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Snake scale
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Snakes, like other reptiles, have skin covered in scales.[1] 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[2] 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
[edit]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.[3] 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).[4]
Evolution
[edit]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.[5] 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.[6]
Morphology
[edit]
Snake scales are formed by the differentiation of the snake's underlying skin or epidermis.[7] 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.[8] 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.[9]
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.[9] They are cool and dry to touch.[10]
Surface and shape
[edit]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.[8]
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.[8]
Snake scales occur in variety of shapes. They may be cycloid as in family Typhlopidae,[11] long and pointed with pointed tips, as in the case of the green vine snake Ahaetulla nasuta,[12] broad and leaf-like, as in the case of green pit vipers Trimeresurus spp.[12] or as broad as they are long, for example, as in rat snake Ptyas mucosus.[12] In some cases, scales may be keeled weakly or strongly as in the case of the buff-striped keelback Amphiesma stolatum.[12] They may have bidentate tips as in some spp of Natrix.[12] Some snakes, such as the short seasnake Hydrophis curtus, may have spinelike and juxtaposed scales[8] while others may have large and non-overlapping knobs as in the case of the Javan mudsnake Xenodermus javanicus.[8]
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.[2]
Rattles
[edit]
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.[13]
At birth, a rattlesnake hatchling has only a small button or 'primordial rattle' which is firmly attached to the tip of the tail.[13] The first segment is added when the hatchling sheds its skin for the first time.[14] 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.[15]
Colour
[edit]Scales 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.[9]
Ecdysis
[edit]
The 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.[16] 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.[9]
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.[9][17]
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.[9][18]
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.[18] 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.[9]
Arrangement
[edit]
Scale arrangements are important, not only for taxonomic utility, but also for forensic reasons and conservation of snake species.[19] Excluding the head, snakes have imbricate scales, overlapping like the tiles on a roof.[20] 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.[8] In the case of some aquatic and marine snakes, the scales are granular and the rows cannot be counted.[20]
The number of rows range from ten in Tiger Ratsnake Spilotes pullatus; thirteen in Lycodon, 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.[8][21] The maximum number of rows are in mid-body and they reduce in count towards the head and on the tail.
Nomenclature
[edit]The 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 and a member of Colubridae, the largest snake family.
Head
[edit]
Identification of cephalic scales is most conveniently begun with reference to the nostril, which is easily identified on a snake. There are two scales enclosing the nostril which are called the nasals. In colubrids, the nostril lies between the nasals, while in vipers it lies in the centre of a single nasal scale.[22] 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.[22]
The scales around the eye are called circumorbital scales and are named as ocular scales but with appropriate prefixes. The ocular scale proper is a transparent scale covering the eye which is called the spectacle, brille or eyecap.[9][23] 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 supraocular scales. Circumorbital scales towards the ventral or lower side, if any, are called subocular scales. Between the preocular and the postnasal scales are one or two scales called loreal scales.[22] Loreal scales are absent in elapids.
The scales along the lips of the snake are called labials. Those on the upper lip are called supralabials or upper labials, while those on the lower lip are called infralabials or lower labials. On top of the head, between the eyes, adjacent to the supraoculars is the frontal scale. The prefrontal scales are the scales connected to the frontal towards the tip of the snout which are in contact with the internasals. They may have a scale in between them.[22] The back of the top of the head has scales connected to the frontal scale called the parietal scales. At the sides of the back of the head between the parietals above and the supralabials below are scales called temporal scales.[22]
On the underside of the head, a snake has an anterior scale called the mental[a] scale. Connected to the mental scale and all along the lower lips are the infralabials or lower labials. Along the chin connected to the infralabials is a pair of shields called the anterior chin shields. Next to the anterior chin shields, further back along the chin is another pair of shields called the posterior chin shields. In some texts the chinshields are referred to as submaxillary scales.[22]
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 the large, paired chin shields and continuing between the smaller gular scales.
Body
[edit]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 usually called ventral scales, although several names are used in the (older) literature, including ventralia, gastrosteges (pl. gastrostegi, gastrostegia), scuta subcaudalia[24] or abdominal scales (scutes, plates).[25] Many authors simply abbreviate the ventral scales as "V".[26] The number of ventral scales can be a guide to the species.[22] 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.[citation needed]
Tail
[edit]
At the end of the ventral scales of the snake is a cloacal plate that protects the opening to the cloaca (a shared opening for waste and reproductive material to pass) on the underside near the tail. This scale has also been the anal scale, which is a misnomer since it does not cover an anus but a cloaca. This cloacal scale may be single or paired. Most authors have differentiated between single and divided cloacal scales. However, based on the origin of scales during development, a scale does not spontaneously divide, but it originates as paired structures that subsequently overlap. The part of the body beyond the cloacal scale is considered to be the tail.[13]
Sometimes snakes have enlarged scales, either single or paired, under the tail; these are called subcaudals or urosteges.[22] 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.
Details for this section have been sourced from scale diagrams in Malcolm Smith.[27] Details of scales of Buff-striped Keelback have been taken from Daniels.[28]
Glossary
[edit]
- ag – anterior genials or chin shields
- f – frontal
- in – internasal
- l – loreal
- la – supralabial
- la' – infralabial
- m – mental
- n – nasal
- p – parietal
- pf – prefrontal
- pg – posterior genials or chin shields
- pro – preocular
- pso – presubocular
- pto – postocular
- r – rostral
- so – supraocular
- t – anterior and posterior temporals
- v – first ventral
- Scales on the head
- Rostral
- Nasorostral
- Nasal
- Internasal
- Brille, spectacle, ocular scale, eyecap
- Circumorbital
- Loreal
- Interorbital, intersupraocular
- Frontal
- Prefrontal
- Parietal
- Occipital
- Interoccipital
- Temporal
- Labial
- Supralabial, upper labial
- Sublabial, infralabial, lower labial
- Mental or symphysial
- Chin shield
- Anterior chin shield, anterior genials
- Posterior chin shield, posterior genials
- Intergeneial
- Gular
- Scales on the body
- Scales on the tail
Other terms
[edit]- Canthus, or canthus rostralis – the angle between the supraocular scale and the rostral scale
- Mental groove
Taxonomic importance
[edit]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.[29]
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 venomous snakes from non-venomous snakes.[30][31] In other places with large biodiversity, such as Myanmar, publications caution that venomous and non-venomous snakes cannot be easily distinguished apart without careful examination.[32]
The scales patterning may also be used for individual identification in field studies. Clipping of specific scales, such as the subcaudals, to mark individual snakes is a popular approach to population estimation by mark and recapture techniques.[33]
Distinguishing between venomous and non-venomous snakes
[edit]
There is no simple way of differentiating a venomous snake from a non-venomous one merely by using a scale character. Finding out whether a snake is venomous or not is correctly done by identification of the species of a snake with the help of experts,[34]: 190 or in their absence, close examination of the snake and using authoritative references on the snakes of the particular geographical region to identify it. Scale patterns help to indicate the species and from the references, it can be verified if the snake species is known to be venomous or not.
