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Gecko walking on the wall of a glass enclosure. Photo: <a href="http://www.flickr.com/photos/keithmarshall/3934863305/">Keith Marshall</a>, Creative Commons, some rights reserved. Inset: microstructure of Gecko toe, by <a href="">Douglasy</a>, Creative Commons, some rights reserved.
Gecko walking on the wall of a glass enclosure. Photo: Keith Marshall, Creative Commons, some rights reserved. Inset: microstructure of Gecko toe, by Douglasy, Creative Commons, some rights reserved.

Natural Selections: Gecko feet

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Geckos have a remarkable ability to run up vertical surfaces, and even across ceilings. But their feet do not form suction cups, nor are they sticky with any kind of secreted glue. Dr. Curt Stager tells Martha Foley the secret of the lizard's gravity-defying feet, which has as much to with atomic physics as biology.

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Martha Foley: Okay, let’s talk about a little tidbit about geckos. How do they hold on?

Dr. Curt Stager: It’s neat to watch them; I mean they really tear up the walls. They can go up smooth glass; they can walk on the ceiling. And when I go to the tropics, you can sit out at night on your porch with the light on. All the bugs are coming, and the geckos immediately congregate around the light to get the insects.

MF: And they walk so comfortably upside down.

CS: And it looks like they’ve got pads on their toes, so I always thought it was suction cups, like you were saying there, or maybe glue or something. But of course there’s some scientists who decided to investigate this.

MF: You never know what you’re going to find. So is it little suction cups, or a little bit of adhesive stuff on there?

CS: No, and after reading the article that was in Natural History, it makes sense that it wouldn’t be those things. If it was suction cups, then you’d have to spend a lot of energy pulling your foot away. And it would slow you down. And if you watch geckos, they definitely stick there, but they’re moving pretty smoothly.

And same thing would be true if it was glue. It still would be taking energy to pull it off every time, and maybe eventually you’d start hurting your toes by doing it, too.

MF: So?

CS: So these folks at multiple universities that are into atomic physics and all this stuff have gone down to the atomic level to explain this. And it’s actually kind of neat. First they look at the toes of these things, and okay, each toe does have a little flat pad on it, which gives it a lot of area. But then if you look really close on the underside of the pad, there are all these thin leaves of tissue in there.

MF: Kind of folded?

CS: Folded, and they’re really thin like sheets of paper. And then if you magnify those, those little sheets of tissue are covered by little hair-like things. Not exactly hairs, but they look like them. But they’re incredibly thin and numerous. Each one of them has a diameter that’s less than the diameter of a bacterial cell--so they’re really, really small. They figure there’s about two million of these little hair things on each foot of the gecko.

MF: Wow. So that’s a lot of – is it surface area? Or do those go into the surface of even glass?

CS: It’s both of those things. It’s like, if we go up to a pane of glass and push our hand against it, we say, “Okay, now my hand is totally flat and up tight against the glass as it’ll ever get.” But if you go down to the size scale that these people are looking at, it’s actually really rough surfaces on the glass, and on your hand. It’s like mountain ranges and valleys, and so they’re actually not that much in contact. There’s a lot of space between them.

MF: So there would be some friction there, some grip, but, but really, you know, relatively speaking, not that much.

CS: Right. And so with these geckos, with these little hair-like things on such a small scale, they go into every nook and cranny of the surface of what they’re walking on. And so it really is about as close contact as you could possibly get. And it turns out when you put two solid objects made out of atoms right up against each other, so the atoms are really just about touching, then these strange atomic forces start to happen, which have to do with the different parts of the individual atoms. Because the inside of the atoms, the nucleus, has a positive charge on it. And the outside, the shell, is basically negative electrons whipping around. And so, because opposites attract, that is why the electrons hang around the nucleus. But they’re also attracted to neighboring atoms’ positive centers.

MF: So the atoms in the little hair-like things on the pads of the foot of the gecko are attracted to the atoms of the surface of the glass or the wall or whatever? I mean it’s actually down to the atomic level, this attraction?

CS: Just by pushing the sets of atoms close enough, it’s a technique to get the atoms up close enough that they can actually interact with each other’s nuclei and electrons, and have almost like an “opposites attract"--almost electrical connection there.

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