How do geckos stick wherever they are? The answer could make glue a thing of the past.
Words: Huw Cordey
Images: Paul D Stewart and Pete Oxford
Adhesive power
Scientists have recently discovered that, theoretically, a gecko, hanging upside-down from the ceiling, can support a 40kg backpack. The ultimate 'cling-ons', geckos can stick to virtually any surface on the planet. So how on Earth do they perform this feat with their feet?
Most people think that geckos use suction. Their rounded toe pads even look like little suction cups. But, a gecko's foot cannot generate the forces needed to create a vacuum and, besides, their stickability works even in a vacuum.
Stuck like glue
Another possibility is some form of glue, or more precisely, capillary adhesion. A cockroach, for example, secretes a thin film of fluid from glands on its feet, which forms a bond between itself and the surface. If it walks up glass, it leaves faint smudges behind. But gecko adhesion is perfectly dry.
Examine a gecko's foot under a microscope, and you'll see countless thousands of hairs. These could act like miniature crampons and hook onto a wall's rough surface. But, how could they stick to impossibly smooth surfaces such as polished glass? In fact, geckos can achieve 600 times more adhesive force than friction could provide.
Not static...
Lateral thinkers might guess that geckos use static electricity, the principle by which a rubbed balloon sticks to a wall. This was disproved in a rather simple scientific experiment. An anti-static gun was fired at a hanging gecko, which remained entirely unmoved.
It was Professor Kellar Autumn who, along with a colleague, finally revealed the truth behind the gecko's amazing adhesive powers. The solution is connected to a discovery made by an eighteenth century Dutch scientist.
Molecular level
Van der Waal was the first to describe the weak attracting forces that exist between molecules - the forces that hold the molecules of a water droplet together. To understand how geckos make use of them, you need to see their feet under an electron microscope.
One foot contains half a million hairs less than a tenth of the thickness of a human hair. Each 'seta' bears perhaps a thousand smaller, mushroom-shaped fibres called 'spatulae'. This intricacy is key to the gecko's success. The spatulae get so close to the substrate that molecules in both the foot and the surface generate a tiny charge that briefly draws one to the other. Geckos use molecular bonding.
Now, it doesn't take a scientist to realise that if we could harness these forces, gecko style, the possible applications might be enormous: goodbye Sellotape, Blu-Tack and Velcro, hello gecko glue! Given NASA's interest in this work, it might even help us explore distant planets. If a Geckobot can move around half as well as its earthly inspiration, the rest of the universe shouldn't be too much of a problem.
Geckos lost and found
Their diversity, wide range and camouflage means that new species of gecko are discovered every year.
The smallest species of gecko (indeed, the smallest known reptile) Sphaerodactylus ariasae was discovered last year on Beata island, off the southernmost point of the Dominican Republic. Adults are 16mm from the snout to the base of the tail.
The largest gecko, Delcourt's gecko from New Zealand, was discovered when a 62cm, stuffed specimen was noticed in the collection of the Musée d'Histoire Naturelle of Marseille in France. The lizard had been collected sometime in the early 1800s and then overlooked. It was formally described and named Hoplodactylus delcourti in 1986. Sadly, it was probably extinct long before then, a victim of invading rats.
The largest surviving geckos are the tokay Gekko gecko and the bulkier New Caledonian giant gecko Rhacodactylus leachianus contender. Both can reach 40cm in total body length.
Geckos have many tricks to keep themselves off the menu
As with most lizards, tail-loss occurs at special junctions between vertebrae, with minimum blood loss and trauma. This helps the gecko to break away from a predator - the writhing tail serving as a distraction long after its owner has gone to cover. Some geckos have evolved a loose bi-layered skin that pulls off without long-term harm. Both tails and skin re-grow.
When a tail does not entirely detach, another may bud by its side. Geckos with up to three tails have been observed in the wild.
