Snakes are elongated, legless, carnivorous reptiles ofthe suborder Serpentes. Like all square mates, snakes are ectothermic, amniotevertebrates 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 elongate bodies
without limbs or with greatly reduced limbs about twenty five times
independently via convergent evolution, leading to many lineages of legless lizards and
snakes. Legless lizards resemble snakes, but several common groups of legless
lizards have eyelids and external ears, which snakes lack, although this rule
is not universal (see Amphisbaenia, Dibamidae, and Pygopodidae).
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, and many small islands of the Atlantic
and central Pacific oceans. Additionally, sea
snakes are
widespread throughout the Indian and Pacific Oceans. More than 20 familiesare
currently recognized, comprising about 520 genera and about 3,600 species. They range in size from the tiny,
10.4 cm (4.1 in)-long thread snake to the reticulated python of
6.95 meters (22.8 ft) in length. The fossil species Titanoboa 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 period (c 66 to 56 Ma ago). The oldest
preserved descriptions of snakes can be found in the Brooklyn Papyrus.
Most species are nonvenomous and those
that have venom use it primarily to kill and subdue prey rather than for
self-defense. Some possess venom potent enough to cause painful injury or death
to humans. Nonvenomous snakes either swallow prey alive or kill by constriction.
Etymology
The
English word snake comes from Old English snaca, itself from Proto-Germanic *snak-an- (cf. Germanic Schnake "ring snake", Swedish snok "grass snake"), from Proto-Indo-European root *(s)nēg-o- "to crawl", "to creep",
which also gave sneak as well as Sanskrit nāgá "snake". The word ousted adder,
as adder went on to narrow in meaning,
though in Old English næddre was
the general word for snake. The other term, serpent, is from French, ultimately from
Indo-European *serp- (to
creep), which also gave Ancient Greek hérpō (ἕρπω) "I crawl".
Evolution
The fossil record of snakes is
relatively poor because snake skeletons are typically small and fragile
making fossilization uncommon. Fossils readily identifiable as snakes
(though often retaining hind limbs) first appear in the fossil record during
the Cretaceous period. The earliest known true snake fossils
(members of the crown group Serpentes) come from the marine simoliophiids,
the oldest of which is the Late Cretaceous (Cenomanian age) Haasiophis terrasanctus, dated to between 112 and 94
million years old.
Based on comparative anatomy, there is consensus that snakes descended from lizards. Pythons and boas—primitive groups among modern
snakes—have vestigial hind limbs: tiny, clawed digits known as anal
spurs, which are used
to grasp during mating.The families Leptotyphlopidae and Typhlopidae also possess remnants of the
pelvic girdle, appearing as horny projections when visible.
Front limbs are nonexistent in all known
snakes. This is caused by the evolution of Hox
genes, controlling
limb morphogenesis. The axial skeleton of the snakes’ common ancestor, like
most other tetrapods, had regional specializations consisting of cervical
(neck), thoracic (chest), lumbar (lower back), sacral (pelvic), and caudal
(tail) vertebrae. Early in snake evolution, the Hox gene expression in the
axial skeleton responsible for the development of the thorax became dominant.
As a result, the vertebrae anterior to the hindlimb buds (when present) all
have the same thoracic-like identity (except from the atlas, axis, and 1–3
neck vertebrae). In other words, most of a snake's skeleton is an extremely
extended thorax. Ribs are found exclusively on the thoracic vertebrae. Neck,
lumbar and pelvic vertebrae are very reduced in number (only 2–10 lumbar and
pelvic vertebrae are present), while only a short tail remains of the caudal
vertebrae. However, the tail is still long enough to be of important use in
many species, and is modified in some aquatic and tree-dwelling species.
Many modern snake groups originated
during the Paleocene, alongside the adaptive radiation of
mammals following the extinction of (non-avian) dinosaurs. The expansion of grasslands in North
America also led to an explosive radiation among snakes. Previously,
snakes were a minor component of the North American fauna, but during the
Miocene, the number of species and their prevalence increased dramatically with
the first appearances of vipers and elapids in North America and the
significant diversification of Colubridae (including the origin of many
modern genera such as Nerodia, Lampropeltis, Pituophis, and Pantherophis).
