Gecko

This article is about the type of reptile. For other uses, see Gecko (disambiguation).

Geckos are lizards belonging to the infraorder Gekkota, found in warm climates throughout the world. They range from 1.6 to 60 cm (0.64 to 24 inches). 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 to let in more light.

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.[2] The New Latin gekko and English "gecko" stem from the Indonesian-Malay gēkoq, which is imitative of the sound the animals make.[3]

All geckos, excluding the Eublepharidae family, lack eyelids and instead have a transparent membrane, which they lick to clean.[4] Nocturnal species have an excellent night vision; their color vision is 350 times more sensitive than human color vision.[5] The nocturnal geckos evolved from diurnal species which had lost the eye rods. The gecko eye therefore modified its cones that increased in size into different types both single and double. Three different photopigments have been retained and are sensitive to UV, blue, and green. They also use a multifocal optical system that allows them to generate a sharp image for at least two different depths.[6][7]

Most gecko species can lose their tails in defense, a process called autotomy.[8] 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. Geckos 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 (for example the house gecko) become part of the indoor menagerie and are often welcomed, as they feed on insects, including moths and mosquitoes. Unlike most lizards, geckos are usually nocturnal.

The largest species, the kawekaweau, is only known from a single, stuffed specimen found in the basement of a museum in Marseille, France. This gecko was 60 cm (24 in) long and it was likely endemic to New Zealand, where it lived in native forests. It was probably wiped out along with much of the native fauna of these islands in the late 19th century, when new invasive species such as rats and stoats were introduced to the country during European colonization. The smallest gecko, the Jaragua sphaero, is a mere 1.6 cm long and was discovered in 2001 on a small island off the coast of the Dominican Republic.[9]

Common traits

Oligocene-era gecko trapped in amber

Geckos are selectively bred. Geckos occur in various patterns and colors, and are among the most colorful 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. Like other reptiles, geckos are ectothermic,[10] producing very little metabolic heat. Essentially a gecko's body temperature is dependent on its environment. Also, in order to accomplish their main functions—such as locomotion, feeding, reproduction, etc.—geckos must have a relatively elevated temperature.[10]

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 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.[11]

Adhesion ability

See also technology designed to mimic gecko adhesion and Gecko feet.

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

Gecko toes have special adaptations that allow them to adhere to most surfaces without the use of liquids or surface tension. About 60% of gecko species have adhesive toe pads; such pads have been gained and lost repeatedly over the course of gecko evolution.[12] Adhesive toepads evolved independently in about 11 different gecko lineages and were lost in at least 9 lineages.[12] The spatula-shaped setae arranged in lamellae on gecko footpads enable attractive van der Waals' forces between the β-keratin lamellae/setae/spatulae structures and the surface.[13][14] 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.[15][16] The setae on the feet of geckos are also self-cleaning and will usually remove any clogging dirt within a few steps.[17][18] Teflon, which has very low surface energy,[19] is more difficult for geckos to adhere to than many other surfaces.

Increasing humidity typically fortifies gecko adhesion,[15][16][20][21][22] even on hydrophobic surfaces, yet is reduced if completely immersed 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.[15][16][20][21][22] Moreover, the elastic properties of the b-keratin change with water uptake.[15][16][20] Friction experiments with gecko toes—torn parallel to surfaces—have shown to be influenced also by electrostatic forces.[23]

Gecko toes seem to be "double jointed", but this is a misnomer and is properly called digital hyperextension.[24] 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, 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.

Uroplatus fimbriatus clinging to glass

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 μm. Human hair varies from 18 to 180 μm, so a human hair could hold between 12 and 1300 setae. Each seta is in turn tipped with between 100 and 1,000 spatulae.[17] Each spatula is 0.2 μm long[17] (one five-millionth of a meter), or just below the wavelength of visible light.[25]

The setae of a typical mature 70 g (2.5 oz) gecko would be capable of supporting a weight of 133 kg (293 lb):[26][27] each spatula can exert an adhesive force of 5 to 25 nN.[20][28] The exact value of the adhesion force of a spatula varies with the surface energy of the substrate to which it adheres. Recent studies [22][29] 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.[30] These lipids lubricate the setae and allow the gecko to detach its foot before the next step.

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.

Teeth

Geckos are polyphyodonts and able to replace each of their 100 teeth every 3 to 4 months.[31] Next to the full grown tooth there is a small replacement tooth developing from the odontogenic stem cell in the dental lamina.[32] 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.

