User:AshLin/Snake scales

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Image:Coralillo.jpg
Smooth shiny scales of a Scarlet kingsnake Lampropeltis spp from Peru.

Scales are important for snakes - they are deemed to be reptiles by the presence of scales, amongst other things. [1] Snakes are covered with horny scales or scutes over the entire body. These come in many shapes and sizes and are intimately connected with the snake's biology and natural history. Scales in their myriad variety cover the skin, protect the body of the snake, allow moisture to be retained within and give simple or complex colouration patterns which help in camouflage and antipredator display. Modifications of the scales serve other functions such as 'eyelash' fringes, rattles, and protective covers for the eyes.

Contents

[edit] Morphology of scales

Keeled scales of Buff-striped keelback Amphiesma stolata, a Colubrid.
Keeled scales of Buff-striped keelback Amphiesma stolata, a Colubrid.

Snake scales are formed from the epidermis. Each scale has an outer surface and an inner surface. The skin from the inner surface hinges back and forms a free area which overlaps the base of the next scale which emerges below this scale. [2]

A snake is born with the a fixed number of scales. These are not added to or lost as it matures. The scales however grow larger in size and/or change shape with each moult.[3]

Snake skin and scales help keep moisture in the animal's body. [3] Snakes can also 'hear' by sensing vibrations with their lower jaw and belly scales.

[edit] Surface and shape

Snake scales are of different shapes and sizes. Snake scales may be granular, have a smooth surface or have a longitudinal ridge or keel on it. Often, snake scales have pits, tubercles and other structures which may be visible to the eye or microscopic. Snake scales may be modified to form fringes, as in the case of the Eyelash Bush Viper Etharis ceratophorus, or rattles as in the case of the rattlesnakes of North America.[4]

Certain snakes such as boas, pythons and certain vipers have small scales arranged irregularly on the head. Other more advanced snakes have special large symmetrical scales on the head called shields or plates. [2]

Modified tail scales form a rattle on a Western Diamondback Rattlesnake Crotalus atrox.
Modified tail scales form a rattle on a Western Diamondback Rattlesnake Crotalus atrox.

Snakes have smaller scales around the mouth and sides of the body which allow expansion so that a snake can consume prey of much larger width than itself. The ventral scales or belly scales are large and oblong. They protect the soft underside of the snake and also grip surfaces allowing the snake to move. The large scales (called 'shields') on the snake's head play a similar role.[3]

Snake scales are cool and dry and not slimy. [3] That is because scales are made of keratin, the same material that nails and fingernails are made of.[3] Scales, more specifically, consist of mostly hard ? keratins, and so are basically transparent. The colours of the scale are actually due to pigments in the inner layers of the skin and not due to the scale material itself. Scales are hued for all colours in this manner except for blue and green. Blue is caused by crystalline substances on the scale surface. By itself, such a scale surface diffracts light and gives a blue hue, while, in combination with yellow from the inner skin it gives a beautiful iridiscent green.

Some snakes have the ability to change the hue of their scales slowly. This is typically seen in cases where the snake becomes lighter or darker with change in season. In some cases, this change may take place between day and night.[3] Snake scales occur in variety of shapes. They may be :-

Cycloid scales on Leptotyphlops humilis.
Cycloid scales on Leptotyphlops humilis.
  • cycloid as in Family Typhlopidae. [5]
  • long and pointed with pointed tips, as in the case of the Green Whip Snake Ahaetulla nasutus. [6]
  • broad and leaf-like, as in the case of green pit vipers Trimeresurus spp. [6]
  • as broad as they are long, for example, as in Rat snake Ptyas mucosus. [6]
  • keeled weakly or strongly as in the case of the Buff-striped keelback Amphiesma stolatum. [6]
  • with bidentate tips as in some spp of Natrix. [6]
  • spinelike, juxtaposed as in the Short Seasnake Lapemis curtus. [2]
  • large, non-overlapping knobs as in the case of the Javan Mudsnake Xenodermis javanicus. [2]
  • modified tail scales to form a rattle as in the rattlesnake family (Crotalinae).

Another example of differentiation of snake scales is a transparent scale called the brille or spectacle which covers the eye of the snake. The snake has no eyelids and the brille protects the eye. It is shed as part of the old skin during moulting.[3]

[edit] Shedding of scales

Cascavel Crotalus durrisus, a rattlesnake, seen moulting.
Cascavel Crotalus durrisus, a rattlesnake, seen moulting.

