Sticky Times: How to Hang On Under a Waterfall
Adhesion, or stickiness, is a powerful and valuable property that results from both physical and chemical interactions. Think of all the sticky things we use day to day: tape, post-its, glue, stickers, and so on. Life might be annoyingly inconvenient without the ability to tape a sign to a wall, or a cover a wound with a Band-Aid; but for a little frog living in the constant stream of a waterfall, stickiness is vital to survival.
The torrent frog, picture above on the right and native to Trinidad, has evolved an ability to cling to rough, wet surfaces so well that it seems to defy gravity. In an attempt to better understand how they maintain adhesion despite external forces and angles that would dislodge most any other creature, a group of scientists from a recent PLOS ONE article “challenged” both tree and torrent frogs to cling to a variety of smooth and coarse surfaces on a rotating platform under both dry and wet conditions. They found that although both species could cling to dry, smooth surfaces, the torrent frogs outperformed the tree frogs on adhering to rough, wet surfaces, both at low- and high-flow water volumes.
Tree frogs, they observed, spread their limbs out sideways and hang on by their pads alone—check out the video below of these frogs’ toes lighting up an inverted glass surface:
Torrent frogs, on the other hand, used their entire bellies and thighs to stick to the surface while water rushed over them, even having better adhesion when the water flow increased. The video below shows how the experiment was set up:
Using a scanning electron microscope on the pads of the frogs revealed that the structure of the cells on the toe pads of torrent frogs are elongated and have straighter channels between them (B and C in the image below) than the pads of the tree frogs (A in the image below). This design potentially allows better drainage of excess fluid beneath the pad, and may help explain the torrent frogs excellent sticking ability.
The incredible adhesive abilities these frogs have allows them to thrive in an environment that would otherwise be very difficult to inhabit. Understanding how they stick so well could contribute to better development of sticky things that are useful in our own lives.
Citation: Endlein T, Barnes WJP, Samuel DS, Crawford NA, Biaw AB, et al. (2013) Sticking under Wet Conditions: The Remarkable Attachment Abilities of the Torrent Frog, Staurois guttatus. PLoS ONE 8(9): e73810. doi:10.1371/journal.pone.0073810
Image: Frogs – Figure 1 from the manuscript, Toe pads – Figure 6 from the manuscript