Some of the reasons paleontologist Robert T. Bakker (1945–) gave for dinosaurs being endothermic homeotherms, or warm-blooded, are:

1. Dinosaurs had complex bone structures (with evidence of constant remodeling), a feature of modern mammals, not reptiles.

2. Dinosaurs had an upright structure similar to birds and modern mammals.

3. Dinosaurs, at least the small theropods, had, based on the evidence to date, active lifestyles.

4. Predator-versus-prey ratios were closer to that of modern mammals than reptiles.

5. Dinosaurs have been found in polar regions, a climate that cold-blooded animals would not likely inhabit.