Origins
There is fossil evidence to suggest that
snakes may have evolved from burrowing lizards, such as the varanids (or
a similar group) during the Cretaceous
Period. An early
fossil snake relative, Najash rionegrina, was a two-legged burrowing animal with a sacrum, and was fully terrestrial. One extant analog of these putative ancestors
is the earless monitor Lanthanotus of Borneo (though it also is semiaquatic).[23] Subterranean species
evolved bodies streamlined for burrowing, and eventually lost their
limbs. According to this hypothesis, features such as the transparent, fused eyelids (brille) and loss of external ears evolved to
cope with fossorial difficulties, such as scratched corneas and dirt in the ears. Some
primitive snakes are known to have possessed hindlimbs, but their pelvic bones
lacked a direct connection to the vertebrae. These include fossil species
like Haasiophis, Pachyrhachis and Eupodophis, which are slightly older than Najash.
This hypothesis was strengthened in 2015
by the discovery of a 113m year-old fossil of a four-legged snake in Brazil
that has been named Tetrapodophis amplectus. It has many snake-like features, is
adapted for burrowing and its stomach indicates that it was preying on other
animals. It is currently uncertain if Tetrapodophis is a snake or another species, in
the squamate order, as a snake-like body has independently evolved
at least 26 times. Tetrapodophis does not have distinctive snake features in its spine and skull.
An alternative hypothesis, based
on morphology, suggests the ancestors of snakes were related to mosasaurs—extinct aquatic reptiles
from the Cretaceous—which in turn are thought to have derived from varanid
lizards. According to this hypothesis, the fused, transparent eyelids of
snakes are thought to have evolved to combat marine conditions (corneal water
loss through osmosis), and the external ears were lost through disuse in an
aquatic environment. This ultimately led to an animal similar to today's sea
snakes. In the
Late Cretaceous, snakes recolonized land, and continued to diversify into
today's snakes. Fossilized snake remains are known from early Late Cretaceous
marine sediments, which is consistent with this hypothesis; particularly so, as
they are older than the terrestrial Najash rionegrina. Similar skull structure, reduced or absent limbs,
and other anatomical features found in both mosasaurs and snakes lead to a
positive cladisticalcorrelation, although some of these features are shared with
varanids.
Genetic studies in recent years have
indicated snakes are not as closely related to monitor lizards as was once
believed—and therefore not to mosasaurs, the proposed ancestor in the aquatic
scenario of their evolution. However, more evidence links mosasaurs to snakes
than to varanids. Fragmented remains found from the Jurassic and Early Cretaceous indicate
deeper fossil records for these groups, which may potentially refute either
hypothesis.
In 2016 two studies reported that limb
loss in snakes is associated with DNA mutations in the Zone of Polarizing
Activity Regulatory Sequence (ZRS), a regulatory region of the sonic hedgehog gene
which is critically required for limb development. More advanced snakes have no
remnants of limbs, but basal snakes such as pythons and boas do have traces of
highly reduced, vestigial hind limbs. Python embryos even have fully developed
hind limb buds, but their later development is stopped by the DNA mutations in
the ZRS.
Distribution
There are over 2,900 species of snakes
ranging as far northward as the Arctic Circle in Scandinavia and southward
through Australia. Snakes can be found on every continent except
Antarctica, in the sea, and as high as 16,000 feet (4,900 m) in the Himalayan Mountains of Asia. There are numerous islands from which
snakes are absent, such as Ireland, Iceland, and New
Zealand (although
New Zealand's waters are infrequently visited by the yellow-bellied sea snake and the banded sea krait).
Taxonomy
See also: List of snake genera
All modern snakes are grouped within
the suborder Serpentes in Linnean taxonomy,
part of the order Squamata, though their precise placement within squamates remains
controversial.
The two infraorders of Serpentes are: Alethinophidia and Scolecophidia. This
separation is based on morphological characteristics and mitochondrial DNA sequence
similarity. Alethinophidia is sometimes split into Henophidia and Caenophidia, with the latter consisting of
"colubroid" snakes (colubrids, vipers, elapids, hydrophiids, and atractaspids)
and acrochordids, while the other alethinophidian families comprise
Henophidia. While not extant today, the Madtsoiidae, a family of giant, primitive,
python-like snakes, was around until 50,000 years ago in Australia, represented
by genera such as Wonambi.
There are numerous debates in the
systematics within the group. For instance, many sources classify Boidae and Pythonidae as one family, while some keep
the Elapidae and Hydrophiidae (sea snakes) separate for
practical reasons despite their extremely close relation.
