Snakes, like other reptiles, have skin covered in scales. Snakes are entirely covered with scales or scutes of various shapes and sizes, known as snakeskin as a whole. A scale protects the body of the snake, aids it in locomotion, allows moisture to be retained within, alters the surface characteristics such as roughness to aid in camouflage, and in some cases even aids 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, art and films.
Functions The 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. 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).
Evolution Reptiles evolved from amphibious ancestors which left the water and became terrestrial. To prevent loss of moisture, reptilian skin lost the softness and moisture of amphibian skin and developed a thick stratum corneum with multiple layers of lipids, which served as an impermeable barrier, as well as providing protection from ultraviolet light. Over time, reptilian skin cells became highly keratinised, horny, sturdy and desiccated. The surfaces of the dermis and epidermis of all reptilian scales form a single contiguous sheet, as can be seen when the snake sheds its skin as a whole.
Morphology
Snake 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 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 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 cycloid as in family Typhlopidae, long and pointed with pointed tips, as in the case of the green vine snake Ahaetulla nasuta, broad and leaf-like, as in the case of green pit vipers Trimeresurus spp. or as broad as they are long, for example, as in rat snake Ptyas mucosus. In some cases, scales may be keeled weakly or strongly as in the case of the buff-striped keelback Amphiesma stolatum. They may have bidentate tips as in some spp of Natrix. Some snakes, such as the short seasnake Hydrophis curtus, may have spinelike and juxtaposed scales while others may have large and non-overlapping knobs as in the case of the Javan mudsnake Xenodermus javanicus. 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.
Rattles The 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 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.
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