Species identification using scales requires a fair degree of knowledge about snakes, their taxonomy, snake-scale nomenclature as well as familiarity with and access to scientific literature. Distinguishing by using scale diagrams whether a snake is venomous or not in the field cannot be done in the case of uncaught specimens. It is not advisable to catch a snake to check whether it is venomous or not using scale diagrams.[34]: 190 Most books or websites provide an array of traits of the local herpetofauna, other than scale diagrams, which help to distinguish whether a snake in the field is venomous or not.[29][35]: 52
In certain regions, presence or absence of certain scales may be a quick way to distinguish non-venomous and venomous snakes, but used with care and knowledge of exceptions. For example, in Myanmar, the presence or absence of loreal scales can be used to distinguish between relatively harmless Colubrids and lethally venomous Elapids.[32] The rule of hand for this region is that the absence of a loreal scale between the nasal scale and pre-ocular scale indicates that the snake is an Elapid and hence lethal.[32] This rule-of-thumb cannot be used without care as it cannot be applied to vipers, which have a large number of small scales on the head. A careful check would also be needed to exclude known poisonous members of the Colubrid family such as Rhabdophis.[32]
In South Asia, it is advisable to take the snake which has bitten a person, if it has been killed, and carry it along to the hospital for possible identification by medical staff using scale diagrams so that an informed decision can be taken them as to whether and which anti-venom is to be administered. However, attempts to catch it or kill the venomous snake are not advised as the snake may bite more people.[36]
Cultural significance
[edit]
Snakes 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 tessellated mosaic patterns which show great similarity to snake-scale patterns.[37]
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.[37] The use of snakeskin has however endangered snake populations[38] and resulted in international restrictions in trade of certain snake species and populations in the form of CITES provisions.[39] Animal lovers in many countries now promote the use of artificial snakeskin instead, which are easily produced from embossed leather, patterned fabric, plastics and other materials.[37]
Snake scales occur as a motif regularly in computer action games.[40][41][42][43] A snake scale was portrayed as a clue in the 1982 film Blade Runner.[44] 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.[45]
See also
[edit]Notes
[edit]- ^ from Latin mentum 'chin', not mens 'mind'
References
[edit]- ^ Boulenger, George A. 1890 The Fauna of British India. p. 1
- ^ a b The Snakes of Indiana Archived 2012-04-19 at the Wayback Machine at The Centre for Reptile and Amphibian Conservation and Management, Indiana. Retrieved 14 August 2006.
- ^ Barnes, Thomas G. Snakes: Information for Kentucky Homeowners. University of Kentucky.
- ^ Hartline, PH (1971). "Physiological basis for detection of sound and vibration in snakes". The Journal of Experimental Biology. 54 (2): 349–71. Bibcode:1971JExpB..54..349H. doi:10.1242/jeb.54.2.349. PMID 5553415.
- ^ Cheng Chang; Ping Wu; Ruth E. Baker; Philip K. Maini; Lorenzo Alibardi; Cheng-Ming Chuong (21 May 2010). "Reptile scale paradigm: Evo-Devo, pattern formation and regeneration". International Journal of Developmental Biology. 53 (5–6): 813–826. doi:10.1387/ijdb.072556cc. PMC 2874329. PMID 19557687.
- ^ "integument". Encyclopædia Britannica Online. 2014. Retrieved 23 September 2014.
- ^ Alibardi, Lorenzo (2005). "Differentiation of snake epidermis, with emphasis on the shedding layer". Journal of Morphology. 264 (2): 178–90. doi:10.1002/jmor.10326. PMID 15761820. S2CID 7506873.
- ^ a b c d e f g Greene, Harry W. (2004) Snakes – The Evolution of Mystery in Nature. University of California Press, pp. 22–23 ISBN 0520224876.
- ^ a b c d e f g h Are snakes slimy? at Singapore Zoological Garden's Docent. Retrieved 14 August 2006.
- ^ Herpetology FAQ at San Diego Museum of Natural History. Retrieved 14 August 2006.
- ^ Boulenger, George A. The Fauna of British India... page 234
- ^ a b c d e Smith, Vol III, p. 6
- ^ a b c Reptiles – Snake facts. Columbus Zoo & Aquarium. Retrieved on 2013-01-21.
- ^ Young Snake Rattles! Ask a scientist! (Zoology archive). Newton BBS, Argonne National Laboratory. Newton.dep.anl.gov. Retrieved on 2013-01-21.
- ^ Rhoades, Dusty. Spring rattles in! Desert USA website.
- ^ Smith, Vol I, p. 30
- ^ ZooPax Scales Part 3. Whozoo.org. Retrieved on 2013-01-21.
- ^ a b General Snake Information. Division of Wildlife, South Dakota
- ^ Baker, Barry W (2006). "Forensic implications of dorsal row counts on Puff-faced Water-snakes (Colubridae: Homalopsinae: Homalopsis buccata)" (PDF). Herpetological Review. 37 (2): 171–173.
- ^ a b Smith, Vol III, p. 5
- ^ Smith, Vol III, p. 7
- ^ a b c d e f g h Identifying snakes by scalation and other details. Wildsideholidays
- ^ Evolution of snakes. Arachnophiliac.co.uk (2007-02-12). Retrieved on 2013-01-21.
- ^ Königlich Preussische Akademie der Wissenschaften zu Berlin; Berlin, Königlich Preussische Akademie der Wissenschaften zu (1866). Monatsberichte der Königlichen Preussische Akademie des Wissenschaften zu Berlin. Vol. 1866. Berlin: Königliche Akademie der Wissenschaften.
- ^ Hallowell, E. (1854). "Remarks on the geographical distribution of reptiles, with descriptions of several species supposed to be new, and corrections of former papers". Proc. Acad. Nat. Sci. Phila. 1854: 98–105.
- ^ Smith, M.A. (1943). The Fauna of British India, Ceylon and Burma. London: Taylor and Francis. pp. 583 pp.
- ^ Smith, Vol III, p. 29
- ^ Daniels, J.C. (2002). Book of Indian Reptiles and Amphibians. BNHS. Oxford University Press. Mumbai, pp. 116–118 ISBN 0195660994.
- ^ a b How To Identify Snakes. kentuckysnakes.org.
- ^ North Carolina State Wildlife Damage Notes – Snakes Archived 2015-01-15 at the Wayback Machine. Ces.ncsu.edu. Retrieved on 2013-01-21.
- ^ Pennsylvania State University – Wildlife Damage Control 15 (pdf) Archived 2009-01-16 at the Wayback Machine. (PDF) . Retrieved on 2013-01-21.
