Because there's so much confusion about what it means for any creature-not just a dinosaur-to be “cold-blooded” or “warm-blooded,” let's start our analysis of this issue with some much-needed definitions.
Biologists use a variety of words to describe a given animal's metabolism (that is, the nature and speed of the chemical processes taking place inside its cells). In an endothermic creature, cells generate heat that maintains the animal's body temperature, while ectothermic animals absorb heat from the surrounding environment.
There are two more terms of art that further complicate this issue. The first is homeothermic, describing animals that maintain constant internal body temperature, and the second is poikilothermic, which applies to animals whose body temperature fluctuates according to the environment. (Confusingly, it's possible for a creature to be ectothermic, but not poikiothermic, if it modifies its behavior in order to maintain its body temperature when faced with an adverse environment.)
What Does it Mean to Be Warm-Blooded and Cold-Blooded?
As you may have surmised from the above definitions, it doesn't necessarily follow that an ectothermic reptile literally has colder blood, temperature-wise, than an endothermic mammal. For example, the blood of a desert lizard basking in the sun will temporarily be warmer than that of a similar-sized mammal in the same environment, though the lizard's body temperature will drop with nightfall.
Anyway, in the modern world, mammals and birds are both endothermic and homeothermic (i.e., “warm-blooded”), while most reptiles (and some fish) are both ectothermic and poikilothermic (i.e., “cold-blooded”). So what about dinosaurs?
For a hundred or so years after their fossils began to be dug up, paleontologists and evolutionary biologists assumed that dinosaurs must have been cold-blooded. This assumption seems to have been fueled by three intertwined lines of reasoning:
1) Some dinosaurs were very big, which led researchers to believe that they had correspondingly slow metabolisms (since it would take a huge amount of energy for a hundred-ton herbivore to maintain high body temperature).
2) These same dinosaurs were assumed to have extremely small brains for their large bodies, which contributed to the image of slow, lumbering, not-particularly-awake creatures (more like Galapagos turtles than speedy Velociraptors).
3) Since modern reptiles and lizards are cold-blooded, it made sense that “lizard-like” creatures like dinosaurs must have cold-blooded, too. (This, as you may have guessed, is the weakest argument in favor of cold-blooded dinosaurs.)
This received view of dinosaurs began to change in the late 1960s, when a handful of paleontologists, chief among them Robert Bakker and John Ostrom, began to promulgate a picture of dinosaurs as fast, quick-witted, energetic creatures, more akin to modern mammalian predators than the lumbering lizards of myth. The problem was, it would be extremely difficult for a Tyrannosaurus Rex to maintain such an active lifestyle if it was cold-blooded--leading to the theory that dinosaurs may, in fact, have been endotherms.
Arguments in Favor of Warm-Blooded Dinosaurs
Because there are no living dinosaurs around to be dissected (with one possible exception, which we'll get to below), most of the evidence for warm-blooded metabolism stems from modern theories about dinosaur behavior. Here are the five main arguments for endothermic dinosaurs (some of which are challenged below, in the "Arguments Against" section).
- At least some dinosaurs were active, smart, and fast. As mentioned above, the main impetus for the warm-blooded dinosaur theory is that some dinosaurs exhibited “mammalian” behavior, which entails a level of energy that (presumably) can only be maintained by a warm-blooded metabolism.
- Dinosaur bones show evidence of endothermic metabolism. Microscopic analysis has shown that the bones of some dinosaurs grew at a rate comparable to modern mammals, and have more features in common with the bones of mammals and birds than they do with the bones of modern-day reptiles.
- Many dinosaur fossils have been found at high latitudes. Cold-blooded creatures are much more likely to evolve in warm regions, where they can use the environment to maintain their body temperatures. Higher latitudes entail colder temperatures, so it's unlikely that dinosaurs were cold-blooded.
- Birds are endotherms, so dinosaurs must have been too. Many biologists consider birds to be “living dinosaurs,” and reason that the warm-bloodedness of modern birds is direct evidence for the warm-blooded metabolism of their dinosaur ancestors.
- Dinosaurs' circulatory systems required a warm-blooded metabolism. If a giant sauropod like Brachiosaurus kept its head in a vertical position, like a giraffe, that would have put enormous demands on its heart--and only an endothermic metabolism could fuel its circulatory system.
Arguments Against Warm-Blooded Dinosaurs
According to a few evolutionary biologists, it's not enough to say that because some dinosaurs may have been faster and smarter than previously assumed, all dinosaurs had warm-blooded metabolisms--and it's especially tricky to infer metabolism from presumed behavior, rather than from the actual fossil record. Here are the five main arguments against warm-blooded dinosaurs.
- Some dinosaurs were too large to be endotherms. According to some experts, a 100-ton sauropod with a warm-blooded metabolism would likely have overheated and died. At that weight, a cold-blooded dinosaur could have been what's called an “inertial homeotherm”--that is, it warmed up slowly and cooled off slowly, allowing it to maintain more-or-less constant body temperature.
- The Jurassic and Cretaceous periods were hot and muggy. It's true that many dinosaur fossils have been found at high altitudes, but 100 million years ago even a 10,000-foot-high mountain peak might have been relatively balmy. If the climate was hot year-round, that would favor cold-blooded dinosaurs that relied on outside temperatures to maintain their body heat.
- We don't know enough about dinosaur posture. It's not certain that Barosaurus elevated its head to forage for grub; some experts think large, herbivorous dinosaurs held their long necks parallel to the ground, using their tails as q counterweight. This would weaken the argument that these dinosaurs needed warm-blooded metabolisms to pump blood to their brains.
- The bone evidence is overrated. It may be true that some dinosaurs grew at a faster clip than previously believed, but this may not be evidence in favor of a warm-blooded metabolism. One experiment has shown that modern (cold-blooded) reptiles can quickly generate bone under the right conditions.
- Dinosaurs lacked respiratory turbinates. To supply their metabolic needs, warm-blooded creatures breathe about five times as often as reptiles. Land-dwelling endotherms have structures in their skulls called “respiratory turbinates,” which help retain moisture during the respiration process. To date, no one has found conclusive evidence of these structures in dinosaur fossils-hence, dinosaurs must have been cold-blooded (or, at least, definitely not endotherms).
Where Things Stand Today
So, what can we conclude from the above arguments for and against warm-blooded dinosaurs? Many scientists (who are unaffiliated with either camp) believe that this debate is based on false premises--that is, it's not the case that dinosaurs needed to be either warm-blooded or cold-blooded, with no third alternative.
The fact is, we don't know enough yet about how metabolism works, or how it can potentially evolve, to draw any definite conclusions about dinosaurs. It's possible that dinosaurs were neither warm-blooded nor cold-blooded, but had an “intermediate” type of metabolism that has yet to be pinned down. It's also possible that all dinosaurs were warm-blooded or cold-blooded, but some individual species developed adaptations in the other direction.
If this last idea sounds confusing, bear in mind that not all modern mammals are warm-blooded in exactly the same way. A fast, hungry cheetah has a classic warm-blooded metabolism, but the relatively primitive platypus sports a tuned-down metabolism that in many ways is closer to that of a comparably sized lizard than to that of other mammals. Further complicating matters, some paleontologists claim that slow-moving prehistoric mammals (like Myotragus, the Cave Goat) had true cold-blooded metabolisms.
Today, the majority of scientists subscribe to the warm-blooded dinosaur theory, but that pendulum could swing the other way as more evidence is unearthed. For now, any definite conclusions about dinosaur metabolism will have to await future discoveries.