The shedding of scales is called moulting. In the case of snakes, the complete outer layer of skin is shed in one layer. [7] Snake scales are not discrete but extensions of the epidermis hence they are not shed separately, but are ejected as a complete contiguous outer layer of skin during each moult, akin to a sock being turned inside out.[3]

Moulting serves a number of functions - firstly, the old and worn skin is replaced, secondly, to get rid of parasites such as mites and ticks. Renewal of the skin by moulting is supposed to allow the growth of the animal due to the new skin achieving larger dimensions than the old one, [8] though this view has been disputed.[3]

Before a moult, the snake stops eating and often hides or moves to a safe place. The inner surface of the old outer skin liquefies. This causes the old outer skin to separate from the new inner skin. When the snake moults the old skin breaks near the mouth and the snake wriggles out of the old skin by using a rough surface to rub against so that the skin comes off like an old sock. The discarded skin gives a perfect imprint of the scale pattern and it is usually possible to identify the snake if this discard is reasonably complete and intact.[3]

[edit] Taxonomic importance

Nomenclature of scales (top view of head)
Nomenclature of scales (top view of head)

Scales do not play an important role in distinguishing between the families but are important at generic and specific level. There is an elaborate scheme of nomenclature of scales. Scales patterns, by way of scale surface or texture, pattern and colouration and the division of the anal plate, in combination with other morphological characteristics, are the principal means of classifying snakes down to species level. [3]

In certain areas in North America, where the diversity of snakes is not too large, easy keys based on simple identification of scales have been devised for the lay public to distinguish poisonous snakes from non-poisonous snakes.[3] [3] In other places with large biodiversity, such as Myanmar, publications caution that venomous and non-venomous snakkes cannot be easily distinguished apart without careful examination.[3]

Identification of snakes by reference to scales seems an arcane art to the amateur naturalist because the nomenclature of snake scales seems esoteric and, more importantly, the snakes need to be caught and the head and body examined closely in hand for identification. The advent of high definition digital cameras means that images taken of snakes, if of appropriate definition and of the correct position, would show scales that can be examined, distinguished and counted.

[edit] Nomenclature of scales

This part of the article provides an easy photographic guide cum glossary of scale names on a snake's head and body for easy assimilation by the amateur naturalist. Please examine the annotated photographs of Buff-striped Keelback Amphiesma stolata (a common grass-snake of South Asia) while following the text.

[edit] Head scales

Nomenclature of scales (underside view of head)
Nomenclature of scales (underside view of head)

Let us start learning about snake scales with the nostril which is easily identified on the snake. There are two scales enclosing the nostril which are called the NASALs. The outer nasal (near the snout) is called the PRENASAL. The inner nasal (near the eye) is called the POSTNASAL. Along the top of the snout connecting the nasals on both sides of the head are scales called INTERNASALs.

Between the two prenasals is a scale at the tip of the snout. This is called the ROSTRAL scale.

The scales around the eye are called OCULAR scales. Those towards the front are called PREOCULAR scales, those towards the rear POSTOCULAR scales and those towards the upper or dorsal side are called as SUPRAOCULAR scales.

Between the preocular and the postnasal scales are the LOREAL scales.

The scales along the lips of the snake are called as LABIALs. Those on the upper lip are called SUPRALABIALs while those on the lower labial are called INFRALABIALs.

Between the eyeballs on top of the head, adjacent to the supraoculars are the FRONTAL scales. The PREFRONTAL scales are the scales connected to the frontals towards the tip of the snout which are in contact with the rostrals.

The back of the top of the head has scales connected to the frontal scales called as the PARIETAL scales. At the sides of the back of the head are scales called TEMPORAL scales.

On the underside of the head, a snake has an anterior scale called as the MENTAL scale. Connected to the mental scales and all along the lower jaws are the INFRALABIALs. Along the lower jaw connected to infralabials are a pair of shields called the ANTERIOR CHIN SHIELDs. Next to the anterior chin shields, further back along the jaw are another pair of shields called the POSTERIOR CHIN SHIELDs.

[edit] Scales on the body

Nomenclature of scales (underside view of body)
Nomenclature of scales (underside view of body)

The scales on the body of the snake are called the DORSAL or COSTAL scales. Sometimes there is a special row of large scales along the top of the back of the snake, i.e, the uppermost row, called the VERTEBRAL scales.

The enlarged scales on the belly of the snake are called VENTRAL scales or GASTROSTEGEs.

The snake has a vent on the underside near the tail which is covered by a ANAL scale which may be single or divided into a pair. The part of the body beyond the anal scale is considered the tail. Sometimes snakes have enlarged scales, either single or paired, under the tail; these are called SUBCAUDALs or UROSTEGEs.

