Have you ever noticed that geckos are actually the last animals up the size scale, who can successfully scale a wall? Latest research reveals why.
A new study, published today in PNAS, shows that in climbing animals from mites and spiders up to tree frogs and geckos, the percentage of body surface covered by adhesive footpads increases as body size increases, setting a limit to the size of animal that can use this strategy because larger animals would require impossibly big feet. Scientists estimate that a human would need adhesive pads covering 40% of their body surface in order to walk up a wall like Spiderman, and believe their insights have implications for the feasibility of large-scale, gecko-like adhesives.
Dr David Labonte and his colleagues in the University of Cambridge’s Department of Zoology discovered that tiny mites use approximately 200 times less of their total body area for adhesive pads than geckos. We humans would require roughly 80% of our front, to be covered in sticky footpads if we wanted to achieve the same.
“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 a blue whale is mostly volume with not much surface area” explains Labonte.
“This poses a problem for larger climbing species because, when they are bigger and heavier, they need more sticking power to be able to adhere to vertical or inverted surfaces, but they have comparatively less body surface available to cover with sticky footpads. This implies that there is a size limit to sticky footpads as an evolutionary solution to climbing – and that turns out to be about the size of a gecko.”
These investigations also gave the researchers greater insights into how the size of adhesive footpads is influenced and constrained by the animals’ evolutionary history.
“We were looking at vastly different animals – a spider and a gecko are about as different as a human is to an ant- but if you look at their feet, they have remarkably similar footpads,” says Labonte.
“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.”
“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 do looking into the strategies that animals have developed in order to maintain the ability to scale smooth walls, which would likely also have very useful applications in the development of large-scale, powerful yet controllable adhesives.”
Thus, we can see two different evolutionary solutions to the problem of scaling walls. One is to get the adhesive pads bigger, and second is to get them stickier. “Across all species the problem is solved by evolving relatively bigger pads, but this does not seem possible within closely related species, probably since there is not enough morphological diversity to allow it. Instead, within these closely related groups, pads get stickier. This is a great example of evolutionary constraint and innovation.”
Author: Technology Vista
