Aug. 5, 2009 -- Geckos have an amazing ability to defy gravity and make it look easy. They climb up walls, scurry across ceilings, and even remain calm in free-fall situations.
Now, it appears that gravity is actually the trigger that tells geckos when to turn on their sticky-footed skills. The lizards cling only when running up a slope, found a new study, not when the surface they're on is slippery.
While scientists have known for a while that geckos engage tiny hairs on their feet to cling to steep slopes, the new study is the first to explain how they know when to do it, said study author Tony Russell, a zoologist at the University of Calgary, Alberta.
The finding might help lead to better wall-climbing robots, car tires that don't slip, and other gecko-inspired products.
At the root of the research are microscopic hairs called setae. Millions of them line the toe pads of about half of the world's 1,000-odd gecko species. With the right angle and the right amount of pressure, the hairs drag across a surface, causing the animal to stick tightly to it.
Geckos that have setae don't always use them. One way to picture this, Russell said, is to put your elbows on the table in front of you, with your fingers pointing to the ceiling.
Imagine that the part of your arm from your shoulder to your elbow is base of a gecko's toe. The section from your elbow to your fingers is the toe pad that holds adhesive hairs.
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A gecko can walk on the base of its toes, like you could crawl on your elbows. Or, if it needs to stick to something, it can roll out its sticky pads like one of those whistling party favors.
To find out how geckos know when to bother rolling out the pads, Russell and a colleague tested six pad-equipped geckos (and five pad-free geckos for comparison) on a three-foot long track. In some cases, the track's surface was covered with smooth, slippery Plexiglas. In other cases, it was lined with rough, sticky sandpaper.
When the tracks were flat, the scientists report this week in the journal Proceedings of the Royal Society B, none used their sticky pads, even though their feet slipped around on the Plexiglas.
When the tracks were raised to an angle of 10 degrees, half of the geckos put their pads down, even on the rough surface that they could have stuck to without the extra help. At an angle of 30 degrees, all of them did it.
While the pad-free geckos were able to get up the rough slopes just fine, the geckos that rolled out their pads were faster.
"It's not the slipperiness of the surface but the body orientation that triggers when geckos use this adhesive system," Russell said. "We think 10 degrees is the tipping point."
Those results surprised Kellar Autumn, a biophysicist at Lewis & Clark College in Portland, Ore., who first discovered how geckos stick.
"If I were a gecko, I would certainly think pretty strongly about turning on the adhesion if my feet were slipping and there was a predator on my tail," he said. "This paper raises so many interesting questions."
Russell suspects that geckos have structures in their ears that tell them when they need to risk damaging their setae for the sake of hanging on. No one has studied that yet.
The work also suggests that geckos have an extremely nuanced way of using gravity to determine how to move, said Duncan Irschick, a biologist at the University of Massachusetts at Amherst.
While Stanford scientists have already invented "stickybot," the next generation of sticky robots should incorporate a similar kind of sensory feedback system that, at the very least, senses the angle of the surface beneath the robot.
"This is a really nice step forward," Irschick said. "It will give robot scientists some fat to chew."
The new study points out just how unique geckos are, Russell added. In previous work, scientists took reptiles on rides in the Vomit Comet, an airplane that goes into free-fall to simulate microgravity in space.
"Most lizards and snakes go nuts in microgravity," Russell said. These animals flailed around, desperately seeking contact with something. Only the ones that found their own tails calmed down.
Geckos, on the other hand, simply stretched their limbs out like they were skydiving.
"It didn't phase them at all that there was no gravity around," Russell said. "The way they read their body positioning is very different from other animals."
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