Walking on water: tree frog's foot uses dual method to stick.

Tree frogs' feet aren't nearly as powerful as those of the well-studied gecko, but their traction is good enough that they can grip the underside of a wet, slick leaf. Now, researchers have evidence that the tree frog's foot may be surprisingly sophisticated.

Unlike a gecko's toe, which uses dry, sticky hairs to clutch a surface (SN: 8/31/02, p. 133), the pad on the bottom of a tree frog's toe is coated with a mucus film. This layer of fluid led scientists to think that the pads cling to a surface by wet adhesion--the force that makes a damp piece of paper stick to a window, for example.

But it turns out that wet adhesion is only part of the picture. Microscopic bumps on the toe pad jut through the film and make direct, dry contact with a surface, researchers report in an upcoming Journal of the Royal Society Interface. This arrangement enables the tree frog to toggle between wet adhesion, which is useful on rough surfaces, and dry friction, which gives the frog a grip on smooth terrain.

"It seems to be a clever solution to use both advantages," according to study leader Walter Federle of the University of Cambridge in England.

Each toe pad consists of hexagonal skin cells that are covered in cleatlike bumps. Mucus-filled channels separate the cells. By studying images of frogs walking on glass, Federle and his colleagues determined the thickness of the pad's mucus film.

In many areas, there appeared to be no film at all. "It seems pretty clear that the tops of all these bumps will actually be touching the surface" says coauthor Jon Barnes of the University of Glasgow in Scotland.

In another test, the researchers discovered that the mucus has a watery consistency, causing it to flow away quickly so that a pad can directly contact a surface, Federle says.

The mucus channels not only provide the mucus film but also serve an important role in frog traction, the new findings indicate. On wet surfaces, they funnel away excess fluid. On dry or uneven surfaces, or when a frog hangs upside down, the mucus creates surface tension and viscosity--in other words, extra dinginess. The channels also allow the hexagonal cells to conform to contoured terrain, like that of a leaf, Barnes says.

The new understanding is "a break-through because it adds a mechanism to what was thought to be a solved problem," says Robert J. Full of the University of California, Berkeley, who has studied adhesion in gecko feet.

Full says the toe-pad structure could lend biological inspiration" for designing car tires that can wick away water while maintaining traction. The design might also be applied to improvements in the holding capability of microsurgical tools, Barnes says.