In focus: Raise a TOEst to geckos!

This week’s post is from Dr. Emily Naylor, a postdoctoral scientist at the Department of Biological Sciences, George Washington University (DC, USA). This post is about her ongoing research. If you would like to write for Anatomy to You, get in touch via Facebook or Twitter. Happy and safe holidays to you!

Have you ever seen a gecko run up a wall or hang upside down on a ceiling?

As a group, the geckos’ claim to fame (other than GEICO) are their sticky toe pads (Figure 1). One pad has tens of thousands of microscopic hairs called setae (pronounced “see-tee”), each about as long as a human hair is wide, which branch into many tiny surface-contacting tips. When a gecko takes a step, the molecules of these tips and the molecules of the surface become so close and cozy that they form temporary adhesive (‘resistance to separation’) bonds; you can think of this in a similar way to how your hair sticks to a balloon when static electricity has built up. The tips of the setae also generate friction (‘resistance to movement force’) against the surface until the gecko peels back its toes to detach and get ready for its next step.

from https://makeagif.com/i/ObAS-o

With millions of setae across all feet creating billions of potential contact points, geckos can generate enough attachment force to support themselves and then some; a Tokay gecko sticking to smooth Plexiglass with its front feet could support the weight of five other geckos! Imagine dangling from a skyscraper with five of your friends hanging on to you!

Figure 1. Gecko toe anatomy using the Tokay gecko (A) as a model. Gecko feet (B) typically have claws, and many have multiple plates (i.e., the stacked lines) that hold the microscopic hairs, or setae, and make up the toe pads. When we cut a toe in half and turn it on its side (C), we can view the setae with a scanning electron microscope, a device that allows us to see external structures in detail that are invisible to the naked eye (D). A and B are modified from Niewiarowski et al. 2016; images C and D are from E.R. Naylor

This specialized anatomy for strong, repeated sticking without glue or suction cups, continues to inspire engineering of materials with similar properties for human use, such as long-lasting bandages, and gripper pads for people and robots to scale buildings. Move over, Spiderman, we’re entering the age of Geckohumans and Geckobots!

However, geckos have more than one trick up their scales for climbing; they also have claws. Claws help animals grip by puncturing into soft surfaces (like cleats on a soccer shoe) or by locking onto bumps on hard surfaces (like a grappling hook; Figure 2). We know from experiments that if you trim a gecko’s claws (the same way you would trim a cat’s), it can’t hold on as well to things like soft leaves or rough sandpaper.

Figure 2. Cartoon drawing of a gecko toe (side view). The left lower image depicts how fewer setae make contact on an irregular, bumpy surface. The lower right image depicts how the tip of a claw can lock onto relatively large surface bumps. Modified from Naylor & Higham 2019

So, are all geckos unstoppable daredevils armed with sticky pads and claws? Not exactly. About one-third of the 1900+ known gecko species don’t actually have toe pads and several don’t have claws – a handful have neither (not counting the legless geckos)! Moreover, toes and toe pads come in a variety of shapes (e.g., Figure 3). What’s up with that?

Figure 3. Gecko feet galore! Feet from six different gecko species provide a glimpse of the different toe anatomies observed among this group of lizards. Much remains to be understood as to what these shape differences mean for how geckos move on surfaces within their particular environments. Image from Autumn et al. 2002

Geckos live on nearly every continent (Antarctica is too cold) and in many types of habitats where they may encounter a variety of surfaces, including rocks, leaves, bark, sand, and humanmade materials (e.g., concrete). Some species specialize on one surface, while others use many. Furthermore, not all geckos are climbers; many species move on the ground and even burrow (Figure 4). So do different toe anatomies in geckos represent adaptations, or inherited features that help them survive in their environments?

Figure 4. Geckos live in many habitats and use a variety of surfaces. From the top: Some are rock-climbers, such as the padless Banded Bent-toed Gecko (Cyrtodacytylus pulchellus; Malaysia), others prefer plant surfaces but don’t mind artificial ones, such as the clawless Gold Dust Day Gecko (Phelsuma laticauda; Hawaiian Islands, non-native), and some live on the ground and bury themselves in soft sand, such as the padless and clawless Web-footed Gecko (Pachydactylus rangei; Namibia). Top image from A.J. Cobos; middle and bottom images from E.R. Naylor

To begin addressing this question, I measured toe pad and claw features in 112 species to test if geckos reported to use the same sorts of habitats share certain features compared to geckos using other habitats – all while accounting for the potential of closer relatives on the gecko family tree to look more like each other than more distantly related species.

So far, my results show that rock-climbing species tend to have longer setae than other climbers and ground-dwellers, which may point to a challenge in getting setae to make contact on bumpy, cracked surfaces (Figure 2). Furthermore, species that climb vegetation and/or rock tend to have larger toe pads and more curved claws relative to species that are more ground-dwelling. Larger toe pads mean more setae and more sticky bonds, and curved claws are better for gripping than flat claws; both seem helpful to avoid falling while chasing food or escaping a predator! 

Now that we have an idea of what differences in toe anatomy are found in geckos living in different kinds of habitat, we can begin investigating how these patterns are related to biological function, or purpose, and why we see some toe shapes in certain environments. For example, are longer setae actually better at sticking on rocky surfaces? We can also dig deeper to study how irregular, squishy, wet, sloped, dusty, and crumbly the surfaces within their habitats are to better understand how they use their toes to move around. The fact that geckos don’t follow a “one style fits all” policy makes them a fun and exciting group to study for engineers and biologists alike!

Check out these resources to learn more about geckos and gecko-inspired technology!

Bauer, A. M. (2013). Geckos: the animal answer guide. JHU Press. Baltimore, MD.

You can follow updates on this work and other projects investigating neat aspects of animal anatomy on my website!

References (*figures used above)

Autumn, K., Liang, Y. A., Hsieh, S. T., Zesch, W., Chan, W. P., Kenny, T. W., Fearing, R., & Full, R. J. (2000). Adhesive force of a single gecko foot-hair. Nature405(6787), 681-685. https://doi.org/10.1038/35015073

*Autumn, K., Sitti, M., Liang, Y. A., Peattie, A. M., Hansen, W. R., Sponberg, S., Kenny, T.W., Fearing, R., Jacob, N., & Full, R. J. (2002). Evidence for van der Waals adhesion in gecko setae. Proceedings of the National Academy of Sciences99(19), 12252-12256. https://doi.org/10.1073/pnas.192252799

Gorb, S. N. (2008). Biological attachment devices: exploring nature’s diversity for biomimetics. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences366(1870), 1557-1574. https://doi.org/10.1098/rsta.2007.2172

Irschick, D. J., Austin, C. C., Petren, K., Fisher, R. N., Losos, J. B., & Ellers, O. (1996). A comparative analysis of clinging ability among pad-bearing lizards. Biological journal of the Linnean Society59(1), 21-35.

*Naylor, E. R., & Higham, T. E. (2019). Attachment beyond the adhesive system: The contribution of claws to gecko clinging and locomotion. Integrative and comparative biology59(1), 168-181. https://doi.org/10.1093/icb/icz027

*Niewiarowski, P. H., Stark, A. Y., & Dhinojwala, A. (2016). Sticking to the story: outstanding challenges in gecko-inspired adhesives. Journal of Experimental Biology219(7), 912-919. doi: 10.1242/jeb.080085 https://jeb.biologists.org/content/219/7/912

Russell, A. P. (2002). Integrative functional morphology of the gekkotan adhesive system (Reptilia: Gekkota). Integrative and Comparative Biology42(6), 1154-1163. https://doi.org/10.1093/icb/42.6.1154

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