The diminutive gecko is capable of some extraordinary feats of locomotion, zipping along vertical walls with ease and even running short distances across water. Precisely how they accomplish these feats has long interested scientists. A new paper in Proceedings of the Royal Society B reports that geckos' ability to reorient their flexible toes is a major factor, enabling them to realign and adjust to shifts in gravity (load). The work may one day help to improve the design of bio-inspired robots.
This work builds on a 2018 study from the laboratory of University of California, Berkeley biophysicist Robert Full. Geckos are known for being expert climbers, able to stick to any surface thanks to the tiny hair-like structures on the bottoms of their feet. The little lizards can also zip along the surface of water at high speeds to elude predators. They can't do it for very long; the energy expenditure required is too great. But it's amazing that they can do it at all.
As we reported in 2018, those creatures in nature capable of walking on water employ different mechanisms depending on their size. Small, lightweight water striders, for instance, rely entirely on surface tension to stay afloat, while the larger, heavier basilisk lizards employ a slapping motion with their feet that creates pockets of air bubbles to keep from sinking. The standard theoretical calculations set very strict boundaries for how small an animal has to be to use surface tension and how large it needs to be before the surface slapping mechanism is viable.
Geckos fall somewhere in between. They are too large to rely solely on surface tension and too small to generate sufficient force to run along the surface of water without sinking. And yet they can still somehow accomplish the feat at lightning speed—almost one meter per second. When Full and his collaborators investigated, they found that the mouse-sized lizards use a combination of surface tension and a slapping motion to run across water.
Amazing gecko toes
The spotted belly of a Tokay gecko used by UC Berkeley biologists to understand how the animal's five sticky toes help it climb on many types of surface. Yi Song Taking advantage of a phenomenon called frustrated total internal reflection, the researchers were able tell which parts of the toe pad (bright spots) were in contact with the surface and supporting the geckos weight. Yi Song Close-up look at the toe pads of a Tokay gecko. They have many tiny hairs per foot, each of which has split ends that maximize contact with the surface and support the animals weight by interacting with surface molecules via van der Waals forces. Yi Song A gecko in motion: sideways rotated upward climbing. YouTube/UC Berkeley A gecko in motion: running over a slippery patch. YouTube/UC Berkeley A gecko in motion: running across rods. YouTube/UC Berkeley Hanging on: gecko grabs rod as its foot is pulled. YouTube/UC Berkeley
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