We fear from them as they have the potential to kill us by their venom. They are snakes. Snakes belong to the suborder Serpents and are legless, crawling and elongating reptiles. They are different from lizards as they lack eyelid and external ears. They are ectothermic, amniotes covered by overlapping scales like all other squamates. The skull bones of snakes are demarcated by presence of a large number of joints that enable them to feed on a prey which is much larger than them. Jaws are highly movable. They have a single functional lung. Some species bear a pair of vestigial claws on either side of the cloaca. They are present in all continents except Antarctica. Fifteen families have been recognized comprising of 456 genera and 2,900 species. They range in size from 10 cm like that of thread snake to the larger pythons and anacondas which are about 7.6 meters long. The recently discovered fossil of snake Titanoboa was about 15 meters long.
Snakes are thought to have evolved from their terrestrial or aquatic lizard ancestors during the Cretaceous period. Modern snakes appeared during the Paleocene period. Majority of the species are non-poisonous and the venomous species use their venom as a measure of self-defence. Some venomous snakes have the potential to kill even humans. Non-venomous snakes either swallow the prey alive or kill it by constriction. The modern English word snake comes from the Old English word snaca which means to crawl or to creep. Fossil record of snakes is relatively poor as the skeletons are relatively small making fossilization impossible. 150 million year old specimens of snakes have been discovered in South America and South Africa. Fossil records suggest that the snakes have evolved from the burrowing lizards. Primitive snakes among the modern ones are the pythons and the boas.
Snake skeleton is chiefly composed of skull, hyoid, vertebral column, ribs and vestiges of pelvis and hind limbs in the henophidian snakes. Skull consists of a solid braincase to which other bones are loosely articulated making the jaws flexible so that effective feeding and prey capture can commence. The left and right sides of the lower jaw are joined to a ligament while the posterior end of lower jaw is articulated with the quadrate making the jaw more mobile. Mandibles and quadrate also help in picking sound borne vibrations. Hyoid is a small bone located in the neck region serving for the attachment of the tongue muscles. Vertebral column comprises 200-400 vertebrae. Tail vertebrae fewer and lack ribs. Vertebrae of body bear two ribs. Vertebrae have strong articulation with muscles in order to bring about effective locomotion in absence of limbs. Autotomy is absent in snakes. Pythons and boas have a pair of vestiges of hind limbs in the form of pelvic spurs present on either side of the cloaca made up of the vestiges of ilium and femur.
The heart is enclosed in a membranous sac called pericardium present at the bifurcation of the bronchi. Heart can move from its position as diaphragm is absent. This property protects heart from damage when large prey ingested passes through the oesophagus. Spleen is found attached to gall bladder and pancreas and helps in blood filtration. Thymus is located just above heart and is responsible for the generation of immune cells. Cardiovascular system of snakes is unique in having a renal portal system where the blood from snake’s tail passes through kidneys before returning back to heart. Left lung is vestigial and is even absent in some species. In majority of species only one lung is functional. Anterior portion of lung is highly vascularized participating in gaseous exchange while the posterior portion fails to do so. Saccular lung provides buoyancy to snake’s body. Kidneys and the reproductive organs are paired. Lymph nodes are absent.
Skin is covered by overlapping scales. Belly scales are used for gripping on the surface. Eyelids are transparent and are often known as brille. Shedding of scales is called as ecdysis. In snakes the outer layer of skin is shed as one layer. Scales are actually the derivatives of epidermis. Scales on head, back and belly are important from taxonomic point of view. Scales are named on the basis of their position in the body. Moulting is very important in snakes it serves two fold functions. First it helps the snake from old and worn skin and second it helps to get rid of parasites and mites. Shedding of skin in the form of moulting helps to grow in size and it occurs periodically. Before moulting a snake stops feeding and hides itself in protective places like underground burrows. Just before moulting skin becomes rough, dry and eyes become cloudy. Inner surface of skin liquefies helping the old skin to easily leave the body. Old skin breaks near the mouth and the snake rubs itself against the surface and the old skin leaves the body in one layer. An older snake sheds its skin once or twice a year. A younger growing snake may shed its skin four times a year in order to grow. Discarded skin gives an imprint of scale arrangement on the snake’s body and helps in identification.
Snake vision varies considerably. They can only distinguish light and dark objects and the vision is not sharp but adequate so they can only trace movements. Vision is best in arboreal snakes but poor in the burrow dwellers. Asian vine snake has a binocular vision so both the eyes can focus on the same point. Most snakes focus objects by back and forward movement of lens in relation to the retina while in others lens is stretched. Smell is used by snakes in prey tracking. Airborne vibrations are picked up by the bifid or forked tongue and are then passed to the vomeronasal organ or the Jacobson’s organ for evaluation. Forked tongues help in olfaction as well as tasting prey. Tongues are kept in constant motion and particles from air, water and ground are used for prey capture. The part of bodyin contact with ground is very sensitive to vibrations so they can sense any animal approaching near even with very faint vibration. Pit vipers, pythons and some boas have infra-red sensitive receptors in deep grooves between the eyes and the nostrils.
