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SNAKE,
common name for reptiles in the order Squamata, which also
includes lizards, and of the suborder Serpentes. The approximately
2500 different species known display a great range in appearance
and behavior, from the antics of the harmless hognose to the stealth
of the deadly mamba. The largest family in the order, Colubridae,
includes the most common snakes on all continents except Australia, where
the family Elapidae outnumbers all others. This latter group includes
some of the most poisonous snakes in the world, such as the cobra
and coral snake. Another important family of venomous snakes is Viperidae,
which includes the pit vipers, such as the rattlesnake and the fer-de-lance,
and the true vipers, such as the adder. The largest snakes in the
world are members of the family Boidae, which includes the boa and the
python. Some members of this family never attain a length of more
than 0.6 m (2 ft), but the largest may grow to more than 9 m (30
ft).
The body of the snake is long and usually cylindrical, although the
bodies of sea snakes and many tree-dwelling snakes are flattened
on the sides. The body is covered with horny scales arranged in
regular rows and usually overlapping, like tiles on a roof. The
scales on the back and sides are generally smaller than those on
the top of the head and along the under, or ventral, surface. The scales
on the upper, or dorsal, surface of the snake may be smooth or may
have a long raised ridge or keel; the scales may also have small
knobs and tiny pits. The skin and outer covering of the horny scales
are shed periodically and usually in one piece, including the hard,
transparent covering of the eye known as the spectacle (snakes lack
movable eyelids, and the spectacle protects the constantly open
eyes). The frequency of shedding varies with different species and
within species, according to the size and age of the individual. Young,
rapidly growing snakes shed their skins more frequently than the
slow-growing adults. In some species the skin is shed about every
20 days; in others, only once a year.
The long, slender body of the snake contains no fewer than 100
vertebrae and sometimes more than 300. Except for the first two,
which connect with the head, each vertebra is connected to a pair
of ribs. The skeleton is light and provides great freedom of movement.
The skull in particular is loosely built and can be stretched in
several directions, permitting the snake to swallow large meals
in relation to the size of the head and body. This mobility can
be seen especially in the two bones of the lower jaw, which are
attached to the skull by a short, movable bone and which are united
at the front end merely by an elastic ligament. Both jaws have a
large number of sharp, needlelike teeth, all curved toward the rear
of the mouth. The teeth are arranged in six rows, two rows on each
side of the upper jaw and one on each side of the lower jaw. Except
in venomous species, the teeth are solid and are replaced periodically.
When the snake catches its prey, the recurved teeth enable it to
retain a firm hold. The prey is killed quickly and swallowed by
alternate movement of the rows of teeth, which work to pull the
food into the mouth. As the food passes through the mouth, it is
covered with saliva; contrary to popular belief, snakes do not cover
their food with saliva before taking it into the mouth. Most meals
are swallowed easily and quickly, but a large meal may take several
hours. The big pythons can eat animals that weigh up to about 68
kg (150 lb), but swallowing such a meal is a difficult process.
Venomous snakes have two hollow, hypodermiclike teeth, known
as fangs, in the front of the upper jaw that, like the other teeth,
are replaced periodically. The new fang moves into place before
the old one is lost, so that, for a short time, the snake may have
two fangs on each side of the jaw. In the solenoglyphs, a large group
of snakes that includes the vipers, the fang-bearing bone is attached
to the skull in such a way that the fangs can be moved back up against
the roof of the mouth when not in use. The cobras and coral snakes represent
another large group of snakes, the proteroglyphs, in which the fangs
are not movable but are constantly erect. The fangs of both groups
are connected by the venom duct to the two venom glands, which are
modified salivary glands situated on each side of the head behind
the eye.
The snake must bite to inject its venom; no snake has a stinger
in its tail. Snakes can bite at any time and from any position,
including underwater. They normally strike out from a defensive
coil, projecting the head and front part of the body at the victim.
As soon as the head leaves the coil, the mouth is wide open. The solenoglyphs
stab the erect fangs into the target and usually withdraw them immediately,
either with or without a biting movement. The proteroglyphs usually
bite, and the bite may be retained for several seconds. Three species
of snake can spit or eject the venom in a fine spray, which is aimed
at the eyes of an enemy and projected for distances up to 2.4 m
(8 ft). If the venom gets into the eyes, it may cause blindness.
The spitting is used only in defense and never to obtain food.
Snake venom damages various tissues of the body, immobilizing
or killing the snake’s prey. Venoms are usually classified
in two major categories, neurotoxic, or nerve-affecting, and hemotoxic,
or blood-affecting. Because of local damage to circulatory cells,
hemotoxic venom spreads more slowly and has a slower action than
the neurotoxins.
Although snakes lack a true voice, they often hiss loudly.
The bull snakes and their relatives, for example, have a small flap
in front of the opening of the windpipe that enables them to produce
an extremely loud hiss.
Vision is well developed in most snakes, but many burrowing
snakes are virtually blind. Snakes have a strong sense of smell,
which is relied on to a large extent in hunting food. They also
have an extra chemical sense that is widely used during most activities.
