Photograph by Robert Clark, National Geographic
Published April 1, 2013
Scientists have long known that geckos can scurry up walls, Spider-Man style.
But less well understood is how these reptiles cling to wet surfaces, which are common in their rainy tropical habitats.
The answer, a new study reveals, is that a gecko's sticky toes enable the animal to walk across wet surfaces that don't get uniformly wet, like waxy leaves—but not on easily wettable surfaces, like glass.
Watch a video of geckos clinging to wet surfaces.
This is "an interesting question that the field of gecko biology hasn't quite looked at before" now, said study leader Alyssa Stark, a biologist at the University of Akron in Ohio.
Why does it matter? Because figuring out a gecko's grip could help people make adhesives that will work in water, Stark said. (Also see "Gecko, Mussel Powers Combined in New Sticky Adhesive.")
Imagine, for instance, putting a Band-Aid on underwater and having it stick just as well as it would on land.
To examine a gecko's cling, Stark and colleagues put harnesses on six tokay geckos (Gekko gecko) and put them on four surfaces which varied in their wettability, or their degree of water resistance.
The reptiles' feet were submerged in water on glass, plexiglass, a transparent plastic often used as a glass alternative, and Teflon.
Plexiglass and the transparent plastic "mimic the surface chemistry of the leaves geckos are really walking on in their natural environments," Stark said.
As a gecko moved across each surface, the team applied a force in the opposite direction until the animal slipped, which allowed them to measure the animal's grip.
The results showed that on glass, a film of water developed between the geckos' toes and the surface, reducing their ability to stick to the glass. But on plexiglass and the plastic, the geckos' toes create air pockets that allows their feet to stay dry—preserving the stickiness.
These results are similar "to the contact made by a terrestrial beetle underwater, where trapped air bubbles actually allow dry contact to occur on [water-resistant] surfaces," according to the study, published this week in Proceedings of the National Academy of Sciences.
A "very obvious application," Stark said, would be to use this data to make a synthetic tape that can stick to wet surfaces.
Stark suspects that the geckos' toepads evolved to negotiate wet rain forests. (Read about a new species: the strikingly striped bumblebee gecko.)
For instance, if a gecko is caught in a tree during a sudden rainstorm, its surroundings likely won't dry for a long while—which means the reptile has to be able to cling to wet leaves in order to escape predators or find food.
Airborne Fecal Matter
Because they don't usually run away, tokay geckos have become the lab gecko of choice, Stark added. But working with the 14-inch-long (35-centimeter-long) critters isn't easy: They don't like to be handled, and can be quite aggressive. (Watch a video of gregarious geckos.)
"Part of their defense mechanism is to basically shoot fecal matter at you," she said with a laugh. It's "pretty high velocity—I've definitely gotten nailed during experiments."
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