Venomous snake

From New World Encyclopedia
Western bush viper, A. chlorechis

Venomous snake is any of a large and diverse number of snakes that is able to inject venom (modified saliva) into another organism, essentially for purposes of capturing prey or self-defense. Venom normally is delivered by means of a bite or stab using highly specialized teeth such as retractable or fixed hollow fangs, while a poisonous organism involves ingestion of the animal or contracting the poisonous agent through the skin. Whereas venomous snakes use venom to immobilize prey, non-venomous species either constrict their prey, or simply overpower it with their jaws.

Venomous snakes include several families of snakes and do not form a single taxonomic group. Two of the most well-known families of venomous snakes are Viperidae (vipers, such as rattlesnakes and puff adders) and Elapidae (such as cobras and sea snakes). ) Atractaspididae (such as burrowing asps and mole vipers) also is a family of venomous snakes, and venomous members are found in Colubridae, such as the boomslang. The history of venom appears to be ancient and the number of venomous snakes may be more common than previously thought, with more than 2,000 species identified and a suggestion that all snakes may be venomous to a certain degree (Fry et al. 2006; Zimmer 2005).

Although the majority of venomous snakes are small innocuous creatures, many are capable of causing painful injury or death to humans. Venom in snakes is more for killing and subduing prey than it is for self-defense.

Overview

Venomous snakes are often said to be poisonous, although this is not the correct term, as venoms and poisons are different. Poisons can be absorbed by the body, such as through the skin or digestive system, while venoms must first be introduced directly into tissues or the blood stream through mechanical means. It is, for example, therefore harmless to drink snake venom as long as there are no lacerations inside the mouth or digestive tract (Klauber 1997). While the term "poisonous snake" is mostly incorrect—poison is inhaled or ingested whereas venom is injected (Freiberg 1984, 125)—there are, however, poisonous snakes known to exist.

The fangs of vipers (Viperidae and elapids (Elapidae)are hollow in order 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, its role in self-defense is secondary (Mehrtens 1987, 243).

Venom, like all salivary secretions, is a pre-digestant that initiates the breakdown of food into soluble compounds allowing for proper digestion, and even "non-venomous" snake bites (like any animal bite) will cause tissue damage (Mehrtens 1987, 209). Snake venoms are complex mixtures of proteins and are stored in poison glands at the back of the head (Freiberg 1984, 123). In all venomous snakes, these glands open through ducts into grooved or hollow teeth in the upper jaw (Mehrtens 1987, 243; Freiberg 1984, 5). 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 (Frieberg 1984, 125). Almost all snake venom contains hyaluronidase, an enzyme that ensures rapid diffusion of the venom (Mehrtens 1987, 243).

Venomous snakes that use hemotoxins usually have the fangs that secrete the venom in the front of their mouths, making it easier for them to inject the venom into their victims (Frieberg 1984, 125). Some snakes that use neurotoxins, such as the 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 (Frieberg 1984, 125). Elapid snakes, however, such as cobras and kraits, are proteroglyphous, possessing hollow fangs that cannot be erected toward the front of their mouths and cannot "stab" like a viper; they must actually bite the victim (Mehrtens 1987, 242).

Certain birds, mammals, and other snakes such as kingsnakes that prey on venomous snakes, have developed resistance and even immunity to certain venom (Mehrtens 1987, 243).

Families of venomous snakes

Over 2,000 species are known to be venomous, about two-thirds of all snake species (Fry et al. 2006; Zimmer 2005). The following groups of snakes can be aggressive and inflict dangerous, even potentially lethal bites.




Family Description
Atractaspididae (atractaspidids) Burrowing asps, mole vipers, stilleto snakes.
Colubridae (colubrids) Most are harmless, but others have toxic saliva and at least five species, including the boomslang (Dispholidus typus), have caused human fatalities.
Elapidae (elapids) Cobras, coral snakes, kraits, mambas, sea snakes, sea kraits and Australian elapids.
Viperidae (viperids) True vipers and pit vipers, including rattlesnakes.
There is a third family containing the opistoglyphous (rear-fanged) snakes as well as the majority of other snake species (Freiberg 1984; 126; Mehrtens 1987, 209):
  • Colubrids - boomslangs, tree snakes, vine snakes, mangrove snakes, although not all colubrids are venomous.

Evolution

Snakes may have evolved from a common lizard ancestor that was venomous, from which venomous lizards like the gila monster and beaded lizard may have also derived. This hypothesis suggests that all snakes 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, curling out only when the snake strikes.

The fact that venomous snakes are spread over several families has been interpreted to mean that venom in snakes originated more than once as the result of convergent evolution. Evidence has recently been presented for the Toxicofera hypothesis however; if correct, venom was present (in small amounts) in the ancestor 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 'non-venomous' 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 assist in small prey capture, but not normally cause harm to humans if bitten.

References
ISBN links support NWE through referral fees

  • Freiberg, M., and J. Walls. 1984. The World of Venomous Animals. New Jersey: TFH Publications. ISBN 0876665679.
  • Fry, B. G., N. Vidal, J. A. Norman, F. J. Vonk, H. Scheib, R. Ramjan, and S. Kuruppu. 2006. Early evolution of the venom system in lizards and snakes. Nature (Letters) 439: 584-588.

[1]


External Links

Credits

New World Encyclopedia writers and editors rewrote and completed the Wikipedia article in accordance with New World Encyclopedia standards. This article abides by terms of the Creative Commons CC-by-sa 3.0 License (CC-by-sa), which may be used and disseminated with proper attribution. Credit is due under the terms of this license that can reference both the New World Encyclopedia contributors and the selfless volunteer contributors of the Wikimedia Foundation. To cite this article click here for a list of acceptable citing formats.The history of earlier contributions by wikipedians is accessible to researchers here:

The history of this article since it was imported to New World Encyclopedia:

Note: Some restrictions may apply to use of individual images which are separately licensed.

  1. Klauber LM. 1997. Rattlesnakes: Their Habitats, Life Histories, and Influence on Mankind. Second Edition. First published in 1956, 1972. University of California Press, Berkeley. ISBN 0-520-21056-5.