I can feel beads of sweat forming between my fingertips and the rock, but I don’t dare upset my balance by reaching around my back to dip into my chalk bag. “Come on Linz, stick it!” one of my friends calls from below. Awkwardly leaning over the top of a huge, egg-shaped boulder, I search for any irregularity in the water-polished sandstone and see nothing. In a desperate last effort, I slap at the top of the egg and feel my feet cut loose from the tiny footholds. I hit the thick foam crash pad below with a dull thud, my fall controlled by the hands of several spotters.
“Damn!” I giggle, jumping up and down on the pad in frustration, clutching my raw fingertips. I step aside and watch one of my other friends try the boulder problem—a tricky sequence of dynamic moves up small edges on the face of the egg. On my next attempt, I try a foothold that seems unlikely to help, but it works, and I haul myself to the top of the boulder, almost giddy with adrenaline and surprise.
Bouldering—climbing short combinations of difficult moves (called problems) close to the ground without the need for ropes—is just one of several forms of rock climbing that have exploded in popularity in recent years. Modern humans climb mostly for fun, but climbing may have been a crucial ability for our early human ancestors and remains important to our close primate relatives, many of which brachiate through the tree-top environments they inhabit. While it’s common to say that someone “climbs like a monkey” and the image of chimps swinging from branch to branch in the forest may be the first things that come to mind when one thinks about good climbers, primates are hardly nature’s only—or even best—climbers.
Climbing ability has evolved hundreds of times in hundreds of species, ranging from insects to sloths to reptiles. Even some birds have interesting climbing abilities; the White-Breasted Nuthatch, for example, often climbs down tree trunks with its head pointed to the ground. For the curious rock climber, nature offers an array of subtle yet obvious climbing adaptations to both fascinate us and make us jealous. Once, while on a route at the Gunks (short for the Shawangunks, a popular climbing area near New Paltz, NY), I took a long rest on a ledge to let a Walking Stick insect make its way across my intended path. Later on the same climb, I was shocked to see a small snake negotiating a nearly featureless face off to the side of the crack system I was ascending. I’ve lain in my sleeping bag in Adirondack lean-tos watching spiders nimbly spin webs—dropping down and climbing back up, over and over again—above my head, and I’ve been captivated watching my friend’s chameleon, Maggie, make her way from branch to branch with an exquisite sense of balance unsurpassed by any of the most talented human climbers I’ve ever seen.
Out of all of the earth’s climbing organisms, however, by far the award for “best climber” goes to a group of unassuming little reptiles. Climbing geckos can cling upside down to glass surfaces with just one toe, an uncanny and, until recently, little-understood ability that would make just about any rock climber, from the total beginner to Chris Sharma (who is arguably the best climber in the world), jealous.
Geckos are members of the reptile order Squamata and the superfamily Gekkota, which is comprised of four families, one of which, Gekkonidae, includes the climbing geckos. Of the approximately 930 species of Gekkonids, most are between 1.5 and 20 centimeters long, nocturnal, and insectivorous. They range in distribution from South America to Siberia and live in habitats ranging from trees and cliffs to underground caves and burrows. While Gekkonids are known in the scientific community mostly for their climbing abilities, they are also unique among reptiles for their ability to vocalize, producing a variety of sounds from chirps to growls. Many geckos use these noises to establish their territories and to aid in mating, and some, like the Barking gecko (found in southern Africa) actually participate in large choruses. Even in a family so diverse that its species may burrow, use flaps of skin to glide between trees, or sing, the species that climb have attracted the most scientific attention. The most widely studied among the Gekkonids is the Tokay Gecko (Gekko gecko) from Southeast Asia, which is known to have the most highly developed climbing abilities of the geckos.
At first glance, Gekkonids seem unlikely climbers. They lack the strong claws found in other climbing lizards, particularly Anoles (members of the iguana family), and they don’t even superficially resemble species in the chameleon family with their characteristic long limbs, zygodactylous toes, narrow bodies, and prehensile tails, all features that contribute to a superb sense of balance. Even among all of the members of the superfamily Gekkonidae, climbing geckos have few features that distinguish them from their non-climbing counterparts. Upon closer study, it turns out that climbing geckos don’t show any of the typical climbing-specific body structure—shorter limbs to keep the center of gravity closer to the surface being climbed (though chameleons also break this rule) and powerful forelimbs—found in other climbing reptiles such as some species of Anoles and one type of Fence Lizard
Scientists have suggested that geckos may have experienced a unique form of evolution in which the possession of one very successful trait supersedes the need for several more common traits that work together to accomplish the same function. Though this seems like an unlikely concept, our own species is our best example. Our highly intelligent brains have allowed us to overcome a huge number of other shortcomings that should preclude us from being the most successful species—biologically speaking—on the planet. What we lack in traits that would allow us to survive running around naked in all parts of the world, we make up for with the intelligence to create clothing, shelter, and tools.
