Lizards running on their two hind legs require no special anatomical tricks, according to a new study; rather the posture is a fluke of biomechanics akin to popping a wheelie. Australian researchers collected specimens of 16 dragon lizard species from the outback and filmed them running flat-out on a treadmill in the lab. Instead of falling neatly into two groups—bipedal runners and quadrupedal scurriers—as the scientists expected, the lizards were highly variable in the amount of time they spent sprinting on two legs—from nearly none to almost all. When researchers drew up an evolutionary tree showing the relationships between lizard species, there was no logical pattern to the variation. Using computer models, they discovered that the peppier the critter's acceleration, the more likely its center of gravity would shift toward the hips and raise the torso like a motorcycle rearing up on its hind wheel. Writing in The Journal of Experimental Biology, the authors say that running in lizards is therefore a natural consequence of acceleration that may have become more prevalent in certain species because it benefited maneuverability—or for reasons still unknown.