Recent molecular studies support
the monophyly of the clades of modern snakes, scolecophidians,
typhlopids + anomalepidids, alethinophidians, core alethinophidians, uropeltids
(Cylindrophis, Anomochilus, uropeltines), macrostomatans, booids,
boids, pythonids and caenophidians.
Legless lizards
While snakes are limbless reptiles,
which evolved from (and are grouped with) lizards, there are many other species
of lizards which have lost their limbs independently and superficially look
similar to snakes. These include the slow
worm and glass
snake.
Size
The now extinct Titanoboa cerrejonensis snakes found were 12.8 m (42 ft) in
length. By comparison, the largest extant snakes are the reticulated python,
which measures about 6.95 m (22.8 ft) long, and the 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 carlae, 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,
which allow 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.
Snakes use smell to track their prey.
They smell by using their forked tongues to
collect airborne particles, then passing them to the vomeronasal organ or Jacobson's organ in the mouth for
examination. The fork in the tongue gives snakes a sort of directional
sense of smell and taste simultaneously. They keep their tongues
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 is very sensitive
to vibration. This allows snakes to be able to sense approaching animals by
detecting faint vibrations in the ground.
Snake vision varies greatly, from only
being able to distinguish light from dark to keen eyesight, but the main trend
is that their vision is adequate although not sharp, and allows them to track
movements. Generally, vision is best in arboreal snakes and weakest in
burrowing snakes. Some snakes, such as the Asian vine snake (genus Ahaetulla), have binocular vision,
with both eyes capable of focusing on the same point. Most snakes focus by
moving the lens back and forth in relation to the retina, while in the other amniote groups, the lens is stretched.
Many nocturnal snakes have slit pupils while diurnal snakes have round pupils.
Skin
Main article: Snake
scales
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, gripping surfaces. The body
scales may be smooth, keeled, or
granular. The eyelids of a snake are transparent "spectacle" scales,
which remain permanently closed, also known as brille.
The shedding of scales is called ecdysis (or
in normal usage, molting or sloughing). In the case of snakes, the complete
outer layer of skin is shed in one layer. 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.
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 scientists to count the vertebrae without dissection.
Snakes' eyes are covered by their clear
scales (the brille) rather than movable eyelids. Their eyes are always open, and for
sleeping, the retina can be closed or the face buried among the folds of the
body.
Molting
Molting, or ecdysis, serves a number of functions. Firstly,
the old and worn skin is replaced; secondly, it helps get rid of parasites such
as mites and ticks. Renewal of the skin by moulting is supposed to allow growth
in some animals such as insects; however, this has been disputed in the case of
snakes.
Molting occurs periodically
throughout the snake's life. Before a 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 eyes become cloudy or blue-colored. The inner surface of
the old skin liquefies. This causes the old skin to separate from the new skin
beneath it. After a few days, the eyes clear and the snake "crawls"
out of its old skin. The old skin breaks near the mouth and the snake wriggles
out, aided by rubbing against rough surfaces. 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. A new, larger, brighter layer of skin has formed
underneath.
An older snake may shed its skin only
once or twice a year. But a younger snake, still growing, may shed up to four
times a year. The discarded skin gives a perfect imprint of the scale
pattern, and it is usually possible to identify the snake if the discarded skin
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
tell the sex of a snake when the species is not distinctly sexually dimorphic.
A probe is inserted into the cloaca until it can go no further. The
probe is marked at the point where it stops, removed, and compared to the
subcaudal depth by laying it alongside the scales. The scalation count
determines whether the snake is a male or female as hemipenes 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 henophidian snakes retain vestiges of the pelvis and rear limbs.
The skull
of the snake consists
of a solid and complete neurocranium, to which many of the other bones are
only loosely attached, particularly the highly mobile jaw bones, which
facilitate manipulation and ingestion of large prey items. The left and right
sides of the lower jaw are joined only by a flexible ligament at the anterior
tips, allowing them to separate widely, while the posterior end of the lower
jaw bones articulate with a quadrate bone, allowing further mobility. The bones
of the mandible and quadrate bones can also pick up ground borne
vibrations. Because the sides of the jaw can move independently of one
another, snakes resting their jaws on a surface have sensitive stereo hearing
which can detect the position of prey. The jaw-quadrate-stapes pathway is
capable of detecting vibrations on the angstromscale, despite the absence of an outer
ear and the ossicle mechanism of impedance matching used
in other vertebrates to receive 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 muscles of the snake's tongue, as it does in all other tetrapods.