What geckos lack in climbing-specific body form, they make up for in their feet. Looking closely at Gekkonids, one notices few distinctions between overall body shapes, but striking differences in toes. In some species of burrowing Gekkonids, the toes have extra bones and skin flaps that make the feet into a more effective shovel for digging, and geckos that live on sand often have fringed toes to help them walk on a shifting surface. Climbing geckos, on the other hand, have toes that are wide and flat, with unique pads on the underside. These toe pads, made up of broad, modified scales called lamellae, have for centuries been a source of intrigue for scientists, who have suggested a number of possibilities from suction to electrostatic interaction to explain how the little lizards “stick.” Only in the last several years have scientists come to understand the secret behind geckos’ climbing abilities, and they’ve discovered that they use the most unlikely of mechanisms.
Even the Peterson Field Guide to Reptiles and Amphibians of Eastern Central North America (Third Edition, 1998) incorrectly says that geckos use “suction cups” for their climbing ability. In fact, each of the gecko’s lamellae contains an array of tiny hairs called setae, which themselves branch into 100-1000 smaller structures aptly called spatulae, which have flat triangular ends. The setae, which are composed of beta-keratin, are only about 100 microns long and five microns in diameter (about one-tenth the diameter of a human hair), and the spatulae are each about 200 nm across at their widest point, making them visible only with a scanning electron microscope. Rather than acting as suction cups, the setae and spatulae allow geckos to achieve extremely intimate contact—down to a molecular level—with the surfaces they climb.
Scientists have known about the presence of setae in gecko toe pads for over a century, and it has taken about that long to figure out just what makes the setae work. Because geckos don’t have any glands on their feet, scientists quickly realized sticky secretions weren’t a possibility. Suction was ruled out next, and the gecko’s ability to stick to smooth glass demonstrated that the setae could not be relying on friction to interlock with surface irregularities. With all of the likely possibilities exhausted, scientists were left to focus on intermolecular forces.
The most obvious possible intermolecular force seemed to be capillary adhesion, a method used by many climbing insects, but scientists already knew that geckos don’t secrete fluid from their feet. Therefore, for capillary adhesion to occur, a layer of water molecules would already have to be present on the surface for a gecko to stick, making its climbing ability humidity-dependent. For geckos living in even a mildly wet environment, this type of climbing would be plausible, but because desert geckos can cling to surfaces just as well as those living in the rain-forest, scientists ruled out capillary adhesion and turned instead to the smallest of molecular forces: van der Waals dispersion forces.
Van der Waals forces are intermolecular attractions that occur as a result of the location of electrons in a molecule. Electrons are constantly moving, giving molecules temporary moments of polarity (meaning that one area on the molecule is somewhat positively charged while another area is somewhat negatively charged). When molecules are close enough together, areas with different charges can come into contact, resulting in weak attractive forces that function in much the same way that the positive end of a magnet attracts the negative end of another magnet, though on a much smaller, much weaker scale. In geckos, the tiny surfaces of the spatulae allow for such close contact that the molecules in the spatulae can interact with the molecules in whatever surface the gecko happens to be climbing. Although individual van der Waals forces are imperceptible on their own, the sum of the forces exerted by millions of setae is plenty to give geckos their surreal sticking abilities.
Within the last two years, researchers have found evidence to support the van der Waals hypothesis of setal adhesion by studying the anatomy of single seta and looking carefully at the way setae attach to surfaces. A team of scientists at Berkeley led by Dr. Kellar Autumn placed setae at different angles on a surface and found not only that orientation is crucial (an idea consistent with the van der Waals hypothesis because attraction depends on how the molecules come into contact with each other), but also that setae must first be pushed against the surface and then slide an imperceptibly small distance in order to achieve adhesive contact. After calculating the forces exerted by a single seta and a single spatula, researchers found that the values fell within the predicted range for van der Waals forces based on the molecules in the proteins making up the setae. Using these measurements, it turns out that an average-sized (50 gram) Tokay Gecko, which has about 6.5 million setae, can support its own weight on a vertical surface with less than half a percent of its setae attached! That’s roughly equivalent to me holding myself up on a climb by just the tip of my pinky finger!
While it might seem, then, that geckos’ feet are “overbuilt” for their purpose, the “extra” setae are crucial anytime a gecko needs to negotiate a surface that is rough enough to prevent uniform contact, which is often the case in the natural world. The additional sticking power gained by having so many setae also allows climbing geckos to jump easily from surface to surface.
