Snake
From Wikipedia, the free encyclopedia
Snakes are elongated, limbless, carnivorous reptiles of
the suborder Serpents /sɜːrˈpɛntiːz/. Like all
other squamates, snakes are ectothermic, amniote vertebrates covered
in overlapping scales. Many species of snakes have skulls with
several more joints than their lizard ancestors, enabling them to
swallow prey much larger than their heads with their highly mobile jaws.
To accommodate their narrow bodies, snakes' paired organs (such as kidneys)
appear one in front of the other instead of side by side, and most have only
one functional lung. Some species retain a pelvic girdle with a
pair of vestigial claws on either side of the cloaca. Lizards
have evolved elongated bodies without limbs or with greatly reduced limbs about
twenty-five times independently via convergent evolution, leading to many
lineages of legless lizards. These resemble snakes, but several
common groups of legless lizards have eyelids and external ears, which snakes
lack, although this rule is not universal (see Amphisbaenian, Diamide,
and Pygopodid).
Living snakes are found on every continent except Antarctica, and on
most smaller land masses; exceptions include some large islands, such as
Ireland, Iceland, Greenland, the Hawaiian archipelago, and the islands of
New Zealand, as well as many small islands of the Atlantic and central Pacific
oceans. Additionally, sea snakes are widespread throughout the Indian
and Pacific oceans. More than twenty families are currently
recognized, comprising about 520 genera and about 3,900 species. They
range in size from the tiny, 10.4 cm-long (4.1 in) Barbados thread snake to the reticulated python of 6.95 meters
(22.8 ft.) in length. The fossil species Titan boa cerrejonensis was 12.8 meters (42 ft.) long. Snakes are
thought to have evolved from either burrowing or aquatic lizards, perhaps
during the Jurassic period, with the earliest known fossils dating to
between 143 and 167 Ma ago. The diversity of modern snakes
appeared during the Paleocene epoch (c. 66 to 56 Ma ago,
after the Cretaceous–Paleogene extinction event). The oldest preserved
descriptions of snakes can be found in the Brooklyn Papyrus.
Most species of snake are nonvenomous and those that have venom use
it primarily to kill and subdue prey rather than for self-defense. Some possess the venom that is potent enough to cause painful injury or death to humans.
Nonvenomous snakes either swallow prey alive or kill by constriction.
Biology
Size
The now extinct Titan boa cerrejonensis snakes
were 12.8 m (42 ft) in length. By comparison, the largest extant snakes
are the reticulated python, measuring about 6.95 m (22.8 ft.)
long, and the green anaconda, which measures about 5.21 m
(17.1 ft) long and is considered the heaviest snake on Earth at
97.5 kg (215 lb.).
At the other end of the scale, the smallest extant snake is Leptotyphlops Carlee, with a length of about 10.4 cm (4.1 in). Most snakes are fairly small animals, approximately 1 m (3.3 ft.) in length.
Perception
Pit vipers, pythons, and some boas have infrared-sensitive
receptors in deep grooves on the snout, allowing them to "see"
the radiated heat of warm-blooded prey. In-pit vipers, the grooves are located
between the nostril and the eye in a large "pit" on each side of the
head. Other infrared-sensitive snakes have multiple, smaller labial pits lining
the upper lip, just below the nostrils.
A snake tracks its prey using smell, collecting airborne
particles with its forked tongue, then passing them to the vomeronasal
organ or Jacobson's organ in the mouth for examination. The
fork in the tongue provides a sort of directional sense of smell and taste
simultaneously. The snake's tongue is constantly in motion, sampling
particles from the air, ground, and water, analyzing the chemicals found and
determining the presence of prey or predators in the local environment. In
water-dwelling snakes, such as the anaconda, the tongue functions efficiently
underwater.
The underside of a snake is very sensitive to vibration,
allowing the snake to detect approaching animals by sensing faint vibrations in
the ground.
Snake vision varies greatly between species. Some have keen
eyesight and others are only able to distinguish light from dark, but the
important trend is that a snake's visual perception is adequate enough to track
movements. Generally, vision is best in tree-dwelling snakes and weakest
in burrowing snakes. Some have binocular vision, where both eyes are
capable of focusing on the same point, an example of this being the Asian
vine snake. Most snakes focus by moving the lens back and forth in
relation to the retina. Diurnal snakes have round pupils and many
nocturnal snakes have slit pupils. Most species possess three visual pigments
and are probably able to see two primary colors in daylight. It has been
concluded that the last common ancestors of all snakes had UV-sensitive
vision, but most snakes that depend on their eyesight to hunt in daylight have
evolved lenses that act like sunglasses for filtering out the UV light, which
probably also sharpens their vision by improving the contrast.
