A final related piece of evidence comes from fossilised dinosaur tracks. According to McNeill Alexander's Dynamics of Dinosaurs and Other Exinct Giants, there theropod tracks in Texas which indicate running speeds on the order of 12 meters per second (about 27 miles per hour.) This compares favourably with the best human sprinters (10 m/s) and respectably with zebras, giraffes and various antelopes as measured in the wild (11 to 14 m/s), although it's not as fast as specially bred race-horses (17 m/s) and greyhounds (16 m/s). Nevertheless, an animal that runs as fast as an antelope might be expected to have a metabolism able to sustain that pace for long enough to be worthwhile.
(The value of the evidence from fossil trackways is made less clear by the confusion that seems to exist concerning reptilian stamina levels. Some sources imply that reptiles can sustain fast movement for only a matter of seconds: development of cursorial adaptation that could be used only for such short durations would be surprising. On the other hand, there are other sources which describe crocodiles strenuously resisting capture for up to half and hour before becoming exhausted: An ectothermic cursorial dinosaur seems a reasonable proposition if it could run for that long before needing to stop and rest.)
If we accept, as most palaeontologists now do, that birds are descended from coelurosuarian dinosaurs, then we have to consider at what stage in the evolution of birds endothermy appeared. It must surely have been in place for the first flyers, since by universal consent, a high metabolic rate is absolutely necessary in order to sustain powered flight. So if Archaeopteryx was endothermic, why should not its immediate ancestors have been?
This line of reasoning has been considerably strengthened in recent years with the discovery of feathered skin impressions from five separate dinosaurs (although, slightly suspiciously, all from the same locality in China):
The significance of feathers is that they make sense only for endothermic animals, as they are an insulation. For animals which generate their own heat internally, feathers help to prevent that heat radiating away; but for ectotherms, feathers (like fur) would only hinder the basking process. So we can conclude that all feathered dinosaurs were endothermic. (Or at least that they maintained a high, constant body temperature, perhaps by inertial homeothermy in the larger species - but while that may be an option for Beipiaosaurus, it would have been impossible in smaller animals such as Caudipteryx.)
Most experts believe that dinosaurs and pterosaurs evolved from a common ornithodiran ancestor. If we accept that pterosaurs must have been endothermic in order to sustain powered flight (and the smaller species could not have been gliders, even if the larger ones were), then we may consider that the common ancestor was likely also to have been endothermic, so that the most primitive dinosaurs shared this property.
It's tempting to go further, and say that therefore all dinosaurs must have been endothermic, but it doesn't necessarily follow: there is some evidence that certain prehistoric crocodiles were endotherms, but that condition has ben lost in modern crocodiles, perhaps because of their aquatic lifestyles. It's possible, then, that even if primitive dinosaurs were endothermic, their descendents may not have been.
The pterosaur line of argument is not a strong one, having about it a strong whiff of ``if this, then that''. Nevertheless, it is one more shovelful of evidence to fling onto the endothermy bandwagon.
fibro-lamellar (dense primary) => fast growth => high metabolism
but some bones show growth rings (rare but not unknown in mammals/birds)
haversian bone (rich in blood vessels) (but also in large ectotherms such as crocodiles and turtles; absent in some small endotherms)
crocs have this but with hole between ventricles
needed for long necks and heads held high
Stop press: literally as I write this (28th November 2000), I read news of a newly-published paper in which ### compare growth-rings in three theropod species and in contemporary crocodilians, observing that the rings are more pronounced in the crocs. From this, they conclude that the dinosaurs had more stable metabolism than crocodiles (though this doesn't tell us anything about how how they compare with contemporary mammals - surely a fruitful follow-up study for someone!)
A six-tonne african elephant eats 250kg of food per day (1/24th of its body mass) and spends thirteen hours of every twenty-four gathering it, despite its huge head.) Lions need to eat their body mass every nine days, wild dogs every seven days, and Komodo dragons only every ninety days.
Huh? You want a conclusion? :-)
Just kidding. Of course I'll give you a conclusion. The important thing to realise is that any conclusion in a debate as wide-ranging as this one, taking into account so many factors, so many of which are still ill-understood, must necessarily pass out of the domain of fact into that of opinions.
With that disclaimer out of the way, my feeling is that dinosaur skeletons tell us enough for us to be pretty sure that they led active lifestyles, which entails a high, more or less constant, body temperature. Whether they achieved this temperature by means of mammal-like endothermy or inertial homeothermy seems to me a very much less important question. The answer is probably ``both and neither'' - no doubt dinosaur metabolism made use of their mass to maintain stability of body temperature; but for smaller dinosaurs, and especially those which gave rise to the birds, it seems perverse to argue that they didn't also maintain body temperature internally.
Informed opinion is that the smaller theropods and ornithopods were the dinosaurs most likely to be endothermic in the manner of modern mammals and birds. It happens that these kinds of dinosaur also have the largest relative brain size. It's interesting to speculate on whether there's a causal link between these two properties, or whether it's just a coincidence (but I'm not going to do it here.)
In the end, there's no reason to assume that animals which in so many ways are so different from anything alive today had a metabolic strategy exactly like any of the extant groups.