How do frogs' toe pads
stay sticky to stop them falling? Why don't they get clogged with dirt and stop sticking? Tree frogs' sticky toe pads remain clean after thousands of uses while sticky tape is useless after one. It turns out that the simple act of walking is sufficient to clean their toe pads.
First scientists tested how well contaminated and uncontaminated White's tree frogs clung to a glass plate as they rotated it from horizontal, through vertical to upside down. Because the frogs tended to want to jump off when they began to feel insecure, they gently encouraged the frogs to hold tight by shielding them with their hands and caught them when the plate became too steep and the frog's hold failed. Monitoring the plate's angle, the team found that the frogs with uncontaminated feet only began to slip as the plate tipped over (106 deg) and finally lost their grip at 142 deg. However, when they dusted the frog's feet with microscopic glass beads, the animals began slipping soon after the glass plate began to tilt. They had lost adhesion, so how could they recover?
Holding a tree frog on a computer-controlled stage and carefully applying a single layer of glass beads to one of its toe pads, they carefully pressed the contaminated toe onto a glass coverslip and then pulled it free. Measuring the adhesion force as they pulled the frog away, they found that it had fallen to zero: the frog was completely incapable of clinging on to the smooth surface with its contaminated feet. Simply dabbing the toe onto a surface was not sufficient to clean it. However, when they simulated real tree frog footsteps, by gently dragging the toe pad sideways after contact with the coverslip, the situation was completely different. Over the course of eight simulated footsteps the toe pad gradually recovered adhesion, slowly at first, returning to normal by the final contact.
So the keys to the tree frog's self-cleaning success are the sliding motion – which shears particles away from the toe pad and increases the contact area with the surface – and the sticky mucous secretions – which help flush away contaminants.
Self-cleaning in tree frog toe pads; a mechanism for recovering from contamination without the need for grooming. (2012) The Journal of experimental biology, 215(22), 3965-3972
Tree 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 White's 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 'flushing' effect of the secreted mucus play an important role in shedding particles/contaminants.