A snake is a scaly, limbless, elongate reptile from the order Squamata. A literary word for snake is serpent (a Middle English word which comes from Old French, and ultimately from *serp-, "to creep"[1]); in modern usage this usually refers to a mythic or symbolic snake, and information about such creatures can be found under serpent (symbolism). This article deals mostly with the biology of snakes.

Venomous Snakes

A venomous snake is a snake that uses modified saliva, venom, delivered through fangs in its mouth, to immobilize or kill its prey. Venomous snakes include several families of snakes and do not constitute a formal classification group used in taxonomy. The term poisonous snake is false - poison is inhaled or ingested whereas venom is injected. (In contrast, a few non-venomous species are constrictors such as pythons, anacondas, and boa constrictors which suffocate their prey.) Snake venom can contain many different active agents, and 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. Snake venom is never a single type of toxin[citation needed].
Venomous snakes that use hemotoxins usually have their fangs to secrete the venom in the front of their mouths, making it easier for them to inject the venom into their victims. Snakes that use neurotoxins, such as the mildly venomous mangrove snake, have their fangs located in the back of their mouths, with the fangs curled backwards. This makes it both difficult for the snake to use its venom and for scientists to milk them.[citation needed]
It has recently been suggested that all snakes are in fact venomous to some degree.[citation needed] Snakes all evolved from a common lizard ancestor that was venomous, from which venomous lizards like the gila monster and beaded lizard also derived. The research suggests that snakes all have venom glands, even species thought totally harmless such as the Corn Snake, commonly kept as a pet. What differentiates 'venomous' from 'non-venomous' is the evolution of a venom delivery system, the most advanced being that of vipers, with fangs that are hinged to prevent self envenomation, and curl out as the snake strikes.

Internal Organs

The left lung is very small or sometimes even absent, as snakes' tubular bodies require all of their organs to be long and thin. To accommodate them all, only one lung is functional. This lung contains a vascularized anterior portion and a posterior portion which does not function in gas exchange. This 'saccular lung' may be used to adjust buoyancy in some aquatic snakes and its function remains unknown in terrestrial species. Also, many organs that are paired, such as kidneys or reproductive organs, are staggered within the body, with one located ahead of the other. Snakes have no urinary bladder.

Internal Organs

The left lung is very small or sometimes even absent, as snakes' tubular bodies require all of their organs to be long and thin. To accommodate them all, only one lung is functional. This lung contains a vascularized anterior portion and a posterior portion which does not function in gas exchange. This 'saccular lung' may be used to adjust buoyancy in some aquatic snakes and its function remains unknown in terrestrial species. Also, many organs that are paired, such as kidneys or reproductive organs, are staggered within the body, with one located ahead of the other. Snakes have no urinary bladder.

Internal Organs

The left lung is very small or sometimes even absent, as snakes' tubular bodies require all of their organs to be long and thin. To accommodate them all, only one lung is functional. This lung contains a vascularized anterior portion and a posterior portion which does not function in gas exchange. This 'saccular lung' may be used to adjust buoyancy in some aquatic snakes and its function remains unknown in terrestrial species. Also, many organs that are paired, such as kidneys or reproductive organs, are staggered within the body, with one located ahead of the other. Snakes have no urinary bladder.

Perception

While snake vision is unremarkable (generally being best in arboreal species and worst in burrowing species), it is able to detect movement. Some snakes, like the Asian vine snake, have binocular vision. In most snakes, the lens moves back and forth within the eyeball to focus. In addition to their eyes, some snakes (pit vipers, pythons, and some boas) have infrared-sensitive receptors in deep grooves between the nostril and eye which allow them to "see" the radiated heat.
Snakes have no external ears, but they do have a bone called the quadrate under the skin on either side of the head which focuses sound into the cochlea. [3] Their sense of hearing is most sensitive to frequencies around 200–300 Hz.
A snake smells by using its forked tongue to collect airborne particles then passing them to the Jacobson's organ or the Vomeronasal organ in the mouth for examination. The fork in the tongue gives the snake a sort of directional sense of smell. The part of the body which is in direct contact with the surface of the ground is very sensitive to vibration, thus a snake is able to sense other animals approaching.

Skin

The skin is covered in scales. Many people are surprised to find that snakeskin has a smooth, dry texture, instead of a slimy texture as might be expected. Some people are afraid to touch them because they confuse snakes with worms. Most snakes use specialized belly scales to travel, gripping surfaces. The body scales may be smooth, keeled, or granular. Their eyelids are transparent "spectacle" scales which remain permanently closed, called brille. They shed their skin periodically. Unlike other reptiles, this is done in one piece, like pulling off a sock, with the snake rubbing its nose against something rough, like a rock, for instance, creating a rip in the skin around the nose and the mouth until the skin is completely removed.[2] The primary purpose of shedding is to grow; shedding also removes external parasites. This periodic renewal has led to the snake being a symbol of healing and medicine, as pictured in the Rod of Asclepius. 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. If there is not enough humidity in the air while snakes are shedding their skin, it can be very dangerous for the snake, because the dry skin does not shed. Skin that remains attached to the snake can harbour diseases and parasites. A tail tip that is not removed can constrict as the snake grows, cutting off the blood supply to the end of the tail causing it to drop off. A retained spectacle can cause blindness in the affected eye.

Digestion & Diet

All snakes are carnivorous, eating small animals including lizards and other snakes, rodents and other small mammals, birds, eggs or insects. Some snakes have a venomous bite, which they use to kill their prey before eating it. Other snakes kill their prey by constriction. Still others swallow their prey whole and alive. 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[2]. Most snakes are very easy to feed in captivity.

