A group of scientists from Cambridge University have discovered that Spiderman is an impossible fiction and that geckos are the largest animal that would be able to scale a smooth vertical wall with no toeholds. The scientists have estimated that the amount of adhesive needed in order for Spiderman to achieve the same feat would involve 40% of their body to be covered with the sticky substance.
Scientists have been researching the feasibility of creating large-scale adhesives similar to those used by geckos to scale vertical objects. They studied all types of climbing animals, from tiny mites, spiders, tree frogs and geckos. They discovered that the bigger the body size, the more adhesive was required in order for them to be able to climb. As the body size increases, so does the amount of volume which makes the animal heavier to lift. This in turn requires stronger sticking power in order to be able to walk up a wall or across a ceiling. However greater volume does not allow for the greater surface area which would enable a bigger animal to climb.
The scientists found that geckos use about 200 times more of their body surface area for adhesive pads than mites. They discovered that for Spiderman to be able to climb up a vertical wall, that he would need very large feet or have 80% of his front covered in sticky substance. The scientists compared the size of the animals with their footpad size and found that although the animals were all different, the sticky feet were similar. The scientists surmised that these adhesive pad must have been evolved separately in the different animal species but that the results of the evolution were similar. This suggested to the scientists that the evolution had created a successful solution to the problem.
The other way for a larger animal to be able to climb would be to make the pads more sticky. The scientists observed this phenomenon in some of the animals they studied which were subspecies. The bigger the animal, the more sticky the footpads became.
“Our study emphasises 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,” commented Dr David Labonte, the lead author of the study.
Labonte, D et al., Extreme positive allometry of animal adhesive pads and the size limits of adhesion-based climbing. PNAS 18 January 2016. DOI: 10.1073/pnas.1519459113