GECKOS: THEIR AMAZING FEET, WALKING ON WALLS AND VAN DER WAALS FORCE

GECKOS


Geckos are soft bodied lizards that can reach lengths of about 18 centimeters and have the unique ability to walk on almost any surface at any angle, even upsidedown. They are usually brown or grey in color and have patterns of speckles on their backs. Most geckos are nocturnal and operate at lower body temperatures than diurnal lizards.

Geckos are familiar to travelers to tropical regions. They hang out at night around lights on the walls and ceilings in restaurants and hotel rooms in jungles, beach areas, islands and deserts around the world. There are about 1,000 different species of gecko. Only a few live in colder climates. Their name is derived from clicking "gecko" sound they make.

Most geckos are carnivorous. They feed on insects like moths and winged termites and like to hang around light sources that attract insects. They sometimes fight among one another for the best positions around the light. Some geckos consumer nectar, pollen and fruit. A few species eat prey as large as mall mammals and birds. Females often feed side by side while males fight off intruders that approach their territory. Geckos are fed upon by mongooses, cats, and birds. Sometimes thy are eaten by giant centipedes.

Websites and Resources on Animals: ARKive arkive.org Animal Info animalinfo.org ; Animal Picture Archives (do a Search for the Animal Species You Want) animalpicturesarchive ; BBC Animals Finder bbc.co.uk/nature/animals ; Animal Diversity Web animaldiversity.ummz.umich.edu ; International Field Guides media.library.uiuc.edu ; animals.com animals.com/tags/animals-z ; Encyclopedia of Life eol.org ; World Wildlife Fund (WWF) worldwildlife.org ; National Geographic National Geographic ; Animal Planet animal.discovery.com ; Wikipedia article on Animals Wikipedia ; Animals.com animals.com ; Endangered Animals iucnredlist.org ; Endangered Species Resource List ucblibraries.colorado.edu ; Biodiversity Heritage Library biodiversitylibrary.org

Websites and Resources on Reptiles: Reptile Database reptile-database.org ; Reptileweb reptilesweb.com ; Reptile Channel reptilechannel.com/reptile-species ; Wikipedia article Wikipedia ; Reptile Phylogeny whozoo.org/herps/herpphylogeny Geckos Global Gecko Association gekkota.com ; Leopard Gecko Guide leopardgeckoguide.com ; Gecko Care geckocare.net ; Gecko and Lizard Photos mongabay.com ; Wikipedia article Wikipedia ;

Gecko Characteristics


Geckos have a large head. Nocturnal species see with slit- or elliptical-shaped pupils that work well in the dark. Scientists can find geckos easily at night by shining flashlights which illuminate the gecko’s eyes. Diurnal geckos have large bulging eyes and round pupils.

Almost all gecko species lack eyelids and unable to close their eyes. Like snakes they have transparent scales over their eyes that offer protection and are periodically shed. The tongues serves as a windshield wiper, keeping the eyes clean. Many desert species lick their eyes to remove windblown material and lick their heads to collect dew that has condensed on their skin.

Gecko tails contain fat the geckos can use if food supplies run low. When attacked by bird or a tree snake a gecko can grow back its. If the tail is split two tails may grow. In Australia a gecko was found with five tails and it was concluded that its tail had been split five ways.

Geckos have a good sense of smell. They pick up pheromones of the geckos and can discriminate between different prey on the basis of chemical signals.

Geckos can shed their tails like other kinds of lizards. Special cells at the base of their tail contract so no blood is lost. The tail contains bone and nerves but does receive signals from the brain. After it comes off it jumps and twitch until it "dies." By that time the gecko has had ample time to make an escape.

Gecko Walk on Walls and Ceilings


Most species of geckos are as comfortable scampering across walls, ceilings and even windows as the are running on the ground. They can run across ceilings at speeds up to three feet a second ; hold the weight of their entire body with one toe; cling upside down on window panes; and never lose their grip even if he scamper through dirt. What is more, they do all this without using any kind of adhesive or suction cups.

Geckos can jump from tree to tree by catching a leaf on the second tree with one toe. Some desert geckos have fringes of scales on their feet that prevent them from sinking in the sand. The Vaseline-like chemical fluon can applied to surfaces to make geckos fall off.

Gecko feet come in an amazing variety. They are not sticky but dry and smooth to the touch. It was long thought that gecko feet splayed out and had little claws and they moved on vertical and upside down surfaces similar to way cockroaches do by wedging parts of their feet their into minute cracks and using some sort of suction devise. But this is not the case.

Gecko Feet Adhesion

Geckos employ a method called contact splitting. Their feet have millions ultrafine fine hairs and hair tips that temporarily rearrange electrons on the walking surface, creating an electrodynamic attraction.

Gecko toes are lined with plates that are covered with hairs called setae that branch into hundreds of spatula-shaped microhairs. There are about 500,000 nano-size bristles on each of the gecko’s four feet. Each bristle is split into 100 to 1,000 spatula-shaped mini-bristles, hairs or filaments. This works out to two billion filament be square centimeter and 6.5 million on each toe. Each filament is only a hundred nanometer thick and are so small they interact in the molecular level with the surface in which the gecko walks.

In the nanoworld of the microhairs a faint intermolecular attraction called the van der Waals force pulls objects together. Multiplied millions of times this force creates enough adhesion that holds the feet of a gecko to surface of glass.

