| Snakes Temporal range: Early Cretaceous – Recent, 112–0 Ma | |
|---|---|
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| Coast garter snake, Thamnophis elegans terrestris | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Order: | Squamata |
| Superfamily: | Varanoidea |
| (unranked): | Pythonomorpha |
| Suborder: | Serpentes Linnaeus, 1758 |
| Subgroups | |
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Snakes are elongate, legless, carnivorous reptiles of the suborder Serpentes that can be distinguished from legless lizards by their lack of eyelids and external ears. Like allsquamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales. Many species of snakes have skulls with many more joints than their lizard ancestors, enabling them to swallow prey much larger than their heads with their highly mobile jaws. To accommodate their narrow bodies, snakes' paired organs (such as kidneys) appear one in front of the other instead of side by side, and most have only one functional lung. Some species retain a pelvic girdle with a pair of vestigial claws on either side of the cloaca.
Living snakes are found on every continent except Antarctica and on most islands. Fifteenfamilies are currently recognized, comprising 456 genera and over 2,900 species.[1][2] They range in size from the tiny, 10 cm-long thread snake to pythons and anacondas of up to 7.6 metres (25 ft) in length. The fossil species Titanoboa cerrejonensis was 15 metres (49 ft) long. Snakes are thought to have evolved from either burrowing or aquatic lizards during the mid-Cretaceous period, and the earliest known fossils date to around 112 Ma ago. The diversity of modern snakes appeared during the Paleocene period (c 66 to 56 Ma ago).
Most species are nonvenomous and those that have venom use it primarily to kill and subdue prey rather than for self-defense. Some possess venom potent enough to cause painful injury or death to humans. Nonvenomous snakes either swallow prey alive or kill by constriction.
Etymology
The English word snake comes from Old English snaca, itself from Proto-Germanic *snak-an- (cf. German Schnake "ring snake," Swedishsnok "grass snake"), from Proto-Indo-European root *(s)nēg-o- "to crawl, creep," which also gave sneak as well as Sanskrit nāgá "snake."[3]The word ousted adder, as adder went on to narrow in meaning, though in Old English næddre was the general word for snake.[4] The other term, serpent, is from French, ultimately from Indo-European *serp- (to creep),[5] which also gave Greek érpo (ερπω) "I crawl."
Evolution
A phylogenetic overview of the extant groups
Note: the tree only indicates relationships, not evolutionary branching times.[6] |
The fossil record of snakes is relatively poor because snake skeletons are typically small and fragile, makingfossilization uncommon. Fossils readily identifiable as snakes (though often retaining hind limbs) first appear in the fossil record during the Cretaceous period.[7] The earliest known snake fossils come from sites in Utahand Algeria, represented by the genera Coniophis andLapparentophis, respectively. These fossil sites have been tentatively dated to the Albian or Cenomanian age of the late Cretaceous, between 112 and 94 Ma ago. However, an even older age has been suggested for one of the Algerian sites, which may be as old as the Aptian, 125-112 Ma ago.[8]
Based on comparative anatomy, there is consensus that snakes descended from lizards.[9]:11[10] Pythons andboas—primitive groups among modern snakes—have vestigial hind limbs: tiny, clawed digits known as anal spurs, which are used to grasp during mating.[9]:11[11]The Leptotyphlopidae and Typhlopidae groups also possess remnants of the pelvic girdle, sometimes appearing as horny projections when visible.
Frontal limbs are nonexistent in all known snakes. This is caused by the evolution of Hox genes, controlling limbmorphogenesis. The axial skeleton of the snakes’ common ancestor, like most other tetrapods, had regional specializations consisting of cervical (neck), thoracic (chest), lumbar (lower back), sacral (pelvic), and caudal (tail) vertebrae. Early in snake evolution, the Hox gene expression in the axial skeleton responsible for the development of the thorax became dominant. As a result, the vertebrae anterior to the hindlimb buds (when present) all have the same thoracic-like identity (except from the atlas, axis, and 1–3 neck vertebrae). In other words, most of a snake’s skeleton is an extremely extended thorax. Ribs are found exclusively on the thoracic vertebrae. Neck, lumbar and pelvic vertebrae are very reduced in number (only 2–10 lumbar and pelvic vertebrae are present), while only a short tail remains of the caudal vertebrae. However, the tail is still long enough to be of important use in many species, and is modified in some aquatic and tree-dwelling species.
Modern snakes greatly diversified during the Paleocene. This occurred alongside the adaptive radiation of mammals, following the extinction of (non-avian) dinosaurs. The colubrids, one of the more common snake groups, became particularly diverse due to preying on rodents, an especially successful mammal group. There are over 2,900 species of snakes ranging as far northward as the Arctic Circle in Scandinaviaand southward through Australia and Tasmania.[10] Snakes can be found on every continent (with the exception of Antarctica), in the sea, and as high as 16,000 feet (4,900 m) in the Himalayan Mountains of Asia.[10][12]:143 There are numerous islands from which snakes are absent, such as Ireland, Iceland, and New Zealand.[12]:143
Origins
The origin of snakes remains an unresolved issue. There are two main hypotheses competing for acceptance.
- Burrowing Lizard Hypothesis
There is fossil evidence to suggest that snakes may have evolved from burrowing lizards, such as the varanids (or a similar group) during theCretaceous Period.[13] An early fossil snake, Najash rionegrina, was a two-legged burrowing animal with a sacrum, and was fullyterrestrial.[14] One extant analog of these putative ancestors is the earless monitor Lanthanotus of Borneo (though it also is semiaquatic).[15]Subterranean species evolved bodies streamlined for burrowing, and eventually lost their limbs.[15] According to this hypothesis, features such as the transparent, fused eyelids (brille) and loss of external ears evolved to cope with fossorial difficulties, such as scratched corneasand dirt in the ears.[13][15] Some primitive snakes are known to have possessed hindlimbs, but their pelvic bones lacked a direct connection to the vertebrae. These include fossil species like Haasiophis, Pachyrhachis and Eupodophis, which are slightly older than Najash.[11]
- Aquatic Mosasaur Hypothesis
An alternative hypothesis, based on morphology, suggests the ancestors of snakes were related to mosasaurs—extinct aquatic reptiles from the Cretaceous—which in turn are thought to have derived from varanid lizards.[10] According to this hypothesis, the fused, transparent eyelids of snakes are thought to have evolved to combat marine conditions (corneal water loss through osmosis), and the external ears were lost through disuse in an aquatic environment. This ultimately lead to an animal similar to today's sea snakes. In the Late Cretaceous, snakes recolonized land, and continued to diversify into today's snakes. Fossilized snake remains are known from early Late Cretaceous marine sediments, which is consistent with this hypothesis; particularly so, as they are older than the terrestrial Najash rionegrina. Similar skull structure, reduced or absent limbs, and other anatomical features found in both mosasaurs and snakes lead to a positive cladistical correlation, although some of these features are shared with varanids.
Genetic studies in recent years have indicated snakes are not as closely related to monitor lizards as was once believed—and therefore not to mosasaurs, the proposed ancestor in the aquatic scenario of their evolution. However, more evidence links mosasaurs to snakes than to varanids. Fragmented remains found from the Jurassic and Early Cretaceous indicate deeper fossil records for these groups, which may potentially refute either hypothesis.
Taxonomy
All modern snakes are grouped within the suborder Serpentes in Linnean taxonomy, part of the order Squamata, though their precise placement within squamates is controversial.[1]
There are two infraorders of Serpentes: Alethinophidia and Scolecophidia.[1] This separation is based on morphological characteristics andmitochondrial DNA sequence similarity. Alethinophidia is sometimes split into Henophidia and Caenophidia, with the latter consisting of "colubroid" snakes (colubrids, vipers, elapids, hydrophiids, and attractaspids) and acrochordids, while the other alethinophidian families comprise Henophidia.[16] While not extant today, the Madtsoiidae, a family of giant, primitive, python-like snakes, was around until 50,000 years ago in Australia, represented by genera such as Wonambi.
There are numerous debates in the systematics within the group. For instance, many sources classify Boidae and Pythonidae as one family, while some keep the Elapidae and Hydrophiidae (sea snakes) separate for practical reasons despite their extremely close relation.
