|English: Indian Spectacled Cobra, Naja Naja Family, one of India's venomous snakes. (Photo credit: Wikipedia)|
Venomous snakes are species of the suborder Serpentes that produce venom. Members of the families Elapidae, Viperidae, Atractaspididae and Colubridae are major venomous snakes.
Venomous snakes use modified saliva, and snake venom, usually delivered through highly specialized teeth, such as hollow fangs, for the purpose of prey immobilization and self-defense. In contrast, nonvenomous species either constrict their prey, or overpower it with their jaws.
Venomous snakes include several families of snakes and do not form a single taxonomic group. This has been interpreted to mean venom in snakes originated more than once as the result of convergent evolution. Evidence has recently been presented for the Toxicofera hypothesis, but venom was present (in small amounts) in the ancestors of all snakes (as well as several lizard families) as 'toxic saliva' and evolved to extremes in those snake families normally classified as venomous by parallel evolution. The Toxicofera hypothesis further implies that 'nonvenomous' snake lineages have either lost the ability to produce venom (but may still have lingering venom pseudogenes), or actually do produce venom in small quantities, likely sufficient to help capture small prey capture but causing no harm to humans when bitten.
Most venomous snakes
Lists or rankings of the world's most venomous snakes are tentative and differ greatly because of numerous factors. The high variability of LD50 tests is a major problem. This includes the age and reliability of the data, the number of species analyzed, and the testing methods and toxicity scale used. While there have been numerous studies on snake venom, potency estimates can vary, creating overlap and greatly complicating the task. Further, LD50 may be measured through intramuscular, intraperitoneal, intravenous or subcutaneous injections on small rodents, although the latter is the most applicable to actual bites. So, considering the toxicity of a species based on LD50 alone may not accurately estimate the danger of the species to humans since the efficiency of venom delivery is not taken into account. Furthermore, results from different tests may cause confusion since different toxicity scales are in use.
Apart from the high variability of toxicity tests, the physiological differences between the animals tested and humans is another major problem in categorizing the most venomous snakes. Mice are the common indicator used to test venom from venomous snakes in LD50 tests, so the LD50 results may not reflect the actual effects on humans due to the physiological differences between mice and humans. Many venomous snakes are specialized predators on mice, and their venom may be adapted specifically to incapacitate mice. While most mammals have a fairly similar physiology, LD50 results may or may not be directly relevant to humans. Moreover, many of these lists include terrestrial and arboreal snakes only and leave out venomous sea snakes. The venom of some sea snake species is more toxic than that of even the inland taipan, and further investigations of certain species' venom are needed.
Toxicity of snake venom is sometimes used to gauge the extent of their danger to humans, but this is improper. A number of other factors are more critical in determining the potential hazard of any given venomous snake to humans, including the distribution and behavior of each species. For example, while the inland taipan is regarded as the most venomous land snake based on LD50, so-called Big Four snakes cause far more snakebites because they are much more abundant in highly-populated areas. Clinical mortality rate (often determined by measured toxicity on mice) is another commonly used indicator to determine the danger of any given venomous snake, but important to are its efficiency of venom delivery, its venom yield and its behavior when it encounters humans. Black mamba and coastal taipan bites, when untreated, have a mortality rate of almost 100%. Other species with clinically proven high mortality rates include the common krait  and the king cobra,  etc. The timespan between envenomation and death is another measure of how dangerous a given venomous species is to humans; e.g., a mamba,  king cobra  or coastal taipan  bite is rapidly fatal if untreated (in contrast to the inland taipan, whose bite has caused no known human fatality).