They can also climb walls. Wait, should we worry about their athletics?
Anyone who has seen a gecko will probably know he can climb the walls. But these ordinary lizards can also pass through water almost as fast as they can move on a rigid basis. Still, as long as we know how the geocaches scale smooth vertical surfaces using countless tiny hairs on their legs called sausages, how they manage to avoid sinking in the water is sort of a mystery – so far. With my colleagues and I recently completed studies that explain how geckos use a combination of techniques to accomplish this incredible achievement.
The ability to walk around with water is recorded in smaller animals, such as waterfalls, which are light enough to keep them from the surface tension of water, the strength between surface water molecules. Meanwhile, larger animals, such as the back, can walk on the water because they are powerful enough to hit the surface with their feet as they flow. Fast movement pushes the water under your feet, creating a pocket around you. The pressure that is generated when this pocket is pushed under water is what holds the animal briefly suspended on the surface.
But geckos usually have a size that falls between these two categories. They are too weak to behave, using superficial self and too heavy to leave the surface of the water. Yet their relative speeds of water movement are close to those of another well-known guide-running lizard, the Basilisk (or "Jesus Lizard") who relies on the technique of knitting.
Initial calculations hinted, and video analysis confirmed that, unlike other species that run on the surface of the water, the geocaches use a combination of techniques to move faster to water than they can by swim through it. By analyzing videos of geckos that move in the water, we find that their gait is similar to that of the basilica. Each step involves pulling the leg in the air, striking the surface and quenching under water.
But unlike basilicas not affected by changes in surface water tension, our experiments have shown that the velocity and height of the geckos are reduced by half when adding a detergent to the water, reducing surface tension. This suggests that they at least partially use the forces between the water molecules to stay above the surface.
We also found that the geocaches use a combination of hydrostatic force (pushing the water known as buoyancy) and hydrodynamic force (the elevator created by motion on the surface of the water, similar to a surfboat motorboat). Together, these forces generate extra lifting for a gecko, a state known as semi-planing.
Sting in the tail
For all the ingenuity of this multitasking approach, the geckos can only hold their heads and torso completely over the water, leaving their tails sliding beneath. Being able to move almost as fast as on land, when nearly half of your body is underwater and facing greater resistance and strength is pretty good – just ask Michael Phelps.
Geckos manage this by using their tail, which has already proven to help them maneuver around obstacles, jump and escape predators. Looking upwards as he walks through the water, the gecko can resemble a crocodile that moves his body and tail with a moving wave to create a propulsion to balance the reverse pulling of the water.
Our studies show that medium animals that move fast on the surface of the water need a complex and clever combination of physical mechanisms previously thought to be only larger and smaller animals. But it can also be used for better projects for animal-inspired robots.
Previous gecko studies have inspired several such "biomimetic" endeavors, from better adhesives to a flexible (and quite fascinating) dangerous robot car, appropriately called Tailbot. A better understanding of the way animals walk through complex terrain, we hope to bring robots who can use these techniques to move both onshore and water with the high metrics observed in geckos.
Jasmin Nirodi, associate professor of postgraduate studies in biophysics, Oxford University
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