Not slippery when wet.

A gecko has amazingly sticky feet (see "How a Gecko Defies Gravity"). In fact, if you pull hard enough on a gecko stuck to a glass plate, you might break the plate. A tree frog's foot doesn't have so powerful a grip, but it still must allow the frog to climb wet, slippery leaves--sometimes while the frog is upside down.

Now, scientists have figured out how a tree frog manages to keep its grip.

A tree frog's foot is covered with a wet film. This layer of fluid led scientists to think that the frog's wet toe pads cling to a surface by the same force that makes a damp piece of paper stick to a window. But this didn't explain how a frog could walk on something wet, such as a rock in a stream or a branch in the rain.

To answer this question, Walter Federle of the University of Cambridge in England and a team of scientists took pictures of tree frogs walking on glass. By magnifying the pictures and making measurements, the researchers found that the wet layer on a frog's foot is very thin. In some places, there's no film at all.

It turns out that a tree frog has tiny bumps on the bottom of its feet, almost like soccer cleats. Because the wet film is so thin, these bumps poke through and stay dry, giving a tree frog better traction when climbing slippery surfaces.

A tree frog's toe pads also have little channels along which fluid can flow. On wet surfaces, the channels funnel away extra fluid. On dry or uneven surfaces, they bring additional fluid to the pads, allowing

the frog to cling more tightly or even hang upside down.

A gecko's feet have inspired a new type of adhesive tape (see "Sticking Around with Gecko Tape"). If engineers can figure out how to imitate a tree frog's foot, we might someday have car tires that stick to the road even when the road's wet.--E. Jaffe http://www.sciencenewsforkids.org/articles/20060614/Note2.asp From Science News for Kids June 14, 2006.