How leaping lizards use their tails

Ever since 1969, when J H Ostrom proposed that theropod dinosaurs used their tails to maintain balance during fast movement, scientists have suggested that a wide range of animals, from primates to rodents, use active motion of appendages such as arms or tails to stabilise their motion. Now a team of scientists from the University of California, Berkeley, has found that by moving their tails, lizards can maintain a near constant pitch when moving through the air after a jump.

In their experiments the group made a number of Red-Headed Agama lizards (known scientifically as Agama agama) jump from a small block, or vault, to a raised platform. A high speed, 1000 frames per second camera was then used to record their motion. The vault was covered with “smooth glossy card stock”, which meant that, as they jumped, the lizards’ legs slipped slightly – imparting an angular momentum to the lizards and causing their angle of flight through the air to change. After analysing the changes to the lizards’ pitch caused by this effect, the scientists found that this was less than would have been expected if the lizards didn’t have a tail and theorised that “by rotating their tails with respect to the body in the sagittal plane, the lizards transferred angular momentum from the body to tail, thereby reducing body angular velocity and ... body rotation”.

To further test it the scientists constructed a wheeled robot with an attached rod to simulate a tail and set it running off a ramp to simulate the lizards’ motion. The team says that while “inertial stabilization of robotic locomotion has been modelled before, [their robot was] the first with a specialized tail-like appendage for continuous inertial stabilization”. When this ‘tail’ was moved in the way the lizards’ tails had been observed to, the angle by which the robot was rotated was 72% less than when the rod was held fixed.

Applying the model developed using this research to velociraptors – 1.5m high theropods that lived in the late Cretaceous, the team found that their extraordinarily flexible tails would have been even more effective in stabilising movement through the air than those of the Agama agama. They conclude that “small theropods like Velociraptor with active tails might have been capable of aerial acrobatics beyond even those displayed by present-day arboreal lizards”.

While this research is of clear interest to biologists, the scientists believe the fact that they managed to replicate the effect seen in lizards in a robot shows that their work can not only “advance our understanding of appendage evolution”, but also “provide biological inspiration for the next generation of manoeuvrable search-and-rescue robots”.

DOI: 10.1038/nature10710