How Do Tree Frogs Stick to Leaves and Glass? Toe Pads Explained

What is on a tree frog’s toe pad?

Look closely at a red-eyed tree frog or any other tree frog and you will see that each finger and toe ends in a rounded, slightly flattened disc. Under a microscope, the surface of this disc is not smooth. It is made of a mosaic of flat-topped skin cells, mostly six-sided like a honeycomb and only about a hundredth of a millimeter across.

Between the cells runs a network of fine channels. Mucous glands in the pad release fluid into these channels, which spreads it over the whole surface. The tops of the cells are covered with even smaller bumps, sometimes called nanopillars, a few hundred nanometers wide. Inside the pad, the soft tissue works like a cushion, so the pad can mold itself to the shape of a leaf or twig.

How do tree frog toe pads stick?

Tree frogs rely mainly on wet adhesion, the same effect that makes a wet glass stick to a table or two wet microscope slides hard to pull apart. A very thin layer of fluid between the pad and the surface pulls the two together through surface tension and resists being pulled apart.

The hexagonal cells and channels help in several ways:

• Spreading the fluid. The channels distribute mucus evenly across the pad, so the film stays thin.
• Draining excess water. On a wet leaf, extra water can flow away through the channels, a little like the grooves of a car tire on a wet road.
• Close contact. Because the fluid film is extremely thin, the tops of the cells and their nanopillars come into near-direct contact with the surface.

That close contact is the second part of the story. Research led by Walter Federle and Jon Barnes, published in 2006, found that tree frog toe pads produce high friction despite being wet, because the cell surfaces touch the substrate almost directly through the thin fluid layer. This “boundary friction” stops the pads from sliding down a slope, while wet adhesion holds them on.

How do tree frogs hang upside down?

Clinging to a vertical leaf mostly needs friction to stop the frog sliding down. Hanging under a leaf or branch is harder, because the frog must resist being pulled straight off by its weight.

Tree frogs manage this by using their whole bodies. On steep overhangs, many species spread their legs out wide and pull them inward, toward the body. That inward pull presses the pads against the surface and turns friction into extra holding power, in the same way that sticky tape is hard to remove when pulled along its length. Small frogs have an advantage here: their weight is small compared with the area of their toe pads, so they can hang on more easily than large frogs.

The red-eyed tree frog makes good use of this ability. It spends the whole day clinging to the underside of broad leaves, pressed flat with its legs tucked under its body, safe from many daytime predators.

How do tree frogs let go?

A grip that is too strong would be a problem for an animal that needs to climb and jump. Tree frogs release their toe pads by peeling them off from one edge rather than pulling them straight away. Peeling breaks the fluid contact a little at a time and takes very little force, a trick also used when removing sticky tape.

Many tree frogs also have an extra small bone or cartilage in each toe, called an intercalary element, which lets the tip of the toe tilt and keep the pad flat against the surface as the frog moves.

Why are tree frogs so good on glass?

Smooth glass is close to perfect for wet adhesion. The soft pad can touch almost every part of the surface, and the thin mucus film fills in the tiny gaps that remain. That is why tree frogs often appear on windows at night, catching insects attracted to the light.

Rough surfaces are a bigger challenge. On bark, frogs combine their pads with their long fingers and toes to grip edges and bumps. Very rough, dusty or crumbly surfaces, and surfaces flooded with water, reduce the grip.

Tree frog toe pads vs gecko feet

Tree frogs and geckos are both famous climbers, but their feet work in very different ways:

Gecko feet generally produce stronger adhesion for their size, but tree frog pads keep working in the wet rainforest conditions where many tree frogs live. You can read more in how do geckos climb walls?.

Which frogs have sticky toe pads?

Adhesive toe pads have evolved several times independently in frogs, wherever species took to climbing. They are found in the tree frogs (Hylidae), the leaf frogs (Phyllomedusidae, including the red-eyed tree frog), the glass frogs, the Old World tree frogs (Rhacophoridae) and some rain frogs, among others. Ground-living frogs and toads have plain toes without pads, one of several differences explored in frog vs toad.

Scientists and engineers study tree frog pads because they grip well on wet surfaces, a problem in fields from tire design to gripping tools. Nature solved it long ago in the leaves above rainforest ponds.