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Evolution, Snakes And HumansAppearance and behavior

Snakes are limbless reptiles with an elongated, cylindrical body, scaly skin, lidless eyes, and a forked tongue. Most species are non-venomous, some are mildly venomous, and others produce a deadly venom. All snakes are carnivores (or meat-eaters). They are also cold blooded (or ectotherms), meaning their body temperature is determined by the environment, rather than being internally regulated (however, snakes will bask in the sun to warm up, and hide in shade to cool down). Because they are ectotherms, snakes are found mainly in tropical and temperate regions throughout the world, and are rare or absent in cold climatic zones.

The 2,700 species of snakes fall into three superfamilies. The Scolecophidia (or Typhlopoidea) comprised the blindsnakes. The Boidea includes relatively primitive (i.e., evolutionarily more ancient) snakes, and includes the family Boidae, consisting of the boas and pythons. The Colubroidea includes the advanced (i.e., more recent) snakes, and includes the family Colubridae (harmless king snakes), the Elapidae (venomous cobras and their relatives), and the Viperidae (adders and pit vipers, which are also venomous).

The family Colubridae is huge, with over 300 genera and 1,400 species, and includes the majority of living species. Most colubrids are harmless (e.g., king snakes). However, the rear-fanged snakes (which lack hollow fangs) have a poison that they inject by chewing on the prey, rather than by a strike. The family Elapidae includes most of the poisonous snakes (cobras, coral snakes, mambas, and kraits), which have fixed, grooved or hollow fangs in the front of the mouth. The base of the fangs is connected to a venom gland, and poison is injected when the victim is bitten. The family Viperidae includes the vipers and pit vipers, which are the most specialized venom injectors. These snakes have long, hollow fangs that fold back when the mouth is closed, and swing forward and down when the mouth is open in the strike position. The pit vipers include some of the most dangerous snakes in the Americas, such as the rattlesnakes, water moccasin, copperhead, bushmaster, and fer-de-lance. In the United States, the venomous snakes include the rattlesnakes, cottonmouth, coral snake, and copperhead. Many other venomous snakes are found in Australia, Africa, Asia, and South America.

The thread snake (4.5 in long; 11.5 cm) is the shortest snake, while the longest is the South American anaconda, measuring up to 37 ft (11 m).


Snakes are covered in dry, glistening scales. Many species have a body pattern of various colors, sometimes quite bright. The dorsal (or back) scales protect the body from friction and dehydration, and the ventral (or belly) scales aid in movement by gripping the surface while powerful muscles propel the body forward, usually with a side-to-side, waving motion. This method of locomotion means that snakes cannot move backward.

Instead of eyelids, the eyes of snakes are covered and protected by a single, clear scale. Several times a year, at intervals determined by the growth rate, age, and rate of metabolism, snakes molt their epidermal skin, shedding it in one complete piece. They do this by rubbing their head against a stick or another rough surface, which starts the shedding at the mouth. Once the first bit of molted skin catches on something, the snake literally crawls out of the rest, which is discarded inside-out.

Hunting and defense

The coloring and patterning of many snakes provides an excellent camouflage from predators and prey. Tree snakes may be green colored as camouflage amongst leaves; ground snakes may be brown or dusty gray to blend with litter and rocks; and sea snakes are dark above and light beneath (this is known as countershading, and is also commonly seen in fish). Some snakes are brightly colored with vivid patterns, such as the highly venomous coral snake with its red (or orange), black, and yellow (or white) rings. Often, poisonous snakes are highly colorful as a way of warning potential predators to leave them alone.

Snakes attack prey only when hungry, and will try to bite a human only if they feel seriously threatened. If possible, a frightened snake will almost always try to flee. However, if there is no time for flight, or if a snake feels cornered, it may try to strike in defense. Venomous snakes have two fangs in the upper jaw that can penetrate the flesh of their prey, while poison glands pump poison through grooves inside or outside of the fangs. When hunting, some poisonous snakes inject their prey with toxin and wait until the animal is no longer struggling before eating it. In this use, snake venom is a feeding aid, serving to both subdue the prey and to aid in its digestion. Snake venoms are cocktails of complex enzyme-like chemicals, and they act on the prey in several different ways. Some venoms are neurotoxins, paralyzing parts of the nervous system. Others prevent the blood from clotting, while yet others cause blood to clot. Some destroy red and white blood cells, and others destroy tissue more generally.

Non-venomous constrictors (such as boas, pythons, and anacondas) simultaneously snatch their prey in their jaws, and rapidly coil their body around the animal, squeezing it to prevent breathing. The prey dies by suffocation; its bones are usually not broken during the constriction.


The teeth of snakes cannot chew and break up a carcass, so they swallow their prey whole. With the aid of elasticized ligaments on their specially hinged lower jaw, the mouth can open to an incredible 150-degree angle, permitting the consumption of animals several times larger than the snake's head. The largest recorded feast was a 130-lb (59 kg) antelope swallowed by an African rock python.

Snakes' teeth curve inward and help prevent their prey from escaping. The strong jaw and throat muscles work the food down the esophagus and into the stomach, where digestion begins. Digestion time varies according to temperature. In one study, a captive python at a temperature of 87°F (30°C) digested a rabbit in four days; at a cooler temperature (64°F; 18°C) digestion took more than two weeks.

The interval between meals also varies, and some snakes may go weeks or even months without food. In temperate climates, snakes fast during the winter hibernation, which may last six months. Pregnant females may hibernate and fast for seven months, and both sexes fast briefly before shedding.

Snakes have extremely poor eyesight and hearing. They detect their prey through vibrations and heat and chemical perceptions, all of which are highly developed and efficient senses in snakes. Pit vipers (such as rattlesnakes) have tiny hollows (or "pits") on the side or top of their snout, which have sensors that can detect the body heat of a bird or mammal at a considerable distance. The flicking, forked tongue of a snake acts as a chemical collector, drawing chemical "smells" into the mouth to be analyzed by sensors (Jacobson's organs) on the palate. This mechanism also allows male snakes to detect the hormones of females in reproductive condition.

Mating and reproduction

Insemination takes place through the vent (or cloaca) of the female, an opening located beneath and near the end of the body, just before the tail. Male snakes lack a true penis, and instead have paired structures called hemipenes, which emerge from their vent during mating. Sperm runs in a groove along each hemipenis. Female snakes may mate with several different males. Gestation time varies widely, from only 30 days in some species to as much as 300 days in others. Most species lay eggs, with the young forcing their way out of the pliable, porous shell when their incubation is over. Other snakes give birth to fully formed young—the eggs are retained in the body of the female until they hatch, so that "live" young are born (this is known as ovovivipary). Some species of pythons incubate their eggs—the female coils around her eggs and shivers to generate heat, keeping them warm until they hatch. In general, however, snake eggs and young receive little or no parental care.

Additional topics

Science EncyclopediaScience & Philosophy: Adam Smith Biography to Spectroscopic binary