Geckos are 'size limit' for sticking to walls - Oh No! Then what about Spider man?
- January 24, 2016
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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 percent 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.
Organismal functions are size-dependent whenever body surfaces supply body volumes. Larger organisms can develop strongly folded internal surfaces for enhanced diffusion, but in many cases areas cannot be folded so that their enlargement is constrained by anatomy, presenting a problem for larger animals.
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. “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.
"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.
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.
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 humanmade adhesives.
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."
Labonte, D et al. Extreme positive allometry of animal adhesive pads and the size limits of adhesion-based climbing. PNAS, January 2016 DOI: 10.1101/033845