Leaf- and frill-tailed geckos have remarkable camouflage - the bordering frills of skin serving to obscure the animal's outline and obliterate its shadow. A further development of these frills is seen in the flying gecko - frills so large that the gecko can glide away from a predator.
City Slickers
Where several geckos are found in the same house, they often have non-overlapping niches in terms of where, when and how they rest and hunt. They divide up their human-made territory like they would any natural environment.
Geckos living in kitchens can be responsible for transmission of the disease salmonella. To even-up the score, they have proved to be effective predators on numerous insect pests, including mosquitoes and cockroaches, which are also responsible for transmitting disease.
The tokay gecko, which was formerly quite abundant in Far-eastern towns and cities, is fast becoming scarce. In some areas the species is heavily collected for use in traditional Chinese medicine; in others it has succumbed to cumulative poisons intended to control insects.
Getting to grip with geckos
There are at least 850 species of geckos and more doubtless await discovery.
The group's range is continent-wide except Antarctica, with the greatest diversity found in tropical and subtropical regions.
They can be broadly divided into two main groups: species with closable eyelids (Aleuroscalabotinae and Eublepharinae) and species with fixed eyelids (Diplodactylinae, Gekkoninae, and Teratoscincinae). The latter group can be further divided into those with sticky footpads (mostly arboreal) and those without footpads (generally terrestrial).
The sticking abilities of adults and their resilient eggs have made geckos effective colonisers of islands. With human transport recently helping new invaders to those islands, many of these established native geckos are being lost. The common house gecko Hemidactylus frenatus has replaced native Lepidodactylus spp. on many Pacific islands.
Clinging feet are not the only high-tech attribute of geckos that has attracted scientific attention. Their eyes are remarkable for the fact that, in bright light, notches in the slit-iris close down to two or four pupils. This creates overlapping images with maximum depth of focus.
Their unusual body chemistry allows geckos to be active at night at temperatures that would leave other reptiles sluggishly inactive.
Geckos have a unique throat structure, which makes them the only lizard with a true voice.
Words: Huw Cordey
Images: Paul D Stewart and Pete Oxford
Adhesive power
Scientists have recently discovered that, theoretically, a gecko, hanging upside-down from the ceiling, can support a 40kg backpack. The ultimate 'cling-ons', geckos can stick to virtually any surface on the planet. So how on Earth do they perform this feat with their feet?
Most people think that geckos use suction. Their rounded toe pads even look like little suction cups. But, a gecko's foot cannot generate the forces needed to create a vacuum and, besides, their stickability works even in a vacuum.
Stuck like glue
Another possibility is some form of glue, or more precisely, capillary adhesion. A cockroach, for example, secretes a thin film of fluid from glands on its feet, which forms a bond between itself and the surface. If it walks up glass, it leaves faint smudges behind. But gecko adhesion is perfectly dry.
Examine a gecko's foot under a microscope, and you'll see countless thousands of hairs. These could act like miniature crampons and hook onto a wall's rough surface. But, how could they stick to impossibly smooth surfaces such as polished glass? In fact, geckos can achieve 600 times more adhesive force than friction could provide.
Not static...
Lateral thinkers might guess that geckos use static electricity, the principle by which a rubbed balloon sticks to a wall. This was disproved in a rather simple scientific experiment. An anti-static gun was fired at a hanging gecko, which remained entirely unmoved.
It was Professor Kellar Autumn who, along with a colleague, finally revealed the truth behind the gecko's amazing adhesive powers. The solution is connected to a discovery made by an eighteenth century Dutch scientist.
Molecular level
Van der Waal was the first to describe the weak attracting forces that exist between molecules - the forces that hold the molecules of a water droplet together. To understand how geckos make use of them, you need to see their feet under an electron microscope.
