Snakes (from Old English snaca, and ultimately from PIE base *snag-
or *sneg-, "to crawl), also known as ophidians, are cold blooded
legless reptiles closely related to lizards, which share the order
Squamata. There are also several species of legless lizard which
superficially resemble snakes, but are not otherwise related to
them. A love of snakes is called ophiophilia, a fear of snakes is
called ophidiophobia (or snakephobia). A specialist in snakes is
An old synonym for snake is serpent (which comes from Old French,
and ultimately from PIE *serp-, "to creep"); in modern
usage this usually refers to a mythic or symbolic snake, and information
about such creatures will be found under serpent (symbolism). This
article deals with the biology of snakes.
All snakes are carnivorous, eating small animals including lizards
and other snakes, rodents and other small mammals, birds, eggs or
insects. 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.
Snakes do not chew their food and 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.
After eating, snakes become torpid while the process of digestion
takes place. Digestion is an intensive activity, especially after
the consumption of very large prey. In species which feed only sporadically,
the entire intestine enters a reduced state between meals to conserve
energy, and the digestive system is 'up-regulated' to full capacity
within 48 hours of prey consumption. So much metabolic energy is
involved in digestion that in Crotalus durissus, the Mexican rattlesnake,
an increase of body temperature to as much as 6 degrees above the
surrounding environment has been observed. Because of this, a snake
disturbed after having eaten recently will often regurgitate the
prey in order to be able to escape the perceived threat. However,
when undisturbed, the digestive process is highly efficient, dissolving
and absorbing everything but hair and claws, which are excreted
along with uric acid waste. Snakes have been known to occasionally
die from trying to swallow an animal that is too big. Snake digestive
acids are unable to digest most plant matter, which passes through
the digestive system mostly untouched.
Snakes do not normally prey on people, but there are instances
of small children being eaten by large constrictors in the jungle.
While some particularly aggressive species exist, most will not
attack humans unless startled or injured, preferring instead to
avoid contact. In fact, the majority of snakes are either non-venomous
or possess venom that is not harmful to humans.
Snakes utilize a variety of methods of movement which allows them
substantial mobility in spite of their legless condition. All snakes
are capable of lateral undulation, in which the body is flexed side-to-side,
and the flexed areas propagate posteriorly, giving the overall shape
of a posteriorly propagating sine wave. In addition, all snakes
are capable of concertina movement. This method of movement can
be used to both climb trees and move through small tunnels. In the
case of trees, the branch is grasped by the posterior portion of
the body, while the anterior portion is extended. The anterior portion
then grasps the branch, and the posterior portion is pulled forward.
In the case of tunnels, instead of grasping, the body loops are
pressed against the tunnel walls to attain traction, but the motion
is otherwise similar. Another common method of locomotion is rectilinear
locomotion, in which the snake remains straight and propels itself
via a caterpillar-like motion of its belly-muscles. This mode is
usually only used by very large, heavy snakes, such as large pythons
and vipers. The most complex and interesting mode is sidewinding,
an undulatory motion used to move across slippery mud or loose sand.
Not all snakes dwell on land; sea snakes live in shallow tropical
Studies of the motion and muscle activity of moving snakes have
shed light on how each of these modes is achieved.
In terrestrial lateral undulation, posteriorly propagating unilateral
waves of muscle contraction occur. The regions of muscle activity
for each side extend from the most concave point on that side posteriorly
to the most convex side. Thus, when a point on the snake's body
is maximally flexed to the right, the right muscles activate, bending
it back to the left until it's maximally right-convex, at which
point the right side muscles turn off, and the left side muscles
turn on. Speed is modulated primarily by alteration of frequency.
Aquatic lateral undulation appears superficially similar, but the
muscle activation pattern is different, with the regions of muscle
activity being 'shifted' posteriorly to where they would be in terrestrial
lateral undulation. The reasons for this difference are not fully
Sidewinding, though it appears complex and confusing, is actually
a simple modification of terrestrial lateral undulation. At the
points of maximal flexion, the dorsalmost muscle group (traversospinalis
group) activates, lifting that portion of the body over the ground,
and resulting in other portions of the body remaining in static
contact. This mode is used to cross slick surfaces such as mud flats
and sand, and has nothing to do with thermoregulation, as is sometimes
erroneously stated. Many species of snake, including species commonly
kept as pets and which do not usually encounter deserts or mud flats,
will sidewind when placed on a slick floor or tabletop and enticed
to move fast.
Concertina locomotion and rectilinear locomotion are less well
understood. Studies of muscle activity have only been done for tunnel
concertina locomotion, which shows that the muscles are unilaterally
active in static regions of bending in order to brace the snake
against the tunnel walls. Rectilinear is believed to rely on different
muscles from the other modes; while they all rely on the large epaxial
muscles, rectilinear locomotion seems to rely upon the small costocutaneous
muscles. However, this has not been verified experimentally, due
to the difficulties in working with these small muscles.
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