In "advanced" (Caenophidian) snakes, the broad belly scales and rows of dorsal scales correspond to the vertebrae, allowing scientists to count the vertebrae without dissection.

Sources. Details for this section have been sourced from scale diagrams in Malcolm Smith.[9] Details of scales of Buff-striped Keelback have been taken from Daniels. [10]

[edit] Arrangement of scales

Snake have imbricate scales, overlapping like the tiles on a roof. [11] Snakes have rows of scales along the whole or part of their length and also many other specialised scales, either singly or in pairs, occurring on the head and other regions of the body.

The dorsal scales on the snake's body are arranged in rows along the length of their bodies. Adjacent rows are diagonally offset from each other. Most snakes have an odd number of rows across the body though certain species have an even number of rows e.g. Zaocis spp. [2] In the case of some aquatic and marine snakes, the scales are granular and the rows cannot be counted . [11]

The number of rows range from ten in Tiger Ratsnake Spilotes pullatus; thirteen in Dryocalamus, Liopeltis, Calamaria and Asian coral snakes of genus Calliophis; 65 to 75 in Python; 74 to 93 in Kolpophis and 130 to 150 in Acrochordus. The majority of the largest family of snakes, the Colubridae have 15, 17 or 19 rows of scales. [2] [12]

The maximum number of rows are in mid-body and they reduce in count towards the head and on the tail.

[edit] Glossary of scales

Nomenclature of scales (side view of head)
Nomenclature of scales (side view of head)


[edit] Cultural significance

The patterns created by scales on snakeskin appeal to people and snakeskin is used to manufacture of many leather articles including fashionable accessories. The use of snakeskin is illegal in certain countries and animal lovers in many countries propagate the use of artificial substances instead, which are easily produced from plactics and other materials.

[edit] Notes

  1. ^ Boulenger, George A. The Fauna of British India... page 1
  2. ^ a b c d e f Greene, Harry W. Snakes - The Evolution of Mystery in Nature, page 22
  3. ^ a b c d e f g h i j k l m n o [www.szgdocent.org/resource/rr/c-slimy.htm Are snakes slimy - Singapore Zoological Garden's Docent site]
  4. ^ Greene, Harry W. Snakes - The Evolution of Mystery in Nature, page 23
  5. ^ Boulenger, George A. The Fauna of British India... page 234
  6. ^ a b c d e Smith, Malcolm A. Fauna of British India...Vol III - Serpentes, page 6
  7. ^ Smith, Malcolm A. Fauna of British India...Vol I - Loricata and Testudines, page 30
  8. ^ ZooPax Scales Part 3
  9. ^ Smith, Malcolm A. Fauna of British India...Vol III - Serpentes, page 29
  10. ^ Daniels, J.C. Book of Indian Reptiles and Amphibians.... page 116 to 118
  11. ^ a b Smith, Malcolm A. Fauna of British India...Vol III - Serpentes, page 5
  12. ^ Smith, Malcolm A. Fauna of British India...Vol III - Serpentes, page 7
  13. ^ Mallow D, Ludwig D, Nilson G. 2003. True Vipers: Natural History and Toxinology of Old World Vipers. Krieger Publishing Company, Malabar, Florida. ISBN 0-89464-877-2.

[edit] References

  • Boulenger, George A., (1890), The Fauna of British India including Ceylon and Burma, Reptilia and Batrachia. Taylor and Francis, London.
  • Daniels, J.C. Book of Indian Reptiles and Amphibians. (2002). BNHS. Oxford University Press. Mumbai.
  • Greene, Harry W. (2004), Snakes - The Evolution of Mystery in Nature. University of California Press, pages 22-23 (excerpted from Google Book Search beta on 07 August 2006).
  • Mallow D, Ludwig D, Nilson G. 2003. True Vipers: Natural History and Toxinology of Old World Vipers. Krieger Publishing Company, Malabar, Florida. ISBN 0-89464-877-2.
  • Smith, Malcolm A. (1943), The Fauna of British India, Ceylon and Burma including the whole of the Indo-Chinese Sub-region, Reptilia and Amphibia. Vol I - Loricata and Testudines, Vol II-Sauria, Vol III-Serpentes. Taylor and Francis, London.
  • Leviton AE, Wogan GOU, Koo MS, Zug GR, Lucas RS, Vindum JV. 2003. The Dangerously Venomous Snakes of Myanmar, Illustrated Checklist with Keys. Proc. Cal. Acad. Sci. 54 (24):407-462. PDF at Smithsonian National Museum of Natural History, Division of Amphibians and Reptiles.

[edit] See also

[edit] External links


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