Gecko's tail helps keep them up

Geckos always seem to be a popular topic when covered on NI, so we always try to bring the latest in cutting-edge gecko related research. The mechanics of exploiting van der Waals forces that geckos rely on has inspired engineers to create new kinds of tape and wall climbing robots. Some new research by the team that originally uncovered the gecko wall walking mechanism has shown that the hairs on their feet are not the whole story.

In an open access article in this week's Early Edition of the Proceeding of the National Academy of Sciences, integrated biology professor Robert J. Full reports on the gecko's reliance on its tail while climbing. In a series of experiments, the researchers looked at the role the gecko's unusually large tail played in its incredible mobility. Using a flat-tailed house gecko—cosymbotus platyurus—they observed that when the gecko ran up a vertical surface with good grip, its tail was held above the surface. When a slippery patch was introduced to the otherwise high-traction surface, the team noted that geckos began to drop their tail about 30 ms after slipping, and the tail touched the surface after a mere 50 ms. This motion, putting the tail down against the surface, prevented the animal from pitching backwards and falling off the wall.

To discern whether this was a "one movement fits all" type of balancing act, the researchers created a device that was capable of measuring the force of a gecko's tail against the surface. Using a surface that resulted in "moderate foot slippage" with each step, the animals kept their tails down at all times. They found two tail actions: a stabilizing impulse moment, and a "natural pitch-back" impulse moment. Through some basic physics, the researchers found that the stabilizing impulse changed in response to the distance and time that the fore feet slipped, suggesting an active level of tail control. They also found that when the animals pitched back too far—greater than 60^o—they would drop the last two-thirds of their tail against the wall to prevent themselves from falling, the tail acting much like a bicycle's kick stand.

In addition to helping hold the geckos to a wall, the researchers found that their tails can help control them when they fall. Scientists, mathematicians, and engineers have long been interested in how falling creatures can right themselves—falling cats were first studied in 1894. It is known that mammals often correct themselves and turn in space thanks to spinal twists and flexions, but geckos mostly just use their tail. In over 70 percent of the tests, the geckos were able to turn themselves (140 to 180 degrees) without moving their limbs or spine. Researchers also found that geckos in windtunnels (pictured above) were able to control their decent and steer towards a solid surface using their tails. When on their stomachs, gliding in a pose similar to Superman's, the position "allowed them to use their tails to turn or control yaw and pitch," Full said. "In the wild, this might allow a gecko escaping a predator to just go off the end of a branch and maneuver to another place."

PNAS, 2008. DOI:10.01073/pnas.0711944105 (PDF)