Skin
The skin of a snake is covered in scales. Contrary to the
popular notion of snakes being slimy (because of possible confusion of snakes
with worms), snakeskin has a smooth, dry texture. Most snakes
use specialized belly scales to travel, allowing them to grip surfaces. The
body scales may be smooth, keeled, or granular. The eyelids of a snake are
transparent "spectacle" scales, also known as braille, which
remain permanently closed.
The shedding of scales is called ecdysis (or in
normal usage, molting, or sloughing). Snakes shed
the complete outer layer of skin in one piece. Snake scales are not
discrete, but extensions of the epidermis—hence they are not shed
separately but as a complete outer layer during each molt, akin to a sock being
turned inside out.
Snakes have a wide diversity of skin coloration patterns which
are often related to behavior, such as the tendency to have to flee from
predators. Snakes that are at a high risk of predation tend to be plain or
have longitudinal stripes, providing few reference points to predators, thus
allowing the snake to escape without being noticed. Plain snakes usually adopt
active hunting strategies, as their pattern allows them to send little
information to prey about motion. Blotched snakes usually use ambush-based
strategies, likely because it helps them blend into an environment with
irregularly shaped objects, like sticks or rocks. Spotted patterning can
similarly help snakes to blend into their environment.
The shape and number of scales on the head, back, and belly are
often characteristic and used for taxonomic purposes. Scales are named mainly
according to their positions on the body. In "advanced" (Caenophidian)
snakes, the broad belly scales, and rows of dorsal scales correspond
to the vertebrae, allowing these to be counted without the need for dissection.
Molting
Molting (or "ecdysis") serves a number of
purposes. Firstly, the old and worn skin is replaced, and secondly, it helps
get rid of parasites such as mites and ticks. Renewal of the
skin by molting supposedly allows growth in some animals such as insects, but
this has been disputed in the case of snakes.
Molting occurs periodically throughout the life of a snake.
Before each molt, the snake stops eating and often hides or moves to a safe
place. Just before shedding, the skin becomes dull and dry-looking and the
snake's eyes turn cloudy or blue-colored. The inner surface of the old skin
liquefies, causing it to separate from the new skin beneath it. After a few
days, the eyes become clear and the snake "crawls" out of its old skin,
which splits close to the snake's mouth. The snake rubs its body against rough
surfaces to aid in the shedding of its old skin. In many cases, the cast skin
peels backward over the body from head to tail in one piece, like pulling a
sock off inside-out, revealing a new, larger, brighter layer of skin that has
formed underneath.
A young snake that is still growing may shed its skin up to four
times a year, but an older snake may shed only once or twice a year. The
discarded skin carries a perfect imprint of the scale pattern, so it is usually
possible to identify the snake from the cast skin if it is reasonably intact. This
periodic renewal has led to the snake being a symbol of healing and medicine,
as pictured in the Rod of Asclepius.
Scale counts can sometimes be used to identify the sex of a
snake when the species is not distinctly sexually dimorphic. A probe is
fully inserted into the cloaca, marked at the point where it stops, then
removed and measured against the subcaudal scales. The scalation
count determines whether the snake is a male or female, as the hemipodes of
a male will probe to a different depth (usually longer) than the cloaca of a
female.
Skeleton
The skeleton of most snakes consists solely of the
skull, hyoid, vertebral column, and ribs, though hemophilia snakes
retain vestiges of the pelvis and rear limbs.
The skull consists of a solid and complete neurocranium, to
which many of the other bones are only loosely attached, particularly the
highly mobile jawbones, which facilitate manipulation and ingestion of large
prey items. The left and right sides of the lower jaw are joined together only
by a flexible ligament at the anterior tips, allowing them to separate widely,
and the posterior end of the lower jawbones articulate with a quadrate
bone, allowing further mobility. The mandible and quadrate bones can
pick up ground-borne vibrations; because the sides of the lower jaw can
move independently of one another, a snake resting its jaw on a surface has
sensitive stereo auditory perception, used for detecting the position of prey.
The jaw–quadrate–stapes pathway is capable of detecting vibrations on
the angstrom scale, despite the absence of an outer ear and the lack
of an impedance matching mechanism—provided by the icicles in
other vertebrates—for receiving vibrations from the air.
