Were Dinosaurs Warm-Blooded or Cold-Blooded? Let's explore the clues they left behind to see if we can solve this mystery. Dinosaurs had relatively small brains compared to their body size — a mark of a cold-blooded animal. They were also the ancestors of birds — warm-blooded animals. In this episode, the Fossil Huntress looks at posture, brain size, predator vs. prey relationships to unravel the warm-blooded vs. cold-blooded debate.