Why Spiderman can't exist: Geckos are 'size limit' for sticking to walls

January 18, 2016
A gecko and ant. Credit: Image courtesy of A Hackmann and D Labonte

Latest research reveals why geckos are the largest animals able to scale smooth vertical walls - even larger climbers would require unmanageably large sticky footpads. 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.

A new study, published today in PNAS, shows that in climbing from mites and spiders up to tree frogs and geckos, the percentage of body surface covered by adhesive footpads increases as body increases, setting a limit to the size of animal that can use this strategy because larger animals would require impossibly big feet.

Dr David Labonte and his colleagues in the University of Cambridge's Department of Zoology found that tiny mites use approximately 200 times less of their total body area for adhesive pads than , nature's largest adhesion-based climbers. And humans? We'd need about 40% of our total body surface, or roughly 80% of our front, to be covered in sticky footpads if we wanted to do a convincing Spiderman impression.

Once an animal is big enough to need a substantial fraction of its body surface to be covered in sticky footpads, the necessary morphological changes would make the evolution of this trait impractical, suggests Labonte.

"If a human, for example, wanted to walk up a wall the way a gecko does, we'd need impractically large sticky feet - our shoes would need to be a European size 145 or a US size 114," says Walter Federle, senior author also from Cambridge's Department of Zoology.

How much of your body surface area needs to be covered by sticky footpads? Credit: Image courtesy of David Labonte

The researchers say that these insights into the size limits of sticky footpads could have profound implications for developing large-scale bio-inspired adhesives, which are currently only effective on very small areas.

"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 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."

Larger animals have evolved alternative strategies to help them climb, such as claws and toes to grip with.

The researchers compared the weight and footpad size of 225 climbing animal species including insects, frogs, spiders, lizards and even a mammal.

"We compared animals covering more than seven orders of magnitude in weight, which is roughly the same as comparing a cockroach to the weight of Big Ben, for example," says Labonte.

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."

The researchers believe we can learn from these evolutionary solutions in the development of large-scale manmade adhesives.

"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," says Labonte.

There is one other possible solution to the problem of how to stick when you're a large animal, and that's to make your sticky footpads even stickier.

"We noticed that within closely related species pad size was not increasing fast enough to match body size, probably a result of evolutionary constraints. Yet these animals can still stick to walls," says Christofer Clemente, a co-author from the University of the Sunshine Coast.

"Within frogs, we found that they 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," says Clemente.

"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."

Explore further: How the stick insect sticks (and unsticks) itself

More information: Labonte, D et al "Extreme positive allometry of animal adhesive pads and the size limits of adhesion-based climbing." PNAS 18 January 2016. www.pnas.org/cgi/doi/10.1073/pnas.1519459113

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5 comments

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SciTechdude
3.7 / 5 (3) Jan 18, 2016
I think they just contradicted the title of this article with the conclusion that pads become sticker to counter-act size increases that couldn't be compensated by larger pads. What that means is, Spiderman COULD exist, if he had REALLY sticky pads. The Dream survives! =p
BartV
1 / 5 (2) Jan 18, 2016
Larger animals have evolved alternative strategies....


Please, omit such irrational statements based on an unproven conjecture of evolution.
The proper scientific sentence would be:

Larger animals have alternative strategies..

Thanks.

Valentiinro
5 / 5 (3) Jan 19, 2016
Larger animals have evolved alternative strategies....


Please, omit such irrational statements based on an unproven conjecture of evolution.
The proper scientific sentence would be:

Larger animals have alternative strategies..

Thanks.


If you don't believe anything scientists say, get off the science news sites.
mreda14
not rated yet Jan 19, 2016
Sir Isaac Newton Pub - Cambridge, United Kingdom. This bar is located at 84 Castle Street
Cambridge CB3 0AJ, United Kingdom. It is the place where Sir Isaac Newton used to go for a beer.Sir Isaac Newton had two beers when he developed the famous newton laws of physics when he saw the apple falls down.
AGreatWhopper
1 / 5 (2) Jan 24, 2016
Larger animals have evolved alternative strategies....


Please, omit such irrational statements based on an unproven conjecture of evolution.
The proper scientific sentence would be:

Larger animals have alternative strategies..

Thanks.


If you don't believe anything scientists say, get off the science news sites.


Better yet, the planet. You're surplus to requirements.

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