Geckos: a look at
nature’s best climbers
With reflections from a rock climber
Lindsay Watkins
Herpetology (BioEE 470)
April 28, 2003
On an unseasonably
nice early November day last fall, I excused myself from a day of classes and
cruised across the state with three of my friends to the Gunks, one of the east
coast’s best rock climbing areas. As I
sat on a ledge over a hundred feet off the ground, dangling my feet above the
blanket of late fall colors below me, I tried to justify missing three lectures
and a field biology lab.
“This is field bio,” I told myself, ticking off the scientific names of the three trees on another nearby ledge.
Moments later my best friend pulled himself up on the ledge next to me and clipped into the anchor. “Sweet,” he said, “this is so much better than field bio!”
“Pinus rigida,” I said in agreement, pointing to the sling of webbing around the Pitch Pine that made up one point of our anchor.
We reorganized our gear, and soon I was 20 feet above the ledge, fumbling with my cams (spring loaded devices that are inserted into cracks in the rock to make artificial anchors) and focusing on nothing but getting to the top of the pitch without taking a fall. The moves were easy, but being on lead (where instead of the rope coming down to you from the top of the cliff, you pull it up with you, clipping into cams and other pieces of protection that you place in the rock as anchors every few feet) made them seem terrifyingly difficult.
I finally got the cam into the rock and clipped in, and as I turned to the side to attach the rest of my gear back to my harness, I noticed a small snake making its way up the considerably less-featured rock next to me (unfortunately I can’t name the species… we hadn’t covered snakes in field bio yet). At the time, watching something with no limbs so easily out-climb me only caused me even more frustration than I already felt, but I looking back on the entry I later wrote in my field bio journal, I feel nothing but fascination at the snake’s ability to negotiate the nearly vertical cliff face.
Rock climbing has been an important catalyst for my interest in nature, environmental issues, and biology, not least because I have become fascinated by watching other organisms climb. Whether it’s a spider nimbly spinning a web in the top of my tent somewhere out on the trail, or my friend Jerry’s chameleon, Maggie, methodically making her way from branch to branch in her tree with exquisite balance that tops every one of the best human climbers I’ve ever seen, organisms that climb captivate me with their subtle and more obvious adaptations for climbing.
Out of everything I’ve ever watched climb, nothing I’ve seen from a human, an insect, or another reptile rivals the climbing abilities of geckos, which can cling upside down to a glass surface from just one toe. I’m certainly not the only climber who is fascinated by the gecko either; more than a few rock climbing gyms and schools use geckos in their names or logos, and all kinds of climbing gear from harnesses to shoes has the name “gecko” associated with it.
Much more importantly than the average rock-jock’s interest in the little reptiles, though, is the scientific community’s interest in learning about geckos, their evolution, and their amazing ability to climb. Scientists since Aristotle have wondered at the gecko’s ability to “run up and down a tree in any way, even with the head downwards,” and researchers have been working earnestly since the late 1800’s to find out just what it is that make geckos stick (Aristotle/Thompson, 1918; Autumn and Peattie, 2002; Russell, 2002).
Geckos (Squamata: superfamily Gekkota)
Geckos are members of the order Squamata and the superfamily Gekkota, which includes the families Gekkonidae (True Geckos), Diplodactylidae (Soutwest Pacific Geckos), Eublepharidae (Eyelid Geckos), and Pygopodidae (Flap-footed Lizards) (Bauer et al., 2002). Though these families share some general characteristics, it is Gekkonidae that is famous for its members’ climbing abilities. Gekkonidae includes 930 species of small (1.5-20cm), mostly nocturnal, predominantly insectivorous lizards that range in distribution from South America to Siberia and in habitat from trees to cliffs to underground (Bauer et al., 2002). In addition to being known for their climbing abilities, Gekkonids are also unique in their ability to vocalize, and can produce a variety of sounds from chirps to growls (Bauer et al., 2002). Of the geckos, the Tokay gecko of Southeast Asia is the most accomplished climber and most widely studied by scientists wishing to understand more about the climbing morphology and adhesion mechanisms used by geckos (Zaff and Van Damme, 2001; Autumn et al., 2000).
Built to climb?
At first glance, climbing geckos may not appear to be remarkable climbers any more than terrestrial geckos or other climbing lizards. While geckos generally have somewhat depressed bodies and short limbs, they lack the strong claws of climbing iguanids, and they certainly do not compare to the chameleons with their long limbs, zygodactylous toes, prehensile tails, compressed bodies, and slow, precise balance (Bauer et al., 2002). Even within the superfamily Gekkonidae, climbing geckos and terrestrial geckos show very few differences in body structure, limb proportions, and gait that should distinguish climbers from non-climbers (Zaaf and Van Damme, 2001; Zaaf et al., 1999; Zaaf et al., 2001).