The geckos’ unique adhesive ability was discovered by the Gecko Team led by Kellar Autumn of Lewis and Clark College and Robert Full of Berkeley. For a more detailed description on gecko adhesion see The Gecko’s Foot by Peter Forbes (W.W, Morton, 2006)

Van der Waals Force


Van de Waals force used gecko feet adhesion is not gravity, electricity, or magnetism, nor is it chemical attraction. It is an adhesive force of quantum physics caused by an interaction between the positive and negative charges of different atoms that cling together briefly as their electron clouds share electric charges.

Van der Waals force is generated by molecules’s fleeting positive and negative charges, which pull any two adjacent objects together. It can be felt in spaces no larger than two nanometers (billionths of a meter). It works on the molecules on whatever surface the gecko is walking on and doesn’t require any special chemistry and theocratically could be applied to any material

Van der Waals force works between the atoms on gecko's feet and the atoms on the surface it is climbing or hanging on to because there are so many setae and microhairs on gecko feet allows so many atoms to interact with the surface. The force created by one gecko is enough to lift 127 kilograms. There is Van der Waals force between a person's hand and a wall but it is relatively weak because not so many atoms on the hand and surface come in contact.

Physics Behind Gecko Movement

Geckos walk in such a way that their micro hairs roll on the surface and release as easily as they cling. If a gecko’s feet were covered with suction cups, a great deal of energy would be required to break the bond when a gecko lifted its foot. Instead, geckos change the angle of their setae and this allows them easily lift their feet.

But adhesion is only part of the process. For a gecko to scamper around on walls and ceilings---which it can do at speeds up to 1 meter per second---it has to be able to unstick its feet quickly and effortlessly. Scientists that have studied this---namely biologists Bob Full and Kellar Autumn---found that gecko adhesion is highly directional. If the toes are pushed forward they stick if the are pulled into the opposite direction they release.

What make feet function so well is not just the nano hairs but the way those hairs grow on large hairs and then in turn grow on toe ridges which are part of bigger toe parts, creating a seven part hierarchy that give the gecko all the capabilities it has.


The setae are also believed to be to be behind the fact that geckos are extraordinary clean even though they never groom themselves. Even if they are immersed in the most god awful dirt and grim geckos will shed whatever is covering them in few quick steps. Setae repel dirt even when it is isolated from the gecko. Autumn’s team calculated that a particle of dirt is too small to be attracted by Van de Waals force and the setae are too close for the dirt to penetrate and are thus quickly repelled.

Geckos do fall or slip from time to time. When this happens their tail plays an important role in correcting mistakes. When a gecko is climbing a vertical wall and starts to slip the tail arches and its tip pushes against the wall surface, which usually prevents the head and the upper body from pitching back, away from the wall . If it doesn’t , then most of the tail flattens against the wall. These movements, researchers say, stop the gecko from within a quarter of a second. When geckos fall they always land right-side-up like a cat. But unlike most other animals that right themselves by twisting their spine, geckos right themselves by swinging their tails around when the are upside down, to produce a counter-rotation that takes about a tenth of a second.

Gecko Robots and Uses of Gecko Adhesion

Scientists led by Mark Cutkosky of Stanford University have created a robot called Stickybot that can climb up walls like a gecko. The robot weighs 500 grams and has pads on it feet made of a urethane fabric with tiny bristles that end in 30-micrometers points. These are not as flexible and adherent as a real gecko’s but the robot does stick to a wall. At this stage of the game---despite the use of high tech sensor and actuators and space age polymers to make it flexible and stiff and branching tendons to distributed weight evenly like on a real gecko---the robot is very slow but designers hope to speed it up and use it in search and rescue missions and other uses.

This gluelike effect of gecko adhesion can have innumerable uses. Scientists are using geckos as a model to make better adhesive tapes that will work in outer space and vacuums. The stickiness of ordinary adhesive tapes is caused in part by tiny bubbles of trapped air, which are not existent in space or a vacuum. Scientists are also studying the process to adapt it to an adhesive tape that can be used over and over; fumble-free football gloves; robotic rovers that can move quickly over the Martian surface; Spiderman-like climb equipment and material that can replace the screw.

Gecko Behavior


Most lizards make few vocal sounds other than hisses. In contrast geckos make a wide a variety of sounds including coughs, clucks and barks. When they bark they move their heads up and down. When threatened some geckos "scream" loudly in an attempt to startle their attacker long enough to make an escape.

Fighting geckos arch their backs make clicking and growling noises, turn side to side and then clamp their jaws onto each other’s head or body and wrestle from that position. Sometimes the dominant combatant dangles the loser over the ground. Sometimes they both fall to the ground.

Some geckos reproduce asexually. Males of those who don’t usually ignore females. When they are in the mood they wag their tails briefly and copulate Females lay their eggs in one or two clutches often in crevices or walls. The eggs hatch after about 50 days.

Image Sources: Wikimedia Commons

Text Sources: Mostly National Geographic articles. Also Life on Earth by David Attenborough (Princeton University Press), New York Times, Washington Post, Los Angeles Times, Smithsonian magazine, Natural History magazine, Discover magazine, Times of London, The New Yorker, Time, Newsweek, Reuters, AP, AFP, Lonely Planet Guides, Compton’s Encyclopedia and various books and other publications.

Last updated March 2011


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