Animals do the most amazing things. Read about them in this series by Janaki Lenin.
A gecko and an ant. Credit: A. Hackmann and D. Labonte
Could Spider Man climb up skyscrapers like he does in the comics? Scientists say that normal-sized sticky hands and feet alone won’t help him climb.
Researchers from the U.K., Australia, and the U.S., studied the gummy feet of 225 climbing species, ranging from insects and spiders to geckos to bats. The largest creature they studied was more than seven times heavier than the smallest.
“We were looking at vastly different animals – a spider and a gecko are about as different as a human is to an ant,” says David Labonte of the University of Cambridge, U.K. “But if you look at their feet, they have remarkably similar footpads.”
Adhesive feet evolved again and again in insects, spiders, reptiles, amphibians, and mammals. With their sticky soles, these creatures climb smooth vertical surfaces or crawl upside down. They hunt prey and shelter from predators in places inaccessible to others. But sticky legs can also be a liability. They can’t selectively turn off the stickiness when it’s not needed. It might prevent them from running faster.
Small shield bugs have two tiny pads on each of the six legs. In the larger tree frogs, the four enlarged toe tips on each leg are sticky. In geckos, the setae-covered soles of their toes help them climb. As animals get larger, the sticky area of their feet also becomes larger. Geckos’ adhesive pads are 200 times larger than mites.
“As animals increase in size, the amount of body surface area per volume decreases,” explains Labonte. “An ant has a lot of surface area and very little volume, and a blue whale is mostly volume with not much surface area.”
How much of your body surface area needs to be covered by sticky footpads? Credit: David Labonte
But there’s a limit to how large these creatures can grow and still retain their glue pads. They can’t proportionately increase the size of their feet. For example, an eight-gram shrew has 4% bone mass. If we scaled up this ratio for an eight-tonne elephant, it would have 400% bone mass. In reality, an elephant’s bone mass is only 13%. Similarly, the size of toe pads can’t be scaled upwards indiscriminately.
“This poses a problem for larger climbing species because they are bigger and heavier,” says Labonte. “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.”
These large animals rely on alternative strategies. One way to compensate for the loss in sticky area is to make their pads stickier. By comparing 17 species of tree frogs, the researchers found adhesive force increases with body mass. So larger species have more efficient pads.
The diversity of adhesive pads in a range of creatures. Credit: David Labonte
“Within frogs, we found that they have switched to making pads stickier rather than bigger,” says Christofer Clemente, a co-author of the study. “It’s remarkable that we see two different evolutionary solutions to the problem of getting big and sticking to walls.”
Some geckos grow far larger than any frog. And the researchers say geckos are the upper size limit for creatures with sticky feet.
To be functional, “geckos are ‘overdesigned,’ and their toe pads can support far more weight than their own bodies,” Bauer told The Wire. “This is because a moving gecko will only have a fraction of its setae in contact with the surface at any one time.” Bauer shared his data with the research team but wasn’t involved in the analysis.
The largest in the world is the 25-cm long Leach’s giant gecko found in New Caledonia. Herpetologists measure geckos from nose tip to cloaca. These reptiles drop their tails when alarmed. Re-grown tails are shorter and some may have no tails at the time of measurement. Has this gecko reached the optimum size to adhesiveness ratio?
“For all we know, Leach’s gecko is the largest animal to have adhesive pads, and the ability to climb smooth surfaces,” Labonte told The Wire. “This does not mean that this is the absolute size limit, but it implies that it is very close to it. This size limit is not only determined by the size and performance of the adhesive pads, but also by the fact that other climbing strategies become increasingly feasible as animals grow larger. To an ant, a twig is a relatively flat, i.e., two dimensional surface, while for larger animals it will be a 3D object that they can grasp. Hence, hands, toes and claws become increasingly useful, and are indeed the tools that larger climbing animals, such as monkeys, use for climbing. However, these animals are not able to climb on smooth surfaces, and they likely wouldn’t need to, anyway.”
It’s hard to say what size prehistoric geckos reached since they don’t fossilise well. An ancient gecko found preserved in amber in Myanmar was a tiny fellow. But there was one even bigger than Leach’s giant gecko – the kawekaweau, or Delcourt’s sticky-toed gecko, that went extinct from New Zealand.
Alain Delcourt, custodian of the collection at Muséum d’histoire naturelle de Marseille, holding the only specimen of the gecko named after him. Credit: Aaron Bauer
The only known museum specimen is 37 cm long. “It certainly weighed well more than one kg. and is the largest padded gecko that ever lived, as far as we know,” says Aaron Bauer, a professor at Villanova University, who first described the species.
Delcourti’s gecko probably didn’t have large enough sticky area. “If only a few toes were in contact with the surface, it might have been able to hang on,” says Bauer. “But it may not have been able to climb and certainly not run upside down like smaller geckos can.”
Historical records say that this reptile lived on trees and hid under bark, just like its closest relative, Leach’s giant gecko. So it would have had use of claws as well as gummy feet.
But claws are no help in climbing glass walls. For Spiderman to walk lightly like geckos, with a fraction of his hands and feet in touch with a glass wall, he’d need to carry much more than his body weight. “A 75-kg. human might need a sufficient surface area to support 1000 kg. or more,” says Bauer.
In fact, he would need 40% of his total body surface or 80% of his front to be adhesive. Or, he’d need shoes of European size 145 or US size 114, says Walter Federle, the senior author of the paper. The Guinness Record for the world’s largest feet is European size 59 or US size 26.
“This is a direct result of simple physical constraints, arising from the different rates at which body weight and body surface area grow with animal size,” says Labonte.
Just as tree frogs increase the stickiness quotient, why couldn’t Spiderman have gummier palms and feet?
“Our data suggests that the variation in stickiness is relatively small, at least in comparison to the variation in pad area per unit body mass,” replies Labonte. “This implies there might be a limit to how much stickier you can or want to make your pads. This limit likely depends on a variety of factors. For example, there’s a trade-off between stickiness and running speed.
In short, there are certainly alternative strategies that might allow humans to climb smooth walls and colleagues at Stanford have demonstrated this. But these will be distinctly different from what climbing animals do, and will likely have a negative impact on other important aspects, such as controllability, running speed etc.”
Labonte and his team are currently investigating how some animals, such as tree frogs or ants, are able to increase the stickiness of their pads as they grow bigger. “We hope this will yield further insights into the biomechanics of controllable attachment, which could then allow us to design improved bio-inspired adhesives, which may also work on larger scales.”
The study illustrates the difficulty for larger climbing animals to have big enough sticky pads.
Bucking the ratio of body size to sticky surface area makes Spiderman a super-hero in many more ways.
The study was published in the Proceedings of National Academy of Sciences on January 18, 2016.
Janaki Lenin is the author of My Husband and Other Animals. She lives in a forest with snake-man Rom Whitaker and tweets at @janakilenin.
Featured image source: marvel.com.
