An analysis of the mechanism of adhesion in the toe pads of tree frogs.
This paper examines the group of arboreal anurans known as tree frogs, which possess the characteristic, expanded, terminal digital pads that assist in adhesion to the steep surfaces present in their natural environment. It looks at how the unique morphology and physical mechanisms of these structures have been studied for over a century and how it is only recently with the advances in transmission and scanning electron microscopy that it has become possible to study the pads in detail. It discusses how, even with this new insight into the ultrastructure of the pads, debate continues among zoologists over the nature and roles played by the various adhesive mechanisms of the toe pad and how the distribution of digital pads over five families suggests that the feature evolved independently many times in an interesting example of convergent evolution.
“Wet or capillary adhesion was first proposed by Nachtigall as “a mechanism of direct adhesion through surface tension of a liquid intermediary fluid and whole pad suction” (1974), whereby toe pads adhere to a surface using the surface tension of the watery mucus exude. A modified version of this system was suggested by Hannah and Barnes in 1991 after conducting research on attachment and detachment mechanisms of the pads. The forces induced when toe pads were detached were measured with a force platform, and the results supported wet adhesion attachment, which was demonstrated through the existence of a concave meniscus at the interface between the pad and surface, showing connection via a fluid filled joint, and a velocity dependent resistance to shear forces of the pads, which is typical of systems using fluid as an adhesive.”