Recent molecular studies support the monophyly of the clades of modern snakes, scolecophidians, typhlopids + anomalepidids, alethinophidians, core alethinophidians, uropeltids (Cylindrophis, Anomochilus, uropeltines), macrostomatans, booids, boids, pythonids and caenophidians.[6]
Families
| Infraorder Alethinophidia 15 families | |||||
| Family[1] | Taxon author[1] | Genera[1] | Species[1] | Common name | Geographic range[17] |
|---|---|---|---|---|---|
| Acrochordidae | Bonaparte, 1831 | 1 | 3 | Wart snakes | Western India and Sri Lanka through tropical Southeast Asia to the Philippines, south through the Indonesian/Malaysian island group to Timor, east through New Guinea to the northern coast of Australia to Mussau Island, the Bismark Archipelago andGuadalcanal Island in the Solomon Islands. |
| Aniliidae | Stejneger, 1907 | 1 | 1 | False coral snake | Tropical South America. |
| Anomochilidae | Cundall, Wallach, 1993 | 1 | 2 | Dwarf pipe snakes | West Malaysia and on the Indonesian island ofSumatra. |
| Atractaspididae | Günther, 1858 | 12 | 64 | Burrowing asps | Africa and the Middle East.[9][18][19] |
| Boidae | Gray, 1825 | 8 | 43 | Boas | Northern, Central and South America, the Caribbean, southeastern Europe and Asia Minor, Northern, Central and East Africa, Madagascar and Reunion Island, the Arabian Peninsula, Central and southwestern Asia, India and Sri Lanka, the Moluccas and New Guinea through to Melanesia and Samoa. |
| Bolyeriidae | Hoffstetter, 1946 | 2 | 2 | Splitjaw snakes | Mauritius. |
| Colubridae | Oppel, 1811 | 304[2] | 1938[2] | Typical snakes | Widespread on all continents, except Antarctica.[20] |
| Cylindrophiidae | Fitzinger, 1843 | 1 | 8 | Asian pipe snakes | Sri Lanka east through Myanmar, Thailand, Cambodia, Vietnam and the Malay Archipelago to as far east asAru Islands off the southwestern coast of New Guinea. Also found in southern China (Fujian, Hong Kong and on Hainan Island) and in Laos. |
| Elapidae | Boie, 1827 | 61 | 235 | Elapids | On land, worldwide in tropical and subtropical regions, except in Europe. Sea snakes occur in the Indian Ocean and the Pacific.[21] |
| Loxocemidae | Cope, 1861 | 1 | 1 | Mexican burrowing snake | Along the Pacific versant from Mexico south to Costa Rica. |
| Pythonidae | Fitzinger, 1826 | 8 | 26 | Pythons | Subsaharan Africa, India, Myanmar, southern China, Southeast Asia and from the Philippines southeast through Indonesia to New Guinea and Australia. |
| Tropidophiidae | Brongersma, 1951 | 4 | 22 | Dwarf boas | From southern Mexico and Central America, south to northwestern South America in Colombia, (Amazonian) Ecuador and Peru, as well as in northwestern and southeastern Brazil. Also found in the West Indies. |
| Uropeltidae | Müller, 1832 | 8 | 47 | Shield-tailed snakes | Southern India and Sri Lanka. |
| Viperidae | Oppel, 1811 | 32 | 224 | Vipers | The Americas, Africa and Eurasia. |
| Xenopeltidae | Bonaparte, 1845 | 1 | 2 | Sunbeam snakes | Southeast Asia from the Andaman and Nicobar Islands, east through Myanmar to southern China, Thailand, Laos, Cambodia, Vietnam, the Malay Peninsula and the East Indies to Sulawesi, as well as the Philippines. |
| Infraorder Scolecophidia 3 families | |||||
| Family[1] | Taxon author[1] | Genera[1] | Species[1] | Common name | Geographic range[17] |
|---|---|---|---|---|---|
| Anomalepidae | Taylor, 1939 | 4 | 15 | Primitive blind snakes | From southern Central America to northwestern South America. Disjunct populations in northeastern and southeastern South America. |
| Leptotyphlopidae | Stejneger, 1892 | 2 | 87 | Slender blind snakes | Africa, western Asia from Turkey to northwestern India, on Socotra Island, from the southwestern United States south through Mexico and Central to South America, though not in the high Andes. In Pacific South America they occur as far south as southern coastal Peru, and on the Atlantic side as far as Uruguay and Argentina. In the Caribbean they are found on the Bahamas,Hispaniola and the Lesser Antilles. |
| Typhlopidae | Merrem, 1820 | 6 | 203 | Typical blind snakes | Most tropical and many subtropical regions around the world, particularly in Africa, Madagascar, Asia, islands in the Pacific, tropical America and in southeastern Europe. |
Biology
Skeleton
The skeleton of most snakes consists solely of the skull, hyoid, vertebral column, and ribs, though henophidian snakes retain vestiges of the pelvis and rear limbs. The skull of the snake consists of a solid and complete braincase, to which many of the other bones are only loosely attached, particularly the highly mobile jaw bones, which facilitate manipulation and ingestion of large prey items. The left and right sides of the lower jaw are joined only by a flexible ligament at the anterior tips, allowing them to separate widely, while the posterior end of the lower jaw bones articulate with a quadrate bone, allowing further mobility. The bones of the mandible and quadrate bones can also pick up ground borne vibrations.[22] The hyoid is a small bone located posterior and ventral to the skull, in the 'neck' region, which serves as an attachment for muscles of the snake's tongue, as it does in all other tetrapods.
The vertebral column consists of anywhere between 200 to 400 (or more) vertebrae. Tail vertebrae are comparatively few in number (often less than 20% of the total) and lack ribs, while body vertebrae each have two ribs articulating with them. The vertebrae have projections that allow for strong muscle attachment enabling locomotion without limbs. Autotomy of the tail, a feature found in some lizards is absent in most snakes.[23] Caudal autotomy in snakes is rare and is intervertebral, unlike that in lizards, which is intravertebral—that is, the break happens along a predefined fracture plane present on a vertebra.[24][25]
In some snakes, most notably boas and pythons, there are vestiges of the hindlimbs in the form of a pair of pelvic spurs. These small, claw-like protrusions on each side of the cloaca are the external portion of the vestigial hindlimb skeleton, which includes the remains of an ilium and femur.
Internal organs
The snake's heart is encased in a sac, called the pericardium, located at the bifurcation of thebronchi. The heart is able to move around, however, owing to the lack of a diaphragm. This adjustment protects the heart from potential damage when large ingested prey is passed through the esophagus. The spleen is attached to the gall bladder and pancreas and filters the blood. The thymus gland is located in fatty tissue above the heart and is responsible for the generation of immune cells in the blood. The cardiovascular system of snakes is also unique for the presence of a renal portal system in which the blood from the snake's tail passes through the kidneys before returning to the heart.[26]
The vestigial left lung is often small or sometimes even absent, as snakes' tubular bodies require all of their organs to be long and thin.[26] In the majority of species, only one lung is functional. This lung contains a vascularized anterior portion and a posterior portion that does not function in gas exchange.[26] This 'saccular lung' is used for hydrostatic purposes to adjust buoyancy in some aquatic snakes and its function remains unknown in terrestrial species.[26] Many organs that are paired, such as kidneys or reproductive organs, are staggered within the body, with one located ahead of the other.[26]
Snakes have no lymph nodes.[26]
Size
The now extinct Titanoboa cerrejonensis snakes found were 12–15 meters (39–49 ft) in length. By comparison, the largest extant snakes are the reticulated python, which measures about 9 meters (30 ft) long, and the anaconda, which measures about 7.5 meters (25 ft) long[27] and is considered the heaviest snake on Earth.
At the other end of the scale, the smallest extant snake is Leptotyphlops carlae, with a length of about 10 centimeters (4 in).[28] Most snakes are fairly small animals, approximately 3 feet in length.[29]
Skin
Main article: Snake scales
The skin of a snake is covered in scales. Contrary to the popular notion of snakes being slimy because of possible confusion of snakes with worms, snakeskin has a smooth, dry texture. Most snakes use specialized belly scales to travel, gripping surfaces. The body scales may be smooth,keeled, or granular. The eyelids of a snake are transparent "spectacle" scales, which remain permanently closed, also known as brille.
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.[30] Snake scales are not discrete, but extensions of the epidermis—hence they are not shed separately but as a complete outer layer during each moult, akin to a sock being turned inside out.[31]
The shape and number of scales on the head, back, and belly are often characteristic and used for taxonomic purposes. Scales are named mainly according to their positions on the body. 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.
Moulting
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 has been disputed in the case of snakes.[31][32]
Moulting occurs 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 skin liquefies. This causes the old skin to separate from the new skin beneath it. 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 pulling a sock off inside-out. A new, larger, brighter layer of skin has formed underneath.[31][33]
An older snake may shed its skin only once or twice a year. But a younger snake, still growing, may shed up to four times a year.[33] The discarded skin gives a perfect imprint of the scale pattern, and it is usually possible to identify the snake if the discarded skin is reasonably intact.[31] This periodic renewal has led to the snake being a symbol of healing and medicine, as pictured in the Rod of Asclepius.[34]
Perception
- Eyesight
- Snake vision varies greatly, from only being able to distinguish light from dark to keen eyesight, but the main trend is that their vision is adequate although not sharp, and allows them to track movements.[35] Generally, vision is best in arboreal snakes and weakest in burrowing snakes. Some snakes, such as the Asian vine snake (genus Ahaetulla), have binocular vision, with both eyes capable of focusing on the same point. Most snakes focus by moving the lens back and forth in relation to the retina, while in the other amniotegroups, the lens is stretched.
