Study Reveals Geckos as the ‘Size Limit’ for Sticking to Walls

A recent study discusses why the largest animal capable of scaling vertical and smooth walls is the gecko. According to scientists, humans can walk on walls like Spiderman, but only with the help of adhesive pads covering 40% of the body’s surface. Scientists believe that their discovery provide a feasibility for large-scale, gecko-like adhesives.

Vallgatan 21D, Gothenburg, Sweden (photo by Gudbjörn Valgeirsson)

The study, published in PNAS, explains that in climbing animals - such as mites and geckos - the larger the body size, the higher percentage of the body surface covered in adhesive footpads. This strategy is suitable for small animals as bigger animals would need impossibly big feet.

Dr David Labonte, and his colleagues from the Department of Zoology in the University of Cambridge, discovered that in comparison with geckos, small mites use 200 times less body surface area for their adhesive pads. Humans would need up to 40% of their total body surface, or around 80% of their front covered in sticky footpads.

Labonte suggests that for a large animal a huge portion of its body surface will need sticky pads and the morphological changes needed would make the development of this trait impractical.

“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 also from Cambridge’s Department of Zoology.

As animals increase in size, the amount of body surface area per volume decreases – an ant has a lot of surface area and very little volume, and an elephant is mostly volume with not much surface area. This poses a problem for larger climbing animals because, when they are bigger and heavier, they need more sticking power, but they have comparatively less body surface available for sticky footpads. This implies that there is a maximum size for animals climbing with sticky footpads – and that turns out to be about the size of a gecko. We covered a range of more than seven orders of magnitude in body weight, which is roughly the same weight difference as between a cockroach and Big Ben. Although we were looking at vastly different animals – a spider and a gecko are about as different as a human is to an ant – their sticky feet are remarkably similar. Adhesive pads of climbing animals are a prime example of convergent evolution – where multiple species have independently, through very different evolutionary histories, arrived at the same solution to a problem. When this happens, it’s a clear sign that it must be a very good solution.

Dr David Labonte, University of Cambridge

It is possible to increase the strength of the footpads by making them stickier.

We noticed that within some groups of closely related species pad size was not increasing fast enough to match body size yet these animals could still stick to walls. We found that tree frogs have switched to this second option of making pads stickier rather than bigger. It’s remarkable that we see two different evolutionary solutions to the problem of getting big and sticking to walls. Across all species the problem is solved by evolving relatively bigger pads, but this does not seem possible within closely related species, probably since the required morphological changes would be too large. Instead within these closely related groups, the pads get stickier in larger animals, but the underlying mechanisms are still unclear. This is a great example of evolutionary constraint and innovation.

Christofer Clemente, University of the Sunshine Coast.

Based on these insights, the researchers felt that the size limits of adhesive pads would have extreme implications when making large-scale adhesives that are also bio-inspired, and currently effective only on surface areas that are very small.

Our study emphasizes the importance of scaling for animal adhesion, and scaling is also essential for improving the performance of adhesives over much larger areas. There is a lot of interesting work still to be done looking into the strategies that animals use to make their footpads stickier - these would likely have very useful applications in the development of large-scale, powerful yet controllable adhesives.

Dr David Labonte, University of Cambridge

This study was supported by research grants from the UK Biotechnology and Biological Sciences Research Council (BB/I008667/1), the Human Frontier Science Programme (RGP0034/2012), the Denman Baynes Senior Research Fellowship, and a Discovery Early Career Research Fellowship (DE120101503).

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