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Snake scales
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Snakes, like other reptiles, have a skin covered in scales. Snakes are entirely covered with scales or scutes of various shapes and sizes. Scales protect the body of the snake, aid it in locomotion, allow moisture to be retained within, alter the surface characteristics such as roughness to aid in camouflage, and in some cases even aid in prey capture (such as Acrochordus). The simple or complex colouration patterns (which help in camouflage and anti-predator display) are a property of the underlying skin, but the folded nature of scaled skin allows bright skin to be concealed between scales then revealed in order to startle predators.
Scales have been modified over time to serve other functions such as 'eyelash' fringes, and protective covers for the eyes with the most distinctive modification being the rattle of the North American rattlesnakes.
Snakes periodically moult their scaly skins and acquire new ones. This permits replacement of old worn out skin, disposal of parasites and is thought to allow the snake to grow. The arrangement of scales is used to identify snake species.
Snakes have been part and parcel of culture and religion. Vivid scale patterns have been thought to have influenced early art. The use of snake-skin in manufacture of purses, apparel and other articles led to large-scale killing of snakes, giving rise to advocacy for use of artificial snake-skin. Snake scales are also to be found as motifs in fiction, video games and films.
Functions of scalesThe scales of a snake primarily serve to reduce friction as it moves, since friction is the major source of energy loss in snake locomotion. The ventral (or belly) scales, which are large and oblong, are especially low-friction, and some arboreal species can use the edges to grip branches. Most snakes have at least some large scales (called 'shields') on their head, which can be used to distinguish different species.
Snake skin and scales help retain moisture in the animal's body.
Snakes pick up vibrations from both the air and the ground, and can differentiate the two, using a complex system of internal resonances (perhaps involving the scales) .
Morphology of scalesSnake scales are formed by the differentiation of the snake's underlying skin or 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.
A snake hatches with a fixed number of scales. The scales do not increase in number as the snake matures nor do they reduce in number over time. The scales however grow larger in size and may change shape with each moult.
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.
Snake scales are made of keratin, the same material that hair and fingernails are made of. They are cool and dry to touch.
Surface and shapeSnake 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 fine structures which may be visible to the naked eye or under a microscope. Snake scales may be modified to form fringes, as in the case of the Eyelash Bush Viper, Atheris ceratophora, or rattles as in the case of the rattlesnakes of North America.
Certain primitive snakes such as boas, pythons and certain advanced snakes such as 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.
Snake scales occur in variety of shapes. They may be :
Another example of differentiation of snake scales is a transparent scale called the brille or spectacle which covers the eye of the snake. The brille is often referred to as a fused eyelid. It is shed as part of the old skin during moulting.
RattlesThe most distinctive modification of the snake scale is the rattle of rattlesnakes, such as those of the genera Crotalus and Sistrurus. The rattle is made up of a series of loosely linked, interlocking chambers that when shaken, vibrate against one another to create the warning signal of a rattlesnake. Only the bottom button is firmly attached to the tip of the tail.
At birth, a rattlesnake hatchling has only a small button or 'primordial rattle' which is firmly attached to the tip of the tail. The first segment is added when the hatchling sheds its skin for the first time. A new section is added each time the skin is shed until a rattle is formed. The rattle grows as the snake ages but segments are also prone to breaking off and hence the length of a rattle is not a reliable indicator of the age of a snake.
ColourScales, more specifically, mostly consist of hard beta keratins which are basically transparent. The colours of the scale are 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 the ultrastructure of the scales. 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 iridescent 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.
EcdysisThe shedding of scales is called ecdysis, or, in normal usage moulting or sloughing. In the case of snakes, the complete outer layer of skin is shed in one layer. 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.
Moulting serves a number of functions - firstly, the old and worn skin is replaced, secondly, it helps get rid of parasites such as mites and ticks. Renewal of the skin by moulting is supposed to allow growth in some animals such as insects, however this view has been disputed in the case of snakes.
