Perception

Perception

Eyesight

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.^[33] 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.

Smell

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 orJacobson's organ in the mouth for examination.^[34] The fork in the tongue gives snakes a sort of directional sense of smell and taste simultaneously.^[34] 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.^[34]

Thermographic image of a snake eating a mouse

Vibration sensitivity

The part of the body in direct contact with the ground is very sensitive to vibration; thus, a snake can sense other animals approaching by detecting faint vibrations in the air and on the ground.^[34]

Infrared sensitivity

Pit vipers, pythons, and some boas have infrared-sensitive receptors in deep grooves between the nostril and eye, although some have labial pits on their upper lip just below the nostrils (common in pythons), which allow them to "see" the radiated heat of warm-blooded prey mammals.^[34]

Venom

See also: Snake venom

Milk snakes are often mistaken for coral snakes, whose venom is deadly to humans.

Cobras, vipers, and closely related species use venom to immobilize or kill their prey. The venom is modified saliva, delivered through fangs.^[7]:243 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.^[7]:243 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.^[7]:209
Certain birds, mammals, and other snakes such as kingsnakes that prey on venomous snakes have developed resistance and even immunity to certain venoms.^[7]:243 Venomous snakes include three families of snakes, and do not constitute a formal classification group used in taxonomy. The term poisonous snake is mostly incorrect; poison is inhaled or ingested, whereas venom is injected.^[35] 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 population of garter snakes in Oregon retains enough toxin in their liver from the newts they eat to be effectively poisonous to local small predators such as crows and foxes.^[36]
Snake venoms are complex mixtures of proteins, and are stored in poison glands at the back of the head.^[36] In all venomous snakes, these glands open through ducts into grooved or hollow teeth in the upper jaw.^[7]:243[35] 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.^[35] Almost all snake venom contains hyaluronidase, an enzyme that ensures rapid diffusion of the venom.^[7]:243
Venomous snakes that use hemotoxins usually have the fangs that secrete the venom in the front of their mouths, making it easier for them to inject the venom into their victims.^[35] Some snakes that use neurotoxins, such as the mangrove snake, have their fangs located in the back of their mouths, with the fangs curled backwards.^[37] This makes it both difficult for the snake to use its venom and for scientists to milk them.^[35] Elapid snakes, however, such as cobras and kraits are proteroglyphous, possessing hollow fangs that cannot be erected toward the front of their mouths and cannot "stab" like a viper; they must actually bite the victim.^[7]:242
It has recently been suggested that all snakes may be venomous to a certain degree, with harmless snakes having weak venom and no fangs.^[38] Most snakes currently labelled “nonvenomous” would still be considered harmless according to this theory, as these snakes either lack a delivery method for the venom or are simply incapable of delivering enough to endanger a human. This theory postulates snakes may have evolved from a common lizard ancestor that was venomous, from which venomous lizards like the gila monster and beaded lizard may also have derived, as well as the monitor lizards and now extinct mosasaurs. 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.^[37]
• Viperids – vipers, rattlesnakes, copperheads/cottonmouths, adders and bushmasters.^[37]

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.^[7]:209[37]

Behavior

Feeding and diet

Snake eating a rodent.

Carpet python constricting and consuming a chicken.

All snakes are strictly carnivorous, eating small animals including lizards, other snakes, small mammals, birds, eggs, fish, snails or insects.^{[7][1][8][39]} 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.

African egg-eating snake.

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,^[39] as snakes do not chew. For example, the African egg-eating snake has flexible jaws adapted for eating eggs much larger than the diameter of its head.^[7]:81 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.^[7]:81
While the majority of snakes eat a variety of prey animals, there is some specialization by some species. King cobras and 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^[7]:184[40]
Some snakes have a venomous bite, which they use to kill their prey before eating it.^[39][41] Other snakes kill their prey by constriction.^[39] Still others swallow their prey whole and alive.^[7]:81[39]
After eating, snakes become dormant while the process of digestion takes place.^[42] 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.^[43] 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.^[44][45]

Lateral undulation

Main article: Undulatory locomotion

Lateral undulation is the sole mode of aquatic locomotion, and the most common mode of terrestrial locomotion.^[45] In this mode, the body of the snake alternately flexes to the left and right, resulting in a series of rearward-moving "waves."^[44] While this movement appears rapid, snakes have rarely been documented moving faster than two body-lengths per second, often much less.^[46] This mode of movement has the same net cost of transport (calories burned per meter moved) as running in lizards of the same mass.^[47]