- ^ a b c d Leviton AE, Wogan GOU, Koo MS, Zug GR, Lucas RS, Vindum JV (2003). "The Dangerously Venomous Snakes of Myanmar, Illustrated Checklist with Keys" (PDF). Proc. Calif. Acad. Sci. 54 (24): 407–462.
- ^ Resources Inventory Branch, Ministry of Environment, Lands and Parks Resources Inventory Branch for the Terrestrial Ecosystems Task Force Resources Inventory Committee . (1998). Inventory Methods for Snakes Standards for Components of British Columbia's Biodiversity No. 38 Archived 2012-03-03 at the Wayback Machine.
- ^ a b Thorpe, Roger S.; Thorpe, R. S.;Wüster, Wolfgang & Malhotra, Anita (1997). Venomous snakes: ecology, evolution, and snakebite. Vol. 70 of Symposia of the Zoological Society of London. Oxford University Press, London. ISBN 0-19-854986-5.
- ^ Berger, Cynthia. (2007). Venomous Snakes. Stackpole books. ISBN 0-8117-3412-9.
- ^ Directorate General Armed Forces Medical Services, India. Memorandum No 102 : Snakebite. Undated.pdf available [ online]. Accessed on 21 Feb 2010.
- ^ a b c Voland, Eckart and Grammer, Karl (2003) Evolutionary Aesthetics, Springer, pp. 108–116 ISBN 3-540-43670-7.
- ^ The Endangered Species Handbook – Trade (chapter) Reptile Trade – Snakes and Lizards (section) Archived 2006-03-06 at the Wayback Machine – accessed on 15 August 2006
- ^ Species in Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) – accessed on 14 August 2006
- ^ Gabriel Knight – Sins of the Father. Gameboomers.com. Retrieved on 2013-01-21.
- ^ Snake Rattle 'n Roll. consoleclassix.com. Retrieved on 2013-01-21.
- ^ Allahkazam's Magical Realm. Everquest.allakhazam.com. Retrieved on 2013-01-21.
- ^ Monsters/Pets : Reptile. Legend of Mana. qrayg.com
- ^ Encyclopaedia. Brmovie.com. Retrieved on 2013-01-21.
- ^ Quynh-Nhu, Daphne (April 2006). Jade Green and Jade White. teenink.com. Retrieved on 2013-01-21.
Bibliography
[edit]- 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.
Further reading
[edit]- Boulenger, George A., (1890), The Fauna of British India including Ceylon and Burma, Reptilia and Batrachia. Taylor and Francis, London.
- Leviton A. E., Wogan G. O. U., Koo M. S., Zug G. R., Lucas R.S., Vindum J. V. (2003) The Dangerously Venomous Snakes of Myanmar, Illustrated Checklist with Keys. Proc. Calif. Acad. Sci. 54 (24):407–462. PDF at Smithsonian National Museum of Natural History, Division of Amphibians and Reptiles.
- Mallow D., Ludwig D., Nilson G. (2003). True Vipers: Natural History and Toxinology of Old World Vipers. Krieger Publishing Company, Malabar, Florida. 359 pp. ISBN 0-89464-877-2.
- Gray, Brian S. (2005) The Serpent's Cast: A Guide to the Identification of shed skins from snakes of the Northeast and Mid-Atlantic States. The Center for North American Herpetology Monograph Series no. 1.Serpent's Tale Natural History Book Distributors, Lanesboro, Minnesota.
External links
[edit]
- Are snakes slimy – Singapore Zoological Garden's Docent site
- Microscopic structure of smooth and keeled scales in snakes
- General Snake Information – Division of Wildlife, South Dakota
- Reptiles – Snake facts. Columbus Zoo & Aquarium.
- North Carolina State Wildlife Damage Notes – Snakes
- Pennsylvania State University – Wildlife Damage Control 15 (pdf) Archived 2009-01-16 at the Wayback Machine
- ZooPax Scales Part 3
- Species in Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) – accessed on 14 August 2006.
- The Endangered Species Handbook – Trade (chapter) Reptile Trade – Snakes and Lizards (section) – accessed on 15 August 2006.
Snake scale
View on GrokipediaMorphology
Surface and Microstructure
The outer surface of snake scales is composed primarily of β-keratins, which form a hard, protective corneous layer, interspersed with α-keratins that contribute to the structural integrity of the epidermis.[4][5] This keratin-based surface exhibits species-specific micro-ornamentation, including microridges, denticles, spinules, and small pits, which are visible under scanning electron microscopy (SEM).[6][7] These microstructures often form comb-like patterns on dorsal scales, enhancing frictional anisotropy and reducing light reflection in some taxa.[8][9] At the microstructural level, the epidermis of snake scales comprises stratified layers: an outer β-layer of hardened keratin followed by an inner α-layer, with the oberhautchen (a thin, spinous precursor layer) contributing to the scale's texture during development and shedding.[10][11] Ventral scales, adapted for locomotion, display gradients in these features, such as aligned denticles that vary in density and orientation, while dorsal scales may feature nanoscale gratings up to 353 species surveyed, promoting hydrophobic properties or camouflage through ultra-black appearances.[12][9] Variations in these elements, including cysteine-glycine-proline-rich β-proteins, correlate with mechanical properties like hardness and wear resistance, differing between arboreal, terrestrial, and burrowing species.[13][14]Shape and Types
Snake scales on the body primarily consist of imbricate, overlapping plates formed from beta-keratin layers, with an asymmetric structure featuring a rigid outer surface and flexible posterior hinge region to accommodate body flexion.[2] These scales vary in shape from rhomboidal to rectangular, arranged in longitudinal rows typically numbering 13 to 21 in most species, though ranging from 10 in some boas to over 30 in certain blindsnakes.[15] Surface texture defines major types: smooth scales lack ridges, yielding a glossy finish that reduces drag in arboreal or swift terrestrial species such as colubrids like the California kingsnake (Lampropeltis californiae).[11] Keeled scales possess a central longitudinal ridge extending along the midline, imparting a rougher texture; this form predominates in viperids and many natricine colubrids, potentially enhancing frictional grip on substrates during lateral undulation.[16] Granular scales, small and rounded without pronounced overlap, characterize specialized lineages like file snakes (Acrochordus spp.), where the loose, velvety integument suits aquatic or semi-fossorial habits by maximizing flexibility over rigidity.[15] Ventral scutes differ markedly, forming broad, transverse rectangles that broaden mid-body before narrowing toward the tail, enabling purchase against ground surfaces for propulsion; these lack keels but may exhibit micro-denticulations for adhesion.[11] Subcaudal scales, narrower and often paired or single-rowed, vary by taxon—single in viperids, divided in many colubrids—reflecting phylogenetic divergence rather than ecological adaptation alone.[17] Apical pits, shallow depressions at the scale apex, occur in select viperid and atractaspidid species, possibly serving mechanosensory functions though their precise role remains debated.[18]Coloration and Patterns
