Australian tree frogs are able to climb a variety of surfaces with the help of their sticky toes. The toes have four digits with pads on the ends, and each pad is made up of layers of hexagonal cells separated by grooves that allow fluid to pass through. These grooves secrete mucus that forms a thin layer of fluid between the pad and the surface, allowing the pads to stick to the surface by wet adhesion.
The mucus also allows the frog to maintain clean and functional toe pads by passively self-cleaning whilst climbing. Contaminants, such as soil and plant particles that stick to the pads, are removed through continual movement and repositioning of the pads during travel. A ‘flushing’ action is also used via the secretion of mucus on the pads; contaminants become trapped in the mucus and are pushed to the tips of the pads. They are then removed entirely and are left on the mucus footprint.Edit Summary
“The ability to climb is a significant advantage for animals, as it makes available habitats not accessible to non-climbers or non-flyers. Indeed, many animal phyla and groups are represented in arboreal habitats. These animals are characterised by mechanisms that facilitate climbing and reduce the risk of falling. On rough surfaces, friction pads and claws can be effective, but on smooth surfaces and significant overhangs, some mechanism of adhesion is essential. Adhesion allows an organism to remain attached to an inclined, vertical or even an upside down surface whilst resisting falling or slipping. Animals have evolved two different mechanisms for adhesion – dry and wet. Dry adhesion, typified by geckos, involves toe pads covered in very large numbers of finely branching setae, each ending in a flattened spatula. These spatulae make such close contact with the surface that van der Waals forces, which hold molecules together in solids, form the adhesive bonds. In contrast, tree frogs adhere by wet adhesion. Mucus, secreted by glands ending on the surface of the toe pads, makes an adhesive bond by a combination of capillarity and viscous forces” (Crawford et al 2012:3965)
Self-cleaning in tree frog toe pads; a mechanism for recovering from contamination without the need for groomingThe Journal of Experimental Biology 215, 3965-3972
“…tree frog toe pads are soft and patterned with a regular hexagonal microstructure of approximately 10 μm diameter epidermal cells separated by approximately 1 μm wide channels; the flattened surface of each cell features a similar but much finer microstructure of approximately 0.1–0.4 μm diameter pegs which originate from hemidesmosomes” (Federle et al. 2006:689)