How geckos land on their feet
Posted: Monday, March 17, 2008 5:00 PM by Alan Boyle
Geckos use amazing sticky pads on their feet to walk on walls, but not even a gecko can stick to the wall all the time. Now scientists have analyzed high-speed video to figure out how the lizard, like a cat, always lands on its feet.
Unlike cats, geckos owe their landing prowess primarily to their tails - which can also keep them from falling in the first place.
The active tail of a gecko is the focus of a research paper published online this week in the Proceedings of the National Academy of Sciences. The genesis of the study was actually an unusual case of robots imitating life, biologist Robert Full of the University of California at Berkeley told me today.
"This is an outstanding example of how important interdisciplinary collaborations can be in research," said Full, the research paper's principal author.
For some time now, engineers at Berkeley and elsewhere have been working with Full and his fellow biologists on a robot that can mimic a gecko's wall-climbing ability. (Check out this video from the gecko-bot project, known as Robots in Scansorial Environments or RiSE.)
Full said the RiSE engineers came back and told him, "We need to use a bar, kind of like a tail, to make the robot more stable."
That suggested that a gecko's tail was more important than the biologists had thought. So Full and the other researchers behind the paper in the Proceedings - Ardian Jusufi, Daniel Goldman and Shai Revzen - set up a series of experiments to study how geckos used their tails in slippery situations.
"We inspired the engineers, and they came back with a good, testable biological hypothesis," Full said.
The first thing the biologists noticed is that the geckos tended to keep their tails off the wall - except when the going got tough. Then they pressed their tails against the wall, to keep their front feet from sliding off a slippery surface. "Imagine that it's kind of like a teeter-totter, with the fulcrum being the hind legs of the animal," Full explained.
When the geckos started to lean backward off the wall, they avoided a fall "by placing their tail in a posture where the last two-thirds of the tail pressed against the wall similar to that of a bicycle's kickstand," the researchers wrote. They could pitch backward as much as 60 degrees and still manage to regain their hold.
To make sure the tail was the deciding factor in a gecko's performance, the researchers put tailless geckos through the same vertical course and found that they fell down far more often.
Then the researchers set up an even tougher task: They placed the lizards upside-down on the bottom of a light, loosely mounted platform that mimicked the underside of a leaf. The geckos with tails almost always landed right side up, and the high-speed video showed how they did it.
Within about a tenth of a second, the geckos flipped their tails around to induce body rotation. Then they spread out their tails as well as their feet into a "belly-down skydiving posture" position to stabilize the fall. All of the geckos that used their tails in this way landed on their feet, even in wind-tunnel tests - while none of the tailless geckos could do the same trick. In less than 10 percent of the tailed-gecko trials, the animals didn't rotate their tails - and those hapless lizards landed as badly as their tailless kin.
Full said the experiments demonstrated that the self-righting technique for geckos is different from the one used by cats. Over the course of more than a century, cat-flipping experiments have shown that tailless kitties land on their feet as well as tabbies with tails. "A cat does it by twisting its body around, but the gecko does in just by using its tail," Full said.
But wait ... there's more: Full said the geckos could actually move forward as they fell by oscillating their tails up and down, "kind of like a dolphin kick." That trick might help a gecko choose a good place to land as it's falling from a tree in the jungle.
"We really don't know how they use the tail as a control device in nature," Full said. "And that's what we want to find out next."
Full said all this gecko lore is being shared with the engineers so they can design better gecko-bots.
"They've built an active tail on RiSE now," he told me. "It doesn't do as much as the gecko tail does, but they're working with it. Maybe we can realize the dream, to ultimately have a search-and-rescue robot that can go anywhere. ... I'm much more optimistic that the future is not as far away as we thought."