Zaaf and Van Damme (2001) explored the idea that body structure and limb proportions should vary to some extent with an organism’s lifestyle, due to different demands for climbing, running, or other functions. In general, climbing lizards should have shorter limbs that are equal in size, in order to keep the lizard’s center of gravity closer to the surface being climbed. Alternatively, terrestrial lizards should have longer hind limbs with a longer tibia than femur to create a more effective lever to provide thrust for running. In addition, climbers should have powerful forelimbs, while terrestrial lizards should have shorter forelimbs (relative to their hindlimbs) so they do not get in the way as the lizard is running (Zaaf and Van Damme, 2001; Zaaf et al., 2001). Organisms in the genus Anolis and Sceloporus occidentalis (both members of the Iguanid family), and several species of chameleons have all clearly demonstrated these types of morphological relationships between body form and function, so it seems likely that geckos should show similar relationships (Zaaf and Van Danne, 2001).
Geckos, however, show no clear differences in limb proportions between climbing species and terrestrial species, a finding that could be related to several factors. For one, Zaaf and Van Danne (2001) argue that ideas about biomechanics may be too narrowly focused on species that are more arboreal and spend time climbing on branches and twigs rather than on the wide, vertical faces that are often home to climbing geckos. If this is the case, it is not surprising then that chameleons, some Anolis species, and Sceloporus occidentalis all show the expected form-function relationships while geckos do not (Zaaf and Van Danne, 2001). Alternatively, natural selection may have simply failed to develop the expected form-function relationships in limb proportions in geckos either because gecko species diverged too recently (which is unlikely) or because the suggested limb proportions are not as critical to success as a climber or runner for geckos as they are for the other lizards mentioned (Zaaf and Van Danne, 2001).
In addition to showing little difference between body structure and limb proportions, climbing geckos and terrestrial geckos also show remarkably little differences in gait characteristics despite their different habitats and locomotion habits (Zaaf et al., 2001). Zaaf and his colleagues (2001) studied the spatio-temporal gait characteristics of stride length, step length, stride frequency, duty factor, and relative phase in Eublepharis macularius (an ancestral, ground-dwelling species) and Gekko gecko (the Tokay gecko, a species that lives almost exclusively on vertical surfaces). The researchers expected that because accuracy is much more important in climbing, G. gecko would only adjust step frequency and duty factor (temporal aspects) and not step length or stride (spatial aspects) when adjusting speed, while E. macularius would adjust both temporal and spatial aspects of its gait since accuracy would not be as important (Zaaf et al., 2001). In addition, E. macularius was expected to show a “floating phase,” where none of its limbs are touching the surface when at maximum speed. G. gecko was not expected to show a floating phase because having a limb in contact with the surface to maintain upward momentum would be much more important in climbing (Zaaf et al., 2001). Surprisingly, E. macularius did not alter the spatial aspects of its gait, nor did it incorporate a floating phase when reaching maximum speed. Since E. macularius is the ancestral species, it seems that the locomotor behavior that it shares with G. gecko does not actually represent a specialization for climbing, but is perhaps related to some other shared characteristic, such as nocturnality, that might require a more precise locomotor strategy without modifications to step length or stride or the addition of a floating phase (Zaaf et al., 2001).
Overall, E. macularius and G. gecko differ much less than would be expected for two species with such drastically different habitats. While certain climbing-specific functions can be found at the muscular level for G. gecko, the lack of expected variation in limb proportions and gait characteristics that can be found in other specialized climbers like chameleons and anoles suggests that in geckos, another key adaptation may have eliminated the need for a more climbing specific body type and gait (Zaff et al., 1999; Zaff and Van Damme, 2001; Zaff et al., 2001). In geckos, this key adaptation is undoubtedly the adhesive toe pads found in some variation in all of Gekkonidae’s 88 genera (Bauer et al., 2002).
Gecko adhesion: all in the feet
Geckos’ unique toe pads have long been a source of intrigue for scientists, who have suggested adhesive mechanisms from suction to electrostatic interaction as the secret behind the lizards’ “stickiness” (Autumn and Peattie, 2002). Only in the last several years have researchers been able to pinpoint the forces at work as a gecko walks effortlessly across the underside of a piece of glass, and their findings suggest that the clinging force exerted by a gecko’s toe pads is far greater than previously believed, making it even clearer why unique structures may have eliminated the need for other climbing-specific adaptations in climbing geckos (Zaaf and Van Danne, 2001; Autumn and Peattie, 2002).
