Wednesday, 12 October 2011

KING COBRA

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, which preys chiefly on other snakes, is found predominantly in forests from India through Southeast Asia to the Philippines and Indonesia. Despite the word "cobra" in its name, this snake is not a member of Naja ("true cobras") but belongs to its own genus.

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 generally 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 Asian cobras. A foolproof method of identification is if on the head, 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.


Habitat

The king cobra is distributed across South Asia, Southeast Asia, and the southern areas of East Asia (southern China) where it 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]

Behaviour

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 diurnal species.


South Indian king cobra

Defense

Although the king cobra usually avoids confrontation with humans, it can be aggressive if provoked.[9] If threatened, it rears up the anterior portion of its body when extending the neck, showing the fangs and hissing loudly.[10] It can be easily irritated by closely approaching objects or sudden movements. The snake strikes rapidly and the attack range can be as far as 2 meters. It is also known to advance to the enemy with a certain distance due to its far strike range where people can easily misjudge the safe zone. The king cobra can deliver multiple bites in a single attack but adults are known to bite and hold on.[11] In spite of being a highly dangerous snake, it prefers to escape first unless it is cornered or provoked.[9] Since this species is secretive and tends to inhabit less-populated forested regions and dense jungle, it is rarely encountered.

If a king cobra encounters a natural predator, such as the mongoose, which has 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.


king cobra snake attack
 Venom

The venom of the king cobra consists primarily of neurotoxins, but it also contains cardiotoxic and some other compounds.[8] Toxic constituents are mainly proteins and polypeptides.[14]

During a bite, venom is forced through the snake's 1.25 to 1.5 centimeters (0.49 to 0.59 in) fangs into the wound, and the toxins begin to attack the victim's central nervous system. Symptoms may include severe pain, blurred vision, vertigo, drowsiness, and paralysis. Envenomation progresses to cardiovascular collapse, and the victim falls into a coma. Death soon follows due to respiratory failure. Moreover, king cobra envenomation is clinically known to cause renal failure.[15]

A 1990 book makes a passing statement of a LD50 of 0.34 mg/kg for this species[16], however this value is inconsistent with most toxicological studies.[17][18][19] For example, a recent study lists the LD50 of the king cobra venom as 1.6 mg/kg – 1.8 mg/kg[20], making it's venom one of the least potent among the elapids.[15] This value is further backed up by another toxicological study which lists the LD50 of the king cobra at 1.7 mg/kg.[21] A similar mean LD50 value of 1.93 mg/kg was obtained from the venom of five wild caught king cobras in Southeast Asia (Meier et al 1995).[22]

This species is capable of delivering a large quantity of venom, injecting a dose anywhere from 200-500 milligrams on average, but it can potentially deliver much more than 500 mg.[10][16][23] Though the venom is weak compared to most other elapids,[15] it can still deliver a bite which can potentially kill a human due to the massive amount of venom it delivers in a single bite. Mortality can vary sharply with amount of venom involved, most bites involve nonfatal amounts.[24] According to a research report from the University of Adelaide Department of Toxinology, an untreated bite has a mortality rate of 50-60%[25]

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.[26] Ohanin, a protein component of the venom, causes hypolocomotion and hyperalgesia in mammals.[27] Other components have cardiotoxic,[28] cytotoxic and neurotoxic effects.[29] 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.[30]

The haditoxin in the king cobra venom was discovered by Singaporean scientists to be structurally unique and can have unique pharmacological properties.[31] Biochemical studies confirmed that it existed as a non-covalent dimer species in solution. Its structural similarity to short-chain α-neurotoxins and κ-neurotoxins notwithstanding, haditoxin exhibited unique blockade of α7-nAChRs (IC50 180 nM), which is recognized by neither short-chain α-neurotoxins nor κ-neurotoxins.[14]


Reproduction

The king cobra is unusual 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,[32] for roughly 60 to 90 days.

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.[33] The baby king cobras, with an average length of 45 to 55 centimeters (18 to 22 in), have venom which is as potent as that of the adults. They may be brightly marked but these colours often fade as they mature. They are alert and nervous, being highly aggressive if disturbed.

king cobra eating snake
Diet
The king cobra's genus name, Ophiophagus, means "snake-eater", and its diet consists primarily of other snakes, including ratsnakes, small pythons and even other venomous snakes (smaller members of its own species, true cobras (of the genus Naja), and even the much more venomous members of the krait family.[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.

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