Snake scale coloration arises primarily from pigments housed in chromatophores within the dermis beneath the translucent keratinous scales, allowing colors to be visible through the overlay, while structural features on the scale surfaces contribute iridescence and light manipulation.[19] Melanin in melanophores produces black, brown, and gray hues, carotenoids in xanthophores yield yellows and oranges, and pteridines in other cells enable reds and additional tones; these pigments are distributed unevenly to form patterns.[20] Structural coloration, independent of pigments, occurs via nanostructures such as nanoridges and leaf-like microstructures on scales that scatter or absorb light, as seen in the intensely black scales of Bitis rhinoceros, where hierarchical patterns enhance broadband light absorption for camouflage in low-light environments.[19] Patterns on snake scales emerge from developmental processes governing chromatophore migration and differentiation, often modeled by reaction-diffusion mechanisms or cell-chemotaxis, resulting in longitudinal stripes, transverse bands, dorsal blotches, or speckling that align with scale rows.[21] Genetic factors play a key role; for instance, in corn snakes (Pantherophis guttatus), a single gene influences the arrangement and localization of chromatophores to generate diverse patterns including stripes, blotches, and saddles, as identified in a 2025 study.[22] The CLCN2 gene has been implicated in patterning across snake species, affecting pigment cell distribution to produce adaptive motifs.[23] Evolutionary pressures shape these traits, with darker melanistic patterns linked to thermal advantages in cooler climates via increased solar absorption, as evidenced in North American ratsnakes where dorsal coloration correlates with latitude and temperature.[24] Ultraviolet reflectance, detectable via iridophores, appears widespread in Western Hemisphere snakes and supports predator deterrence over sexual signaling.[25] Examples include the alternating yellow-and-black bands of the banded krait (Bungarus fasciatus), formed by scale pigmentation that creates warning coloration, and iridescent sheens in boas like the Peruvian rainbow boa (Epicrates cenchria), derived from light-interfering nanostructures rather than pigments alone.[26]Functions
Protection and Defense
Snake scales provide mechanical protection through their keratinized structure, which forms a durable barrier against abrasion, punctures, and environmental hazards. Composed mainly of beta-keratin, these scales possess high hardness and elastic modulus, enabling them to withstand compressive and tensile forces encountered during predator encounters or terrain navigation.[4] The imbricate overlapping of scales distributes localized stresses, preventing deep penetration by distributing force over a broader area and maintaining body integrity during defensive coiling or evasion maneuvers.[3] This arrangement, combined with the scales' layered microstructure featuring an outer stratum corneum, confers toughness comparable to other keratin-based armors in reptiles.[1] In defensive contexts, scales contribute to passive armor by resisting tearing and scraping from claws or teeth, as evidenced by their role in shielding vital organs during constriction by predators like birds of prey or mammals. Species-specific adaptations, such as keeled dorsal scales in many colubrids and vipers, add ridges that increase rigidity and deflect glancing blows, enhancing overall resilience without sacrificing flexibility.[27] Nanomechanical analyses indicate that scale surfaces exhibit hydrophobic properties and variable hardness gradients, with harder exteriors optimized for impact absorption.[28] While scales alone do not deter aggression—snakes often rely on complementary behaviors like hissing or striking—their material properties empirically reduce injury severity, as observed in field studies of wild snake survivorship post-attack.[29] Ventral scales, though specialized for locomotion, indirectly support defense by enabling rapid undulatory movement to escape threats, with their broader, overlapping design providing underbody protection against substrate hazards or inverted attacks. In burrowing or aquatic species, scales may thicken or smoothen to counter specific abrasions, reflecting evolutionary tuning for habitat-specific threats. Limitations exist, as scales can fracture under extreme force, but periodic ecdysis renews compromised layers, sustaining long-term protective efficacy.[3][30]Locomotion and Friction
Snake ventral scales generate directional friction essential for locomotion, exhibiting pronounced frictional anisotropy that minimizes sliding resistance when moving forward while maximizing grip during backward propulsion.[31][32] This property arises from the scales' macrostructure, which features backward-oriented ridges or denticle-like projections resembling those on shark skin, enabling a preferred direction of motion akin to specialized surfaces on skis or treads.[33][34] Ventral scales display the highest anisotropy at approximately 26%, far exceeding the 4–5% observed in lateral or dorsal scales, allowing snakes to redistribute weight and exploit substrate friction for efficient slithering across varied terrains.[32][35] In modes such as lateral undulation and sidewinding, the anisotropic friction of ventral scales facilitates thrust generation by anchoring posterior body segments against the substrate while permitting anterior segments to advance with reduced drag.[31][36] Microstructural features, including micron-sized fibrils with tail-oriented nanoscale steps, heighten this effect by increasing posterior-directed resistance through mechanical interlocking with rough surfaces.[34][37] Snakes actively modulate friction by actuating scales—erecting, extending, or sliding them into contact—which can double frictional forces and prevent slippage, as observed in species like the corn snake during uphill or concertina movement.[35][38][39] This dynamic control, driven by muscular action beneath the scales, contrasts with passive friction in other animals and enables adaptation to inclines, where scales are oriented to "dig" into the substrate.[38] Rectilinear locomotion relies less on lateral friction but still benefits from ventral scale traction for straight-line advancement, with scales providing uniform posterior grip to support slow, deliberate progression.[36] Empirical measurements confirm that this scale-mediated friction scales with body weight redistribution, allowing snakes to traverse smooth or granular substrates without legs by converting muscular waves into directed force.[33][40] Variations in scale microstructure along the body, such as step height gradients, further tune anisotropy regionally, optimizing performance across different locomotion demands.[34]Sensory and Thermoregulatory Roles
Snake scales incorporate specialized sensory structures, primarily scale sensilla, which are small, dome-shaped mechanoreceptors embedded in the scale surface. These sensilla detect tactile stimuli, vibrations, and mechanical deformations, enabling snakes to sense prey movements, substrate vibrations, and environmental textures during locomotion and foraging.[41] In terrestrial species, such as elapids, sensilla density can reach thousands per snake, functioning as distributed tactile sensors across the body to facilitate prey localization and obstacle avoidance.[41] Aquatic elapids, like sea kraits and sea snakes, exhibit evolved scale sensilla adapted for hydrodynamic sensing, where they respond to water flow and pressure changes, enhancing prey detection in low-visibility marine environments.[42] Vibration-sensitive nerve endings beneath scales further amplify this capability, allowing snakes to perceive distant prey vibrations through ground or water conduction, independent of visual or olfactory cues.[43] Thermoregulation via scales primarily involves pigmentation and surface properties rather than insulation, as reptilian integument prioritizes permeability for rapid heat exchange over retention. Darker, melanistic dorsal scales absorb solar radiation more efficiently than lighter ones, elevating body temperature in cooler or shaded habitats—a pattern observed across snake species where pigmentation correlates with latitude and climate, with higher melanin content in populations from temperate zones.[44] For instance, melanic morphs of garter snakes (Thamnophis sirtalis) maintain higher body temperatures under insolation compared to striped morphs, demonstrating a direct thermal benefit from scale color.[45] Scale microstructure, including keeling or smoothness, influences emissivity and convective heat loss minimally, but the keratinized barrier reduces evaporative water loss, indirectly supporting thermoregulatory stability by preventing desiccation during basking or exposure to fluctuating ambient temperatures.[3] Behavioral adjustments, such as body flattening to maximize exposed scale surface area, exploit these properties for passive solar heating, with snakes achieving preferred body temperatures of 28–32°C in many species through such integument-mediated exchange.[46]Ecdysis and Scale Renewal