While there is considerable variation in the feet of the 930 species of Gekkonids, nearly all show some specialization related to their method of locomotion (Bauer et al., 2002). The adhesive toe pads that give geckos their climbing abilities are made up of broad modified scales called lamellae, or scansors. Each of these lamellae contains an array of tiny hairs called setae, which themselves branch into 100-1000 smaller structures called spatulae, which have triangular shaped ends attached to a stalk (Autumn and Peattie, 2002; Russell, 2002). The setae, which are composed of beta-keratin, are about 100 microns long and five microns in diameter (about one-tenth the diameter of a human hair), and each spatula is about 200 nm across at its widest point (Autumn et al., 2000, Autumn and Peattie, 2002). The setae and spatulae allow geckos to achieve extremely intimate contact—down to the molecular level—with the surfaces they climb (Autumn et al. 2002; Autumn and Peattie, 2002; Russel, 2002).
While the presence of setae in gecko toe pads has been known for over a century, their function has proved more difficult to understand. After easily ruling out sticky secretions since geckos do not have glandular tissue on their toes, scientists first suggested in the late 1800’s that setae might act as very tiny suction cups (Autumn and Peattie, 2002). By the 1930’s, however, this theory had been invalidated and scientists had moved on to the suggestion that electrostatic interaction or friction and microinterlocking might explain gecko adhesion (Autumn and Peattie, 2002). Experiments conducted in ionized air, however, proved that electrostatic attraction could not account for setal adhesion, and the gecko’s ability to stick to glass helped to disprove the idea that setae relied solely on friction to lock into surface irregularities (Autumn and Peattie, 2002). By the late 1960’s, researchers were leaning toward intermolecular forces as yet another possibility to explain setal adhesion (Autumn and Peattie, 2002).
Most early experiments testing intermolecular forces as the adhesive mechanism focused on capillary forces, the same mechanism used by many insects to cling to smooth substrates (Autumn and Peattie, 2002). However, since geckos do not have the ability to secrete fluid from the skin on their feet, capillary adhesion would require a layer of water molecules to already be present in the environment, implying that humidity would be an important factor in a gecko’s climbing ability (Autumn and Peattie, 2002). This does not seem to be the case, however, since geckos living in very dry desert habitats have shown climbing abilities equal to geckos living in tropical rainforests (Autumn and Peattie, 2002). In addition, since there is an inverse relationship between the hydrophobicity of the surface and the strength of adhesion when capillary forces are present, presumably geckos would not be able to cling to hydrophobic surfaces if capillary adhesion were the mechanism (Autumn and Peattie, 2002). In fact, tests of geckos’ abilities to cling to a hydrophilic semiconductor (Si02) and a hydrophobic semiconductor (GaAs) showed that the adhesive forces were similar under each condition (Autumn and Peattie, 2002; Autumn et al., 2002).
With logic and scientific evidence refuting the theory of capillary adhesion, researchers turned their attention to another possibility: van der Waals dispersion forces. Although weaker than any other proposed mechanism, van der Waals forces do not depend on the hydrophobicity of the surface and therefore can explain the gecko’s ability to exert similar adhesive forces on both hydrophilic and hydrophobic semiconductors (Autumn and Peattie, 2002; Autumn et al., 2002).
Researchers first found evidence to support the van der Waals hypothesis in studies of the function of single seta and the method of setal attachment to a substrate (Autumn et al., 2000). Autumn and his colleges (2000) hypothesized that geckos’ unique toe curling behavior, which they described as being “much like blowing up an inflating party favor,” implied that the orientation and geometry of the setae as they are placed on the substrate is important in determining the adhesive force, and idea consistent with the van der Waals hypothesis. Tests of the forces exerted by setae placed at different angles on a substrate showed not only that orientation is important, but also demonstrated the mechanism by which setae attach to the substrate (Autumn et al., 2000). Rather than just sticking on to the substrate, setae are first preloaded—pushed against the surface—and then slide an imperceptibly small distance parallel to the surface of the substrate to establish adhesive contact (Autumn et al., 2000). The orientation of the setae also seems to be important for detachment, which may help explain geckos’ unique toe peeling behavior in which the toe peels from the tip toward the center of the foot during detachment and running (Autumn et al., 2000, Autumn and Peattie., 2002).