- Smell
- Snakes use smell to track their prey. They smell by using their forked tongues to collect airborne particles, then passing them to thevomeronasal organ or Jacobson's organ in the mouth for examination.[36] The fork in the tongue gives snakes a sort of directional sense of smell and taste simultaneously.[36] They keep their tongues constantly in motion, sampling particles from the air, ground, and water, analyzing the chemicals found, and determining the presence of prey or predators in the local environment. In water-dwelling snakes, such as the Anaconda, the tongue functions efficiently under water.[36]
- Vibration sensitivity
- The part of the body in direct contact with the ground is very sensitive to vibration; thus, a snake can sense other animals approaching by detecting faint vibrations in the air and on the ground.[36]
- Infrared sensitivity
- Pit vipers, pythons, and some boas have infrared-sensitive receptors in deep grooves between the nostril and eye, although some have labial pits on their upper lip just below the nostrils (common in pythons), which allow them to "see" the radiated heat of warm-blooded prey mammals.[36]
Venom
Cobras, vipers, and closely related species use venom to immobilize or kill their prey. The venom is modified saliva, delivered through fangs.[9]:243 The fangs of 'advanced' venomous snakes like viperids and elapids are hollow to inject venom more effectively, while the fangs of rear-fanged snakes such as the boomslang merely have a groove on the posterior edge to channel venom into the wound. Snake venoms are often prey specific, their role in self-defense is secondary.[9]:243
Venom, like all salivary secretions, is a predigestant that initiates the breakdown of food into soluble compounds, facilitating proper digestion. Even nonvenomous snake bites (like any animal bite) will cause tissue damage.[9]:209
Certain birds, mammals, and other snakes (such as kingsnakes) that prey on venomous snakes have developed resistance and even immunity to certain venoms.[9]:243Venomous snakes include three families of snakes, and do not constitute a formalclassification group used in taxonomy.
The term poisonous snake is mostly incorrect. Poison is inhaled or ingested, whereas venom is injected.[37] There are, however, two exceptions: Rhabdophis sequesters toxins from the toads it eats, then secretes them from nuchal glands to ward off predators, and a small population of garter snakes in Oregon retains enough toxin in their liver from the newts they eat to be effectively poisonous to small local predators (such as crows and foxes).[38]
Snake venoms are complex mixtures of proteins, and are stored in poison glands at the back of the head.[38] In all venomous snakes, these glands open through ducts into grooved or hollow teeth in the upper jaw.[9]:243[37] These proteins can potentially be a mix of neurotoxins(which attack the nervous system), hemotoxins (which attack the circulatory system), cytotoxins, bungarotoxins and many other toxins that affect the body in different ways.[37] Almost all snake venom contains hyaluronidase, an enzyme that ensures rapid diffusion of the venom.[9]:243
Venomous snakes that use hemotoxins usually have fangs in the front of their mouths, making it easier for them to inject the venom into their victims.[37] Some snakes that use neurotoxins (such as the mangrove snake) have fangs in the back of their mouths, with the fangs curled backwards.[39] This makes it both difficult for the snake to use its venom and for scientists to milk them.[37] Elapids, however, such ascobras and kraits are proteroglyphous—they possess hollow fangs that cannot be erected toward the front of their mouths, and cannot "stab" like a viper. They must actually bite the victim.[9]:242
It has recently been suggested that all snakes may be venomous to a certain degree, with harmless snakes having weak venom and no fangs.[40] Most snakes currently labelled “nonvenomous” would still be considered harmless according to this theory, as they either lack a venom delivery method or are incapable of delivering enough to endanger a human. This theory postulates that snakes may have evolved from a common lizard ancestor that was venomous—and that venomous lizards like the gila monster, beaded lizard, monitor lizards, and the now-extinct mosasaurs may also have derived. They share this venom clade with various other saurian species.
- Elapids – cobras including king cobras, kraits, mambas, Australian copperheads, sea snakes, and coral snakes.[39]
- Viperids – vipers, rattlesnakes, copperheads/cottonmouths, adders and bushmasters.[39]
There is a third family containing the opistoglyphous (rear-fanged) snakes (as well as the majority of other snake species):
- Colubrids – boomslangs, tree snakes, vine snakes, mangrove snakes, although not all colubrids are venomous.[9]:209[39]
Behavior
Feeding and diet
All snakes are strictly carnivorous, eating small animals including lizards, other snakes, small mammals, birds, eggs, fish, snails or insects.[9][1][10][41] Because snakes cannot bite or tear their food to pieces, they must swallow prey whole. The body size of a snake has a major influence on its eating habits. Smaller snakes eat smaller prey. Juvenile pythons might start out feeding on lizards or mice and graduate to small deer or antelope as an adult, for example.
The snake's jaw is a complex structure. Contrary to the popular belief that snakes can dislocate their jaws, snakes have a very flexible lower jaw, the two halves of which are not rigidly attached, and numerous other joints in their skull (see snake skull), allowing them to open their mouths wide enough to swallow their prey whole, even if it is larger in diameter than the snake itself,[41] as snakes do not chew. For example, the African egg-eating snake has flexible jaws adapted for eating eggs much larger than the diameter of its head.[9]:81 This snake has no teeth, but does have bony protrusions on the inside edge of its spine, which it uses to break shells when it eats eggs.[9]:81
While the majority of snakes eat a variety of prey animals, there is some specialization by some species.King cobras and the Australian bandy-bandy consume other snakes. Pareas iwesakii and other snail-eating colubrids of subfamily Pareatinae have more teeth on the right side of their mouths than on the left, as the shells of their prey usually spiral clockwise[9]:184[42]
Some snakes have a venomous bite, which they use to kill their prey before eating it.[41][43] Other snakes kill their prey by constriction.[41] Still others swallow their prey whole and alive.[9]:81[41]
After eating, snakes become dormant while the process of digestion takes place.[44] Digestion is an intense activity, especially after consumption of large prey. In species that feed only sporadically, the entireintestine enters a reduced state between meals to conserve energy. The digestive system is then 'up-regulated' to full capacity within 48 hours of prey consumption. Being ectothermic (“cold-blooded”), the surrounding temperature plays a large role in snake digestion. The ideal temperature for snakes to digest is30 °C (86 °F). So much metabolic energy is involved in a snake's digestion that in the Mexican rattlesnake (Crotalus durissus), surface body temperature increases by as much as 1.2 °C (2.2 °F) during the digestive process.[45] Because of this, a snake disturbed after having eaten recently will often regurgitate its prey to be able to escape the perceived threat. When undisturbed, the digestive process is highly efficient, with the snake's digestive enzymes dissolving and absorbing everything but the prey's hair (or feathers) and claws, which are excreted along with waste.
Locomotion
The lack of limbs does not impede the movement of snakes. They have developed several different modes of locomotion to deal with particular environments. Unlike the gaits of limbed animals, which form a continuum, each mode of snake locomotion is discrete and distinct from the others; transitions between modes are abrupt.[46][47]
Lateral undulation
Main article: Undulatory locomotion
Lateral undulation is the sole mode of aquatic locomotion, and the most common mode of terrestrial locomotion.[47] In this mode, the body of the snake alternately flexes to the left and right, resulting in a series of rearward-moving "waves."[46] While this movement appears rapid, snakes have rarely been documented moving faster than two body-lengths per second, often much less.[48] This mode of movement has the same net cost of transport (calories burned per meter moved) as running in lizards of the same mass.[49]
Terrestrial
Terrestrial lateral undulation is the most common mode of terrestrial locomotion for most snake species.[46] In this mode, the posteriorly moving waves push against contact points in the environment, such as rocks, twigs, irregularities in the soil, etc.[46] Each of these environmental objects, in turn, generates a reaction force directed forward and towards the midline of the snake, resulting in forward thrust while the lateral components cancel out.[50] The speed of this movement depends upon the density of push-points in the environment, with a medium density of about 8 along the snake's length being ideal.[48] The wave speed is precisely the same as the snake speed, and as a result, every point on the snake's body follows the path of the point ahead of it, allowing snakes to move through very dense vegetation and small openings.[50]
Aquatic
Main article: Sea snake
When swimming, the waves become larger as they move down the snake's body, and the wave travels backwards faster than the snake moves forwards.[51] Thrust is generated by pushing their body against the water, resulting in the observed slip. In spite of overall similarities, studies show that the pattern of muscle activation is different in aquatic versus terrestrial lateral undulation, which justifies calling them separate modes.[52] All snakes can laterally undulate forward (with backward-moving waves), but only sea snakes have been observed reversing the motion (moving backwards with forward-moving waves).[46]
Sidewinding
See also: Sidewinding
Most often employed by colubroid snakes (colubrids, elapids, and vipers) when the snake must move in an environment that lacks irregularities to push against (rendering lateral undulation impossible), such as a slick mud flat, or a sand dune. Sidewinding is a modified form of lateral undulation in which all of the body segments oriented in one direction remain in contact with the ground, while the other segments are lifted up, resulting in a peculiar "rolling" motion.[53][54] This mode of locomotion overcomes the slippery nature of sand or mud by pushing off with only static portions on the body, thereby minimizing slipping.[53] The static nature of the contact points can be shown from the tracks of a sidewinding snake, which show each belly scale imprint, without any smearing. This mode of locomotion has very low caloric cost, less than ⅓ of the cost for a lizard or snake to move the same distance.[49] Contrary to popular belief, there is no evidence that sidewinding is associated with the sand being hot.[53]
Concertina
Main article: Concertina movement
When push-points are absent, but there is not enough space to use sidewinding because of lateral constraints, such as in tunnels, snakes rely on concertina locomotion.[46][54] In this mode, the snake braces the posterior portion of its body against the tunnel wall while the front of the snake extends and straightens.[53] The front portion then flexes and forms an anchor point, and the posterior is straightened and pulled forwards. This mode of locomotion is slow and very demanding, up to seven times the cost of laterally undulating over the same distance.[49]This high cost is due to the repeated stops and starts of portions of the body as well as the necessity of using active muscular effort to brace against the tunnel walls.