The vertebral column consists of
anywhere between 200 and 400 (or more) vertebrae. Tail vertebrae are
comparatively few in number (often less than 20% of the total) and lack ribs,
while body vertebrae each have two ribs articulating with them. The vertebrae
have projections that allow for strong muscle attachment enabling locomotion
without limbs.
Autotomy of the tail, a feature found in
some lizards is absent in most snakes. Caudal autotomy in snakes is rare
and is intervertebral, unlike that in lizards, which is intravertebral—that is,
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 cloacaare the external portion of the
vestigial hindlimb skeleton, which includes the remains of an ilium and femur.
Snakes are polyphyodonts with teeth that are continuously replaced.
Internal organs
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,
however, 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
gland is located
in fatty tissue above the heart and is responsible for the generation of immune
cells in the blood. The cardiovascular system of snakes is also 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, with one located ahead of
the other.
Snakes have no lymph
nodes.
Venom
Cobras, vipers, and closely related
species use venom to immobilize or kill their prey. The venom is modified saliva, delivered through fangs. The fangs of 'advanced' venomous
snakes like viperids and elapids are hollow to inject venom more effectively,
while the fangs of rear-fanged snakes such as the boomslang merely have a groove on the
posterior edge to channel venom into the wound. Snake venoms are often prey
specific—their role in self-defense is secondary.
Venom, like all salivary
secretions, is a predigestant that initiates the breakdown of food into soluble
compounds, facilitating proper digestion. Even nonvenomous snake bites (like
any animal bite) will cause tissue damage.
Certain birds, mammals, and other snakes
(such as kingsnakes) that prey on venomous snakes have developed resistance and
even immunity to certain venoms. Venomous snakes include three families of
snakes, and do not constitute a formal classification group used in taxonomy.
The colloquial term
"poisonous snake" is generally an incorrect label for snakes. A
poison is inhaled or ingested, whereas venom produced by snakes is injected
into its victim via fangs. There are, however, two exceptions: Rhabdophis sequesters toxins from the toads it eats, then secretes them from
nuchal glands to ward off predators, and a small unusual population of garter snakes in
the U.S. state of Oregonretains enough toxins in their livers from the newts they eat to be effectively
poisonous to small local predators (such as crows and foxes).
Snake venoms are complex mixtures
of proteins, and are stored in venom glands at the back of the
head. In all venomous snakes, these glands open through ducts into grooved
or hollow teeth in the upper jaw. These proteins can potentially be a mix
of neurotoxins(which attack the nervous system), hemotoxins (which attack the circulatory
system), cytotoxins, bungarotoxins and many other toxins that affect
the body in different ways. Almost all snake venom contains hyaluronidase, an enzyme that ensures rapid diffusion of the venom.
Venomous snakes that use hemotoxins
usually have fangs in the front of their mouths, making it easier for them to
inject the venom into their victims. Some snakes that use neurotoxins
(such as the mangrove snake)
have fangs in the back of their mouths, with the fangs curled
backwards. This makes it difficult both for the snake to use its venom and
for scientists to milk them. Elapids,
however, such as cobras and kraits are proteroglyphous—they possess hollow fangs that cannot be erected toward the
front of their mouths, and cannot "stab" like a viper. They must
actually bite the victim.
It has recently been suggested that all
snakes may be venomous to a certain degree, with harmless snakes having weak
venom and no fangs. Most snakes currently labelled "nonvenomous"
would still be considered harmless according to this theory, as they either
lack a venom delivery method or are incapable of delivering enough to endanger
a human. This theory postulates that snakes may have evolved from a common
lizard ancestor that was venomous—and that venomous lizards like the gila
monster, beaded lizard, monitor lizards,
and the now-extinct mosasaurs may also have derived from it.
They share this venom clade with various other saurian species.
Venomous snakes are
classified in two taxonomic families:
- Elapids – cobras including king cobras, kraits, mambas, Australian copperheads, sea snakes, and coral snakes.
- Viperids – vipers, rattlesnakes, copperheads/cottonmouths, and bushmasters.
There is a third family
containing the opistoglyphous (rear-fanged) snakes (as well as the majority of
other snake species):
- Colubrids – boomslangs, tree snakes, vine snakes, mangrove snakes, although not all colubrids are venomous.
