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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 ophiologist.

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 seas.

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 understood.

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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