The elongated, limbless reptiles known as snakes have long been held in fear and awe by humans and have often appeared in art and mythology. Although some species are dangerously venomous, snakes are important in nature and to humans as predators of rodents. The estimated 2,500 to 3,000 species of living snakes are widely distributed throughout the world. They are most common in warm areas and reach their greatest abundance in the tropics, but the common Old World viper, or adder, Vipera berus, occurs above the Arctic Circle (68 deg north latitude) in Scandinavia and is also found in southern Siberia, and a pit viper called the snouted lance-head, Bothrops ammodytoides, is found almost to the southern tip of South America (50 deg south latitude). Snakes may live at greatly diverse elevations: the sidewinder, Crotalus cerastes, for example, occurs in valleys below sea level, whereas the Himalayan pit viper, Agkistrodon himalayanus, has been found at 4,800 m (16,000 ft). Snakes live in almost every habitat exclusive of the deep seas and regions of perpetual snow. Some dwell in burrows beneath the ground; others live most of their lives in trees. Snakes may be found both in humid forests and dry deserts, and some sea-inhabiting snakes are completely marine, never leaving the sea, even to reproduce. The smallest snakes are the slender blind snakes, family Leptotyphlopidae; the adult Texas slender blind snake, Leptotyphlops dulcis, for example, may be only 13 cm (5 in) long. According to verified reports, the world's largest snake is the Asiatic reticulated python, Python reticulatus, which has attained a recorded length of 10 m (33 ft); the South American anaconda, Eunectes murinus, may be even larger, having been described as exceeding 11 m (36 ft) in length, but these measurements have not been authenticated. PHYSICAL CHARACTERISTICS Snakes lack limbs, a sternum (breastbone), a shoulder girdle (skeletal elements for support of the front limbs), external ear openings, and a urinary bladder. Most snakes also lack a pelvic girdle (skeletal elements for support of the hind limbs), but blind snakes, such as those of the family Typhlopidae, may show traces of a pelvis (hipbone), and the boas and pythons, family Boidae, retain a vestigial pelvis and the remnants of the two hind limbs in the form of small, clawlike spurs. Snakes lack internal mechanisms for maintaining body heat and therefore have variable body temperatures (poikilothermy). Body temperatures may be controlled behaviorally, however, by moving in and out of the Sun's rays. The preferred range for activity for most snakes is between 25 deg and 32 deg C (77 deg to 90 deg F). Metabolism varies greatly with body temperature. Temperate-zone snakes are subjected to potentially fatal low winter temperatures. Thus such snakes are obliged to hibernate, finding shelter in deep underground burrows or crevices, where they remain inactive and their metabolism slows dramatically. Conversely, some snakes may estivate during excessively hot and dry periods. During estivation the snakes retire to deep, cool, subterranean recesses and live upon their stored resources of fat until more favorable times return. Skull The snake's skull is one of its most distinctive features. The skull is technically classified as diapsid, meaning it has two openings on each side behind the eye sockets; because of numerous evolutionary changes, however, these diagnostic openings are no longer evident. Partly as an adaptation to permit the swallowing of large food items whole, the snake's skull has been increasingly modified for greater elasticity. In many evolutionarily advanced snakes the only solid portion of the skull is the brain case, the upper skull consisting broadly of the brain case in the rear; small protective bones around the eyes and snout in front; and four rather long tooth-bearing bones below. The four tooth-bearing bones are the two maxillaries, which form the outer rim of the upper jaw, and the two palatopterygoids (joined palatine and pterygoid bones), which run along the roof of the mouth. All four tooth-bearing bones are loosely attached to the skull by elastic ligaments and are independently movable. The bones of the snout are also movable and allow the snake to bend its snout upward. Each side of the lower jaw bears a row of teeth on the forward-positioned dentary bone, and each half of the lower jaw is slightly movable along the line formed between the dentary bone and the rear segments of the jaw. Further, the two halves of the lower jaw are not rigidly fused where they meet in front; rather, they are united only by elastic ligaments that permit the two halves to be rotated or moved apart. Eyes Except for the blind snakes, families Typhlopidae, Anomalepidae, and Leptotyphlopidae, whose rudimentary eyes are situated beneath the head scales, a snake's eyes are permanently covered and protected by the transparent lower eyelid, called the spectacle or brille. The lower eyelid is fused to the vestigial upper eyelid, and snakes, therefore, cannot blink or "close" their eyes, which accounts