A snake is an elongate reptile of the suborder Serpentes. