Gecko

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Gecko
Temporal range: 110–0  Ma
Phelsuma l. laticauda.jpg
Gold dust day gecko
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Squamata
Clade: Gekkonomorpha
Infraorder: Gekkota
Cuvier, 1817
Subgroups

Geckos are small, mostly carnivorous lizards that have a wide distribution, found on every continent except Antarctica. Belonging to the infraorder Gekkota, geckos are found in warm climates throughout the world. They range from 1.6 to 60 centimetres (0.6 to 23.6 inches ).

Contents

Geckos are unique among lizards for their vocalisations, which differ from species to species. Most geckos in the family Gekkonidae use chirping or clicking sounds in their social interactions. Tokay geckos (Gekko gecko) are known for their loud mating calls, and some other species are capable of making hissing noises when alarmed or threatened. They are the most species-rich group of lizards, with about 1,500 different species worldwide. [1]

All geckos, except species in the family Eublepharidae lack eyelids; instead, the outer surface of the eyeball has a transparent membrane, the brille. They have a fixed lens within each iris that enlarges in darkness to let in more light. Since they cannot blink, species without eyelids generally lick their own brilles when they need to clear them of dust and dirt, in order to keep them clean and moist. [2]

Unlike most lizards, geckos are usually nocturnal [3] and have excellent night vision; their colour vision in low light is 350 times more sensitive than human eyes. [4] The nocturnal geckos evolved from diurnal species, which had lost the rod cells from their eyes. The gecko eye, therefore, modified its cone cells that increased in size into different types, both single and double. Three different photo-pigments have been retained, and are sensitive to ultraviolet, blue, and green. They also use a multifocal optical system that allows them to generate a sharp image for at least two different depths. [5] [6] While most gecko species are nocturnal, some species are diurnal and active during the day, which have evolved multiple times independently. [3]

Many species are well known for their specialised toe pads, which enable them to grab and climb onto smooth and vertical surfaces, and even cross indoor ceilings with ease. Geckos are well known to people who live in warm regions of the world, where several species make their home inside human habitations. These, for example the house gecko, become part of the indoor menagerie and are often welcomed, as they feed on insect pests; including moths and mosquitoes. Like most lizards, geckos can lose their tails in defence, a process called autotomy; the predator may attack the wriggling tail, allowing the gecko to escape. [7]

The largest species, Gigarcanum delcourti , is only known from a single, stuffed specimen probably collected in the 19th century found in the basement of the Natural History Museum of Marseille in Marseille, France. This gecko was 600 millimetres (24 inches ) long, and it was likely endemic to New Caledonia, where it lived in native forests. [8] The smallest gecko, the Jaragua sphaero, is a mere 16 millimetres (0.63 inches) long, and was discovered in 2001 on a small island off the coast of Hispaniola. [9]

Etymology

The Neo-Latin gekko and English 'gecko' stem from Indonesian-Malaysian gēkoq, [10] it is a Malay word borrowed from Javanese, [11] from tokek, which imitates the sounds that some species like Tokay gecko make. [10] [12] : 120  [13] : 253 

Common traits

Like other reptiles, geckos are ectothermic, [14] producing very little metabolic heat. Essentially, a gecko's body temperature is dependent on its environment. Also, to accomplish their main functions; such as locomotion, feeding, reproduction, etc., geckos must have a relatively elevated temperature. [14]

Shedding or molting

Video of leopard gecko shedding skin

All geckos shed their skin at fairly regular intervals, with species differing in timing and method. Leopard geckos shed at about two- to four-week intervals. The presence of moisture aids in the shedding. When shedding begins, the gecko speeds the process by detaching the loose skin from its body and eating it. [15] For young geckos, shedding occurs more frequently, once a week, but when they are fully grown, they shed once every one to two months. [16]

Adhesion ability

Close-up of the underside of a gecko's foot as it walks on vertical glass Gecko foot on glass.JPG
Close-up of the underside of a gecko's foot as it walks on vertical glass

About 60% of gecko species have adhesive toe pads which allow them to adhere to most surfaces without the use of liquids or surface tension. Such pads have been gained and lost repeatedly over the course of gecko evolution. [17] Adhesive toepads evolved independently in about eleven different gecko lineages, and were lost in at least nine lineages. [17]