Moulting is repeated periodically throughout a snake's life. Before a moult, the snake stops eating and often hides or moves to a safe place. Just before shedding, the skin becomes dull and dry looking and the eyes become cloudy or blue-colored. The inner surface of the old outer skin liquefies. This causes the old outer skin to separate from the new inner skin. After a few days, the eyes clear and the snake "crawls" out of its old skin. The old skin breaks near the mouth and the snake wriggles out aided by rubbing against rough surfaces. In many cases the cast skin peels backward over the body from head to tail, in one piece like an old sock. A new, larger, and brighter layer of skin has formed underneath.
An older snake may shed its skin only once or twice a year, but a younger, still-growing snake, may shed up to four times a year. 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.
Arrangement of scalesExcepting for the head, snakes have imbricate scales, overlapping like the tiles on a roof. 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 (or body) 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. Zaocys spp. In the case of some aquatic and marine snakes, the scales are granular and the rows cannot be counted.
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 pythons; 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.
The maximum number of rows are in mid-body and they reduce in count towards the head and on the tail.
Nomenclature of scalesThe various scales on a snake's head and body are indicated in the following paragraphs with annotated photographs of Buff-striped Keelback Amphiesma stolata (a common grass-snake of South Asia).
Head scalesIdentification of scales is most conveniently begun with reference to 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 while 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 called the rostral scale.
The scales around the eye are called circumorbital scales and are named as 'ocular' scales but with appropriate prefix. The ocular scale proper is a transparent scale covering the eye which is called the spectacle, brille or eyecap..
The circumorbital scales towards the snout or the front are called preocular scales, those towards the rear are called postocular scales and those towards the upper or dorsal side are called as supraocular scales. Circumorbital scales towards the ventral or lower side, if any, are called as subocular 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 internasals.
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.
Scales in the central or throat region, which are in contact with the first ventral scales of a snake's body and are flanked by the chin shields, are called gular scales.
The mental groove is a longitudinal groove on the underside of the head between large, paired chin shields and smaller gular scales.
Scales on the bodyThe 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.
In "advanced" snakes, the broad belly scales and rows of dorsal scales correspond to the vertebrae, allowing scientists to count the vertebrae without dissection.
Tail scalesAt the end of the ventral scales of the snake is an anal plate which protects the opening to the cloaca (a shared opening for waste and reproductive material to pass) on the underside near the tail. This anal scale may be single or divided into a pair. The part of the body beyond the anal scale is considered to be the tail.
Sometimes snakes have enlarged scales, either single or paired, under the tail; these are called subcaudals or urosteges. These subcaudals may be smooth or keeled as in Bitis arietans somalica. The end of the tail may simply taper into a tip (as in the case of most snakes), it may form a spine (as in Acanthophis), end in a bony spur (as in Lachesis), a rattle (as in Crotalus), or a rudder as seen in many sea snakes.
Sources. Details for this section have been sourced from scale diagrams in Malcolm Smith. Details of scales of Buff-striped Keelback have been taken from Daniels.
Other pertinent terms
Cultural significanceSnakes have been a motif in human culture and religion and an object of dread and fascination all over the world. The vivid patterns of snake scales, such as the Gaboon Viper, both repel and fascinate the human mind. Such patterns have inspired dread and awe in humans from pre-historic times and these can be seen in the art prevalent to those times. Studies of fear imagery and psychological arousal indicate that snake scales are a vital component of snake imagery. Snake scales also appear to have affected Islamic art in the form of tessallated mosaic patterns which show great similarity to snake-scale patterns.
Snakeskin, with its highly periodic cross-hatch or grid patterns, appeals to people's aesthetics and have been used to manufacture many leather articles including fashionable accessories. The use of snakeskin has however endangered snake populations and resulted in international restrictions in trade of certain snake species and populations in the form of CITES provisions. Animal lovers in many countries now propagate the use of artificial snakeskin instead, which are easily produced from embossed leather, patterned fabric, plastics and other materials.
Snake scales occur as a motif regularly in computer action games.
A snake scale was portrayed as a clue in the 1982 film Blade Runner.
Snake scales also figure in popular fiction, such as the Harry Potter series (desiccated Boomslang skin is used as a raw material for concocting the Polyjuice potion), and also in teen fiction.
See also
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