Snakes do not chew their food and 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. It is a common misconception that snakes actually dislocate their lower jaw to consume large prey.

After eating, snakes become torpid while the process of digestion takes place. Digestion is an intensive activity, especially after the consumption of very large prey. In species that feed only sporadically, the entire intestine enters a reduced state between meals to conserve energy, and the digestive system is 'up-regulated' to full capacity within 48 hours of prey consumption. So much metabolic energy is involved in digestion that in Crotalus durissus, the Mexican rattlesnake, an increase of body temperature to as much as 14 degrees Celsius above the surrounding environment has been observed.[3] Because of this, a snake disturbed after having eaten recently will often regurgitate its prey in order to be able to escape the perceived threat. However, when undisturbed, the digestive process is highly efficient, dissolving and absorbing everything but hair and claws, which are excreted along with uric acid waste. Snakes have been known to occasionally die from trying to swallow an animal that is too big. Snake digestive fluids are unable to digest most plant matter, which passes through the digestive system mostly untouched.

Snakes do not normally prey on people, but there are instances of small children being eaten by large constrictors in the jungle.[citation needed] While some particularly aggressive species exist, most will not attack humans unless startled or injured, preferring instead to avoid contact. The majority of snakes are either non-venomous or possess venom that is not harmful to humans.

As a general rule, snakes eat small vertebrates such as rodents, fish, lizards, birds, and frogs. There are exceptions to this, such as the natal green snake, which eats insects. Snakes generally specialise in a few food types (for example, royal pythons will generally eat mice and gerbils in the wild). However, they do not need to hunt every day. A big meal will keep some snakes content for a long time. Anacondas and pythons can live for a year after eating large prey.[1]

Evolution

The phylogeny of snakes is poorly known because snake skeletons are typically small and fragile, making fossilization unlikely. It has however been generally agreed, on the basis of morphology, that snakes descended from lizard-like ancestors. Recent research based on genetics and biochemistry confirms this; snakes form a venom clade with several extant lizard families.

Recent fossil evidence suggests that snakes directly evolved from burrowing lizards, either varanids or some other group. An early fossil snake, Najash rionegrina, was a two-legged burrowing animal with a sacrum, fully terrestrial. One extant analog of these putative ancestors is the earless monitor Lanthanotus of Borneo, although it also is semi-aquatic. As these ancestors became more subterranean, they lost their limbs and became more streamlined for burrowing. Features such as the transparent, fused eyelids (brille) and loss of external ears, according to this hypothesis, evolved to combat subterranean conditions (scratched corneas, dirt in the ears). According to this hypothesis, snakes re-emerged onto the surface of the land much as they are today. Other primitive snakes are known to have possessed hindlimbs but lacked a direct connection of the pelvic bones to the vertebrae, including Haasiophis, Pachyrhachis and Eupodophis) which are slightly older than Najash.

Primitive groups among the modern snakes, pythons and boas, do have vestigial hind limbs, tiny, clawed digits known as anal spurs and used to grasp during mating. Leptotyphlopidae and Typhlopidae are other examples where remnants of the pelvic girdle are still present, in Leptotyphlopidae sometimes as horny projections or not visible at all. The frontal limbs in all snakes are gone because of the evolution of the Hox genes in this area. The axial skeleton of the snakes' common ancestor had like most other tetrapods the familiar regional specializations consisting of cervical (neck), thoracic (chest), lumbar (lower back), sacral (pelvic) and caudal (tail) vertebrae. But the Hox gene expression in the axial skeleton responsible for the development of the thorax became dominant early in snake evolution. 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), meaning most of the snake's skeleton is actually made up of an extremely extended thorax. Ribs are found exclusively on the thoracic vertebrae. The neck, lumbar and pelvic vertebrae are very reduced in number (only 2-10 lumbar and pelvic vertebrae are still present), while only a short tail remains of the caudal vertebrae, although the tail is still long enough to be of good use in many species, and is modified in some aquatic and tree dwelling species. Because the front (thoracic) limbs in tetrapods appear in the area between the neck and the thorax, a location that is now almost absent in snakes, there is simply no longer any room left where they can develop.

The alternative hypothesis, based on morphology, suggests that ancestors were related to mosasaurs — extinct aquatic reptiles from the Cretaceous — which in turn are thought to have derived from varanid lizards. Under this hypothesis, the fused, transparent eyelids of snakes are thought to have evolved to combat marine conditions (corneal water loss through osmosis), while the external ears were lost through disuse in an aquatic environment, ultimately leading to an animal similar in appearance to sea snakes of today. In the Late Cretaceous, snakes re-colonized the land much like they are today. Fossil snake remains are known from early Late Cretaceous marine sediments, which is consistent with this hypothesis, particularly as they are older than the terrestrial Najash rionegrina. Similar skull structure; reduced/absent limbs; and other anatomical features found in both mosasaurs and snakes lead to a positive cladistical correlation, though some features are also shared with varanids. Supposedly similar locomotion for both groups is also used as support for this hypothesis. Genetic studies have indicated that snakes are not especially related to monitor lizards, and (it has been claimed) therefore not to mosasaurs, the proposed ancestor in the aquatic scenario of their evolution. However, there is more evidence linking mosasaurs to snakes than to varanids. Fragmentary remains that have been found from the Jurassic and Early Cretaceous indicate deeper fossil records for these groups, which may eventually refute either hypothesis.

The great diversity of modern snakes appeared in the Paleocene, probably correlated with the adaptive radiation of mammals following the extinction of the dinosaurs.