In Cold (or Warm) Blood: Dinosaur Physiology
While teeth and bones can offer clues about what dinosaurs ate and how they moved, there are a lot of details we don't know about their physiology. One large question, which encompasses several smaller questions, is whether dinosaurs were warm-blooded, like birds and mammals, or cold-blooded, like reptiles.
These aren't scientific terms, and they don't have anything to do with the actual temperature of an animal's blood. Instead, they're descriptions of how an animal uses energy and regulates its body temperature. A warm-blooded animal controls its own body temperature with techniques like sweating and burning stored nutrients. They're endotherms -- their heat comes from within. These animals burn energy quickly, or have a relatively high metabolism. They also maintain a relatively consistent temperature, or are homeothermic.
Cold-blooded animals, on the other hand, are ectothermic, meaning they use their environment to regulate their temperature. Many reptiles, for example, raise their temperature by resting in the sun or on warm surfaces. Cold-blooded animals tend to have a relatively low metabolism. They're also poikilothermic -- their internal temperatures vary depending on their environment and activity.
So, are dinosaurs homeothermic endotherms, or are they poikilothermic ectotherms? Scientific opinion has shifted through the years. In the late 1800s, when scientists began theorizing that dinosaurs evolved into birds, people thought dinosaurs must be warm-blooded like their avian relatives. But beginning in the 1920s, people began to view dinosaurs as reptiles -- and obsolete reptiles at that. The reasons behind this change are murky, and they may have been influenced by public opinion. But the idea that dinosaurs were cold-blooded, slow and not very smart began to overshadow the idea that they were intelligent, swift and agile like birds.
Today, the idea that dinosaurs evolved into birds is back in the scientific forefront -- but there's still ongoing debate about their metabolism. Here's a run-down of some common arguments:
Dinosaurs are endothermic:
- Birds evolved from dinosaurs, so they must have inherited their warm-blooded nature from dinosaurs.
- Relative to their bodies, dinosaurs' limbs are arranged like mammals' limbs, and mammals are warm-blooded. Computer models suggest that dinosaurs could move very quickly, and, in general, the faster an animal moves, the faster its metabolism tends to be. A CT scan on preserved tissue from a dinosaur skeleton found in South Dakota appeared to reveal that the dinosaur's heart had four chambers, like a bird or mammal, rather than three chambers, like a reptile [source: Fisher].
Dinosaurs are ectothermic:
- Extremely large dinosaurs could maintain a constant body temperature through inertia, so they wouldn't need internal body processes to regulate temperature.
- The climate during most of the time that dinosaurs existed was warmer than it is today, making endothermic ability unnecessary.
- Dinosaurs don't seem have structures called respiratory turbinates, which are common in endothermic mammals.
Since no one can study dinosaurs in the wild, it's unlikely that scientists will find conclusive proof any time soon.
Next, we'll look into how the bird-versus-mammal question also applies to dinosaurs' reproduction and parenting.