The hyoid is a small bone located posterior and ventral to the skull,
in the 'neck' region, which serves as an attachment for the muscles of the
snake's tongue, as it does in all other tetrapods.
The vertebral column consists of between 200 and 400 vertebrae,
or sometimes more. The body vertebrae each have two ribs articulating with
them. The tail vertebrae are comparatively few in number (often less than 20%
of the total) and lack ribs. The vertebrae have projections that allow for
strong muscle attachment, enabling locomotion without limbs.
Caudal autotomy (self-amputation of the tail), a feature found in some lizards, is absent in most snakes. In the rare cases
where it does exist in snakes, caudal autotomy is intervertebral (meaning the
separation of adjacent vertebrae), unlike that in lizards, which is
intravertebral, i.e. the break happens along a predefined fracture plane
present on a vertebra.
In some snakes, most notably boas and pythons, there are
vestiges of the hindlimbs in the form of a pair of pelvic spurs. These
small, claw-like protrusions on each side of the cloaca are the external portion of the vestigial hindlimb skeleton, which includes the remains of an
ilium and femur.
Snakes are polyphyodonty with teeth that are
continuously replaced.
Internal organs
Snakes and other reptiles have a three-chambered heart that
controls the circulatory system via the left and right atrium and
one ventricle. Internally, the ventricle is divided into three
interconnected cavities: the cavum arteriosus, the cavum pulmonale, and the
cavum venous. The cavum venous receives deoxygenated blood from
the right atrium and the cavum arteriosus receives oxygenated blood from the
left atrium. Located beneath the cavum venous is the cavum pulmonale, which
pumps blood to the pulmonary trunk.
The snake's heart is encased in a sac, called the pericardium,
located at the bifurcation of the bronchi. The heart is able to
move around, owing to the lack of a diaphragm; this adjustment protects the
heart from potential damage when large ingested prey is passed through
the esophagus. The spleen is attached to the gall bladder and pancreas and
filters the blood. The thymus, located in fatty tissue above the heart, is
responsible for the generation of immune cells in the blood. The cardiovascular
system of snakes is unique for the presence of a renal portal system in which
the blood from the snake's tail passes through the kidneys before returning to
the heart.
The vestigial left lung is often small or
sometimes even absent, as snakes' tubular bodies require all of their organs to
be long and thin. In the majority of species, only one lung is functional.
This lung contains a vascularized anterior portion and a posterior portion that
does not function in gas exchange. This 'saccular lung' is used for hydrostatic purposes
to adjust buoyancy in some aquatic snakes and its function remains unknown in
terrestrial species. Many organs that are paired, such as kidneys or reproductive
organs, are staggered within the body, one located ahead of the other.
Reproduction
Although a wide range of reproductive modes is used by snakes,
all employ internal fertilization. This is accomplished by means of
paired, forked hemipodes, which are stored, inverted, in the male's tail. The hemipodes are often grooved, hooked, or spine—designed to grip the walls of
the female's cloaca.
Most species of snakes lay eggs that they abandon shortly after laying. However, a few species (such as the king cobra) construct nests and stay in the vicinity of the hatchlings after incubation. Most pythons coil around their egg clutches and remain with them until they hatch. A female python will not leave the eggs, except to occasionally bask in the sun or drink water. She will even "shiver" to generate heat to incubate the eggs.
Some species of snake are ovoviviparous and retain the
eggs within their bodies until they are almost ready to hatch. Several
species of snake, such as the boa constrictor and green anaconda, are
fully viviparous, nourishing their young through a placenta as
well as a yolk sac; this is highly unusual among reptiles, and normally
found in requiem sharks or placental mammals. Retention of
eggs and live birth are most often associated with colder environments.
Sexual selection in snakes is demonstrated by the 3,000 species that each use different tactics in acquiring mates. Ritual combat between males for the females they want to mate with includes topping, a behavior exhibited by most viperids in which one male will twist around the vertically elevated fore body of its opponent and force it downward. It is common for neck biting to occur while the snakes are entwined.
Behavior
Winter dormancy
In regions where winters are too cold for snakes to tolerate
while remaining active, local species will enter a period of brumation.
Unlike hibernation, in which the dormant mammals are actually asleep, bromating reptiles are awake but inactive. Individual snakes may brumate in
burrows, under rock piles, or inside fallen trees, or large numbers of snakes
may clump together in hibernacula.