Rectilinear
Main article: Rectilinear locomotion
The slowest mode of snake locomotion is rectilinear locomotion, which is also the only one where the snake does not need to bend its body laterally, though it may do so when turning.[55] In this mode, the belly scales are lifted and pulled forward before being placed down and the body pulled over them. Waves of movement and stasis pass posteriorly, resulting in a series of ripples in the skin.[55] The ribs of the snake do not move in this mode of locomotion and this method is most often used by large pythons, boas, and vipers when stalking prey across open ground as the snake's movements are subtle and harder to detect by their prey in this manner.[53]
Other
The movement of snakes in arboreal habitats has only recently been studied.[56] While on tree branches, snakes use several modes of locomotion depending on species and bark texture.[56] In general, snakes will use a modified form of concertina locomotion on smooth branches, but will laterally undulate if contact points are available.[56] Snakes move faster on small branches and when contact points are present, in contrast to limbed animals, which do better on large branches with little 'clutter'.[56]
Gliding snakes (Chrysopelea) of Southeast Asia launch themselves from branch tips, spreading their ribs and laterally undulating as they glide between trees.[53][57][58] These snakes can perform a controlled glide for hundreds of feet depending upon launch altitude and can even turn in midair.[53][57]
Reproduction
Although a wide range of reproductive modes are used by snakes, all snakes employ internal fertilization. This is accomplished by means of paired, forked hemipenes, which are stored, inverted, in the male's tail.[59] The hemipenes are often grooved, hooked, or spined in order to grip the walls of the female's cloaca.[59]
Most species of snakes lay eggs, but most snakes abandon the eggs shortly after laying. However, a few species (such as the King cobra) actually construct nests and stay in the vicinity of the hatchlings after incubation.[59] Most pythons coil around their egg-clutches and remain with them until they hatch.[60] A female python will not leave the eggs, except to occasionally bask in the sun or drink water. She will even “shiver” to generate heat to incubate the eggs.[60]
Some species of snake are ovoviviparous and retain the eggs within their bodies until they are almost ready to hatch.[61][62] Recently, it has been confirmed that several species of snake are fully viviparous, such as the boa constrictor and green anaconda, nourishing their young through a placenta as well as a yolk sac, which is highly unusual among reptiles, or anything else outside of placental mammals.[61][62]Retention of eggs and live birth are most often associated with colder environments, as the retention of the young within the female.[59][62]
Interactions with humans
Bite
Main article: Snakebite
Snakes do not ordinarily prey on humans, and most will not attack humans unless the snake is startled or injured, preferring instead to avoid contact. With the exception of large constrictors, nonvenomous snakes are not a threat to humans. The bite of nonvenomous snakes is usually harmless because their teeth are designed for grabbing and holding, rather than tearing or inflicting a deep puncture wound. Although the possibility of an infection and tissue damage is present in the bite of a nonvenomous snake, venomous snakes present far greater hazard to humans.[9]:209
Documented deaths resulting from snake bites are uncommon. Nonfatal bites from venomous snakes may result in the need for amputation of a limb or part thereof. Of the roughly 725 species of venomous snakes worldwide, only 250 are able to kill a human with one bite. Australia averages only one fatal snake bite per year. In India, 250,000 snakebites are recorded in a single year, with as many as 50,000 recorded initial deaths.[66]
The treatment for a snakebite is as variable as the bite itself. The most common and effective method is through antivenom (or antivenin), a serum made from the venom of the snake. Some antivenom is species specific (monovalent) while some is made for use with multiple species in mind (polyvalent). In the United States for example, all species of venomous snakes are pit vipers, with the exception of the coral snake. To produce antivenom, a mixture of the venoms of the different species of rattlesnakes, copperheads, and cottonmouths is injected into the body of a horse in ever-increasing dosages until the horse is immunized. Blood is then extracted from the immunized horse; the serum is separated and further purified and freeze-dried. It is reconstituted with sterile water and becomes antivenom. For this reason, people who are allergic to horses are more likely to suffer an allergic reaction to antivenom.[67] Antivenom for the more dangerous species (such asmambas, taipans, and cobras) is made in a similar manner in India, South Africa, and Australia, although these antivenoms are species-specific.
Snake charmers
Main article: Snake charming
In some parts of the world, especially in India, snake charming is a roadside show performed by a charmer. In such a show, the snake charmer carries a basket that contains a snake that he seemingly charms by playing tunes from his flutelike musical instrument, to which the snake responds.[68] Snakes lack external ears, though they do have internal ears, and respond to the movement of the flute, not the actual noise.[68][69]
The Wildlife Protection Act of 1972 in India technically proscribes snake charming on grounds of reducing animal cruelty. Other snake charmers also have a snake and mongoose show, where both the animals have a mock fight; however, this is not very common, as the snakes, as well as the mongooses, may be seriously injured or killed. Snake charming as a profession is dying out in India because of competition from modern forms of entertainment and environment laws proscribing the practice.[68]
Trapping
The Irulas tribe of Andhra Pradesh and Tamil Nadu in India have been hunter-gatherers in the hot, dry plains forests, and have practiced the art of snake catching for generations. They have a vast knowledge of snakes in the field. They generally catch the snakes with the help of a simple stick. Earlier, the Irulascaught thousands of snakes for the snake-skin industry. After the complete ban on snake-skin industry in India and protection of all snakes under the Indian Wildlife (Protection) Act 1972, they formed the Irula Snake Catcher's Cooperative and switched to catching snakes for removal of venom, releasing them in the wild after four extractions. The venom so collected is used for producing life-saving antivenom, biomedical research and for other medicinal products.[70] The Irulas are also known to eat some of the snakes they catch and are very useful in rat extermination in the villages.
Despite the existence of snake charmers, there have also been professional snake catchers or wranglers. Modern-day snake trapping involves a herpetologist using a long stick with a V- shaped end. Some television show hosts, like Bill Haast, Austin Stevens, Steve Irwin, and Jeff Corwin, prefer to catch them using bare hands.
Consumption
While not commonly thought of as food in most cultures, in some cultures, the consumption of snakes is acceptable, or even considered a delicacy, prized for its alleged pharmaceutical effect of warming the heart. Snake soup of Cantonese cuisine is consumed by local people in autumn, to warm up their body. Western cultures document the consumption of snakes under extreme circumstances of hunger.[71] Cookedrattlesnake meat is an exception, which is commonly consumed in parts of the Midwestern United States. In Asian countries such as China, Taiwan, Thailand,Indonesia, Vietnam and Cambodia, drinking the blood of snakes—particularly the cobra—is believed to increase sexual virility.[72] The blood is drained while the cobra is still alive when possible, and is usually mixed with some form of liquor to improve the taste.[72]
In some Asian countries, the use of snakes in alcohol is also accepted. In such cases, the body of a snake or several snakes is left to steep in a jar or container of liquor. It is claimed that this makes the liquor stronger (as well as more expensive). One example of this is the Habu snake sometimes placed in the Okinawan liquor Awamori also known as "Habu Sake."[73]
U.S. Army Special Forces trainees are taught to catch, kill, and eat snakes during their survival course; this has earned them the nickname "snake eaters," which the video game Metal Gear Solid 3: Snake Eater may be inferred to draw from.
Snake wine (蛇酒) is an alcoholic beverage produced by infusing whole snakes in rice wine or grain alcohol. The drink was first recorded to have been consumed in China during the Western Zhou dynasty and considered an important curative and believed to reinvigorate a person according to Traditional Chinese medicine.[74]
Pets
In the Western world, some snakes (especially docile species such as the ball python and corn snake) are kept as pets. To meet this demand a captive breeding industry has developed. Snakes bred in captivity tend to make better pets and are considered preferable to wild caught specimens.[75] Snakes can be very low maintenance pets, especially compared to more traditional species. They require minimal space, as most common species do not exceed five feet in length. Pet snakes can be fed relatively infrequently, usually once every 5–14 days. Certain snakes have a lifespan of more than 40 years if given proper care.
