Toe Pads Adhere and Clean Themselves

Biological Strategy

Australian green treefrog

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Image: William Kreijkes / CC BY SA - Creative Commons Attribution + ShareAlike

Attach Temporarily

Living systems must sometimes, temporarily, stay in one place, climb or otherwise move around, or hold things together. This entails attaching temporarily with the ability to release, which minimizes energy and material use. Some living systems repeatedly attach, detach, and reattach for an extended time, such as over their lifetimes. Despite being temporary, these attachments must withstand physical and other forces until they have achieved their purpose. Therefore, living systems have adapted attachment mechanisms optimized for the amount of time or number of times they must be used. An example is the gecko, which climbs walls by attaching its toes for less than a second. Other examples include insects that attach their eggs to a leaf until they hatch, and insects whose wings temporarily attach during flight but separate after landing.

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Protect From Dirt/solids

When dirt and other small solids adhere to living systems, they can slow them down, create blockages, reduce their ability to carry out vital functions, or cause surface wear and tear. Due to electrostatic forces, it’s easy for dirt and other solids to adhere to surfaces, so living systems must overcome those forces. An earthworm, for example, uses a small electric current to keep soil particles from adhering to its body as it moves through the soil. This enables it to move more efficiently by reducing drag.

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Amphibians

Class Amphibia (“to live a double life”): Frogs, salamanders, toads

Amphibians live a “double life” by being able to survive both on land and in water. Unlike reptiles with dry, scaly skin, amphibians have smooth skin that needs to be kept moist. Many live in the water through development and start life with gills, and then transition to breathing with lungs. They can get even more oxygen through cutaneous respiration, or breathing through their skin. This class represents the first animal group to be able to efficiently walk on land—aided by the well-defined limbs that gave them a “leg up” on their finned competitors.

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The feet of the Australian green tree frog stick to surfaces and stay clean due to mucus secreted between the toe pads

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.

Image: Puneet Garg, Prerana Ghatmale, Kirtan Tarwadi and Sachin Chavan / CC BY - Creative Commons Attribution alone

Adhesive surface of the frog toe pad. (a) Immature White’s tree frog (Litoria caerulea) (b) Low resolution scanning electron microscopy (SEM) image of the whole pad of a juvenile frog; (c) Medium resolution SEM image showing the mucous pore and (mainly) hexagonal epithelial cells separated by channels at their distal ends; (d) High resolution SEM image showing the presence of nanostructures on the surface of the epithelial cells.

Image: David M. Green / CC BY SA - Creative Commons Attribution + ShareAlike

SEM of tree frog toe pads displaying hexagonal pattern separated by grooves.

Image: William Kreijkes / CC BY SA - Creative Commons Attribution + ShareAlike

Australian green tree frog climbs a tree

Last Updated January 8, 2019

References

“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)

Journal article

Self-cleaning in tree frog toe pads; a mechanism for recovering from contamination without the need for grooming

The Journal of Experimental Biology 215, 3965-3972 | Niall Crawford, Thomas Endlein and W. Jon P. Barnes

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Reference

“…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)

Journal article

Wet but not slippery: boundary friction in tree frog adhesive toe pads

Journal of the Royal Society Interface | W Federle, W.J.P Barnes, W Baumgartner, P Drechsler, J.M Smith

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Reference

Journal article

Elastic modulus of tree frog adhesive toe pads

Journal of Comparative Physiology A, 197:969 | W. Jon. P. Barnes, Pablo J. Perez Goodwyn, Mohsen Nokhbatolfoghahai, Stanislav N. Gorb

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