Process of Shedding
The process of ecdysis in snakes entails the periodic detachment and sloughing of the outer epidermal generation as a single coherent sheet, distinguishing it from the fragmentary shedding observed in lizards and geckos. The epidermis comprises stratified keratinized layers—oberhautchen, beta, mesos, alpha, lacunar, and clear—that collectively form a protective barrier; during renewal, keratinocytes proliferate in the basal layer and differentiate suprabasally to generate a new inner epidermal generation beneath the existing outer one.[47] Separation initiates via enzymatic and lymphatic mechanisms that loosen the interface between generations, often preceded by 4–7 days of behavioral quiescence, skin dulling, and ocular opacity from sub-spectacular fluid accumulation, which impairs vision and prompts retreat to sheltered microhabitats.[48][49] The shedding phase commences 4–7 days after ocular clearing, with the snake rubbing its rostral region against abrasive substrates to fissure the old skin along the labial margins of the mouth. Propulsion through the split proceeds caudally as the snake advances, inverting the discarded integument into a continuous, tubular cast that mirrors the body contour and includes eye caps from the spectacles.[48] This synchrony ensures uniform renewal across the body, facilitated by hormonal regulation involving thyroid influences that modulate timing, though excessive thyroid activity can inhibit the cycle. Post-ecdysis, the refreshed skin exhibits heightened luster and permeability resistance, restoring sensory and barrier functions essential for terrestrial adaptation.[49]Frequency and Factors Influencing Renewal
Juvenile snakes typically shed their skin more frequently than adults due to higher growth rates, with shedding occurring every 2–4 weeks in rapidly growing individuals, while adults may shed only 3–6 times annually or less in temperate species.[50][51] In free-ranging timber rattlesnakes (Crotalus horridus), ecdysis happens 1–2 times per year, often aligning with seasonal activity peaks rather than strict growth cycles.[52] Overall shedding frequency across snake species ranges from 4–12 cycles per year, modulated by intrinsic and extrinsic variables.[51] The primary driver of renewal frequency is the snake's growth rate, which correlates directly with food intake, nutritional quality, and metabolic demands; faster metabolism from ample prey availability accelerates epidermal turnover to accommodate bodily expansion.[53][48] Age exerts a causal influence, as juveniles prioritize somatic growth over reproduction or maintenance, leading to more frequent ecdysis that diminishes post-maturity when linear growth slows.[50] Species-specific traits, such as viviparity or habitat, further differentiate patterns—for instance, some oviparous snakes exhibit synchronized post-hatching sheds within hours to days, timed to embryonic development cues.[50] Environmental conditions, particularly temperature and humidity, regulate shedding tempo through their effects on metabolic rate and skin hydration; suboptimal humidity delays ecdysis by impeding stratum corneum loosening, while elevated temperatures can shorten inter-shed intervals by boosting physiological activity.[50] Health status influences renewal, with pathologies like hyperthyroidism increasing frequency to every 10–14 days via hormonal overstimulation, whereas malnutrition or parasitism prolongs cycles by curtailing growth.[54] These factors interact causally: for example, seasonal food scarcity in wild populations reduces metabolic drive, extending shed intervals beyond what age alone predicts.[48] Empirical studies confirm that while growth remains the dominant correlate, exogenous variables like photoperiod and stress can impose variability, underscoring ecdysis as an adaptive response rather than a fixed ontogenetic schedule.[52][50]Arrangement on the Body
Head Scales
Head scales in snakes consist of enlarged, plate-like epidermal structures that cover the cranium, providing rigid protection to underlying bones and sensory organs without the imbrication seen in body scales. These scales are typically smooth, symmetrical, and immobile relative to each other, formed from keratinized folds of the epidermis and dermis, and exhibit low variation within species but diagnostic differences across taxa for herpetological identification.[3] In most alethinophidian snakes, head scales are large and distinct, contrasting with the smaller, undifferentiated or granular scales in scolecophidian blindsnakes, where burrowing adaptations favor reduced prominence.[55] Dorsal head scales include the rostral scale at the snout's anterior tip, which is usually rounded or wedge-shaped and contacts the upper labials; paired nasals enclosing the nostrils; internasals separating the nasals medially; prefrontals posterior to the internasals; a single or paired frontal centrally; supraoculars overlying the eyes; and posterior parietals bordering the neck scales.[55] Lateral scales encompass preoculars and postoculars framing the eye, loreals (when present) between the nasal and preocular, and a series of temporals behind the postoculars. Ventral head scales feature the mental scale at the lower jaw's tip, infralabials along the lower lip, and paired chinshields (genials) posterior to the mental.[55] These arrangements vary by family—for instance, viperids often possess a loreal pit between the loreal scale and nasal, while colubrids may show fused or fragmented plates.[56] The structural integrity of head scales derives from their direct epidermal origin, lacking the dermal bony cores (osteoderms) common in lizards or crocodilians, though a 2023 histological analysis identified rare dermal armor in the head of a colubroid snake, suggesting sporadic reinforcement in select lineages.[57] [58] Functionally, these scales shield the braincase during locomotion and predation, accommodate openings for nostrils, eyes, and mouth without impeding jaw kinesis, and in pitvipers, integrate heat-sensing loreal pits for infrared detection.[3] Scale counts and shapes, such as the number of upper labials (typically 6-9 per side), serve as reliable taxonomic characters, with asymmetries or fusions noted in about 5-10% of specimens due to developmental anomalies.[55]Body Scales
The body scales of snakes, encompassing both dorsal and ventral scutes, form a flexible, imbricated covering along the trunk that excludes the head and tail regions. Dorsal scales encircle the body in longitudinal rows, typically numbering 13 to 25 at midbody depending on the species and family, with arrangements often expressed as a formula indicating row counts at the neck, midbody, and vent, such as 15-17-15 in many colubrids.[59][60][61] Adjacent dorsal rows are offset diagonally, and the total row count excludes the ventral scale row, providing a diagnostic trait for species identification in herpetology.[62][55] Dorsal scales are generally smaller and more numerous than ventrals, arranged in an odd number of rows that may reduce toward the head and cloaca, with the vertebral row along the midline sometimes enlarged or distinctly keeled.[63] These scales vary in texture: smooth scales yield a glossy appearance, while keeled scales feature a central ridge enhancing roughness for camouflage or traction.[18] In viperids, keeling is often pronounced across multiple rows, contributing to a matte dorsal surface, whereas many colubrids exhibit smooth or weakly keeled scales.[64][65] Ventral scales form a single median row of larger, rectangular or hexagonal plates along the underside, each overlapping the subsequent scale posteriorly to facilitate undulating locomotion.[66][67] These scutes are broader than dorsal scales, smooth-surfaced, and staggered relative to adjacent laterals, with their count also serving taxonomic purposes across Serpentes.[59] In advanced snakes (Alethinophidia), laterals between dorsals and ventrals may form distinct rows, but the ventral row remains unpaired and prominent.[66]Tail Scales