for their so-called hypnotic stare. The lenses of a snake's eyes are rounded and are normally adjusted for distance vision. To focus on near objects, the snake moves the lenses forward, much like the focusing of a camera. Some diurnal snakes have yellow-tinted lenses to reduce the glare or chromatic aberration (unequal refraction) caused by the high amount of bluish (short-wavelength) light in daytime. Although the eyes are normally set for distance, a snake's distance vision is typically poor except for detecting movement. Hearing Snakes lack external ear openings, eardrums, and middle-ear cavities. The small, sound-conducting columella bone, however, is still present. In other reptiles the columella is positioned between the inner ear and the eardrum, but in snakes it is situated between the inner ear and the jaw hinge (quadrate bone). When the snake's head is on the ground, earthborne sounds or vibrations are picked up by the lower jaw and skull and are transmitted by bone conduction to the columella and through the columella to the inner ear. Snakes, therefore, can readily detect earthborne sounds. The lack of eardrums and middle-ear cavities implies that snakes cannot hear airborne sounds. Certain experiments, however, have shown that the snake's inner ear does respond to low-frequency airborne vibrations (sounds), particularly at about 300 Hz, which may reach the inner ear by bone conduction. Snakes also have specialized nerve endings in the skin that function as vibration detectors, and the same experiments indicated that these receptors were able to detect low-frequency airborne sounds at vibrations up to about 1,000 Hz. Jacobson's Organ The Jacobson's, or vomeronasal, organ is located in front of the roof of the mouth. It is a chemical receptor capable of detecting tiny chemical particles, including those given off in the body scent of animals. The snake's forked tongue is flicked out to allow the chemical particles in the air to adhere to or dissolve in the moisture on the tongue. The tongue is then brought back to the Jacobson's organ, where its forked tip is inserted into the organ's two openings and the chemical particles identified. Snakes cannot sting or cause any physical harm with their tongues. Heat Detection Snakes can detect heat, or infrared, rays through specialized groups of nerve endings scattered through the skin. These heat receptors are presumably involved in sun basking and other thermoregulatory behaviors. More specialized heat receptors are present as pits along the upper lip margins of many pythons and boas. In the \Lpit viper\ls, such as the rattlesnakes, the specialized heat receptors (here called pit organs) are highly developed and occur as a single pair at the front of the head. The paired frontal placement of the pit organs enables the rattlesnake not only to detect the presence of warm-bodied prey in the dark but also to strike at it accurately up to a distance of about 50 cm (20 in). Respiration In snakes the left lung is either reduced in size or completely lacking. Only the boas and pythons, family Boidae, and the sunbeam snake, family Xenopeltidae, have retained fully functional left lungs. The right lung is a greatly elongated structure, extending through much of the snake's body, and ends in a thin-walled sac that is believed to serve for the storage of air. Some snakes also have a tracheal, or windpipe, "lung." The supporting cartilaginous bands around the windpipe are incomplete rings, an opening being present dorsally, that is, toward the top of the body. The tissue between the open ends of the rings is ballooned out and is structurally modified into a lunglike tube that runs along the windpipe and that functions as a lung. Some sea snakes are able to extract oxygen from the water through the tissues (mucous membranes) lining the insides of the mouth, and several kinds of snakes--the sea snakes, family Hydrophiidae, and file snakes, family Acrochordidae--can use their skin for respiration when submerged. For a description of the three-chambered heart and the circulatory system of snakes, see \Treptile\t. Glands The most important glands that perform endocrine, or internal secretory, functions are the single thyroid, which plays an important role in molting; the parathyroid glands, which appear to be involved in the regulation of blood calcium and phosphate levels; an adrenal gland, which functions in body electrolyte (certain dissolved substances) content and water balance; the pancreas, which controls blood-sugar levels; the pituitary, which secretes hormones that regulate growth and reproductive functions and the activity of the thyroid and adrenal glands; and the gonads, or sex glands, whose hormones produce different bodily characteristics in males and females (sexual dimorphism). Skin and Scales The snake's skin is primarily a protective structure, guarding it against physical injury and the loss of body moisture (desiccation). The scales covering the skin are epidermal, that is, they are mainly derived from a folding of the epidermis, or the upper layer of the