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Sorry, Spiderman. Geckos are the largest animals that can stick to walls, study says

Sorry, Spiderman. Geckos are the largest animals that can stick to walls, study says
A gecko is posed for a photo next to an ant. Both animals are alive. (David Labonte)

For those who have ever watched a spider or gecko scale a wall and dreamed of doing the same, scientists have some disappointing news: You're too big to play Spiderman.

In fact, no creature larger than a gecko has the dimensions suitable for vertical climbs, according to a study published this week in the Proceedings of the National Academy of Sciences.

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Wall-clinging animals owe their gifts to sticky pads on their appendages. The larger they get, the more of their body area they must devote to these pads.

If a human were to climb walls like Spiderman, he would need to have adhesive pads covering 40% of his body, said lead author David Labonte, who studies biomechanics at the University of Cambridge.

Labonte and his colleagues compared the adhesive pads of 225 different climbing species, ranging in size from mites to geckos. Bigger animals had bigger sticky pads, of course, but the proportion of those pads grew as well, the researchers found.

For example, mites use 0.02% of their bodies for sticking to walls, spiders use about 1%, tree frogs use 2% and geckos use 4%.

How much of abody's surface area needs to be covered by sticky footpads to stick to walls. Humans would need to use 40% of their body area.
How much of abody's surface area needs to be covered by sticky footpads to stick to walls. Humans would need to use 40% of their body area. (David Labonte)

In other words, geckos dedicated about 200 times more of their body area to adhesive pads than did mites. For Peter Parker to make that work, he'd need a shoe size of 114. (With great power comes great big feet, apparently.)

It would be impractical for an animal to dedicate that much body area to adhesive pads, "because they need that surface area for something else," Labonte said. In fact, anything more than the 4% used by a gecko "might interfere with other body functions."

Evolution has developed two strategies for allowing animals to stick to walls: bigger feet and stickier feet.

Among closely related species, such as two kinds of spider, the researchers found the pads didn't get much bigger. Instead, they got sticker. This probably has to do with evolutionary constraints, they said.

"There is only so much you can change without having to redesign the whole animal, or somehow 'grow' new features," Labonte said.

Sticky footpads in various species.
Sticky footpads in various species. (David Labonte)

Scientists hope that by understanding how animals climb walls so effectively, they can replicate their abilities with new technology.

Gecko-inspired technology could help scientists develop rescue robots capable of climbing into dangerous areas. But first they'd have to learn more about adhesives that are reusable and don't require much energy to unstick and re-stick.

In 2014, scientists at Stanford University unveiled a concept device that mimics the way geckos adhere to and scale surfaces. However, it worked best on flat, smooth surfaces and was vulnerable to contamination. (Here's a video of it in action.)

The bio-inspired contraption is "impressive but still nothing compared to what geckos can do," Labonte said.

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As for your dreams of becoming Spiderman, Labonte advised patience.

"You might have to wait for a while," he said. "We can't reproduce what geckos have. And if we could, it still wouldn't even be good enough to have a Spiderman-type thing."

Follow me on Twitter @seangreene89

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