Reproduction
See also: Sexual selection in scaled reptiles
Although a wide range of reproductive
modes are used by snakes, all snakes employ internal fertilization. This is accomplished by means of
paired, forked hemipenes, which are stored, inverted, in the male's tail. The
hemipenes are often grooved, hooked, or spined in order to grip the walls of
the female's cloaca.
Most species of snakes lay eggs which
they abandon shortly after laying. However, a few species (such as the king
cobra) actually
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.
Recently, it has been confirmed that several species of snake are fully viviparous, such as the boa constrictor and green anaconda,
nourishing their young through a placenta as well as a yolk
sac, which is highly
unusual among reptiles, or anything else outside of 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 three thousand 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 forcing it
downward. It is common for neck biting to occur while the snakes are entwined.
Facultative
parthenogenesis
Parthenogenesis is
a natural form of reproduction in which growth and development of embryos occur
without fertilization. Agkistrodon contortrix (copperhead) and Agkistrodon piscivorus (cotton mouth) can reproduce by
facultative parthenogenesis. That is, they are capable of switching from a
sexual mode of reproduction to an asexual mode. The type of
parthenogenesis that likely occurs is automixis with terminal fusion, a process in which two terminal
products from the same meiosis fuse to form a diploid zygote. This process leads to genome
wide homozygosity, expression of deleterious recessive alleles and often to
developmental abnormalities. Both captive-born and wild-born A. contortrix and A. piscivorus appear to be capable of this form
of parthenogenesis.
Reproduction in squamate reptiles is almost exclusively
sexual. Males ordinarily have a ZZ pair of sex determining chromosomes, and
females a ZW pair. However, the Colombian Rainbow boa, Epicrates maurus can also reproduce by facultative parthenogenesis
resulting in production of WW female progeny. The WW females are likely
produced by terminal automixis.
Behavior
Winter dormancy
In regions where winters are colder than
snakes can tolerate while remaining active, local species will brumate. Unlike hibernation, in which mammals
are actually asleep, brumating reptiles are awake but inactive. Individual
snakes may brumate in burrows, under rock piles, or inside fallen trees, or
snakes may aggregate in large numbers at hibernacula.
Feeding and diet
All snakes are strictly carnivorous, eating small animals including lizards, frogs, other snakes, small
mammals, birds, eggs, fish,
snails or insects. Because snakes cannot bite or tear their food to
pieces, they must swallow prey whole. The body size of a snake has a major
influence on its eating habits. 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, snakes have a very
flexible lower jaw, the two halves of which are not rigidly attached, and
numerous other joints in their skull (see snake
skull), allowing them
to open their mouths wide enough to swallow their 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 shells when it eats eggs.
While the majority of snakes eat a
variety of prey animals, there is some specialization by some species. King
cobrasand the
Australian bandy-bandy consume other snakes. Pareas iwesakii and other snail-eating colubrids of subfamily Pareatinae have more teeth on the right side
of their mouths than on the left, as the shells of their prey 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. Still others swallow their prey whole and alive.
After eating, snakes become dormant
while the process of digestion takes place. Digestion is an
intense activity, especially after 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 a large role in snake digestion. The ideal
temperature for snakes to digest is 30 °C (86 °F). So much metabolic energy is involved in a snake's
digestion that in the Mexican rattlesnake (Crotalus durissus), surface body temperature increases by as much as
1.2 °C (2.2 °F) during the digestive process. Because of this, a
snake disturbed after having eaten recently will often regurgitate its prey to be able to escape the
perceived threat. When undisturbed, the digestive process is highly efficient,
with the snake's digestive enzymes dissolving and absorbing
everything but the prey's hair (or feathers) and claws, which are excreted along with waste.
Locomotion
The lack of limbs does not impede
the movement of snakes. They have developed several different modes of
locomotion to deal with particular environments. Unlike the gaits of limbed
animals, which form a continuum, each mode of snake locomotion is discrete and
distinct from the others; transitions between modes are abrupt.
Lateral undulation
Lateral undulation is the sole
mode of aquatic locomotion, and the most common mode of terrestrial
locomotion. In this mode, the body of the snake alternately flexes to the
left and right, resulting in a series of rearward-moving "waves". While
this movement appears rapid, snakes have rarely been documented moving faster
than two body-lengths per second, often much less. This mode of movement
has the same net cost of transport (calories burned per meter moved) as running
in lizards of the same mass.