Cobras, vipers and some related ones use venom in order to immobilize or kill prey. Venom is actually the modified saliva injected through teeth called fangs. Certain birds, mammals and some snakes like the kingsnakes prey on venomous snakes and have developed resistance as well as immunity against the snake venom. Snake venom is a complex mixture of proteins stored in poison glands at the back of the head. These glands open into hollow fangs which act as hypodermic needles for injecting the venom into the victim’s body. The venom proteins can be a mixture of neurotoxins, haemotoxins, cytotoxins and bungarotoxins and other toxins. Venom has hyalouronidase enzyme that helps in rapid diffusion of venom in the victim’s body. Cobras and kraits have proteroglyphous fangs that remain permanently erect. Venomous snakes have been classified in three taxonomic families like Elapidae comprising of cobras, kraits, mambas, coral snakes, sea snakes and Viperidae consisting of vipers, rattlesnakes, copperheads, bushmasters. The third family is Colubridae enclosing boomslangs, tree snakes, vine snakes but all colubrids are not poisonous.
All snakes are strictly carnivorous feeding on eggs, fish, insects, small snakes and mammals. They swallow the prey as a whole. Poisonous snakes kill the prey with venom before swallowing it other swallow the prey by constriction. After feeding they become dormant for effective digestion to take place. They are ectothermic so the external temperature helps in digestion. Best temperature for digestion is 30C. Digestive enzymes help in the digestion of nails, claws, feathers and hairs. Although the limbs are absent but snakes have developed unique ways to carry out locomotion. Lateral undulation is the most common mode of locomotion performed by both aquatic and terrestrial snakes. In lateral undulation the body of snake flexes to right and left resulting in the formation of rearward moving waves. The snake moves faster and high amount of energy is required to carry out this mode of locomotion.
Terrestrial lateral undulation is the most common mode of locomotion performed by land snakes. The posteriorly moving waves push the body more closer to the contact points in the environment like the trees, rocks etc. resulting in the formation of a forward thrust. Snake moves faster even in thick or dense vegetation. While swimming the waves become longer and the snake moves faster. Thrust is generated by pushing the body against water. Sea snakes can perform reverse lateral undulation. Side winding is another mode of locomotion adopted by colubrids like vipers, elapids while moving on a slick mud flat or sand dune. It is a modified form of lateral undulation in which all the body segments move in one direction and are in contact with the ground while other segments are lifted up resulting in rolling of body. This mode of locomotion helps in conserving the energy.
When the push-points are absent and the snake is unable to perform lateral undulation as well as sidewinding like in the tunnels and the burrows then it moves by concertina locomotion. In this locomotion snake braces the posterior portion of body against the tunnel wall while the front of head extends and straightens. Front portion is then relaxed and it bends to form an anchor and the posterior portion now straightens and is pulled forwards. This is a very mode of locomotion and demands high amount of energy. The slowest mode of locomotion is the rectilinear locomotion and it is the only mode of locomotion where the snake does not bends its body. The belly scales are lifted up and pulled forward before being placed down and the body is pulled over them. Ribs do not move in this mode of locomotion. It is the most common mode of locomotion in pythons, boas and vipers.
The tree snakes perform locomotion differently which is very much dependent on the bark of the tree as well as on the species. Snakes use modified form of concertina locomotion and may also crawl by lateral undulation of contact points are available. Snakes move faster on small branches if contact points are available. Fertilization in snakes is internal and is carried by means of hemipenes stored and inverted in the tail of male snake. Most species are oviparous. Females of King cobra construct nests for their eggs and guard them until they hatch. Most pythons coil around their eggs until they hatch. Some species are ovoviviparous and keep eggs inside their bodies until they are ready to hatch. Boa constrictor and green anaconda are viviparous nourishing the youngs by means of placentas well as yolk sac like the higher placental mammals.
Snakes in general avoid biting humans. The bite of the non-poisonous snakes is usually harmless. Out of the 725 species of venomous snakes only 250 species bear the potential to kill humans in one bite. Australia averages only one fatal snake bite per year. In India about 250,000 deaths have been recorded in a year by snake bite. Bite of a poisonous snake affects every organ of human body. It causes dizziness, fainting, increased thirst and head ache. Blood pressure becomes low and pulse rate increases. Muscle coordination diminishes. Respiration slows down. Nausea and vomiting are also common. Excessive bleeding occurs at the site of the bite. The snake venom can be treated with the help of anti-venom. Antivenom is species specific and is prepared by collecting the venom from the poison gland of the poisonous snakes and is then injected into the body of horse. The doses of venom are increased later on until the horse is immunized. Blood is extracted from the immunized horse and the serum is separated and purified and is allowed to freeze.
In some parts of the world particularly in India snake charming is a roadside show performed by charmers. In such a show the charmer plays tunes and the snake is seen to perform certain sorts of motions which are actually a sort of defensive action in response to the movements of the flute. According to the Wildlife Protection Act of 1972 has prohibited such types of illegitimate acts. Snakes are also consumed as food in certain parts of world. In China, Thailand, Taiwan drinking the blood of snakes is believed to increase the sexual virility. Snakes are also a famous delicacy in the form of snake soups in some Asian countries. In western world snakes are also reared as pets. They also deserve a special place in the history and religion. Brazil is known as the Snake Island as it houses the largest population of snakes.
Though snakes appear dangerous but they help in keeping the ecological balance.