It was previously thought that the spatula-shaped setae arranged in lamellae on gecko footpads enable attractive van der Waals' forces (the weakest of the weak chemical forces) between the β-keratin lamellae / setae / spatulae structures and the surface. [18] [19] These van der Waals interactions involve no fluids; in theory, a boot made of synthetic setae would adhere as easily to the surface of the International Space Station as it would to a living-room wall, although adhesion varies with humidity. [20] [21] However, a 2014 study suggests that gecko adhesion is in fact mainly determined by electrostatic interaction (caused by contact electrification), not van der Waals or capillary forces. [22]

The setae on the feet of geckos are also self-cleaning, and usually remove any clogging dirt within a few steps. [23] [24] [25] Polytetrafluoroethylene (PTFE), which has very low surface energy, [26] is more difficult for geckos to adhere to than many other surfaces.

Gecko adhesion is typically improved by higher humidity, [20] [21] [27] [28] [29] even on hydrophobic surfaces, yet is reduced under conditions of complete immersion in water. The role of water in that system is under discussion, yet recent experiments agree that the presence of molecular water layers (water molecules carry a very large dipole moment) on the setae, as well as on the surface, increase the surface energy of both, therefore the energy gain in getting these surfaces in contact is enlarged, which results in an increased gecko adhesion force. [20] [21] [27] [28] [29] Moreover, the elastic properties of the b-keratin change with water uptake. [20] [21] [27]

Gecko toes seem to be double-jointed, but this is a misnomer, and is properly called digital hyperextension. [30] Gecko toes can hyperextend 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, by this peeling action, the gecko separates spatula by spatula from the surface, so for each spatula separation, only some force necessary. (The process is similar to removing Scotch Tape from a surface.)

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 force requires very large surface areas; every square millimetre of a gecko's footpad contains about 14,000 hair-like setae. Each seta has a diameter of 5  μm. Human hair varies from 18 to 180 μm, so the cross-sectional area of a human hair is equivalent to 12 to 1300 setae. Each seta is in turn tipped with between 100 and 1,000 spatulae. [23] Each spatula is 0.2 μm long [23] (one five-millionth of a metre), or just below the wavelength of visible light. [31]

The setae of a typical mature 70- gram (2.5- ounce ) gecko would be capable of supporting a weight of 133 kilograms (293 pounds): [32] [33] each spatula could exert an adhesive force of 5 to 25 nN. [27] [34] The exact value of the adhesion force of a spatula varies with the surface energy of the substrate to which it adheres. Recent studies [29] [35] 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.

Apart from the setae, phospholipids; fatty substances produced naturally in their bodies, also come into play. [36] These lipids lubricate the setae and allow the gecko to detach its foot before the next step.

The origin of gecko adhesion likely started as simple modifications to the epidermis on the underside of the toes. This was recently discovered in the genus Gonatodes from South America. [37] [38] Simple elaborations of the epidermal spinules into setae have enabled Gonatodes humeralis to climb smooth surfaces and sleep on smooth leaves.

Biomimetic technologies designed to mimic gecko adhesion could produce reusable self-cleaning dry adhesives with many applications. Development effort is being put into these technologies, but manufacturing synthetic setae is not a trivial material design task.

Skin

Gecko skin does not generally bear scales, but appears at a macro scale as a papillose surface, which is made from hair-like protuberances developed across the entire body. These confer superhydrophobicity, and the unique design of the hair confers a profound antimicrobial action. These protuberances are very small, up to 4 microns in length, and tapering to a point. [39] Gecko skin has been observed to have an anti-bacterial property, killing gram-negative bacteria when they come in contact with the skin. [40]

The mossy leaf-tailed gecko of Madagascar, U. sikorae, has coloration developed as camouflage, most being greyish brown to black, or greenish brown, with various markings meant to resemble tree bark; down to the lichens and moss found on the bark. It also has flaps of skin, running the length of its body, head and limbs, known as the dermal flap, which it can lay against the tree during the day, scattering shadows, and making its outline practically invisible. [41]

Teeth

Geckos are polyphyodonts, and able to replace each of their 100 teeth every 3 to 4 months. [42] Next to the full grown tooth there is a small replacement tooth developing from the odontogenic stem cell in the dental lamina. [43] The formation of the teeth is pleurodont; they are fused (ankylosed) by their sides to the inner surface of the jaw bones. This formation is common in all species in the order Squamata.

Taxonomy and classification

Pores on the skin are often used in classification. LizardFemoralPoresRooij.png
Pores on the skin are often used in classification.