Symbolism
Main article: Serpent (symbolism)
In Egyptian history, the snake occupies a primary role with the Nile cobra adorning the crown of the pharaoh in ancient times. It wasworshipped as one of the gods and was also used for sinister purposes: murder of an adversary and ritual suicide (Cleopatra).
In Greek mythology snakes are often associated with deadly and dangerous antagonists, but this is not to say that snakes are symbolic of evil; in fact, snakes are a chthonic symbol, roughly translated as 'earthbound'. The nine-headed Lernaean Hydra that Hercules defeated and the three Gorgon sisters are children of Gaia, the earth.[76] Medusa was one of the three Gorgon sisters who Perseus defeated.[76]Medusa is described as a hideous mortal, with snakes instead of hair and the power to turn men to stone with her gaze.[76] After killing her, Perseus gave her head to Athena who fixed it to her shield called theAegis.[76] The Titans are also depicted in art with snakes instead of legs and feet for the same reason—they are children of Gaia and Ouranos (Uranus), so they are bound to the earth.
Three medical symbols involving snakes that are still used today are Bowl of Hygieia, symbolizing pharmacy, and the Caduceus and Rod of Asclepius, which are symbols denoting medicine in general.[34]
India is often called the land of snakes and is steeped in tradition regarding snakes.[77] Snakes are worshipped as gods even today with many women pouring milk on snake pits (despite snakes' aversion for milk).[77] The cobra is seen on the neck of Shiva and Vishnu is depicted often as sleeping on a seven-headed snake or within the coils of a serpent.[78] There are also several temples in India solely for cobras sometimes called Nagraj (King of Snakes) and it is believed that snakes are symbols of fertility. There is a Hindu festival called Nag Panchami each year on which day snakes are venerated and prayed to. See also Nāga.
In India there is another mythology about snakes. Commonly known in Hindi as "Ichchhadhari" snakes. Such snakes can take the form of any living creature, but prefer human form. These mythical snakes possess a valuable gem called "Mani", which is more brilliant than diamond. There are many stories in India about greedy people trying to possess this gem and ending up getting killed.
The Ouroboros is a symbol associated with many different religions and customs, and is claimed to be related to Alchemy. The Ouroboros or Oroboros is a snake eating its own tail in a clock-wise direction (from the head to the tail) in the shape of a circle, representing manifestation of one's own life and rebirth, leading to immortality.
Religion
Snakes are a part of Hindu worship. A festival Nag Panchami is celebrated every year on snakes. Most images of Lord Shiva depict snake around his neck. Puranas have various stories associated with Snakes. In the Puranas, Shesha is said to hold all the planets of the Universe on his hoods and to constantly sing the glories of Vishnu from all his mouths. He is sometimes referred to as "Ananta-Shesha," which means "Endless Shesha." Other notable snakes in Hinduism are Ananta, Vasuki, Taxak, Karkotaka and Pingala. The term Nāga is used to refer to entities that take the form of large snakes in Hinduism and Buddhism.
Snakes have also been widely revered, such as in ancient Greece, where the serpent was seen as a healer, and Asclepius carried two intertwined on his wand, a symbol seen today on many ambulances.
In religious terms, the snake is arguably the most important animal in ancient Mesoamerica. “In states of ecstasy, lords dance a serpent dance; great descending snakes adorn and support buildings from Chichen Itza to Tenochtitlan, and the Nahuatl word coatl meaning serpent or twin, forms part of primary deities such as Mixcoatl, Quetzalcoatl, and Coatlicue.”[81] In both Maya and Aztec calendars, the fifth day of the week was known as Snake Day.
In Judaism, the snake of brass is also a symbol of healing, of one's life being saved from imminent death (Book of Numbers 21:6–9).
King Cobra
The king cobra (Ophiophagus hannah) is the world's longest venomous snake, with a length up to 5.6 m (18.5 ft).[1] This species is widespread throughout Southeast Asia and parts of India, and is found mostly in forested areas. The king cobra can be fierce and agile, and can deliver a large quantity of highly potent venom in a single bite. It is one of the most dangerous and feared Asiatic snakes.Profile
The King Cobra is a large and powerful snake, averaging 3.6–4 m (12–13 feet) in length and typically weighing about 6 kg (13.2 lb). A particularly large specimen was kept captive at the London Zoo, and grew to 5.7 m (18.8 ft) before being euthanizedupon the outbreak of World War II.[3] Despite their large size, king cobras are fast and agile.
Characteristics
The skin of this snake is either olive-green, tan, or black, and it has faint, pale yellow cross bands down the length of the body. The belly is cream or pale yellow, and the scales are smooth. Juveniles are shiny black with narrow yellow bands (can be mistaken for a banded krait, but readily identified with its expanded hood). The head of a mature snake can be quite massive and bulky in appearance, though like all snakes, they can expand their jaws to swallow large prey items. It has proteroglyphdentition, meaning it has two short, fixed fangs in the front of the mouth which channel venom into the prey like hypodermic needles. The male is larger and thicker than the female. The average lifespan of a king cobra is about 20 years.
Identification
The king cobra is the sole member of genus Ophiophagus, while most other cobras are members of the genus Naja. They can be distinguished from other cobras by size and hood marks. King cobras are larger than other cobras, and the stripe on the neck is like the symbol "^" instead of a double or single eye(s) shape that may be seen in most of the other cobras. A foolproof method of identification if the head is clearly visible is the presence of a pair of large scales known as occipitals, at the back of the top of the head. These are behind the usual "nine-plate" arrangement typical of colubrids and elapids, and are unique to the king cobra.
Scalation
Dorsal scales: midbody 15 rows; Ventral scales: Males 235-250, females 239-265; Tail: Subcaudal scales single or paired in each row, 83-96 in males and 77-98 in females.[2]
Habitat
The king cobra is distributed across South Asia, Southeast Asia, and the southern areas of East Asia (southern China) but is not common. It lives in dense highland forests,[1][4] preferring areas dotted with lakes and streams. King cobra populations have dropped in some areas of its range because of the destruction of forests. It is listed as an Appendix II Animal within CITES.[5]
Behavior
King cobras, like other snakes, receive chemical information ("smell") via their forked tongues, which pick up scent particles and transfer them to a special sensory receptor (Jacobson's organ) located in the roof of its mouth.[1] When the scent of a meal is detected, the snake flicks its tongue to gauge the prey's location (the twin forks of the tongue acting in stereo); it also uses its keen eyesight (king cobras are able to detect moving prey almost 100 m [300 feet] away), intelligence[6] and sensitivity to earth-borne vibration to track its prey.[7] Following envenomation, the king cobra will begin to swallow its struggling prey while its toxins begin the digestion of its victim. King cobras, like all snakes, have flexible jaws. The jaw bones are connected by pliable ligaments, enabling the lower jaw bones to move independently, enabling the King cobra to swallow its prey whole. The expansion of the jaw enables the snake to swallow prey much larger than its head.[1]
King cobras are able to hunt at all times of day, although it is rarely seen at night, leading most herpetologists to classify it as a diurnalspecies.[1][8]
Defense
The king cobra can be highly aggressive.[9] When threatened, it raises up the anterior portion of its body, flattening the neck, showing thefangs and hissing loudly. (Bioacoustic analysis of the "growl" of the king cobra has shown that it differs significantly from other snakes. Generally a typical snake hiss has a broad-frequency span (~3,000 to 13,000 Hz) with a dominant frequency near 7,500 Hz, whereas the "growl" of the king cobra consists of frequencies below 2,500 Hz, with a dominant frequency near 600 Hz.)[10] It is easily irritated by closely approaching objects or sudden movements. The king cobra attacks quickly, and the strike distance is about 2 m (7 feet); people can easily misjudge the safe distance. The king cobra may deliver multiple bites in a single attack, or bite and hold on.[11] Although it is a highly dangerous snake, it prefers to escape unless it is cornered or provoked.[9]
If a king cobra encounters a natural predator, such as the mongoose, which has some resistance to the neurotoxins,[12] the snake generally tries to flee. If unable to do so, it forms the distinctive cobra hood and emits a hiss, sometimes with feigned closed-mouth strikes. These efforts usually prove to be very effective, especially since it is more dangerous than other mongoose prey, as well as being much too large for the small mammal to kill with ease.
Diet
The king cobra's genus name, Ophiophagus, means "snake-eater", and its diet consists primarily of other snakes, including ratsnakes, sizeable pythons and even other venomous snakes (including kraits, cobras and smaller members of its own species).[8][13] When food is scarce, they may also feed on other small vertebrates, such as lizards, birds, and rodents. In some cases, the cobra may "constrict" its prey, such as birds and larger rodents, using its muscular body, though this is uncommon.[1][13] After a large meal, the snake may live for many months without another one because of its slow metabolic rate.[1] The king cobra's most common meal is the ratsnake; pursuit of this species often brings king cobras close to human settlements.