The tongue flicks out, picking up odors and carrying them to the
roof of the mouth into contact with a sensory receptor called Jacobson’s
organ. The tongue also receives touch stimuli. Snakes are deaf to
airborne sounds. Thus a rattlesnake does not hear the sound of another snake’s
rattle, nor does the cobra hear, as it is believed, the snake-charmer’s
flute. They can, however, feel vibrations through the ground or
whatever they are resting on.
The pit vipers, as well as some boas and pythons, have another
kind of sense organ, a heat receptor that can detect small differences
in temperature. In the pit vipers the heat receptor is located in
facial pits, between the eye and nostril on each side of the face;
in the boas and pythons the receptor is located in the labial pits.
By means of these receptors the snakes can locate and seize warm-blooded
prey at night.
Most snakes lack external limbs, although a few have vestigial
hind legs, the tips of which are visible at the base of the tail
in large boas and pythons. A surprising characteristic of snakes
is their ability to move rapidly without legs. Four different types
of locomotion are used by snakes in getting from place to place,
although not all snakes can use all four methods. The most commonly
used method is the simple, undulating crawl, which is called the
serpentine method. In this type of locomotion, the snake pushes
against the ground on the back side of each curve or undulation
and flows smoothly forward. All snakes use the serpentine method
for maximum speed. Snakes move slower than an adult human can run; the
fastest recorded speed achieved by any snake is about 13 km/hr
(8 mph), but few can go that fast. In another method called caterpillar,
used only by the heavier-bodied snakes, the skin of the ventral
surface is moved forward and backward by strong muscles, and the
broad belly scales grip the ground, moving the snake forward in
a straight line. This method has led to the mistaken belief that
snakes walk on their ribs; actually the ribs do not move forward
and backward in any of the four types of movement. Several desert-dwelling
species use a special type of locomotion, called sidewinding, to
move on loose sand. In this method the snake rolls its body sidewise
along the ground in a looping motion. The fourth method is known
as concertina, because the body is alternately stretched out and
pulled together as the snake moves from one anchor point to another.
The concertina is used in crossing smooth surfaces and in climbing.
Except for sidewinding, any of the methods may be used in climbing,
but snakes swim only by means of the serpentine method. Some species
of the Colubridae family, such as garters, and king snakes, in East
Asia and New Guinea reportedly fly. These snakes do not actually
fly, but they can drop or hurl themselves from fairly high trees
and fall or even partly glide to the ground without injury. It was
once mistakenly believed that the mud snake could take its tail
in its mouth and roll along like a hoop, which gave rise to its
popular name, the hoop snake.
Snakes have a well-developed nervous system and an intelligence intermediate
between that of fish and mammals. They adapt well to captivity,
and many become quite tame. The members of many species are nervous
and excitable when first caught but usually respond quickly to gentle
care. When disturbed in the wild, some snakes, such as the hognose,
go through elaborate bluffing performances; harmless snakes can
even appear more dangerous than venomous ones.
Many snakes vibrate their tails back and forth when excited,
and if they happen to be in dry grass or leaves, a whirring noise
may be heard. The rattlesnake’s horny rattles, as an extreme
example, are vibrated at an average rate of 50 times per second.
The rattling sound, like the hiss, is a warning to enemies; it is
not a lure or a mating sound or a signal to other rattlesnakes.
Contrary to popular opinion, the number of rattles does not indicate
the age of the snake. A new segment or rattle is added every time
the snake sheds its skin, which occurs from one to five times a
year, and many segments are broken off from time to time.
All snakes are carnivorous, eating a variety of animal life,
from insects, spiders, and snails to frogs, mice, and rats. Oddly,
a number of snakes eat only other kinds of snakes.
Snakes reproduce either by laying eggs, which the female usually abandons
in a secluded spot, or by bringing forth living young from eggs
hatched within the body, which are also abandoned. Eggs and young
may number as many as 100 at one time, but much smaller clutches
are usual. Parental care is entirely absent so far as the young
are concerned, but a few species guard the eggs during incubation,
and the female python actually broods them. Growth is fairly rapid,
and the young reach maturity in from one to five years. The greatest
age known for any snake is just under 30 years, attained by both
the anaconda and the black-lipped cobra.
Snakes are found in all the warmer parts of the world, except
Ireland, New Zealand, and a number of oceanic islands. They occur
as far north as the Arctic Circle, but the variety decreases from
the Tropics poleward.
Snakes are the most recent group of reptiles to appear, probably
evolving from lizards or lizardlike animals about 100 million years
ago, toward the end of the Mesozoic era. Human encroachment upon
their habitats, as well as the belief that they are dangerous and
must be exterminated, threatens to diminish their numbers.
Through their carnivorous food habits, however, snakes often
play an important part in preserving an ecological balance, particularly
in controlling such pests as rodents, which tend to multiply at
a rapid rate. They also help to keep down a number of species injurious
to agriculture.
The snake has been held in mythical or religious awe by various
cultures throughout history. The ancient Egyptians worshipped the
asp as a symbol of fertility; the Greeks and Romans associated the
Aesculapian snake with the god of healing and medicine; and the
serpent figures prominently in the Bible. Among some American Indian
tribes, the snake has been associated with rain and fertility. The
cobra has been highly regarded in India, and snakes play a role
in the practice of voodoo in the West Indies, as well as in snake-handling
cults in the U.S. The serpent also appears, usually as a symbol
of evil, throughout art and literature.