The infraorder Gekkota is divided into seven families, containing about 125 genera of geckos, including the snake-like (legless) pygopods. [17] [44] [45] [46] [47] [3] [48]

Legless lizards of the family Dibamidae, also referred to as blind lizards, [49] have occasionally been counted as gekkotans, but recent molecular phylogenies suggest otherwise. [50] [51]

Gekkota

Diplodactylidae

Carphodactylidae

Pygopodidae

Eublepharidae

Sphaerodactylidae

Phyllodactylidae

Gekkonidae

Evolutionary history

Skeleton of Eichstaettisaurus, thought to be an early member of the gecko lineage Eichstaettisaurus schroederi 398858 (cropped).jpg
Skeleton of Eichstaettisaurus, thought to be an early member of the gecko lineage
Fossil of Yantarogekko preserved in Baltic amber Deutsches Bernsteinmuseum 04 (cropped).jpg
Fossil of Yantarogekko preserved in Baltic amber

Several species of lizard from the Late Jurassic have been considered early relatives of geckos, the most prominent and most well supported being the arboreal Eichstaettisaurus from the Late Jurassic of Germany. Norellius from the Early Cretaceous of Mongolia is also usually placed as a close relative of geckos. [52] The oldest known fossils of modern geckos are from the mid-Cretaceous Burmese amber of Myanmar (including Cretaceogekko ), around 100 million years old, which have adhesive pads on the feet similar to those of living geckos. [53] [54] [55]

Species

Mediterranean house gecko Mediterranean house gecko.JPG
Mediterranean house gecko

More than 1,850 species of geckos occur worldwide, [56] including these familiar species:

Reproduction

Most geckos lay a small clutch of eggs. Some are live-bearing, and a few can reproduce asexually via parthenogenesis. Geckos also have a large diversity of sex-determining mechanisms, including temperature-dependent sex determination and both XX/XY and ZZ/ZW sex chromosomes with multiple transitions among them over evolutionary time. [57] Madagascar day geckos engage in a mating ritual in which sexually mature males produce a waxy substance from pores on the back of their legs. Males approach females with a head swaying motion along with rapid tongue flicking in the female. [58]

Obligate parthenogenesis as a reproductive system has evolved multiple times in the family Gekkonidae. [59] It has been shown that oocytes are able to undergo meiosis in three different obligate parthenogenetic complexes of geckos. An extra premeiotic endoreplication of chromosomes is essential for obligate parthenogenesis in these geckos. [59] Appropriate segregation during meiosis to form viable progeny is facilitated by the formation of bivalents made from copies of identical chromosomes.

Related Research Articles

<span class="mw-page-title-main">Gekkonidae</span> Family of lizards

Gekkonidae is the largest family of geckos, containing over 950 described species in 64 genera. The Gekkonidae contain many of the most widespread gecko species, including house geckos (Hemidactylus), the tokay gecko (Gekko), day geckos (Phelsuma), the mourning gecko (Lepidodactylus), and dtellas (Gehyra). Gekkonid geckos occur globally and are particularly diverse in tropical areas.

<span class="mw-page-title-main">Pygopodidae</span> Family of lizards

Pygopodidae, commonly known as snake-lizards, or flap-footed lizards, are a family of legless lizards with reduced or absent limbs, and are a type of gecko. At least 35 species are placed in two subfamilies and eight genera. They have unusually long, slender bodies, giving them a strong resemblance to snakes. Like snakes and most geckos, they have no eyelids, but unlike snakes, they have external ear holes and flat, unforked tongues. They are native to Australia and New Guinea.

<i>Hemidactylus</i> Genus of common geckos

Hemidactylus is a genus of the common gecko family, Gekkonidae. It has 192 described species, newfound ones being described every few years. These geckos are found in all the tropical regions of the world, extending into the subtropical parts of Africa and Europe. They excel in colonizing oceanic islands by rafting on flotsam, and are for example found across most of Polynesia. In some archipelagoes, cryptic species complexes are found. Geckos like to live in and out of houses. They have been introduced to Australia.

<i>Pachydactylus</i> Genus of lizards

Pachydactylus is a genus of insectivorous geckos, lizards in the family Gekkonidae. The genus is endemic to Africa, and member species are commonly known as thick-toed geckos. The genus also displays rich speciation, having 58 distinct species identified when compared to other closely related gecko genera like Rhoptropus, most of which have emerged since 35Ma. It has been suggested that the reason for this rich speciation not from adaptive radiation nor nonadaptive radiation, but that the genus represents a clade somewhere between the two drivers of speciation. P. bibronii geckos have been used by NASA as animal models for experimentation.

