Bearded Dragons in the wild will often stand up on their hind legs to run from danger. This is actually slower than running on all fours, and might be a form of temperature control. The animals produce heat while running, and being cold blooded, have no good way to regulate their body temperature. Running on their hind legs puts their body farther away from the hot ground and improves airflow around them. This should allow them to run farther before having to stop.