skin. Because the scales do not increase in size or because the outer layer becomes worn, or because of reasons not yet understood, a snake must periodically shed its old skin and replace it with a new and larger one. In molting, the old, upper layer of the epidermis becomes loosened and separated from a newer layer of the epidermis developing beneath it. The molting process, or ecdysis, is accomplished by the snake loosening the skin around the lips, pushing this back, and crawling out of the old skin, turning it inside out. Frequency of molting, or shedding, varies greatly. FEEDING All snakes are carnivorous. Many of the smaller burrowing forms feed on earthworms or insects. Some snakes have specialized food habits, preying exclusively on snails or eggs. Others feed on fish, amphibians, reptiles (including other snakes), birds, or small mammals or a combination of these. Snakes find food by sight or by smell (using the Jacobson's organ as an accessory) and by heat-sensitive pits, when present. A few species are active at any time of the day, but most species restrict their activities to either daylight or nighttime. The more slender and agile snakes actively stalk their prey; the heavier-bodied ones remain coiled in ambush. Prey capable of harming the snake are killed by constriction or envenomation before being swallowed. Constriction Some snakes simply suffocate their captured prey by squeezing them with their jaws, but a snake's long, muscular body is ideally suited for applying pressure on a prey animal. Constriction has been defined as the immobilization of prey by the exertion of pressure by two or more points of the snake's body. Constriction does not crush the prey animal but rather prevents it from breathing and suffocates it. In the case of very small animals with a high metabolic rate and the need of an uninterrupted supply of oxygen, death may be very quick. In larger, stronger animals, however, it may be quite slow. It is also relatively slow in cold-blooded prey, such as lizards. Such animals, however, often become torpid and are swallowed alive. Some snakes use one or two body loops to press their prey against the ground or against the inside of a burrow if below ground. Others wind or wrap around their prey. Envenomation Snakes have powerful digestive enzymes (specialized proteins) to break down the hair, feathers, bones, and other parts of their prey. As part of the digestive system, the salivary glands of the mouth also produce powerful enzymes. If saliva containing these enzymes enters the wounds of a prey animal, it not only begins the digestive process but also may cause such serious tissue damage that the prey dies. Such toxic elements have been found in the saliva of many nonpoisonous snakes, and they are generally introduced into the prey's wounds by repeated bitings by the snake. Obviously, a more toxic saliva (venom) and a more effective means of introducing it into a wound would be of great advantage. In the most primitive venomous snakes, the rearmost teeth, usually the last two or three on each side of the upper jaw, are enlarged into fangs. Each fang has an open groove on the front or side. Venom from the modified salivary glands above is discharged at the base of the teeth and conducted by the grooves into the wound. Such snakes are called opisthoglyphs ("rearward" and "carved") or, more commonly, back-fanged snakes. Although an evolutionary improvement, the rear-fang system is not efficient, and because their venom is generally not highly potent, back-fanged snakes are not considered dangerous. The boomslang, Dispholidus typus, and a few other opisthoglyph snakes, however, have caused human fatalities. A more efficient venom-delivery system involves the placement of the fangs in the front of the mouth. Some front-fanged snakes are termed proteroglyphs ("frontward" and "carved"), while others are classified as solenoglyphs ("tube" and "carved"). In proteroglyph snakes, such as the cobras, the poison-conducting groove in each of the two fangs is closed over for most of its length, forming a more efficient venom canal. Proteroglyph venom is also usually quite potent, and consequently most proteroglyph snakes are regarded as dangerous. In solenoglyph snakes, not only are each of the two poison-conducting fangs formed into a hollow, venom-conducting tube, but also each fang can be folded back along the roof of the mouth. This ability has allowed solenoglyph snakes, such as rattlesnakes, to develop very large fangs. Most solenoglyphs also have potent venom and are considered dangerous. Snake venom contains a variety of substances that cause tissue damage in different ways, including nerve destruction (neurotoxic) and blood-tissue destruction (hemotoxic). The neurotoxins paralyze the nervous system and cause heart and respiratory failure; the hemotoxins destroy blood vessels and blood corpuscles and cause internal hemorrhaging. The different substances are not uniformly present in all snake venom but vary with the species and even with individual snakes within a species. The venom of cobras, however, is