Terrestrial lateral undulation is
the most common mode of terrestrial locomotion for most snake species. In
this mode, the posteriorly moving waves push against contact points in the
environment, such as rocks, twigs, irregularities in the soil, etc. Each of
these environmental objects, in turn, generates a reaction force directed
forward and towards the midline of the snake, resulting in forward thrust while
the lateral components cancel out. The speed of this movement depends upon
the density of push-points in the environment, with a medium density of about
8 along the snake's length being ideal. The wave speed is precisely
the same as the snake speed, and as a result, every point on the snake's body
follows the path of the point ahead of it, allowing snakes to move through very
dense vegetation and small openings.
When swimming, the waves become
larger as they move down the snake's body, and the wave travels backwards
faster than the snake moves forwards. Thrust is generated by pushing their
body against the water, resulting in the observed slip. In spite of overall
similarities, studies show that the pattern of muscle activation is different
in aquatic versus terrestrial lateral undulation, which justifies calling them
separate modes. All snakes can laterally undulate forward (with
backward-moving waves), but only sea snakes have been observed reversing the
motion (moving backwards with forward-moving waves).
Sidewinding
Most often employed
by colubroid snakes (colubrids, elapids, and vipers) when the snake must move in an environment
that lacks irregularities to push against (rendering lateral undulation
impossible), such as a slick mud flat, or a sand dune, sidewinding is a
modified form of lateral undulation in which all of the body segments oriented
in one direction remain in contact with the ground, while the other segments
are lifted up, resulting in a peculiar "rolling" motion. This
mode of locomotion overcomes the slippery nature of sand or mud by pushing off
with only static portions on the body, thereby minimizing slipping. The
static nature of the contact points can be shown from the tracks of a
sidewinding snake, which show each belly scale imprint, without any smearing.
This mode of locomotion has very low caloric cost, less than ⅓ of the cost for a
lizard to move the same distance. Contrary to popular belief, there is no
evidence that sidewinding is associated with the sand being hot.
Concertina
When push-points are absent, but
there is not enough space to use sidewinding because of lateral constraints,
such as in tunnels, snakes rely on concertina locomotion. In this mode,
the snake braces the posterior portion of its body against the tunnel wall
while the front of the snake extends and straightens. The front portion
then flexes and forms an anchor point, and the posterior is straightened and
pulled forwards. This mode of locomotion is slow and very demanding, up to
seven times the cost of laterally undulating over the same distance. This high
cost is due to the repeated stops and starts of portions of the body as well as
the necessity of using active muscular effort to brace against the tunnel
walls.
Arboreal
The movement of snakes in
arboreal habitats has only recently been studied. While on tree branches,
snakes use several modes of locomotion depending on species and bark
texture. In general, snakes will use a modified form of concertina
locomotion on smooth branches, but will laterally undulate if contact points
are available. Snakes move faster on small branches and when contact
points are present, in contrast to limbed animals, which do better on large
branches with little 'clutter'.
Gliding snakes (Chrysopelea) of Southeast Asia launch themselves from branch tips, spreading their
ribs and laterally undulating as they glide between trees. These snakes
can perform a controlled glide for hundreds of feet depending upon launch
altitude and can even turn in midair.
Rectilinear
The slowest mode of snake locomotion is
rectilinear locomotion, which is also the only one where the snake does not
need to bend its body laterally, though it may do so when turning. In this
mode, the belly scales are lifted and pulled forward before being placed down
and the body pulled over them. Waves of movement and stasis pass posteriorly,
resulting in a series of ripples in the skin. The ribs of the snake do not
move in this mode of locomotion and this method is most often used by large pythons, boas, and viperswhen stalking prey across open ground as
the snake's movements are subtle and harder to detect by their prey in this
manner.
Interactions with
humans
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 designed 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 far greater hazard to humans. The World Health Organisation
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 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 is 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.
Snake charmers
In some parts of the world, especially
in India, snake charming is a roadside show performed by a charmer. In such a
show, the snake charmer carries a basket that contains a snake that he
seemingly charms by playing tunes from his flutelike musical instrument, to
which the snake responds. Snakes lack external ears, though they do have
internal ears, and respond to the movement of the flute, not the actual noise.