Though Autumn and his colleagues’ (2000) measurements of the force exerted by a single seta and a single spatula fell within the predicted values for van der Waals forces, capillary forces were still not ruled out until the later research with semiconductors (Autumn and Peattie, 2002; Autumn et al., 2002). In these experiments, which used actual gecko setae as well as synthetic models made from silicon and polyester fibers, both types of setae not only held up to the suggestion of the van der Waals hypothesis that geometry is more important than the surface chemistry of the spatulae, but also proved to exert adhesive forces far beyond those previously estimated (Russel, 2002; Autumn and Peattie, 2002; Autumn et al., 2002). Based on single setal measurements taken by Autumn et al. (2000), a 50g Tokay gecko, which has about 6.5 million setae, needs only to attach less than half a percent of its setae to support its own weight on a vertical surface (Autumn et al., 2000, Autumn and Peattie, 2002). Geckos then clearly seem to have more than adequate means for climbing, though more than such a tiny fraction of the setae can account for the gecko’s abilities to catch itself in a fall, jump from one vertical surface to another, and adhere to more textured substrates.
Still climbing toward more answers…
While it seems that the mystery of geckos’ uncanny climbing abilities has finally been solved within the last few years, there is still a great deal to be learned about setal adhesion. Setae seem to have evolved as ornamentation on more generalized spinules present in the skin in most lizards to assist in skin shedding (Russell, 2002). How and why these spinules have evolved into adhesive structures in geckos is still a subject of much speculation. In addition, most of the research on setal adhesion has focused on the Tokay gecko, so while similar scansors and setae are present in other climbing geckos, further research may reveal previously unnoticed differences (Russell, 2002).
The most recent research on gecko adhesion has brought together a multi-disciplinary approach to biology, incorporating scientists and ideas from fields ranging from herpetology to math and physics to chemistry to engineering, and further research will require similar cooperation (Autumn and Peattie, 2002). The results thus far, though, have been inspiring; the question of how geckos stick has been answered after more than two centuries of research, providing insights into evolution and into the possibilities for biologically-inspired adhesive technology. Just three years ago, Autumn et al. (2000) commented that “manufacturing small, closely packed arrays mimicking setae are beyond the limits of human technology,” yet last year, engineers manufactured synthetic gecko setae that have the same adhesive power and use the same molecular forces as a gecko’s scansors (Autumn et al., 2002). With science and technology moving at its current pace, it may be only a matter of a few years before the remaining secrets of setal adhesion are understood and nature’s phenomenal dry adhesive is incorporated into every day products.
* * *
It’s seven o’clock on a Thursday night and I’m where I spend almost every Thursday night; working at Cornell Outdoor Education’s Lindseth Climbing Wall. Some first time climbers are struggling up one of the easier routes. “I’m too short to reach this hold,” one says. “Yeah, but at least you’re strong. I have no forearm strength!” the other responds. Later, one asks me if there’s any “ideal” body type or build for climbing.
“Well,” I say, thinking about it for a minute, “certain body types definitely help. It’s great if you’re built like a chameleon—thin and lanky with an awesome sense of natural balance—but that doesn’t mean you have to have the ‘ideal’ climbing body type to be a good climber. Just look at the geckos… they definitely don’t look like chameleons but they can hang upside down from glass. It’s all in the way they use their hands and feet, just like climbing is more about technique than anything else.”
“Geckos are the lizards with sticky toe pads, right?” the guys asks, “That would be so cool if they could make something like that for climbing!”
The idea isn’t an entirely novel one; a recent issue of Climbing magazine (May 1, 2003) carried a short blurb about Autumn’s research and the development of synthetic gecko hairs under the headline “Rock climbing may no longer involve trying,” suggesting that at some point, gecko technology could be incorporated into gloves and shoes to climb previously impossible routes. While I am certainly intrigued by the idea of being able to effortlessly scale featureless rock or even glass buildings, and I’ll probably jump at the chance to try out “gecko gloves” if and when they are ever manufactured, I also think there’s a more important lesson to be learned from geckos’ climbing techniques. In rock climbing, like in reptiles, climbing success does not necessarily depend on having the body type or strength of an ‘ideal’ climber, but on having the ability to evolve your own techniques and style.
WORKS CITED
Aristotle, Historia Animalium, trans. D. W. Thompson. 1918, http://classics.mit.edu/Aristotle/history_anim.html
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.
Zaaf, A., A. Herrel, P. Aerts, and F. De Vree. 1999. Morphology and morphometrics of the appendicular musculature in geckoes with different locomotor habits (Lepidosauria). Zoomorphology 119:9-22.
Zaaf, A., R. Van Damme, A. Herrel, and P. Aerts. 2001. Spatio-temporal gait characteristics of level and vertical locomotion in a ground dwelling and a climbing gecko. The Journal of Experimental Biology 204:1233-1246.