Researchers have found it difficult to say just why setae may have evolved in the first place, though it’s interesting to note that in all Gekkonids, adaptations for getting around—whether it’s by climbing or burrowing—have evolved specifically in the feet and not elsewhere in the body. Setae likely evolved from growths that assist in skin shedding often found in the skin of lizards.. This hypothesis is supported by the fact that similar, less-developed setae have evolved in a few species of arboreal Anoles, which are only very distantly related to Gekkonids. Still, figuring out why and how such general structures became so highly specialized may take several more years of research, which could lead to even more unlikely and fascinating conclusions.
Though reptiles rarely get positive media attention—if any at all—the recent findings about geckos’ feet have gained recognition in the mainstream media as well as in more specialized publications, particularly because of the possibilities that exist for developing geckos’ technology for human purposes. As part of the research at Berkeley that concluded that gecko setae use van der Waals forces, scientists constructed and studied synthetic gecko hairs in addition to real ones. Since then, they’ve been working with engineers to produce synthetic setal arrays in hopes of producing complete lamellae. Success may be several years away, but there is a definite possibility that in the future, gecko-inspired robots could be created to navigate any surface, opening up limitless possibilities in exploration, safety, convenience, and even entertainment.
Even rock climbers have big dreams for the potential of fake gecko setae. In May 2003, Climbing magazine published a blurb about the Berkeley researchers’ work under the headline “Rock climbing may no longer involve trying,” suggesting that the synthetic lamellae may someday be incorporated into gloves and shoes that would allow climbers to scale previously impossible routes. While such “gear” may give new meaning to the encouraging climbing phrase “stick it,” I think Climbing is missing the point. Making it possible to climb anything would completely destroy the creative physical challenge that makes climbing fun.
In rock climbing, a set of techniques has “evolved” that generally work the best for doing different types of moves and routes. Command of these techniques and a body type favorable for climbing—tall and thin, with little body fat and long arms—are definitely an advantage, but not always. When students in the climbing classes I teach for Cornell Outdoor Education ask if there’s an ideal body type for climbing, my answer is yes, to an extent, and I liken having a “climber’s build” to being like a chameleon. Then I tell them about geckos and Lynn Hill, one of the best climbers in the world, who makes up for her five-foot-two figure with tiny fingers and an incredible strength-to-weight ratio.
We
have a lot to learn from geckos beyond rock climbing, too. Geckos remind us that despite our amazing
brains, nature has a lot of incredible surprises for us, even when we think we
already know the obvious answer.
Scientists were so convinced that geckos stick using suction that even
the most recent Peterson guide, one of the foremost authorities on reptiles,
still has it wrong. When researchers
ventured to explore the possibilities further, it turned out that not only was
suction one of the most obviously incorrect hypotheses, but that the truth
opens up possibilities in science and engineering that were never thought
possible. Ultimately, these little
reptiles are a perfect reminder that when looking to our own brains—as
intelligent as they are—for intuitive answers to life’s questions and
innovative solutions to problems, we shouldn’t forget to look to the rest of
the organisms that share the earth with us.
In science, like in climbing, the best footholds are often the least
obvious.
References
Autumn, K., Y. A. Liang, S. T. Hsieh, W. Zesch, W. P. Chan, T. W. Kenny, R. Fearing and R. J. Full. 2000. Adhesive force of a single gecko foot hair. Nature 405:681-684.
Autumn, K. and A. M. Peattie. 2002. Mechanisms of adhesion in geckos. Integrative Comparative Biology 42:1081-1090.
Autumn, K., M. Sitti, Y. A. Liang, A. M. Peattie, W. R. Hansen, S. Sponberg, T. W. Kenny, R. Fearing, J. N. Israelachvili, and R. J. Full. 2002. Evidence for van der Waals adhesion in gecko setae. Proceedings of the National Academy of Sciences. 99(19):12252-12256, http://www.pnas.org/cgi/doi/10.1073/pnas.192252799
Bauer, A. M., A. G. Kluge, and G. Shuett. 2002. Lizards. In T. Halliday and K. Adler (eds.), Firefly Encyclopedia of Reptiles and Amphibians, pp. 138-169. Firefly Books, Buffalo, New York.
Russell, A. P. 2002. Integrative functional morphology of the Gekkotan adhesive system (Reptilia: Gekkota). Integrative Comparative Biology 42:1154-1163.
Zaaf, A. and R. Van Damme. 2001. Limb proportions in climbing and ground-dwelling geckos (Lepidosauria, Gekkonidae): a phylogenetically informed analysis. Zoomorphology 121:45-53.