Narrator: This is Science Today. Were dinosaurs cold-blooded like modern reptiles or warm-blooded like mammals? We can't go back and check, but according to paleontologist Kevin Padian of the University of California, Berkeley, the fossil evidence points to active, warm-blooded dinosaurs.

Padian: The best evidence for 20, 30 years now has been the evidence from the microstructure of the bones.

Narrator: Under a microscope, dinosaur bones look like mammal bones, not reptile bones. Dinosaurs grew quickly, walked upright, migrated long distances, and could move fast.

Padian: Yes, lizards can run very fast, yes, crocodiles can get going if they want to -- but after just a short while, these guys poop out, they run out of gas, and this is not what happens to dinosaurs. Dinosaurs keep going all day, so do birds, so do mammals. Everything we know about dinosaur structure tells us that. In other words, the animals that you saw in Jurassic Park may have been a little lively, but they were much closer to the mark than the ones that have them lumbering around and falling over and that sort of thing that we saw often when we were kids.

Narrator: For Science Today, I'm Steve Tokar.