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

Abstract

SUMMARYTree frogs use adhesive toe pads for climbing on a variety of surfaces. They rely on wet adhesion, which is aided by the secretion of mucus. In nature, the pads will undoubtedly get contaminated regularly through usage, but appear to maintain their stickiness over time. Here, we show in two experiments that the toe pads of Whiteʼs tree frogs (Litoria caerulea) quickly recover from contamination through a self-cleaning mechanism. We compared adhesive forces prior to and after contamination of (1) the whole animal on a rotatable platform and (2) individual toe pads in restrained frogs mimicking individual steps using a motorised stage. In both cases, the adhesive forces recovered after a few steps but this took significantly longer in single toe pad experiments from restrained frogs, showing that use of the pads increases recovery. We propose that both shear movements and a ʻflushingʼ effect of the secreted mucus play an important role in shedding particles/contaminants. Supplementary material available online at

When the going gets rough – studying the effect of surface roughness on the adhesive abilities of tree frogs

“…But this study goes further: comparable to the drag reduction mechanisms of snake skin [51], the superhydrophobicity and self-cleaning mechanisms of lotus leaves [52], and the adhesive setae of geckos [53] the toe pads of tree frogs exhibit significant biomimetic potential to advance the technology of surface engineering. This is because they combine high friction under wet conditions [3] with self-cleaning [33]. Their main applications will likely be in the medical field, as Chen et al already demonstrated their potential for use as surgical graspers [54].…”

“…There must therefore be a fine balance for frogs with respect to the volume of pad fluid that they produce for effective climbing on a wide variety of surfaces. It is unclear how the fluid production is controlled in frogs, but as fluid is often left behind in steps [33], it must be replenished frequently.…”

“…Using a similar setup to Crawford et al [33] the maximum friction and adhesive forces for individual toe pads were measured. A custom built force transducer, composed of strain gauges connected to a bending beam, was used to measure lateral (friction) and normal (adhesive) forces of the toe pad.…”

“…A force feedback system implemented in LabView was programmed to maintain a constant preload (2 mN) for measurements of friction forces. Simple LabView programs (similar to those used in Crawford et al [33]) were used to move the pad over the force plate so that maximum frictional and adhesive forces could be measured. This involved a proximal lateral drag (5 mm drag at 1 mm s −1 ), followed by a vertical pull off.…”

Sticking under Wet Conditions: The Remarkable Attachment Abilities of the Torrent Frog, Staurois guttatus

“…The experimental set-up was similar to that used in previous single toe pad studies on tree frogs [16]. Held in position within a plastic Petri dish using foam cushioning, frogs were positioned upside-down underneath a two-dimensional force transducer by means of a micromanipulator to allow specific body parts to come into contact with it.…”