generally neurotoxic, whereas that of most rattlesnakes is predominantly hemotoxic. Although modified for high toxicity, venom still retains digestive powers, and the presence of venom in a prey animal begins the digestive process even before the prey is swallowed. Venom in a prey animal may shorten the usual days-long digestive process of a snake by more than half. Less than one-third of the 2,500 to 3,000 snake species are classified as poisonous, and fewer than 300 species may be fatal to humans. In the United States more than twice as many people are killed each year by bee, wasp, and scorpion stings as by snakebites. It should be noted, however, that the presence of toxic elements in the saliva of many so-called nonpoisonous snakes makes it difficult to make a sharp distinction between venomous and nonvenomous types. Swallowing Lacking limbs to force whole prey animals into the throat, snakes are generally required to "walk" their jaws forward over the prey animal rather than simply swallowing it. Because the prey may be in the snake's mouth for a considerable time, blocking the air passage, snakes have developed a modified windpipe whose front end can be projected forward over the tongue so that its opening has access to free air space. LOCOMOTION Snakes use several types of locomotion. These have been grouped in various ways but are often considered to consist of four basic kinds: lateral (horizontal) undulation, concertina movement, sidewinding, and rectilinear movement. Lateral undulation, also called serpentine movement, is the most common form of snake locomotion and is employed by all snakes. By alternately contracting and relaxing a set of muscles down each side of the body, the snake forms itself into a number of rearward-moving horizontal waves. While doing so, the snake maneuvers its body so that the rear of each backward-moving wave pushes against something resistant. Concertina movement, also called earthworm movement, consists of a snake anchoring the forepart of its body and then pulling up the rest of its body behind it in the form of horizontal curves; it then extends out the forepart of its body, anchors it, and repeats the process. Sidewinding is a specialized type of locomotion employed on soft sand or other surfaces that offer no resistance or grip. In sidewinding, the snake loops its body into an S-shape, with only two sections of the body in contact with the ground. It then progressively shifts the two contact points farther back along the body, consequently pushing itself ahead. A new contact point is begun at the head when one contact point reaches the tail. Rectilinear, or caterpillar, movement involves a sliding of the skin back and forth over the body musculature and is therefore possible only in those snakes, such as rattlesnakes and boas, that do not have the skin tightly attached to the underlying musculature. Rectilinear movement is often thought to involve the snake walking on its ribs or digging in with its scales, but the ribs remain essentially motionless, and the scales provide only body-to-ground friction. Speeds attainable by snakes have been greatly overestimated. One of the fastest North American snakes, the whip snake, Masticophis, has a top speed of only 6.4 km/h (4 mph). The black mamba, Dendroaspis polylepis, may be the world's fastest snake and has been clocked at 11.2 km/h (7 mph). REPRODUCTION AND LONGEVITY Courtship among snakes usually involves the male aligning along the female in a series of spasmodic jerks and rapid tongue flicking. Eventually, if the female is receptive, the male inserts one of the paired copulatory organs (hemipenis) and mating ensues, lasting several hours to about a day. In snakes of the family Boidae, the male's spurs may play an important courtship role by stroking or vibrating against the female and thus helping to position her. Strong ridges (keeling) or tubercles are present in males of some species and are thought to be used in courtship. Snakes inhabiting temperate regions usually mate in the spring, but in species in which females can retain viable sperm for long periods (sperm storage), mating may occur during the fall. A few temperate species breed only once every 2 years. Reproduction in tropical snakes is influenced by the amount of rainfall. The vast majority of snakes lay eggs, but in some the eggs are retained in the female until hatched, and the young are born alive (ovoviviparity). Pythons have laid more than 100 eggs in a single clutch, and some live-bearing snakes are remarkably prolific, as demonstrated by a fer-de-lance, Bothrops, specimen that contained 86 embryos. The number of eggs or young is dependent on the species and on the size of the female parent. No true parental care of young is known, but a few snakes brood their eggs until they hatch. Gestation and incubation periods vary according to the species and temperature. Young snakes escape from the egg by splitting the shell with a special structure, the egg tooth, which falls off shortly after hatching. Snakes have relatively long life spans, and many have survived