The Gecko

Jan 19, 2014 Features / Columnists, Interesting Creatures in Guyana

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Geckos are lizards belonging to the infra-order Gekkota, found in warm climates throughout the world. They range from 1.6 to 60 cm. Most geckos cannot blink, but they often lick their eyes to keep them clean and moist. They have a fixed lens within each iris that enlarges in darkness.
Geckos are unique among lizards in their vocalizations. They use chirping sounds in social interactions with other geckos. They are the most species-rich group of lizards, with about 1,500 different species worldwide.

All geckos, excluding the Eublepharidae family, lack eyelids, and instead have a transparent membrane. Nocturnal species have excellent night vision; their eyes are 350 times more sensitive to light than the human eye.  Most gecko species can lose their tails in defence, a process called autotomy. Many species are well known for their specialized toe pads that enable them to climb smooth and vertical surfaces, and even cross indoor ceilings with ease. These antics are well-known to people who live in warm regions of the world, where several species of geckos make their home inside human habitations. These species (for example the house gecko) become part of the indoor menagerie and are often welcomed, as they feed on insects, including mosquitoes. Unlike most lizards, geckos are usually nocturnal and are great climbers.
Geckos occur in various patterns and colours, and are among the most colourful lizards in the world. Some species can change colour and may be lighter in colour at night. Some species are parthenogenic, which means the female is capable of reproducing without copulating with a male. This improves the gecko’s ability to spread to new islands. However, in a situation where a single female gecko populates an entire island, the island will suffer from a lack of genetic variation within the geckos that inhabit it. The gecko’s mating call sounds like a shortened bird chirping which attracts males, when they are nearby.
All geckos shed their skin at fairly regular intervals, with species differing in timing and method. Leopard geckos will shed at about two- to four-week intervals. The presence of moisture aids in the shedding. When shedding begins, the gecko will speed the process by detaching the loose skin from its body and eating it.
The toes of the gecko have a special adaptation that allows them to adhere to most surfaces without the use of liquids or surface tension. Recently, geckos were shown to have a special secretial gland that allows them to absorb and excrete liquid to allow for maximum adherence to surfaces.
Geckos’ toes seem to be “double jointed”, but this is a misnomer. Their toes actually bend in the opposite direction from human fingers and toes. This allows them to overcome the van der Waals force by peeling their toes off surfaces from the tips inward. In essence, this peeling action alters the angle of incidence between millions of individual setae and the surface, reducing the Van der Waals force.
Geckos’ toes operate well below their full attractive capabilities most of the time, because the margin for error is great depending upon the surface roughness, and therefore the number of setae in contact with that surface.
Use of small van der Waals attraction force requires very large surface areas: every square millimeter of a gecko’s footpad contains about 14,000 hair-like setae. Each seta has a diameter of 5 ì s. Human hair varies from 18 to 180 ì, so a human hair could hold between three and 36 setae. Each seta is in turn tipped with between 100 and 1,000 spatulae. Each spatula is 0.2 ì long (one five-millionth of a meter), or just below the wavelength of visible light.[22]
The setae of a typical mature 70 g (2.5 oz) gecko would be capable of supporting a weight of 133 kg (290 lb): each spatula can exert an adhesive force of 5 to 25 nN. The exact value of the adhesion force of a spatula varies with the surface energy of the substrate to which it adheres. Recent studies have moreover shown that the component of the surface energy derived from long-range forces, such as van der Waals forces, depends on the material’s structure below the outermost atomic layers (up to 100 nm beneath the surface); taking that into account, the adhesive strength can be inferred.
Recent studies have also revealed that apart from the setae, phospholipids — fatty substances produced naturally in their bodies — also come into play. These lipids lubricate the setae and allow the gecko to detach its foot before the next step.
About 60 per cent of gecko species have adhesive toe pads; such pads have been gained and lost repeatedly over the course of gecko evolution. Adhesive toe pads evolved independently in about 11 different gecko lineages and were lost in at least nine lineages.
Development of geckel, a super-sticky adhesive that can attach to both wet and dry surfaces, has been inspired by adhesion in gecko feet.
Geckos are polyphyodonts and able to replace each of their 100 teeth every three to four months. Next to the full grown tooth there is a small replacement tooth developing from the odontogenic stem cell in the dental lamina.
(Source: Wikipedia – The Free Online Encyclopedia)

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