The tail of snakes is covered dorsally by scales that generally continue the arrangement of the body scales, forming longitudinal rows that overlap posteriorly in an imbricate pattern. These dorsal caudal scales typically maintain the mid-body row count—often 13 to 21 rows depending on the taxon—but may reduce in number toward the tail tip, eventually converging into fewer rows or a single apical scale series. Ventral to these, the subcaudal scales form the primary feature of tail scalation, consisting of enlarged, transversely elongated plates posterior to the cloaca and anal scute.[56][68] Subcaudal scales exhibit significant variation in configuration across Serpentes. They occur either as a single undivided row, where each tail segment bears one broad scale, or as divided (paired) rows, with two narrower scales per segment aligned side-by-side. The divided condition predominates in many colubroids and viperids, while undivided subcaudals characterize most boas and pythons; classification relies on the majority configuration in the proximal two-thirds of the tail, as distal portions often revert to a single row regardless of proximal pattern. The anal plate immediately preceding the subcaudals may itself be single or divided, correlating loosely with subcaudal arrangement but not universally.[69][56] Numerically, subcaudal counts range widely, from fewer than 10 in short-tailed species to over 150 in elongate forms, with database records spanning 0 to 163 across taxa. Sexual dimorphism is pronounced, particularly in Colubridae, where males possess higher subcaudal counts—often 10-20% more than females—reflecting relatively longer tails adapted for copulatory grasping via hemipenes. For instance, in the lancehead pitviper Bothrops asper, males average more subcaudals than females, with dorsal scale rows varying from 21 to 29. Such meristic traits aid in species identification and phylogenetic analysis but show clinal variation influenced by geography and body size.[68][70][71]Specialized Scales
Rattles and Vibratory Structures
The rattle of rattlesnakes consists of specialized keratinous scales modified into interlocking segments at the tail terminus, unique to genera Crotalus and Sistrurus within the Viperidae family.[72] Each segment forms from the distal portion of shed epidermal scales that are retained and hollowed out during ecdysis, resulting in a multilobed structure that interlocks with adjacent segments via a button-like proximal end and a concave distal cup.[73] Neonatal rattlesnakes possess a single pre-button, with one new segment added per shed cycle, though segments can fracture or be lost, rendering rattle length an imprecise age indicator.[74] Sound production occurs through rapid tail vibration by caudal muscles, causing sequential segment collisions that generate broadband frequencies typically between 500 and 3000 Hz, with no internal loose material akin to a maraca.[75] Empirical studies confirm the rattle's primary function as an aposematic deterrent against predators, eliciting avoidance in species like domestic dogs and sympatric vertebrates, with response intensity correlating to phylogenetic proximity to rattlesnakes.[76] [77] Tail vibration alone, without a rattle, produces substrate-borne buzzes in many non-rattlesnake viperids and colubrids as a defensive mimicry of rattling, suggesting this behavior predates and facilitated rattle evolution.[78] For instance, ground squirrels and burrowing owls respond to rattlesnake rattles with anti-predator behaviors, and similar vibrational cues from vibrating tails in other snakes exploit these learned associations.[79] Evolutionarily, the rattle arose once in crotaline pit vipers through modifications to ancestral tail-shaking, involving two transitions from small, few-segmented precursors to the multi-segmented form observed today, conserved across species despite variations in segment count.[72] This structure enhances vibrational signaling efficiency over bare tail shaking, as rattling produces louder, more sustained airborne and substrate-transmitted sounds without requiring contact with foliage or ground.[78] No other dedicated vibratory organs exist in snakes; defensive tail vibration relies on standard scales rasping against substrates, with efficacy tied to habitat acoustics and predator familiarity rather than morphological specialization.[80]Heat-Sensing Pits and Other Adaptations
Heat-sensing pits, also known as loreal pits or facial pits, are specialized sensory structures present in certain snake taxa, primarily the pit vipers (subfamily Crotalinae within Viperidae) and secondarily in some boas (Boidae) and pythons (Pythonidae). These pits enable the detection of infrared radiation emitted by warm-blooded prey or environmental heat sources, functioning as a form of thermal vision independent of visible light. In pit vipers, each pit is located in a deep depression between the eye and nostril, formed by modifications to the surrounding facial scales, such as the loreal scale, and consists of an outer chamber leading to an inner chamber separated by a thin, suspended membrane approximately 1 mm thick.[81][82] This membrane is richly vascularized and innervated by branches of the trigeminal nerve, with free nerve endings and specialized receptor cells that respond to temperature gradients as small as 0.001–0.003°C across distances of up to 1 meter.[83][81] The mechanism relies on transient receptor potential (TRP) ion channels, particularly TRPA1 variants tuned for warmth detection in the infrared spectrum (wavelengths of 5–30 μm), allowing snakes to construct a thermal map of their surroundings for prey localization, strike guidance, and predator avoidance, especially in nocturnal or low-visibility conditions.[81] In Crotalinae, the pits provide bifocal imaging by integrating with visual input via the optic tectum, enhancing hunting efficiency against endothermic prey like mammals and birds, with behavioral studies showing strikes accurate to within millimeters based on thermal cues alone.[84] Boas and pythons possess less advanced labial pits—smaller depressions on the upper labial scales beneath the snout—offering comparable but lower-resolution infrared sensitivity, likely evolved convergently for similar ecological roles in ambush predation.[85] These structures demonstrate a neural integration where thermal signals project to the lateral descending trigeminal nucleus, distinct from visual pathways, underscoring their role as a dedicated sensory modality rather than an extension of scale thermoregulation.[82] Beyond heat-sensing pits, snake scales exhibit other adaptations that enhance sensory or thermoregulatory functions, often through microstructural modifications. Keeled dorsal scales, featuring longitudinal ridges, increase surface roughness to aid traction during locomotion over varied substrates, indirectly supporting thermoregulatory behaviors like precise basking positioning, as observed in species with keeled scales absorbing solar radiation more effectively due to altered reflectance.[3] Ventral scales in many colubrids and viperids incorporate micro-ornamentation, such as denticles or asperities, which reduce frictional drag during forward gliding while facilitating grip for backward resistance, optimizing energy use in environments where thermal gradients influence activity patterns.[3] Some burrowing species, like those in the Typhlopidae, have highly specialized smooth, polished scales with minimal keeling to minimize soil resistance, coupled with scale-embedded mechanoreceptors for vibration detection that complements thermal cues in locating prey underground.[3] These adaptations, while not as specialized as pits, reflect evolutionary pressures for integrating scale morphology with sensory ecology, though functions like tubercular pits on certain scales remain partially unresolved pending further empirical dissection.[3]Evolutionary Origins