<i>Matoatoa</i> Genus of lizards

Matoatoa is a small genus of geckos, lizards in the family Gekkonidae. The genus is endemic to Madagascar.

In biology, setae are any of a number of different bristle- or hair-like structures on living organisms.

<span class="mw-page-title-main">Crested gecko</span> Species of lizard

The crested gecko or eyelash gecko is a species of gecko native to southern New Caledonia (France). In 1866, the crested gecko was described by French zoologist Alphonse Guichenot. This species was thought to be extinct until it was rediscovered in 1994 during an expedition led by Robert Seipp. Along with several other New Caledonian gecko species, it is being considered for protected status by the Convention on the International Trade in Endangered Species of Wild Flora and Fauna.

<span class="mw-page-title-main">Lamella (surface anatomy)</span> Anatomical structure

In surface anatomy, a lamella is a thin plate-like structure, often one amongst many lamellae very close to one another, with open space between. Aside from respiratory organs, they appear in other biological roles including filter feeding and the traction surfaces of geckos.

<span class="mw-page-title-main">Synthetic setae</span> Artificial dry adhesives

Synthetic setae emulate the setae found on the toes of a gecko and scientific research in this area is driven towards the development of dry adhesives. Geckos have no difficulty mastering vertical walls and are apparently capable of adhering themselves to just about any surface. The five-toed feet of a gecko are covered with elastic hairs called setae and the ends of these hairs are split into nanoscale structures called spatulae. The sheer abundance and proximity to the surface of these spatulae make it sufficient for van der Waals forces alone to provide the required adhesive strength. Following the discovery of the gecko's adhesion mechanism in 2002, which is based on van der Waals forces, biomimetic adhesives have become the topic of a major research effort. These developments are poised to yield families of novel adhesive materials with superior properties which are likely to find uses in industries ranging from defense and nanotechnology to healthcare and sport.

<i>Hemidactylus prashadi</i> Species of lizard

Hemidactylus prashadi, also known commonly as the Bombay leaf-toed gecko or Prashad's gecko, is a species of lizard in the family Gekkonidae. The species is endemic to the Western Ghats of India.

<span class="mw-page-title-main">Eublepharidae</span> Family of lizards

The Eublepharidae are a family of geckos (Gekkota) consisting of 43 described species in six genera. They occur in Asia, Africa, North America, and Central America. Eublepharid geckos lack adhesive toepads and, unlike other geckos, have movable eyelids, thus commonly called eyelid geckos. Like other members of Gekkota, the Eublepharidae exhibits tail autotomy due to the fracture planes near their vent. A new tail will then grow in its place, usually lacking the original color and texture. The muscles in the old tail will continue to flex for up to 30 minutes after the drop to distract predators. Leopard geckos and African fat-tailed geckos are popular pet lizards.

Dry glue is an adhesion product based upon the adaptations of geckos' feet that allow them to climb sheer surfaces such as vertical glass. Synthetic equivalents use carbon nanotubes as synthetic setae on reusable adhesive patches.

<span class="mw-page-title-main">Gecko feet</span> Hairy feature allowing suction

The feet of geckos have a number of specializations. Their surfaces can adhere to any type of material with the exception of Teflon (PTFE). This phenomenon can be explained with three elements:

<span class="mw-page-title-main">Phyllodactylidae</span> Family of geckos

The Phyllodactylidae are a family of geckos (Gekkota) consisting of over 150 species in 10 genera, distributed throughout the New World, North Africa, Europe and the Middle East. The family was first delineated based on a molecular phylogenetic analysis in 2008, and all members possess a unique single codon deletion in the phosducin (PDC) gene. The phyllodactylid genus Bogertia has been recently synonymized with Phyllopezus. The name of the family comes from the leaf shaped fingers.

<span class="mw-page-title-main">Sphaerodactylidae</span> Family of geckos

The Sphaerodactylidae are a family of geckos (Gekkota) distributed in North America, Central America, South America, and the Caribbean, as well as in Southern Europe, North Africa, the Middle East, and into Central Asia. The family contains 12 living genera and over 200 living species. The family name comes from the ball shape of their finger joints.

