snake

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Reptile of the suborder Serpentes of the order Squamata, which also includes lizards. Snakes are characterized by an elongated limbless body, possibly evolved because of subterranean ancestors. However, a team of US and Israeli palaeontologists rediscovered a fossil collection in 1996 which suggested that snakes evolved from sea-dwelling predators.

One of the striking internal modifications is the absence or greatly reduced size of the left lung. The skin is covered in scales, which are markedly wider underneath where they form. There are 3,000 species found in the tropical and temperate zones, but none in New Zealand, Ireland, Iceland, and near the poles. Only three species are found in Britain: the adder, smooth snake, and grass snake.

Locomotion
In all except a few species, scales are an essential aid to locomotion. A snake is helpless on glass where scales can effect no ‘grip’ on the surface; progression may be undulant, ‘concertina’, or creeping, or a combination of these.

Senses
Detailed vision is limited at a distance, though movement is immediately seen; hearing is restricted to ground vibrations (sound waves are not perceived); the sense of touch is acute; besides the sense of smell through the nasal passages, the flickering tongue picks up airborne particles which are then passed to special organs in the mouth for investigation; and some (rattlesnakes) have a cavity between eye and nostril which is sensitive to infrared rays (useful in locating warm-blooded prey in the dark).

Reproduction
Some are oviparous and others ovoviviparous, that is, the eggs are retained in the oviducts until development is complete; in both cases the young are immediately self-sufficient.

Species
The majority of snakes belong to the Colubridae family, and are chiefly harmless, such as the common grass snake of Europe, but this family includes the deadly African boomslang Dispholidus typus. The venomous families include the Elapidae, comprising the true cobras, the New World coral snakes, and the Australian taipan, copper-head, and death adder; the Viperidae (see viper); and the Hydrophiidae, aquatic sea-snakes.

Among the more primitive snakes are the Boidae, which still show links with lizards and include the boa constrictor, anaconda, and python. These kill by constriction but their victims are usually comparatively small animals.

All snakes are carnivorous, and often camouflaged for better concealment in hunting as well as for their own protection.

Treatment of snakebite
There are 50,000–100,000 deaths from snake bites worldwide annually, it was announced in September 2000. The serums used to treat snakebites are called antivenins. Antivenins are produced by injecting animals (horses and sheep are used) with venom, extracting their blood, now containing antibodies to the venom, and removing the red blood cells. However, in addition to the desired venom antibodies, many other antibodies and proteins are contained within the serum. These often cause ‘serum sickness’ in the patient, as a result of a severe allergenic reaction.

As antivenins are expensive to prepare and store, and specific to one snake species, experiments have been carried out using more widely valid treatments, for example, trypsin, a powerful protein-degrading enzyme, effective against the cobra/mamba group.

In 1993 Japanese and Brazilian researchers independently identified a protein in the blood of a venomous snake that neutralizes its own venom. In laboratory tests in Australia, this protein, named Notechis scutatus inhibitor (NSI) after the tiger snake from the whose blood it was isolated, was effective against the venom of six other snakes.

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