Venom
The venom of the king cobra consists primarily of neurotoxins, but it also contains cardiotoxiccompounds.[8] Toxic constituents are mainly proteins and polypeptides.
During a bite, venom is forced through the snake's half-inch (1.25 cm) fangs into the wound, and quickly attacks the victim's central nervous system, inducing severe pain, blurred vision, vertigo,drowsiness, and paralysis.[14] Envenomation progresses to cardiovascular collapse, and the victim falls into a coma. Death soon follows due to respiratory failure.
In the past, the LD50 of the king cobra's venom was treated as 1.6 mg/kg – 1.8 mg/kg (which was one of the least venomous elapids). However, in recent toxicology study the LD50 of Chinese king cobra venom was found to be 0.34 mg/kg[15]. The value is lower than that of many Naja species found in the same habitats (such as the Chinese cobra), showing that the king cobra can actually be more venomous than many other cobras.[15][16] The king cobra is also capable of delivering larger quantities of venom than most other snakes, injecting a 380-600 mg dose in a single bite on average. It was reported that a single bite from this species can kill an adult Asian elephant. A bite from the king cobra can cause the death of a healthy adult human within 15 minutes,[15] though the average death time recorded is between 30–45 minutes after envenomation.[14][16][17] The mortality rate from a bite can be over 75%,[8][18]depending upon treatment details. It is regarded as one of the deadliest snakes in the world.[16][19]
There are two types of antivenom made specifically to treat king cobra envenomations. The Red Cross in Thailand manufactures one, and the Central Research Institute in India manufactures the other; however, both are made in small quantities and are not widely available.[20]Ohanin, a protein component of the venom, causes hypolocomotion and hyperalgesia in mammals.[21] Other components have cardiotoxic,[22] cytotoxic and neurotoxic effects.[23] In Thailand, a concoction of alcohol and the ground root of turmeric is ingested, which has been clinically shown to create a strong resilience against the venom of the king cobra, and other snakes with neurotoxic venom.[24]
Reproduction
The king cobra is unique among snakes in that the female king cobra is a very dedicated parent. She makes a nest for her eggs, scraping up leaves and other debris into a mound in which to deposit them, and remains in the nest until the young hatch.
A female usually deposits 20 to 40 eggs into the mound, which acts as an incubator. She stays with the eggs and guards the mound tenaciously, rearing up into a threat display if any large animal gets too close.[25]
Inside the mound the eggs are incubated at a steady 28 °C (82 °F). When the eggs start to hatch, instinct causes the female to leave the nest and find prey to eat so she does not eat her young.[26]The baby king cobras have a length of 45 to 55 centimeters (18 to 22 in). They are highly aggressive, and their venom is as deadly as that of an adult.
Other culture
In Burma, king cobras are often used by female snake charmers.[13] The charmer is usually tattooed with three pictograms, using an ink mixed with snake venom; superstition holds that it protects the charmer from the snake.[13] The charmer kisses the snake on the top of its head at the end of the show.[13]
Related species
The king cobra belongs to the family Elapidae which includes other well known snakes such as the cobra, the coral snake, the death adderand the black mamba.
A snakebite is an injury caused by a bite from a snake, often resulting in puncture wounds inflicted by the animal's fangs and sometimes resulting in envenomation. Although the majority of snake species are non-venomous and typically kill their prey with constriction rather thanvenom, venomous snakes can be found on every continent except Antarctica.[1] Snakes often bite their prey as a method of hunting, but also for defensive purposes against predators. Since the physical appearance of snakes may differ, there is often no practical way to identify a species and professional medical attention should be sought.[2][3]
The outcome of snake bites depends on numerous factors, including the species of snake, the area of the body bitten, the amount of venom injected, and the health conditions of the victim. Feelings of terror and panic are common after a snakebite and can produce a characteristic set of symptoms mediated by the autonomic nervous system, such as a racing heart andnausea.[4][5] Bites from non-venomous snakes can also cause injury, often due to lacerationscaused by the snake's teeth, or from a resulting infection. A bite may also trigger ananaphylactic reaction, which is potentially fatal. First aid recommendations for bites depend on the snakes inhabiting the region, as effective treatments for bites inflicted by some species can be ineffective for others.
The number of fatalities attributed to snake bites varies greatly by geographical area. Although deaths are relatively rare in Australia, Europe and North America,[1][6][7] the morbidity and mortality associated with snake bites is a serious public health problem in many regions of the world, particularly in rural areas lacking medical facilities. Further, while South Asia, Southeast Asia, and sub-Saharan Africa report the highest number of bites, there is also a high incidence in the Neotropics and other equatorial and subtropical regions.[1][6][7] Each year tens of thousands of people die from snake bites,[1] yet the risk of being bitten can be lowered with preventive measures, such as wearing protective footwear and avoiding areas known to be inhabited by dangerous snakes.
Signs and symptoms
The most common symptoms of all snakebites are overwhelming fear, panic, and emotional instability, which may cause symptoms such as nausea and vomiting, diarrhea, vertigo, fainting,tachycardia, and cold, clammy skin.[4][5] Television, literature, and folklore are in part responsible for the hype surrounding snakebites, and a victim may have unwarranted thoughts of imminent death.
Dry snakebites, and those inflicted by a non-venomous species, can still cause severe injury to the victim. There are several reasons for this: a snakebite which is not treated properly may become infected (as is often reported by the victims of viper bites whose fangs are capable of inflicting deep puncture wounds), the bite may cause anaphylaxis in certain people, and the snake's saliva and fangs may harbor many dangerous microbial contaminants, includingClostridium tetani. If neglected, an infection may spread and potentially kill the victim.
Most snakebites, whether by a venomous snake or not, will have some type of local effect. There is minor pain and redness in over 90% of cases, although this varies depending on the site.[4]Bites by vipers and some cobras may be extremely painful, with the local tissue sometimes becoming tender and severely swollen within 5 minutes.[7] This area may also bleed and blister and can eventually lead to tissue necrosis. Other common initial symptoms of pitviper and viperbites include lethargy, bleeding, weakness, nausea, and vomiting.[4][7] Symptoms may become more life-threatening over time, developing into hypotension, tachypnea, severe tachycardia, severe internal bleeding, altered sensorium, kidney failure and respiratory failure.[4][7]
Interestingly, bites caused by the Mojave rattlesnake, kraits, coral snake, and the speckled rattlesnake reportedly cause little or no pain despite being serious injuries.[4] Victims may also describe a "rubbery," "minty," or "metallic" taste if bitten by certain species of rattlesnake.[4]Spitting cobras and rinkhalses can spit venom in their victims' eyes. This results in immediate pain, ophthalmoparesis, and sometimes blindness.[12][13]
Some Australian elapids and most viper envenomations will cause coagulopathy, sometimes so severe that a person may bleed spontaneously from the mouth, nose, and even old, seemingly-healed wounds.[7] Internal organs may bleed, including the brain and intestines and will causeecchymosis (bruising) of the victim's skin.
Venom emitted from elapids, including sea snakes, kraits, cobras, king cobra, mambas, and many Australian species, contain toxins which attack the nervous system, causingneurotoxicity.[4][7][14] The victim may present with strange disturbances to their vision, including blurriness. Paresthesia throughout the body, as well as difficulty speaking and breathing, may be reported.[4] Nervous system problems will cause a huge array of symptoms, and those provided here are not exhaustive. If the victim is not treated immediately they may die from respiratory failure.
Venom emitted from some types of cobras, almost all vipers, some Australian elapids and some sea snakes causes necrosis of muscle tissue.[7] Muscle tissue will begin to die throughout the body, a condition known as rhabdomyolysis. Rhabdomyolysis can result in damage to the kidneys as a result of myoglobin accumulation in the renal tubules. This, coupled with hypotension, can lead to acute renal failure, and, if left untreated, eventually death.[7]
[edit]Pathophysiology
Since envenomation is completely voluntary, all venomous snakes are capable of biting without injecting venom into their victim. Snakes may deliver such a "dry bite" rather than waste their venom on a creature too large for them to eat.[15] However, the percentage of dry bites varies between species: 50% of bites from the normally timid coral snake do not result in envenomation, whereas only 25% of pitviper bites are dry.[4] Furthermore, some snake genera, such as rattlesnakes, significantly increase the amount of venom injected in defensive bites compared to predatory strikes.[16]
Some dry bites may also be the result of imprecise timing on the snake's part, as venom may be prematurely released before the fangs have penetrated the victim's flesh.[15] Even without venom, some snakes, particularly large constrictors such as those belonging to the Boidae andPythonidae families, can deliver damaging bites; large specimens often cause severe lacerations as the victim or the snake itself pull away, causing the flesh to be torn by the needle-sharp recurved teeth embedded in the victim. While not as life-threatening as a bite from a venomous species, the bite can be at least temporarily debilitating and could lead to dangerous infections if improperly dealt with.