The Wildlife Protection Act of 1972 in India technically proscribes
snake charming on grounds of reducing animal cruelty. Other snake charmers also
have a snake and mongoose show, where both the animals have a mock fight;
however, this is not very common, as the snakes, as well as the mongooses, may
be seriously injured or killed. Snake charming as a profession is dying out in
India because of competition from modern forms of entertainment and environment
laws proscribing the practice.
Trapping
The Irulas tribe of Andhra Pradesh and Tamil
Nadu in India have
been hunter-gatherers in the hot, dry plains forests, and have practiced the
art of snake catching for generations. They have a vast knowledge of snakes in
the field. They generally catch the snakes with the help of a simple stick. Earlier,
the Irulas caught thousands of snakes for the
snake-skin industry. After the complete ban of the snake-skin industry in India
and protection of all snakes under the Indian Wildlife (Protection) Act 1972, they formed the Irula Snake Catcher's
Cooperative and switched to catching snakes for removal of venom, releasing
them in the wild after four extractions. The venom so collected is used for
producing life-saving antivenom, biomedical research and for other medicinal
products. The Irulas are
also known to eat some of the snakes they catch and are very useful in rat
extermination in the villages.
Despite the existence of snake charmers,
there have also been professional snake catchers or wranglers. Modern-day snake trapping involves a herpetologist using
a long stick with a V- shaped end. Some television show hosts, like Bill
Haast, Austin Stevens, Steve
Irwin, and Jeff
Corwin, prefer to catch
them using bare hands.
Consumption
While not commonly thought of as food in
most cultures, in some cultures, the consumption of snakes is acceptable, or
even considered a delicacy, prized for its alleged pharmaceutical effect of
warming the heart. Snake
soup of Cantonese cuisine is
consumed by local people in autumn, to warm up their body. Western cultures
document the consumption of snakes under extreme circumstances of
hunger. Cooked rattlesnake meat is an exception, which is
commonly consumed in parts of the Midwestern United States.
In Asian countries such as China, Taiwan, Thailand, Indonesia, Vietnam and
Cambodia, drinking the blood of snakes—particularly the cobra—is believed to increase sexual
virility. The blood is drained while the cobra is still alive when
possible, and is usually mixed with some form of liquor to improve the taste.
In some Asian countries, the use of
snakes in alcohol is also accepted. In such cases, the body of a snake or
several snakes is left to steep in a jar or container of liquor. It is claimed
that this makes the liquor stronger (as well as more expensive). One example of
this is the Habusnake sometimes placed in the Okinawan liquor Awamori also known as "Habu
Sake".
Snake wine (蛇酒) is an alcoholic beverage produced by infusing
whole snakes in rice wine or grain alcohol. The drink was first recorded to have
been consumed in China during the Western Zhou dynasty and
considered an important curative and believed to reinvigorate a person
according to Traditional Chinese medicine.
Pets
In the Western world, some snakes
(especially docile species such as the ball python and corn
snake) are kept as
pets. To meet this demand a captive breeding industry
has developed. Snakes bred in captivity tend to make better pets and are
considered preferable to wild caught specimens. Snakes can be very low
maintenance pets, especially compared to more traditional species. They require
minimal space, as most common species do not exceed 5 feet (1.5 m) in
length. Pet snakes can be fed relatively infrequently, usually once every 5 to
14 days. Certain snakes have a lifespan of more than 40 years if given proper
care.
Symbolism
In Egyptian history,
the snake occupies a primary role with the Nile cobra adorning the crown of the
pharaoh in ancient times. It was worshipped as
one of the gods and was also used for sinister purposes: murder of an adversary
and ritual suicide (Cleopatra).
In Greek mythology snakes
are often associated with deadly and dangerous antagonists, but this is not to
say that snakes are symbolic of evil; in fact, snakes are a chthonic symbol, roughly translated as
'earthbound'. The nine-headed Lernaean Hydra that Hercules defeated and the three Gorgon sisters are children of Gaia, the earth.[97] Medusa was one of the three Gorgon
sisters who Perseus defeated. Medusa is described as a hideous mortal, with
snakes instead of hair and the power to turn men to stone with her
gaze. After killing her, Perseus gave her head to Athena who fixed it to her shield called
the Aegis. The Titans are
also depicted in art with snakes instead of legs and feet for the same
reason—they are children of Gaia and Uranus, so they
are bound to the earth.
