Climbing is good. It allows for gaining access to habitats that would otherwise be unavailable. And while this access is important (otherwise, why climb in the first place?), equally important is not falling to a gruesome death. This means that your method of adhesion to the surface you are climbing needs to be effective. For example, on rough surfaces, friction pads and claws work rather well. Smooth surfaces and overhangs offer a bit more of a challenge. If you want to climb one of these surfaces you have a couple of adhesion options – dry and wet. If you are a creature that chooses dry adhesion, like a gecko, then you have toe pads covered by large numbers of finely branching setae, each ending in a flattened spatula. When these spatula come into close contact with a surface van der Waals forces allow for the sticking. You remember van der Waals forces from physics class right? Those are the attractive forces that hold together molecules in solids. If you are a creature that chooses wet adhesion, like tree frogs, then you secrete mucus from glands ending on the surface of your toe pads. This mucus makes an adhesive bond by a combination of
capillary and
viscous forces.
Frog toes. They’re squishy, they’re sticky, they’re cute (feel free to say that in a sing-songy voice if you haven’t already). If you look very closely, you will also see that they have a hexagonally patterned surface. In between the pad epithelial cells there are mucus-filled channels that spread the mucus over the surface of the pad, creating a thin layer that allows for wet adhesion to a surface.
Enter a new problem: How do you keep your toe pads clean? Think about any sticky surface you know. Now think about putting that sticky surface on anther surface. What happens? It gets really dirty really quickly, causing it to be less sticky. So now you are a frog that needs sticky feet to climb, and to do that you need to keep your toe pads clean so that they remain sticky. You can’t really groom your toe pads when you are using them and molting/shedding isn’t frequent enough to shed the dirt (or “contamination”). The answer: a passive self-cleaning mechanism.
A study published in
The Journal of Experimental Biology looks at this passive self-cleaning mechanism in frogs, using both single-toe experiments and whole-animal experiments. Their study animal was the Australian green tree frog (
Litoria caerulea), also known as White’s tree frogs (and that dough-boy kind of cute if I must say *grin*). They used five frogs in each of their experiments, first washing them and carefully blotting them dry. (An aside, how do you get the job of tree frog washer?) For the purposes of a lab experiment, they had to create a contaminant. For this they used very small glass beads arranged in a single layer on a glass coverslip for the single toe experiment and as a thin layer in a Petri dish for the whole-animal experiment. For the single-toe experiments, the researchers used a custom-built force transducer consisting of the glass coverslip (the surface attachment) connected to a bending beam and then measured the forces in two dimensions – lateral drag and dab (simply pressed against the surface) – with and without beads. For the whole-frog experiments the animals were put on maneuverable platforms to see at what angles they began to slip and/or fall with and without beads. These experiments allowed them to calculate shear (friction) force and the normal (adhesive) force. Ever wonder when you are going to use trigonometry again? Well, if you know the body weight of the frog and the angle of the slip/fall you can calculate these forces. You're high school math teacher would be proud.
There were computer programs and statistics and equations (lots of equations) that I won’t go in to (as usual, you’re welcome). What they found is rather interesting. The whole-animal experiments showed that the toe pads of frogs will self-clean over time. With the first step, when the toe pad becomes contaminated, the adhesive and friction forces decrease, but then they recover such that by the fourth step 91.9 percent of the original adhesive forces and 98.5 percent of the original friction forces are back. These experiments also found that the more the toe pad is used the greater the recovery of the contaminated toes for both adhesion and friction forces. That is, a moving frog has cleaner toes than a stationary frog. The single-toe experiments shed more light onto why this is so. In the experiments that included the lateral drag movement, the frog toe recovered its adhesive force after about eight trials. Whereas in the dab experiments, there was little if any recovery (except if the pads were only partially contaminated). It seems that this shearing movement is an important feature of the self-cleaning mechanism, and it is one that is common in walking frogs. Frogs use this type of movement a lot on vertical surfaces and will continually re-position slipping pads to incorporate self-cleaning while maintaining adhesion to the surface. When the researchers looked even closer, at the level of pad contact area and the number of glass beads deposited on the glass during each trial, they found a correlation between adhesive force and contact area such that normal stress (adhesive force per unit area) remained constant. Since the number of beads deposited on the glass plate is a measure of self-cleaning, they were able to show a positive correlation between force recovery and contact area, particularly in the drag scenario. Essentially, as the toe pad drags along the beads get left behind, or "flushed," in the mucus footprint. Hmmm…mucus flushing doesn't make frog toes sound quite as cute.
Crawford, N., Endlein, T., & Barnes, W. (2012). Self-cleaning in tree frog toe pads; a mechanism for recovering from contamination without the need for grooming Journal of Experimental Biology, 215 (22), 3965-3972 DOI: 10.1242/jeb.073809
...and if you have access there are some interesting if hard to see videos in the Supplemental Materials
(image via Wikimedia Commons)