While most snakes must open their mouths before biting, African and Middle Eastern snakes belonging to the family Atractaspididae are able to fold their fangs to the side of their head without opening their mouth and jab at victims.[17]
[edit]Snake venom
Main articles: Snake venom and Snake toxins
It has been suggested that snakes evolved the mechanisms necessary for venom formation and delivery sometime during the Mioceneepoch.[18] During the mid-Tertiary, most snakes were large ambush predators belonging to the superfamily Henophidia, which use constriction to kill their prey. As open grasslands replaced forested areas in parts of the world, some snake families evolved to become smaller and thus more agile. However, subduing and killing prey became more difficult for the smaller snakes, leading to the evolution of snake venom.[18] Other research on Toxicofera, a hypothetical clade thought to be ancestral to most living reptiles, suggests an earlier time frame for the evolution of snake venom, possibly to the order of tens of millions of years, during the Late Cretaceous.[19]
Snake venom is produced in modified parotid glands normally responsible for secreting saliva. It is stored in structures called alveoli behind the animal's eyes, and ejected voluntarily through its hollow tubular fangs. Venom is composed of hundreds to thousands of different proteinsand enzymes, all serving a variety of purposes, such as interfering with a prey's cardiac system or increasing tissue permeability so that venom is absorbed faster.
Venom in many snakes, such as pitvipers, affects virtually every organ system in the human body and can be a combination of many toxins, including cytotoxins, hemotoxins, neurotoxins, and myotoxins, allowing for an enormous variety of symptoms.[4][20] Earlier, the venom of a particular snake was considered to be one kind only i.e. either hemotoxic or neurotoxic, and this erroneous belief may still persist wherever the updated literature is hard to access. Although there is much known about the protein compositions of venoms from Asian and American snakes, comparatively little is known of Australian snakes.
The strength of venom differs markedly between species and even more so between families, as measured by LD50 in mice. SubcutaneousLD50 varies by over 140-fold within elapids and by more than 100-fold in vipers. The amount of venom produced also differs among species, with the Gaboon viper able to potentially deliver from 450–600mg of venom in a single bite, the most of any snake. [21] Opisthoglyphous colubrids have venom ranging from life-threatening (in the case of the boomslang) to barely noticeable (as in Tantilla).
[edit]Prevention
Snakes are most likely to bite when they feel threatened, are startled, are provoked, or have no means of escape when cornered. Encountering a snake is always considered dangerous and it is recommended to leave the vicinity. There is no practical way to safely identify any snake species as appearances may vary dramatically.
Snakes are likely to approach residential areas when attracted by prey, such as rodents. Practicing regular pest control can reduce the threat of snakes considerably. It is beneficial to know the species of snake that are common in local areas, or while traveling or hiking. Areas of the world such as Africa, Australia, the Neotropics, and southern Asia are inhabited by many highly dangerous species. Being wary of snake presence and ultimately avoiding it when known is strongly recommended.
When in the wilderness, treading heavily creates ground vibrations and noise, which will often cause snakes to flee from the area. However, this generally only applies to North America as some larger and more aggressive snakes in other parts of the world, such as king cobras andblack mambas, will protect their territory. When dealing with direct encounters it is best to remain silent and motionless. If the snake has not yet fled it is important to step away slowly and cautiously.
The use of a flashlight when engaged in camping activities, such as gathering firewood at night, can be helpful. Snakes may also be unusually active during especially warm nights when ambient temperatures exceed 21 °C (70 °F). It is advised not to reach blindly into hollow logs, flip over large rocks, and enter old cabins or other potential snake hiding-places. When rock climbing, it is not safe to grab ledges or crevices without examining them first, as snakes are cold-blooded and often sunbathe atop rock ledges.
Pet owners of domestic animals or snakes should be aware that a snake is capable of causing injury and that is necessary to always act with caution. When handling snakes it is never wise to consume alcoholic beverages. In the United States more than 40% of snakebite victims intentionally put themselves in harm's way by attempting to capture wild snakes or by carelessly handling their dangerous pets—40% of that number had a blood alcohol level of 0.1 percent or more.[22]
It is also important to avoid snakes that appear to be dead, as some species will actually roll over on their backs and stick out their tongue to fool potential threats. A snake's detached head can immediately act by reflex and potentially bite. The induced bite can be just as severe as that of a live snake.[4][23] Dead snakes are also incapable of regulating the venom they inject, so a bite from a dead snake can often contain large amounts of venom.[24]
[edit]Treatment
It is not an easy task determining whether or not a bite by any species of snake is life-threatening. A bite by a North American copperheadon the ankle is usually a moderate injury to a healthy adult, but a bite to a child's abdomen or face by the same snake may be fatal. The outcome of all snakebites depends on a multitude of factors: the size, physical condition, and temperature of the snake, the age and physical condition of the victim, the area and tissue bitten (e.g., foot, torso, vein or muscle), the amount of venom injected, the time it takes for the patient to find treatment, and finally the quality of that treatment.[4][25]
[edit]Snake identification
Identification of the snake is important in planning treatment in certain areas of the world, but is not always possible. Ideally the dead snake would be brought in with the patient, but in areas where snake bite is more common, local knowledge may be sufficient to recognize the snake. However, in regions where polyvalent antivenoms are available, such as North America, identification of snake is not a high priority item.
The three types of venomous snakes that cause the majority of major clinical problems are vipers, kraits, and cobras. Knowledge of what species are present locally can be crucial, as is knowledge of typical signs and symptoms of envenomation by each type of snake. A scoring systems can be used to try and determine the biting snake based on clinical features,[26] but these scoring systems are extremely specific to particular geographical areas.
[edit]First aid
Snakebite first aid recommendations vary, in part because different snakes have different types of venom. Some have little local effect, but life-threatening systemic effects, in which case containing the venom in the region of the bite by pressure immobilization is highly desirable. Other venoms instigate localized tissue damage around the bitten area, and immobilization may increase the severity of the damage in this area, but also reduce the total area affected; whether this trade-off is desirable remains a point of controversy.
Because snakes vary from one country to another, first aid methods also vary. As always, this article is not a legitimate substitute for professional medical advice. Readers are strongly advised to obtain guidelines from a reputable first aid organization in their own region, and to be wary of homegrown or anecdotal remedies.
However, most first aid guidelines agree on the following:
- Protect the patient (and others, including yourself) from further bites. While identifying the species is desirable in certain regions, do not risk further bites or delay proper medical treatment by attempting to capture or kill the snake. If the snake has not already fled, carefully remove the victim from the immediate area.
- Keep the victim calm. Acute stress reaction increases blood flow and endangers the patient. Keep people near the patient calm. Panicis infectious and compromises judgment.
- Call for help to arrange for transport to the nearest hospital emergency room, where antivenom for snakes common to the area will often be available.
- Make sure to keep the bitten limb in a functional position and below the victim's heart level so as to minimize blood returning to the heart and other organs of the body.
- Do not give the patient anything to eat or drink. This is especially important with consumable alcohol, a known vasodilator which will speed up the absorption of venom. Do not administer stimulants or pain medications to the victim, unless specifically directed to do so by a physician.
- Remove any items or clothing which may constrict the bitten limb if it swells (rings, bracelets, watches, footwear, etc.)
- Keep the victim as still as possible.
- Do not incise the bitten site.
Many organizations, including the American Medical Association and American Red Cross, recommend washing the bite with soap and water. However, do not attempt to clean the area with any type of chemical. Australian recommendations for snake bite treatment strongly recommend against cleaning the wound. Traces of venom left on the skin/bandages from the strike can be used in combination with a snake bite identification kit to identify the species of snake. This speeds determination of which antivenom to administer in the emergency room.[27]
[edit]Pressure immobilization
In 1979, Australia's National Health and Medical Research Council formally adopted pressure immobilization as the preferred method of first aid treatment for snakebites in Australia.[28] As of 2009, clinical evidence for pressure immobilization remains limited, with current evidence based almost entirely on anecdotal case reports.[28] This has led most international authorities to question its efficacy.[28] Despite this, all reputable first aid organizations in Australia recommend pressure immobilization treatment; however, it is not widely adhered to, with one study showing that only a third of snakebite patients attempt pressure immobilization.[28]
Pressure immobilization is not appropriate for cytotoxic bites such as those inflicted by mostvipers,[29][30][31] but may be effective against neurotoxic venoms such as those of mostelapids.[32][33][34] Developed by medical researcher Struan Sutherland in 1978,[35] the object of pressure immobilization is to contain venom within a bitten limb and prevent it from moving through the lymphatic system to the vital organs. This therapy has two components: pressure to prevent lymphatic drainage, and immobilization of the bitten limb to prevent the pumping action of theskeletal muscles.
