For all its blind tinkering, evolution still has to operate within certain physical limits. Size is one of these, and it's the reason why certain traits work for some animals but not others. One such feature is the adhesive pad — a characteristic that's evolved independently in numerous species, including amphibians, reptiles, and mammals. But if the adhesive pad is so useful, why don't we see it in bigger animals, like, I don't know, humans? To get to the heart of the matter, why can't people be Spider-Man?
The answer is pretty simple: as a species, humans are just too big. In a study published in the journal PNAS this week, biologists from the University of Cambridge’s department of zoology explain that the larger an animal is, the greater the proportion of its body surface area is needed to be adhesive for it stick to a wall. In short, if you're big, your sticky feet need to be much bigger.
The bigger you are, the more of your surface area needs to be sticky. (Image credit: Cambridge University)
So, for an animal as small as an ant, only 0.09 percent of the creature's surface area needs to be adhesive for it walk up walls. For a gecko, this percentage rises to 4.3 percent, and for a human, the estimated surface area needed is 40 percent — or 80 percent if you're only using one half of your body as an adhesive.
Humans would need US size 114 shoes (sticky ones) to walk up walls
"If a human, for example, wanted to climb up a wall the way a gecko does, we’d need impractically large sticky feet — and shoes in European size 145 or US size 114," says Walter Federle, senior author from Cambridge’s department of zoology, in a press statement. You could make nearly all of the front of your body one giant adhesive pad, of course, but then how would you move? Any time you tried to raise an arm or leg to do any climbing you'd fall off the wall, as there wouldn't be great enough contact between your body and the surface you were climbing to keep you up there.
The crux of the matter is that the volume and surface area of an animal do not increase at the same rate. As animals get bigger, the ratio of surface area to volume decreases. "This poses a problem for larger climbing animals," says David Labonte, a senior author from the same department. "Because, when they are bigger and heavier, they need more sticking power, but they have comparatively less body surface available for sticky footpads." Labonte says this means there's a natural upper limit for how big animals that make use of adhesive pads can be before it simply becomes too much effort — and it's about the size of a gecko.
We can still climb walls — but only with the right technology
Of course, this study only looked at the adhesive pads found in nature, and with stickier materials, the amount of surface area needed would be decreased. Labonte, Federle, and their colleagues even found that this was true among certain species where animals were getting bigger faster than their adhesive pad size was increasing — instead, their pads got stickier. This means that humans may never be able to climb up walls using just our hands and feet, we might be able to do so with specially engineered sticky pads. So, less Spider-Man, and more Doctor Octopus.
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