Fossil Evidence and Ancestral Transitions
The fossil record of snake integument is sparse, as epidermal scales rarely preserve due to their thin, keratinous composition and the taphonomic biases favoring harder skeletal elements. Direct evidence of scales in early snake fossils is limited to impressions or traces in exceptional Lagerstätten, with no comprehensive series documenting microevolutionary changes in scale morphology. However, preserved skin from basal reptiles provides insight into the ancestral squamate condition, characterized by small, imbricating epidermal scales formed from beta-keratin placodes, a structure conserved in modern snakes.[2] The oldest known fossilized reptile skin, from the Early Permian Captorhinus magnicornis dated to approximately 289 million years ago, reveals pebbly, non-overlapping scales resembling those in extant legless squamates such as snakes and amphisbaenians. This morphology, with uniform micro-ornamentation and lack of large scutes, aligns with the inferred ancestral integument for Squamata, predating snakes by over 100 million years and indicating that snake scales represent a retention rather than a novel derivation. Ancestral state reconstructions in squamate evolution support this continuity, with scale patterning governed by conserved developmental pathways involving Wnt signaling and placode formation, rather than radical innovations tied to limblessness.[86][87] Transitional fossils bridging lizard-like ancestors and modern snakes, such as Najash rionegrina from the Late Cretaceous (approximately 95 million years ago), preserve articulated skeletons with hind limbs and an elongated trunk but lack direct integumental preservation. These forms demonstrate the skeletal preconditions for snake-like scalation—expanded vertebral count and reduced limb girdles—without evidence of dermal armor or scale fusion beyond what's seen in anguimorph lizards, the probable sister group. Earlier Jurassic snake fossils, including cranial and postcranial remains from 167–143 million years ago, similarly imply epidermal scales analogous to those of basal lizards, adapted for burrowing or fossorial habits through increased flexibility and shedding cycles, though soft tissue is absent.[88][89] Empirical data from these fossils refute hypotheses of aquatic origins for snake scales, as terrestrial or semi-fossorial adaptations in transitional taxa align with imbricate, protective epidermal layers suited to abrasion resistance in elongated bodies. Rare osteoderm discoveries in select snake lineages, such as embedded dermal ossicles beneath scales in some fossils, represent secondary elaborations rather than ancestral traits, with parsimony favoring multiple independent origins or losses in Squamata. Overall, the evidence points to causal continuity: snake scales evolved via proportional scaling and row uniformity in response to axial elongation, without requiring de novo integumental transitions beyond those in legged squamates.[57]Competing Hypotheses and Empirical Debates
The evolutionary origins of snake scales remain linked to unresolved questions about the ecological transitions in early snake ancestry, with implications for scale morphology as an adaptation to locomotion and habitat. Three competing hypotheses dominate discussions on proto-snake ecology: fossorial (burrowing), aquatic (marine), and terrestrial (surface-dwelling). Under the fossorial model, ancestral snake scales would have evolved reduced friction surfaces—such as smooth, overlapping dorsal plates and robust ventral scutes—to facilitate soil penetration and undulatory propulsion without limbs, aligning with observations in extant burrowing taxa like scolecophidians where scales exhibit minimal keeling and enhanced ventral ridging for grip.[90] This view posits scale row reduction (from 20+ in lizards to 13–21 in most snakes) as a derived trait minimizing drag in subterranean environments, supported by phylogenetic mapping showing early ophidian fossils with streamlined integument impressions.[91] In contrast, the aquatic hypothesis, drawn from Cretaceous marine fossils like Pachyrhachis, suggests scales adapted for hydrodynamic efficiency, potentially with smoother contours or specialized ventral keels for paddling, though empirical critiques highlight that such forms represent peripheral dollochocephalic lineages rather than stem snakes, lacking broad genomic corroboration for integumentary convergence.[90] Proponents argue for scale microornamentation (e.g., denticle patterns) enhancing water flow, but fossil scale preservation rarely resolves this, and modern aquatic snakes like acrochordids retain generalized squamate scales without radical divergence. The terrestrial hypothesis invokes direct descent from scansorial or ground-foraging lizards (e.g., anguids), with keeled dorsal scales evolving for traction on uneven surfaces and ventral scutes broadening for static contact thrust, yet this faces challenges from disparity analyses indicating skull and body elongation preceded surface adaptations.[90] Empirical debates center on reconciling fossil, phylogenetic, and developmental data. Geometric morphometric studies of cranial evolution favor a fossorial-to-terrestrial shift around 100–128 million years ago, implying scales co-evolved with axial elongation via somitic positional cues that enforce hexagonal patterning, a mechanism conserved from squamate ancestors but accelerated in snakes for modular body scaling.[90][92] Genomic analyses reveal elevated evolutionary rates in keratin-associated genes and Hox clusters in snakes, supporting adaptive bursts in scale beta-keratin composition for flexibility and renewal, yet debates persist on whether ventral microdermatoglyphs (frictional ridges) represent plesiomorphic squamate traits or snake-specific innovations, as convergent patterns appear in limbless lizards like anguids.[93][94] Fossil integument from mid-Cretaceous forms like Dinilysia shows generalized squamate scales without unambiguous fossorial specializations, fueling arguments for multiple microevolutionary origins of scale traits rather than a singular ecological driver.[91] Ongoing phylogenetic supermatrix reconstructions increasingly resolve snakes within Toxicofera, suggesting scale homology to lizards but with clade-specific tuning via Wnt signaling for row reduction, though homology assessments remain tentative due to variable scale nomenclature across squamates.[95]Comparative Anatomy with Other Reptiles