The infraorder Gekkota is divided into seven families, containing numerous genera of gecko species.[12][33][34][35][36]

Dwarf yellow-headed gecko, in Dar es Salaam, Tanzania

Species of geckos

Mediterranean house gecko
video of Japanese gecko (Gekko japonicus) in Tokyo, Japan

About 1,500 species of geckos occur worldwide, including these familiar or notable species:

References

  1. ↑
    • Arnold, E.N.; Poinar, G. (2008). "A 100 million year old gecko with sophisticated adhesive toe pads, preserved in amber from Myanmar (abstract)" (PDF). Zootaxa. Retrieved August 12, 2009.
    • Borsuk-Białynicka, M. (1990). "Gobekko cretacicus gen. et. sp. n., a new gekkonid lizard from the Cretaceous of the Gobi Desert". Acta Palaeontologica Polonica 35 (1–2): 67–76.
    • Conrad, Jack L.; Norell, Mark A. (1 December 2006). "High-resolution X-ray computed tomography of an Early Cretaceous gekkonomorph (Squamata) from Öösh (Övörkhangai; Mongolia)". Historical Biology 18 (4): 405–431. doi:10.1080/08912960600679570.
    • Conrad, Jack L. (3 June 2008). "Phylogeny and Systematics of Squamata (Reptilia) Based on Morphology". Bulletin of the American Museum of Natural History 310: 1–182. doi:10.1206/310.1.
    • Bauer, Aaron M.; Böhme, Wolfgang; Weitschat, Wolfgang (April 2005). "An Early Eocene gecko from Baltic amber and its implications for the evolution of gecko adhesion". Journal of Zoology 265 (4): 327–332. doi:10.1017/S0952836904006259.
  2. ↑ http://reptile-database.reptarium.cz/advanced_search?taxon=gecko&submit=Search Geckos in the Reptile Database
  3. ↑ gecko, n. Oxford English Dictionary Second edition, 1989; online version September 2011. Accessed 29 October 2011. Earlier version first published in New English Dictionary, 1898.
  4. ↑ Badger, David (2006). Lizards: a Natural History of Some Uncommon Creatures. St. Paul, MN: Voyageur Press. p. 47. ISBN 0760325790.
  5. ↑ Roth, L.S.V.; Lundstrom, L.; Kelber, A.; Kroger, R. H. H.; Unsbo, P. (1 March 2009). "The pupils and optical systems of gecko eyes". Journal of Vision 9 (3): 27–27. doi:10.1167/9.3.27. PMID 19757966.
  6. ↑ The pupils and optical systems of gecko eyes
  7. ↑ Gecko-inspired multifocal contact lenses, cameras on the anvil
  8. ↑ Mihai, Andrei (Sep 9, 2009). "Gecko tail has a mind of its own". ZME Science.
  9. ↑ Piper, Ross (2007). Extraordinary Animals: an Encyclopedia of Curious and Unusual Animals. Westport, Conn.: Greenwood Press. p. 143. ISBN 0313339228.
  10. 1 2 Girons, Hubert. "Thermoregulation in Reptiles with Special Reference to the Tuatara and Its Ecophysiology" Tuatara: Volume 24, Issue 2, August 1980. Victoria University of Wellington Library, August 1980. Web. May 31, 2014. http://nzetc.victoria.ac.nz/tm/scholarly/tei-Bio24Tuat02-t1-body-d2.html.
  11. ↑ http://www.geckocare.net/shedding.php
  12. 1 2 3 Gamble, Tony; Greenbaum, Eli; Jackman, Todd R.; Russell, Anthony P.; Bauer, Aaron M.; Castresana, Jose (June 27, 2012). "Repeated Origin and Loss of Adhesive Toepads in Geckos". PLoS ONE 7 (6): e39429. doi:10.1371/journal.pone.0039429. PMC 3384654. PMID 22761794.
  13. ↑ http://www.nisenet.org/scientific-images/gecko_toe
  14. ↑ Santos, Daniel; Matthew Spenko; Aaron Parness; Kim Sangbae; Mark Cutkosky (2007). "Directional adhesion for climbing: Theoretical and practical considerations". Journal of Adhesion Science and Technology 21 (12–13): 1317–1341. doi:10.1163/156856107782328399. 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) anti-self sticking and (7) non-sticky default state. ... The gecko's adhesive structures are made from ß-keratin (modulus of elasticity [approx.] 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 [approx.] 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.
  15. 1 2 3 4 Puthoff, J.B.; Prowse, M.; Wilkinson, M.; Autumn, K. (2010). "Changes in materials properties explain the effects of humidity on gecko adhesion". Journal of Experimental Biology 213 (21): 3699–3704. doi:10.1242/jeb.047654.
  16. 1 2 3 4 Prowse, M.S.; Wilkinson, Matt; Puthoff, Jonathan B.; Mayer, George; Autumn, Kellar (2011). "Effects of humidity on the mechanical properties of gecko setae". Acta Biomaterialia 7 (2): 733–738. doi:10.1016/j.actbio.2010.09.036. PMID 20920615.