in captivity for more than 20 years. ORIGINS The origin of snakes is obscure because their skeletons are fragile and make poor fossils. One of the earliest known snake fossils is the 1.8-m (6-ft) Dinilysia from the Late Cretaceous (about 70 million years ago) of Patagonia. Two schools of thought concerning snake evolution are that (1) they arose from a superfamily of lizards, the Varanoidea, or Platynota, which includes several living families that have snakelike characteristics, or (2) they evolved from limbless, burrowing lizards in which the eyes had degenerated. These views need not be mutually exclusive, because it is possible that, although no living members of the Varanoidea now lack limbs or have degenerate eyes, extinct species may have possessed these distinctive characteristics of burrowers. CLASSIFICATION Snakes are classified in the class Reptilia, subclass Lepidosauria, order Squamata, suborder Serpentes. The suborder Serpentes is divided into three infraorders. The infraorder Scolecophidia is the most primitive, having cylindrical bodies, short tails, traces of the pelvic girdle, and a multilobed liver. The infraorder is divided into two, or sometimes three, families of small burrowing snakes (blind snakes, Typhlopidae; slender blind snakes, Leptotyphlopidae; and, sometimes, anomalepid blind snakes, Anomalepidae) that are highly specialized for a subterranean existence. The infraorder Henophidia includes families possessing both primitive and advanced characteristics. The family Boidae contains some of the world's largest snakes (pythons, boas, anacondas). Most members of this family retain vestiges of the pelvic girdle and also of the hind limbs, which are present in the form of spurs. Members of the family Aniliidae are stout-bodied, short-tailed, cylindrical snakes. They are generally less than a meter (3 ft) in length and are found in South America, Southeast Asia, and the Greater Sunda Islands (East Indies). The Family Uropeltidae is made up of small, secretive burrowers found in damp places in mountainous regions of India and Sri Lanka (Ceylon). These snakes have an enlarged scale at the end of their tail, the function of which has never been convincingly explained. The family Xenopeltidae contains only one small species in Southeast Asia. Its common name is the sunbeam snake, and it is appropriately named, for its scales are highly iridescent. It is secretive and nocturnal. Some of the strangest snakes belong to the family Acrochordidae, variously called file snakes, wart snakes, or the elephant-trunk snakes. These aquatic snakes are covered with small, granular scales. Fish are their main prey. They range over parts of India, Indochina, and the Indo-Australian archipelago. Most snake species are in the advanced infraorder Caenophidia and are characterized by a complete absence of the pelvic girdle. The Colubridae are the largest family of snakes. Most are small and harmless, but a few species possess enlarged teeth (venom fangs) on the posterior portion of their upper jaws (opisthoglyphous condition). The Elapidae are a family containing extremely venomous snakes: coral snakes, cobras, kraits, mambas, and sea snakes (sea snakes are often placed in a family of their own). These have a relatively short, permanently erect fang on the front part of the upper jaw (proteroglyphic condition). The family Viperidae includes the true vipers, which lack heat-detecting pit organs (subfamily Viperinae), and the pit vipers, which possess those organs (subfamily Crotalinae). In some classifications, the pit vipers are considered a separate family. Possibly included in the Viperidae are the mole vipers, Atractaspis, but the relationships of this group have been seriously questioned, and they may represent a highly modified colubrid line. Viperid snakes have long, recurved, erectile fangs (solenoglyphous condition). Bibliography: Bellairs, Angus, The Life of the Reptiles, 2 vols. (1970); Conant, Roger, A Field Guide to Reptiles and Amphibians of Eastern and Central North America, 2d ed. (1975); Ditmars, Raymond L., Snakes of the World (1931; repr. 1966); Dunson, W. A., ed., The Biology of Sea Snakes (1975); Fitzsimons, V., The Snakes of Southern Africa (1962); Goin, C. J., et al., Introduction to Herpetology, 3d ed. (1978); Harrison, Hal H., World of the Snake (1971); Klauber, L. M., Rattlesnakes: Their Habits, Life Histories and Influence on Mankind, 2 vols., rev. ed. (1972); Minton, S. A. and M. R., Venomous Reptiles (1969); Oliver, J. A., The Natural History of North American Amphibians and Reptiles (1955); Parker, H. W., Snakes: A Natural History, 2d ed. (1977); Pope, Clifford H., The Giant Snakes (1961) and Snakes and How They Live (1937); Schmidt, K. P., and Inger, R. F., Living Reptiles of the World (1957); Shaw, Charles E., and Campbell, Sheldon, Snakes of the American West (1974); Stebbins, R. C., A Field Guide to Western Reptiles and Amphibians (1966); Stidworthy, John, Snakes of the World, rev. ed. (1975); Wright, Albert H. and Anna A., Handbook of Snakes of the United States and Canada, 2 vols. (1957).