Feeding and diet
All snakes are strictly carnivorous, preying on
small animals including lizards, frogs, other snakes, small mammals, birds,
eggs, fish, snails, worms, and insects. Snakes cannot bite or tear their
food to pieces so must swallow their prey whole. The eating habits of a snake
are largely influenced by body size; smaller snakes eat smaller prey. Juvenile
pythons might start out feeding on lizards or mice and graduate to small deer
or antelope as an adult, for example.
The snake's jaw is a complex structure. Contrary to
the popular belief that snakes can dislocate their jaws, they have an extremely flexible lower jaw, the two halves of which are not rigidly attached, and
numerous other joints in the skull, which allow the snake to open its mouth
wide enough to swallow prey whole, even if it is larger in diameter than the
snake itself. For example, the African egg-eating snake has
flexible jaws adapted for eating eggs much larger than the diameter of its head. This
snake has no teeth but does have bony protrusions on the inside edge of
its spine, which it uses to break the shell when eating eggs.
The majority of snakes eat a variety of prey animals, but there
is some specialization in certain species. King cobras and the
Australian bandy-bandy consume other snakes. Species of the
family Paradise have more teeth on the right side of their mouths
than on the left, as they mostly prey on snails and the shells usually spiral
clockwise.
Some snakes have a venomous bite, which they use to kill their
prey before eating it. Other snakes kill their prey by constriction, while
some swallow their prey when it is still alive.
After eating, snakes become dormant to allow the process
of digestion to take place; this is an intense activity,
especially after the consumption of large prey. In species that feed only
sporadically, the entire intestine enters a reduced state between
meals to conserve energy. The digestive system is then 'up-regulated to full
capacity within 48 hours of prey consumption. Being ectothermic ("cold-blooded"),
the surrounding temperature plays an important role in the digestion process.
The ideal temperature for snakes to digest food is 30 °C (86 °F).
There is a huge amount of metabolic energy involved in a snake's
digestion, for example, the surface body temperature of the South American
rattlesnake (Crotalus durissus) increases by as much as 1.2 °C
(2.2 °F) during the digestive process. If a snake is disturbed after
having eaten recently, it will often regurgitate its prey to be able
to escape the perceived threat. When undisturbed, the digestive process is
highly efficient; the snake's digestive enzymes dissolve and absorb
everything but the prey's hair (or feathers) and claws, which are excreted
along with the waste.
Bite
Snakes do not ordinarily prey on humans. Unless startled or
injured, most snakes prefer to avoid contact and will not attack humans. With
the exception of large constrictors, nonvenomous snakes are not a threat to
humans. The bite of a nonvenomous snake is usually harmless; their teeth are
not adapted for tearing or inflicting a deep puncture wound, but rather
grabbing and holding. Although the possibility of infection and tissue damage is
present in the bite of a nonvenomous snake, venomous snakes present a far greater hazard to humans. The World Health Organization (WHO)
lists snakebite under the "other neglected conditions"
category.
Documented deaths resulting from snake bites
are uncommon. Nonfatal bites from venomous snakes may result in the need for
amputation of a limb or part thereof. Of the roughly 725 species of venomous
snakes worldwide, only 250 are able to kill a human with one bite. Australia
averages only one fatal snake bite per year. In India, 250,000 snakebites
are recorded in a single year, with as many as 50,000 recorded initial deaths. The
WHO estimates that on the order of 100 000 people die each year as a result of
snake bites, and around three times as many amputations and other permanent
disabilities are caused by snakebites annually.
The treatment for a snakebite is as variable as the bite itself. The most common and effective method is through antivenom (or antivenin), a serum made from the venom of the snake. Some antivenom is species-specific (monovalent) while some are made for use with multiple species in mind (polyvalent). In the United States, for example, all species of venomous snakes are pit vipers, with the exception of the coral snake. To produce antivenom, a mixture of the venoms of the different species of rattlesnakes, copperheads, and cottonmouths is injected into the body of a horse in ever-increasing dosages until the horse is immunized. Blood is then extracted from the immunized horse. The serum is separated and further purified and freeze-dried. It is reconstituted with sterile water and becomes antivenom. For this reason, people who are allergic to horses are more likely to suffer an allergic reaction to antivenom. Antivenom for the more dangerous species (such as mambas, taipans, and cobras) is made in a similar manner in India, South Africa, and Australia, although these antivenoms are species-specific.
0 Comments