Vipers

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Vipers

Vipers are snakes in the family Viperidae, a group of short-tailed, (usually) stout-bodied snakes with long fangs at the front of the mouth, positioned on a short jawbone that can be rotated to bring the fangs from their resting position parallel with the palate to an erect position for striking.

This efficient venom delivery system allows vipers to eat large (and sometimes dangerous) animals without a struggle that might expose them to harm. Vipers make a swift strike in which the long hollow (hypodermic needle like) fangs inject a strong venom deep into the prey’s body. The snakes then wait until the animal dies, tracking it down if necessary, and then calmly swallowing it. The venom also has the effect of initiating digestion even before the prey is swallowed. Many vipers do not find it necessary to eat more than once a month. The venoms of these snakes are diverse, being adapted to quickly kill the preferred prey animals of each species.

Vipers are an old and diverse group. They are generally divided into the Old World “true” vipers (Viperinae) and the pitvipers (Crotalinae), which are found in Asia and the Americas. One unusual viper, (Azemiops) from southern China and northern Myanmar of unknown relationships, is placed in its own sub-family, Azemiopinae.

Old World vipers

Seventy-five species of true vipers (family Viperinae) are found in the Old World and lack facial pits; this distinguishes them from the pit vipers of the Americas. Africa is the home of many species of Old World vipers. Among the African vipers are the Gaboon viper (Bitis gabonicus) and the rhinoceros viper (B. nasicornis) that attain lengths of almost 6 ft (2 m) and have fangs that may be 2 in (5 cm) in length. There are green tree-vipers (Atheris), desert-dwelling sandvipers (Cerastes), and even a little-known worm-eating, burrowing viper (Adorhinos).

The deserts of north Africa and south Asia are home to a number of sandvipers, one of which is the notorious carpet viper (Echis carinatus). Although an adult of this snake may be little more than 1 ft (30 cm) in length, its bite is highly toxic and is responsible for many deaths, particularly in the desert regions of Pakistan and western India.

The small European viper or adder (Vipera berus), by contrast, is not dangerous to humans. Its bite has been described as “a little worse than a bee-sting,” and the few reported deaths are apparently due to over-treatment. This viper is notable in that it is one of very few snakes that ranges above the Arctic Circle in Sweden and Norway.

Some large vipers of the genus Vipera range from the eastern Mediterranean eastward through southern

Asia. The ill-tempered and highly venomous Russell’s viper (Vipera russelii) follows the rats into the rice fields when the fields are drained for harvesting. It is the major cause of fatal snake bites (killing perhaps 10,000 people annually) in Myanmar and Thailand.

Pitvipers

the pitvipers are easily identified by the loreal pit, a heat-sensitive receptor that lies between the nostril and the eye on either side of the face. With this receptor the snakes can detect and accurately strike a warmblooded prey animal in absolute darkness, guided by the infra-red (heat) rays that the prey animal produces. The Viperinae lack this heat-sensitive pit.

The pitvipers range from eastern Europe to the East Indies and Japan, and from Canada to Argentina in the Americas. Although the 174 species of New World pit vipers are much more diverse than the Old World vipers, most are terrestrial, with a few (usually green) arboreal species. The best-known pit vipers are the rattlesnakes (Crotalus and Sistrurus ), which are found only in the Americas and whose modified tail skin vibrates to produce a rattling, warning sound. Most rattlesnakes are North American, ranging from southern Canada to Panama, with the greatest number inhabiting the dry regions of the southwestern United States and northern Mexico. However, a small tropical group (Crotalus durissus and its relatives ) ranges as far as southern Brazil.

There are about 60 species of tropical pitvipers (Bothrops and relatives ) that range from coastal Mexico southward to the Patagonian plains of Argentina. Most of these are relatives of the large terrestrial South American “fer-de-lance” Bothrops atrox, but Central America has a more diverse assemblage composed of several genera, some of which are treevipers.

KEY TERMS

Loreal pit —A sensitive heat receptor found in pitvipers.

Venomous —Having a toxic substance used to subdue prey and in defense.

The largest of all vipers is the bushmaster (Lachesis muta) of northern South America and Panama, which attains a reported length of 12 ft (4 m). Like the large African vipers, it has very long fangs (2 in/5 cm or more) and a large supply of venom. Very few people are bitten by these snakes, however, because bushmasters live in forested areas and are active only at night.

Asia has a large contingent of pitvipers (including green, arboreal species), similar to the American Bothrops, that are placed in the genus Trimeresurus. Some of its members are so similar to some of the American species that there is some doubt that they should be in different genera.

The water moccasins (genus Agkistrodon ) are found both in North America and in Asia. The copperhead (Agkistrodon contortrix) is common in the forests of the eastern United States, and is responsible for most of the venomous snakebites in that region. Fortunately, its venom is not highly toxic to humans, and almost no one dies from these bites.

See also Reptiles.