One foot contains half a million hairs less than a tenth of the thickness of a human hair. Each 'seta' bears perhaps a thousand smaller, mushroom-shaped fibres called 'spatulae'. This intricacy is key to the gecko's success. The spatulae get so close to the substrate that molecules in both the foot and the surface generate a tiny charge that briefly draws one to the other. Geckos use molecular bonding.
Now, it doesn't take a scientist to realise that if we could harness these forces, gecko style, the possible applications might be enormous: goodbye Sellotape, Blu-Tack and Velcro, hello gecko glue! Given NASA's interest in this work, it might even help us explore distant planets. If a Geckobot can move around half as well as its earthly inspiration, the rest of the universe shouldn't be too much of a problem.
Geckos lost and found
Their diversity, wide range and camouflage means that new species of gecko are discovered every year.
The smallest species of gecko (indeed, the smallest known reptile) Sphaerodactylus ariasae was discovered last year on Beata island, off the southernmost point of the Dominican Republic. Adults are 16mm from the snout to the base of the tail.
The largest gecko, Delcourt's gecko from New Zealand, was discovered when a 62cm, stuffed specimen was noticed in the collection of the Musée d'Histoire Naturelle of Marseille in France. The lizard had been collected sometime in the early 1800s and then overlooked. It was formally described and named Hoplodactylus delcourti in 1986. Sadly, it was probably extinct long before then, a victim of invading rats.
The largest surviving geckos are the tokay Gekko gecko and the bulkier New Caledonian giant gecko Rhacodactylus leachianus contender. Both can reach 40cm in total body length.
Geckos have many tricks to keep themselves off the menu
As with most lizards, tail-loss occurs at special junctions between vertebrae, with minimum blood loss and trauma. This helps the gecko to break away from a predator - the writhing tail serving as a distraction long after its owner has gone to cover. Some geckos have evolved a loose bi-layered skin that pulls off without long-term harm. Both tails and skin re-grow.
When a tail does not entirely detach, another may bud by its side. Geckos with up to three tails have been observed in the wild.
Leaf- and frill-tailed geckos have remarkable camouflage - the bordering frills of skin serving to obscure the animal's outline and obliterate its shadow. A further development of these frills is seen in the flying gecko - frills so large that the gecko can glide away from a predator.
City Slickers
Where several geckos are found in the same house, they often have non-overlapping niches in terms of where, when and how they rest and hunt. They divide up their human-made territory like they would any natural environment.
Geckos living in kitchens can be responsible for transmission of the disease salmonella. To even-up the score, they have proved to be effective predators on numerous insect pests, including mosquitoes and cockroaches, which are also responsible for transmitting disease.
The tokay gecko, which was formerly quite abundant in Far-eastern towns and cities, is fast becoming scarce. In some areas the species is heavily collected for use in traditional Chinese medicine; in others it has succumbed to cumulative poisons intended to control insects.
Getting to grip with geckos
There are at least 850 species of geckos and more doubtless await discovery.
The group's range is continent-wide except Antarctica, with the greatest diversity found in tropical and subtropical regions.
They can be broadly divided into two main groups: species with closable eyelids (Aleuroscalabotinae and Eublepharinae) and species with fixed eyelids (Diplodactylinae, Gekkoninae, and Teratoscincinae). The latter group can be further divided into those with sticky footpads (mostly arboreal) and those without footpads (generally terrestrial).
The sticking abilities of adults and their resilient eggs have made geckos effective colonisers of islands. With human transport recently helping new invaders to those islands, many of these established native geckos are being lost. The common house gecko Hemidactylus frenatus has replaced native Lepidodactylus spp. on many Pacific islands.
Clinging feet are not the only high-tech attribute of geckos that has attracted scientific attention. Their eyes are remarkable for the fact that, in bright light, notches in the slit-iris close down to two or four pupils. This creates overlapping images with maximum depth of focus.
Their unusual body chemistry allows geckos to be active at night at temperatures that would leave other reptiles sluggishly inactive.
Geckos have a unique throat structure, which makes them the only lizard with a true voice.