Snake scales, composed primarily of β-keratin with an outer layer of α-keratin, exhibit imbricate overlapping structures on dorsal and lateral surfaces, enabling flexible undulation for limbless locomotion, in contrast to the more rigid, often osteoderm-reinforced scales of many lizards within Squamata.[2] Lizards typically feature smaller, granular or tubercular ventral scales without the broad, transversely elongated scutes of snakes, which provide traction during lateral undulation; additionally, numerous lizard species incorporate dermal osteoderms—bony plates embedded beneath the epidermis—for enhanced armor, a feature absent in snakes to preserve body flexibility.[1] [96] Shedding patterns further diverge: snakes undergo complete ecdysis, molting the entire integument as a single tubular sheet due to synchronized epidermal generation, whereas lizards shed in irregular fragments reflecting asynchronous scale renewal.[2] Turtle (Testudines) scutes differ markedly, forming thick, non-overlapping keratinous plates fused to underlying dermal bone in the carapace and plastron, prioritizing rigid protection over mobility; these lack the dynamic overlap and renewal seen in snake scales, with growth occurring via annular rings rather than periodic molts.[97] [2] In crocodilians, scales integrate with prominent osteoderms forming a dorsal armor of parakeratotic, pebbled surfaces adapted for aquatic ambush and defense, contrasting the smoother, friction-optimized microstructures of snake ventral scales that minimize drag during terrestrial gliding.[2] The tuatara (Sphenodontia), the sole surviving rhynchocephalian, possesses lizard-like granular scales with some osteoderm integration, bridging squamate flexibility and crocodilian rigidity but lacking the specialized ventral propulsion aids unique to snakes.[2] These integumentary variations reflect evolutionary pressures: snakes' scales emphasize elongation and anisotropy for efficient serpentine motion, derived from squamate ancestors, while other reptilian clades favor protective rigidity tied to quadrupedal or shelled lifestyles.[14]Nomenclature and Terminology
Standard Anatomical Terms
Standard anatomical terms for snake scales facilitate precise descriptions in herpetological taxonomy and identification keys, emphasizing positional and morphological features across the head, body, and tail. These terms, rooted in comparative anatomy, distinguish snakes from other reptiles and enable differentiation among species through scale counts and arrangements. For instance, head scales in most snakes are enlarged and symmetrical, contrasting with the smaller, granular scales in more primitive forms.[55] On the head, the rostral scale occupies the snout's anterior tip, contacting the upper labials and often the internasals; it is typically triangular or rounded.[55] [62] Nasal scales enclose the nostril, divided into prenasal (anterior) and postnasal (posterior) in many species.[62] The loreal scale, when present, lies between the postnasal and preocular. Preocular scales border the eye anteriorly, postoculars posteriorly, and supraoculars dorsally cover the eye. Frontal and prefrontal scales form the midline dorsal structure, with parietals at the posterior head. Temporal scales fill the lateral posterior region, while supralabials and infralabials line the upper and lower lips, respectively; infralabials exclude the mental scale at the chin's tip. Chin shields, usually in two pairs, separate the infralabial rows ventrally.[55] [62] Body scales include dorsal (or costal) scales, arranged in longitudinal rows encircling the trunk, excluding ventrals; row counts are taken at midbody, often in odd numbers like 15 or 17, with vertebral scales enlarged along the dorsal midline.[55] [62] Ventral scales, or gastrosteges, are enlarged, transversely elongated plates on the underside from neck to cloaca, counted only if wider than long.[55] Dorsal scales may be smooth or keeled (with a central ridge), influencing texture and locomotion.[55] Tail scales feature the anal plate covering the cloaca, either single or divided, followed by subcaudals on the ventral tail, counted unilaterally and noted as single or divided into paired rows.[55] [98] These terms underpin quantitative metrics, such as ventral scale counts ranging from 140–180 in many colubrids, essential for species delimitation.[55]Taxon-Specific Variations
In scolecophidian snakes, such as those in Typhlopidae and Leptotyphlopidae, dorsal scales are small, smooth, and arranged in 14 or more transverse rows around the body, lacking keels and often appearing uniform and annular, which facilitates burrowing in soil.[99] Head scalation features few enlarged shields, with variations in supralabial count (typically 2–6 per side) and reduced ocular scales covering vestigial eyes, differing markedly from alethinophidian taxa.[100] Among basal alethinophidians like Boidae and Pythonidae, dorsal scales are small, granular, and irregularly overlapping rather than forming neat transverse rows, with counts often exceeding 30–50 rows at midbody; ventral scales are narrow and elongated but not as broad as in advanced snakes.[101] These families exhibit minimal keeling, contrasting with the more structured imbrication in caenophidians, and tail scales include paired subcaudals that are undivided proximally. In Colubridae, the largest snake family, dorsal scales typically occur in 15–21 rows, either smooth (as in many natricines) or weakly keeled (e.g., in some ratsnakes like Pantherophis), with head shields forming large, symmetrical plates such as paired prefrontals, frontal, and parietals.[102] Elapidae show similar large head shields but greater body scale diversity, including smooth scales in hydrophiines or keeled in some Asian species, often with 15–23 dorsal rows.[99] Viperidae display distinctive small, irregular dorsal head scales numbering in dozens, lacking prominent shields beyond loreal and temporal regions, paired with strongly keeled body scales in 21–33 rows that enhance traction and camouflage.[103] Subcaudal scales in viperids are typically single (undivided), aiding in distinguishing them from elapids, which often have divided subcaudals.[60] These patterns reflect adaptations to ambush predation, with keeling providing textural grip on substrates.[28]Taxonomic Importance
Distinguishing Venomous from Non-Venomous Species
Members of the Viperidae family, including pit vipers and true vipers, typically exhibit a crown of the head covered by numerous small, irregularly arranged scales, differing markedly from the large, symmetrical shields—such as the frontal, prefrontal, and parietal plates—found in many Colubridae and Elapidae species. This scale fragmentation in viperids arises from evolutionary divergence and aids in distinguishing them from non-viperid snakes, though some viper species retain partial large shields surrounded by smaller scales.[104][105] Subcaudal scales provide another key differentiator: Viperidae possess a single row of undivided subcaudal scales distal to the cloaca, in contrast to the paired or divided rows typical of Colubridae. This pattern holds reliably for viperids across regions, including North America where it helps identify pit vipers like rattlesnakes from colubrids. However, exceptions occur, such as certain non-venomous colubrids (e.g., mud snakes or Texas long-nosed snakes) with undivided subcaudals, necessitating integration with other traits like head shape or pupil form for accurate identification.[106][60][107] The same subcaudal distinction applies to shed skins, allowing retrospective identification of viperid molts via a single ventral tail scale row versus the double row in colubrid sheds. Elapids, despite being venomous, generally feature divided subcaudals and large head shields akin to colubrids, rendering scale-based separation from non-venomous species less straightforward without fangs or banding patterns. Keeled dorsal scales, prevalent in many viperids, offer supplementary evidence but are not exclusive to venomous taxa.[60][108]| Scale Feature | Viperidae (Venomous) | Colubridae/Elapidae (Mostly Non-Venomous or Venomous Elapids) |
|---|---|---|
| Head Scales | Small, numerous, irregular | Large, symmetrical shields (e.g., frontal, parietals) |
| Subcaudal Scales | Single row, undivided | Paired/divided rows (exceptions in some colubrids) |