<span class="mw-page-title-main">Diplodactylidae</span> Family of lizards

The Diplodactylidae are a family in the suborder Gekkota (geckos), with over 150 species in 25 genera. These geckos occur in Australia, New Zealand, and New Caledonia. Diplodactylids are the most ecologically diverse and widespread family of geckos in both Australia and New Caledonia, and are the only family of geckos found in New Zealand. Three diplodactylid genera have recently been split into multiple new genera.

<span class="mw-page-title-main">Arthropod adhesion</span>

Arthropods, including insects and spiders, make use of smooth adhesive pads as well as hairy pads for climbing and locomotion along non-horizontal surfaces. Both types of pads in insects make use of liquid secretions and are considered 'wet'. Dry adhesive mechanisms primarily rely on Van der Waals' forces and are also used by organisms other than insects. The fluid provides capillary and viscous adhesion and appears to be present in all insect adhesive pads. Little is known about the chemical properties of the adhesive fluids and the ultrastructure of the fluid-producing cells is currently not extensively studied. Additionally, both hairy and smooth types of adhesion have evolved separately numerous times in insects. Few comparative studies between the two types of adhesion mechanisms have been done and there is a lack of information regarding the forces that can be supported by these systems in insects. Additionally, tree frogs and some mammals such as the arboreal possum and bats also make use of smooth adhesive pads. The use of adhesive pads for locomotion across non-horizontal surfaces is a trait that evolved separately in different species, making it an example of convergent evolution. The power of adhesion allows these organisms to be able to climb on almost any substance.

<i>Rhoptropus bradfieldi</i> Species of lizard

Bradfield's Namib day gecko is a species of lizard in the family Gekkonidae. The species is endemic to Namibia. This species was first described in 1935 by the British-born, South African zoologist John Hewitt, who gave it the name Rhoptropus bradfieldi in honour of the South African naturalist and collector R.D. Bradfield (1882–1949).

<span class="mw-page-title-main">Pygopodoidea</span> Superfamily of lizards

Pygopodoidea is a gecko superfamily and the only taxon in the gekkotan subclade Pygopodomorpha. The clade includes three Australasian families: Diplodactylidae, Carphodactylidae, and Pygopodidae. Traditional gekkotan systematics had considered Diplodactylidae and Carphodactylidae as subfamilies of the family Gekkonidae, but recent molecular work have placed Pygopodidae within Gekkonidae making it paraphyletic. These analyses have shown support of Pygopodidae and Carphodactylidae being sister taxa, with Diplodactylidae occupying a basal position in Pygopodoidea.

<span class="mw-page-title-main">Nano tape</span> Synthetic adhesive tape

Nano tape, also called gecko tape; marketed under the name Insanity Tape, is a synthetic adhesive tape consisting of arrays of carbon nanotubes transferred onto a backing material of flexible polymer tape. These arrays are called synthetic setae and mimic the nanostructures found on the toes of a gecko; this is an example of biomimicry. The adhesion is achieved not with chemical adhesives, but via van der Waals forces, which are weak electric forces generated between two atoms or molecules that are very close to each other.So far there is little evidence to support nano tape being recyclable in the same way as plastic bottles, but it is reusable. More data is needed to know how environmentally safe nano tape is.

References

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  19. ↑ Santos, Daniel; Spenko, Matthew; Parness, Aaron; Sangbae, Kim; Cutkosky, Mark (2007). "Directional adhesion for climbing: Theoretical and practical considerations". Journal of Adhesion Science and Technology . 21 (12–13): 1317–1341. doi:10.1163/156856107782328399. S2CID   53470787. Archived from the original on 2012-01-15. Retrieved 2012-02-04. Gecko "feet and toes are a hierarchical system of complex structures consisting of lamellae, setae, and spatulae. The distinguishing characteristics of the gecko adhesion system have been described [as] (1) anisotropic attachment, (2) high pulloff force to preload ratio, (3) low detachment force, (4) material independence, (5) self-cleaning, (6) antiself sticking and (7) nonsticky default state. ... The gecko's adhesive structures are made from ß-keratin (modulus of elasticity [about] 2 GPa). Such a stiff material is not inherently sticky; however, because of the gecko adhesive's hierarchical nature and extremely small distal features (spatulae are [about] 200 nm in size), the gecko's foot is able to intimately conform to the surface and generate significant attraction using van der Waals forces.
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