Most animals that walk up vertical surfaces – from ants and spiders to geckos and sucker-footed bats – are able to do so thanks to sticky pads on their toes or feet. Now, researchers analyzing the adhesive footpads of animals of varying sizes found that the pad area increases disproportionately with weight, limiting the size of climbing animals.

In other words, if humans were able to scale buildings like Spider-Man, 40 percent of our body surface would have to be covered by sticky pads, according to findings published in Proceedings of the National Academy of Sciences this week. 

By allowing animals to climb smooth, vertical or even inverted surfaces, adhesive pads have opened up all sorts of new habitats. In fact, they’ve independently evolved multiple times in arthropods, amphibians, reptiles, and even mammals. But this adaptive trait is subject to size-dependent physical constraints. As animals increase in size, their body surface area per volume decreases; bigger, heavier animals need more adhesive power to support their weight but at some point, the amount of sticky surfaces needed becomes impractical. That’s likely why geckos are the biggest animals capable of scaling walls. 

To see how climbing species produce adhesive forces equivalent to their body weight, a team led by University of Cambridge’s David Labonte compared the sticky footpads of 225 species. These animals varied over seven orders of magnitude in body weight.

^{A sample of adhesive footpads (in orange). D. Labonte}

The team found that, relative to total body surface area, the adhesive area of footpads increased with weight. Tiny mites, for example, use 200 times less of their body area for adhesion compared to geckos. And an 80-kilogram (176-pound) person with a body surface area of two square meters (21.5 square feet) would need disproportionally large adhesive pads that take up 40 percent of the total body surface area – or 80 percent of just the front. 

"If a human, for example, wanted to walk up a wall the way a gecko does, we'd need impractically large sticky feet," study coauthor Walter Federle says in a statement. "Our shoes would need to be a European size 145 or a U.S. size 114." 

However, when the team compared closely related species of frogs, they didn’t find this disproportionate increase of pad area. Because of evolutionary or anatomical constraints, those bigger frogs had stickier pads, not bigger ones: Pad adhesion per contact area increased with size. This suggests that two different strategies have evolved to help adhesion-based climbers deal with the physical challenges of bigger body sizes. 

^{The increase in the relative size of adhesive pads required is illustrated by blue circles. D. Labonte}