Pressure is preferably applied with an elastic bandage, but any cloth will do in an emergency. Bandaging begins two to four inches above the bite (i.e. between the bite and the heart), winding around in overlapping turns and moving up towards the heart, then back down over the bite and past it towards the hand or foot. Then the limb must be held immobile: not used, and if possible held with a splint or sling. The bandage should be about as tight as when strapping a sprained ankle. It must not cut off blood flow, or even be uncomfortable; if it is uncomfortable, the patient will unconsciously flex the limb, defeating the immobilization portion of the therapy. The location of the bite should be clearly marked on the outside of the bandages. Some peripheral edema is an expected consequence of this process.
Apply pressure immobilization as quickly as possible; if you wait until symptoms become noticeable you will have missed the best time for treatment. Once a pressure bandage has been applied, it should not be removed until the patient has reached a medical professional. The combination of pressure and immobilization may contain venom so effectively that no symptoms are visible for more than 24 hours, giving the illusion of a dry bite.[citation needed] But this is only a delay; removing the bandage releases that venom into the patient's system with rapid and possibly fatal consequences.
[edit]Antivenom
Main article: Antivenom
Until the advent of antivenom, bites from some species of snake were almost universally fatal.[36] Despite huge advances in emergency therapy, antivenom is often still the only effective treatment for envenomation. The first antivenom was developed in 1895 by French physicianAlbert Calmette for the treatment of Indian cobra bites. Antivenom is made by injecting a small amount of venom into an animal (usually a horse or sheep) to initiate an immune system response. The resulting antibodies are then harvested from the animal's blood.
Antivenom is injected into the patient intravenously, and works by binding to and neutralizing venom enzymes. It cannot undo damage already caused by venom, so antivenom treatment should be sought as soon as possible. Modern antivenoms are usually polyvalent, making them effective against the venom of numerous snake species. Pharmaceutical companies which produce antivenom target their products against the species native to a particular area. Although some people may develop serious adverse reactions to antivenom, such asanaphylaxis, in emergency situations this is usually treatable and hence the benefit outweighs the potential consequences of not using antivenom.
[edit]Outmoded treatments
The following treatments have all been recommended at one time or another, but are now considered to be ineffective or outright dangerous. Many cases in which such treatments appear to work are in fact the result of dry bites.
- Application of a tourniquet to the bitten limb is generally not recommended. There is no convincing evidence that it is an effective first aid tool as ordinarily applied.[37] Tourniquets have been found to be completely ineffective in the treatment of Crotalus durissus bites,[38] but some positive results have been seen with properly applied tourniquets for cobra venom in thePhilippines.[39] Uninformed tourniquet use is dangerous, since reducing or cutting off circulation can lead to gangrene, which can be fatal.[37] The use of a compression bandage is generally as effective, and much safer.
- Cutting open the bitten area, an action often taken prior to suction, is not recommended since it causes further damage and increases the risk of infection.
- Sucking out venom, either by mouth or with a pump, does not work and may harm the affected area directly.[40] Suction started after 3 minutes removes a clinically insignificant quantity—less than one thousandth of the venom injected—as shown in a human study.[41] In a study with pigs, suction not only caused no improvement but led to necrosis in the suctioned area.[42] Suctioning by mouth presents a risk of further poisoning through the mouth's mucous tissues.[43] The well-meaning family member or friend may also release bacteria into the victim's wound, leading to infection.
- Immersion in warm water or sour milk, followed by the application of snake-stones (also known as la Pierre Noire), which are believed to draw off the poison in much the way a sponge soaks up water.
- Application of potassium permanganate.
- Use of electroshock therapy. Although still advocated by some, animal testing has shown this treatment to be useless and potentially dangerous.[44][45][46][47]
In extreme cases, where the victims were in remote areas, all of these misguided attempts at treatment have resulted in injuries far worse than an otherwise mild to moderate snakebite. In worst case scenarios, thoroughly constricting tourniquets have been applied to bitten limbs, completely shutting off blood flow to the area. By the time the victims finally reached appropriate medical facilities their limbs had to beamputated.
[edit]
Epidemiology
Most snakebites are caused by non-venomous snakes. Of the roughly 3,000 known species of snake found worldwide, only 15 percent are considered dangerous to humans.[1][4][48] Snakes are found on every continent except Antarctica.[1] The most diverse and widely distributed snake family, the colubrids, has approximately 700 venomous species,[49] but only fivegenera—boomslangs, twig snakes, keelback snakes, green snakes, and slender snakes—have caused human fatalities.[49]
Since reporting is not mandatory in many regions of the world,[1] snakebites often go unreported. Consequently, no accurate study has ever been conducted to determine the frequency of snakebites on the international level. However, some estimates put the number at 5.4 million snakebites, 2.5 million envenomings, resulting in perhaps 125,000 deaths.[1] Others estimate 1.2 to 5.5 million snakebites, 421,000 to 1.8 million envenomings, and 20,000 to 94,000 deaths.[1]Many people who survive bites nevertheless suffer from permanent tissue damage caused by venom, leading to disability.[7]
Most snake envenomings and fatalities occur in South Asia, Southeast Asia, and sub-Saharan Africa, with India reporting the most snakebite deaths of any country.[1] In India almost all of these deaths are caused by the Big Four, consisting of the Russell's viper, Indian cobra, saw-scaled viper, and the common krait. In Burma 80 percent of the approximately 1000 deaths each year from snake bite are caused by the Russell's Viper. Tea plantations sometimes are associated with some elapids such as the common cobras and the king cobra.
In the Neotropics, the lance-headed vipers inflict the majority of fatal bites, although of the many known species, only two, the common lancehead and terciopelo, are responsible for most cases.[7][50] The tropical rattlesnake is another important species.
In Africa, the puff adder is responsible for most fatalities,[51] although there are regional differences, with the saw-scaled viper inflicting more bites in Northern Africa, where the puff adder is not normally found.[50] Most bites occur in industrial plantations, which attract many types of snake prey. Banana plantations are associated with vipers such as night adders, while rubber and palm tree plantations attract elapids, including cobras and black mambas.[6] There are also highly venomous colubrids in Africa, such as the boomslang.
In the Middle East, the snakes responsible for most bites tend to be more venomous than European species, but deaths are rare, with some estimating perhaps 100 fatal bites annually.[6] The coastal viper, Palestine viper, and Lebetine viper are the species involved in most bites.[6]Larger and more venomous elapids, such as the Egyptian cobra, can also be found throughout the Middle East.
In Europe, nearly all of the snakes responsible for venomous bites belong to the viper family, and of these, the nose-horned viper, asp viper, and Lataste's viper inflict the majority of bites.[6] Although Europe has a population of some 731 million people, snake bites only kill about 30 people each year, largely due to wide access to health care services and antivenom, as well as the relatively mild potency of many native species' venom.[6]
In Australia, the only continent where venomous snakes constitute the majority of species,[52] the Taipan, tiger snake and Eastern brown snake inflict virtually all reported venomous bites,[6][52] with the latter responsible for perhaps 60% of deaths caused by snakebite.[52]Although Australian snakes are highly venomous, wide access to antivenom has made deaths exceedingly rare, with only a few fatalities each year.
Most of the Pacific Islands are free of terrestrial snakes;[6] however, sea snakes are common in the Indian Ocean and tropical Pacific Ocean, but are not found in the Atlantic Ocean or the Caribbean, Mediterranean or Red Seas.[14] While the majority of species live close to shorelines or coral reefs, the yellow-bellied sea snake can be found in the open ocean.[14] Over 50% of bites inflicted by sea snakes, which are generally not aggressive, occur when fishermen attempt to remove snakes which have become tangled in fishing nets.[14][53] Symptoms may appear in as little as 5 minutes or take 8 hours to develop, depending on the species and region of the body bitten.[14] Although sea snakes are highly venomous, about 80% of reported bites end up being dry.[14][54] The advent of antivenom and advances in emergency medicine have reduced fatalities to about 3% of snakebite cases.[14]
Of the 120 known indigenous snake species in North America, only 20 are venomous to human beings, all belonging to the families Viperidaeand Elapidae.[4] However, in the United States, every state except Maine, Alaska, and Hawaii is home to at least one of 20 venomous snake species.[4] Most snakebite related deaths in the United States are attributed to Eastern and Western diamondback rattlesnake bites.[4]Further, the majority of bites in the United States occur in the southwestern part of the country, in part because rattlesnake populations in the eastern states are much lower.[55] The state of North Carolina has the highest frequency of reported snakebites, averaging approximately 19 bites per 100,000 persons.[20] The national average is roughly 4 bites per 100,000 persons.[20]
Worldwide, snakebites occur most frequently in the summer season when snakes are active and humans are outdoors.[1][56] Agricultural and tropical regions report more snakebites than anywhere else.[1][50] Victims are typically male and between 17 and 27 years of age.[4][56][57]Children and the elderly are the most likely to die.[4][25]
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