  17. 1 2 3 Hansen, W. R.; Autumn, K. (2005). "Evidence for self-cleaning in gecko setae". PNAS 102 (2): 385–389. doi:10.1073/pnas.0408304102. PMC 544316. PMID 15630086. Setae occur in uniform arrays on overlapping lamellar pads at a density of 14,400 per mm2
  18. ↑ How Geckos Stick to Walls.
  19. ↑ Why do the gecko's feet not stick to a teflon surface?.
  20. 1 2 3 4 Huber, G.; Mantz, H.; Spolenak, R.; Mecke, K.; Jacobs, K.; Gorb, S.N. and Arzt, E. (2005). "Evidence for capillarity contributions to gecko adhesion from single spatula nanomechanical measurements". Proceedings of the National Academy of Sciences 102 (45): 16293–6. doi:10.1073/pnas.0506328102. PMC 1283435. PMID 16260737.
  21. 1 2 Chen, B.; Gao, H. (2010). "An alternative explanation of the effect of humidity in gecko adhesion: stiffness reduction enhances adhesion on a rough surface". International Journal of Applied Mechanics 2: 1–9. doi:10.1142/s1758825110000433.
  22. 1 2 3 Loskill, P.; Puthoff, J.; Wilkinson, M.; Mecke, K.; Jacobs, K.; Autumn, K. (September 2012). "Macroscale adhesion of gecko setae reflects nanoscale differences in subsurface composition". J. R. Soc. Interface 10 (78): 20120587. doi:10.1098/rsif.2012.0587. PMC 3565786. PMID 22993246.
  23. ↑ "Role of contact electrification and electrostatic interactions in gecko adhesion". Journal of the Royal Society. April 8, 2014. Retrieved July 10, 2014.
  24. ↑ Russell, A. P. (1975). "A contribution to the functional analysis of the foot of the Tokay, Gekko gecko (Reptilia: Gekkonidae)". Journal of Zoology, London 176: 437–476. doi:10.1111/j.1469-7998.1975.tb03215.x.
  25. ↑ Autumn, Kellar; Sitti, M.; Liang, Y.A.; Peattie, A.M.; Hansen, W.R.; Sponberg, S.; Kenny, T.W.; Fearing, R.; Israelachvili, J.N.; Full, R.J. (2002). "Evidence for van der Waals adhesion in gecko setae". PNAS 99 (19): 12252–12256. doi:10.1073/pnas.192252799. PMC 129431. PMID 12198184.
  26. ↑ "Geckos can hang upside down carrying 40kg". physics.org. Retrieved 2 November 2012.
  27. ↑ Autumn, Kellar (September 29, 2003). "How do gecko lizards unstick themselves as they move across a surface?". Scientific American. Retrieved 23 March 2013.
  28. ↑ Lee, Haeshin; Lee, Bruce P.; Messersmith, Phillip B. (2007). "A reversible wet/dry adhesive inspired by mussels and geckos". Nature 448 (7151): 338–341. doi:10.1038/nature05968. PMID 17637666.
  29. ↑ Loskill, P.; Haehl, H.; Grandthyll, S.; Faidt, T.; Mueller, F.; Jacobs, K. (November 2012). "Is adhesion superficial? Silicon wafers as a model system to study van der Waals interactions". Adv. Coll. Interf. Sci. 179–182: 107–113. doi:10.1016/j.cis.2012.06.006. PMID 22795778.
  30. ↑ Hsu, P. Y.; Ge, L.; Li, X.; Stark, A. Y.; Wesdemiotis, C.; Niewiarowski, P. H.; Dhinojwala, A. (24 August 2011). "Direct evidence of phospholipids in gecko footprints and spatula-substrate contact interface detected using surface-sensitive spectroscopy". Journal of the Royal Society Interface 9 (69): 657–664. doi:10.1098/rsif.2011.0370.
  31. ↑ Mechanism of tooth replacement in Leopard geckos
  32. ↑ Gregory R. Handrigan, Kelvin J. Leung, Joy M. Richman (2010). "Identification of putative dental epithelial stem cells in a lizard with life-long tooth replacement". Development (137): 3545–3549. doi:10.1242/dev.052415.
  33. ↑ Han, D.; Zhou, K.; Bauer, A.M. (2004). "Phylogenetic relationships among gekkotan lizards inferred from c-mos nuclear DNA sequences and a new classification of the Gekkota". Biological Journal of the Linnean Society 83 (3): 353–368. doi:10.1111/j.1095-8312.2004.00393.x.
  34. ↑ Gamble, T.; Bauer, A.M.; Greenbaum, E.; Jackman, T.R. (July 2008). "Out of the blue: A novel, trans-Atlantic clade of geckos (Gekkota, Squamata)". Zoologica Scripta 37 (4): 355–366. doi:10.1111/j.1463-6409.2008.00330.x.
  35. ↑ Gamble, Tony; Bauer, Aaron M.; Greenbaum, Eli; Jackman, Todd R. (21 August 2007). "Evidence for Gondwanan vicariance in an ancient clade of gecko lizards". Journal of Biogeography: 070821084123003. doi:10.1111/j.1365-2699.2007.01770.x.
  36. ↑ Gamble, T.; Bauer, A.M.; Colli, G.R.; Greenbaum, E.; Jackman, T.R.; Vitt, L.J.; Simons, A.M. (February 2011). "Coming to America: Multiple Origins of New World Geckos". Journal of Evolutionary Biology 24 (2): 231–244. doi:10.1111/j.1420-9101.2010.02184.x. PMC 3075428. PMID 21126276.

Further reading

External links

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