The legendary account of the foundation
of Thebes mentioned a monster snake guarding the spring from
which the new settlement was to draw its water. In fighting and killing the
snake, the companions of the founder Cadmus all perished - leading to the term
"Cadmean victory" (i.e. a victory involving one's own ruin).
Three medical symbols involving snakes
that are still used today are Bowl of Hygieia,
symbolizing pharmacy, and the Caduceus and Rod of Asclepius,
which are symbols denoting medicine in general.
India is often called the land of snakes
and is steeped in tradition regarding snakes. Snakes are worshipped as
gods even today with many women pouring milk on snake pits (despite snakes'
aversion for milk). The cobra is seen on the neck of Shiva and Vishnu is depicted often as sleeping on a
seven-headed snake or within the coils of a serpent.There are also several
temples in India solely for cobras sometimes called Nagraj (King of Snakes) and it is
believed that snakes are symbols of fertility. There is a Hindu festival
called Nag Panchami each year on which day snakes are venerated and prayed
to. See also Nāga.
In India there is another mythology
about snakes. Commonly known in Hindi as "Ichchhadhari"
snakes. Such snakes can take the form of any living creature, but prefer human
form. These mythical snakes possess a valuable gem called "Mani",
which is more brilliant than diamond. There are many stories in India about
greedy people trying to possess this gem and ending up getting killed.
The ouroboros is a symbol associated with many
different religions and customs, and is claimed to be related to alchemy. The ouroboros or uroboros is a snake
eating its own tail in a clock-wise direction (from the head to the tail) in
the shape of a circle, representing the cycle of life,
death and rebirth,
leading to immortality.
The snake is
one of the 12 celestial animals of Chinese Zodiac,
in the Chinese calendar.
Many ancient Peruvian cultures
worshipped nature. They emphasized animals and often depicted snakes in
their art.
Religion
Snakes are a part of Hindu worship. A
festival, Nag Panchami, in which participants worship either images of or
live Nāgas (cobras) is celebrated every year. Most images of Lord Shiva depict snake around his neck.
Puranas have various stories associated with snakes. In the Puranas, Sheshais said to hold all the planets of the
Universe on his hoods and to constantly sing the glories of Vishnu from all his
mouths. He is sometimes referred to as "Ananta-Shesha", which means
"Endless Shesha". Other notable snakes in Hinduism are Ananta, Vasuki, Taxak, Karkotaka and Pingala. The term Nāga is used to refer to entities that
take the form of large snakes in Hinduism and Buddhism.
Snakes have also been widely revered,
such as in ancient Greece, where the serpent was seen as a healer. Asclepius carried a serpent wound around his
wand, a symbol seen today on many ambulances.
In religious terms, the snake and jaguar are arguably the most important
animals in ancient Mesoamerica. "In states of ecstasy, lords
dance a serpent dance; great descending snakes adorn and support buildings
from Chichen Itza to Tenochtitlan, and the Nahuatl word coatl meaning serpent or twin, forms
part of primary deities such as Mixcoatl, Quetzalcoatl, and Coatlicue." In both Maya and Aztec calendars,
the fifth day of the week was known as Snake Day.
In Judaism, the snake of brass is also a symbol of
healing, of one's life being saved from imminent death.
In some parts of Christianity, Christ's redemptive work is compared to saving
one's life through beholding the Nehushtan (serpent of brass). Snake handlers use
snakes as an integral part of church worship in order to exhibit their faith in
divine protection. However, more commonly in Christianity, the serpent has been
seen as a representative of evil and sly plotting, which can be
seen in the description in Genesis chapter
3 of a snake in the Garden of Eden tempting Eve. Saint Patrick is
reputed to have expelled all snakes from Ireland while converting the country
to Christianity in the 5th century, thus explaining the absence of snakes
there.
In Christianity and Judaism, the snake
makes its infamous appearance in the first book of the Bible when a serpent
appears before the first couple Adam
and Eve and tempts
them with the forbidden fruit from
the Tree of Knowledge. The snake returns in Exodus when Moses, as a sign of God's power, turns his
staff into a snake and when Moses made the Nehushtan, a bronze snake on a pole that when
looked at cured the people of bites from the snakes that plagued them in the
desert. The serpent makes its final appearance symbolizing Satan in the Book of Revelation:
"And he laid hold on the dragon the old serpent, which is the devil and
Satan, and bound him for a thousand years."
Medicine
The cytotoxic effect of snake venom is being
researched as a potential treatment for cancers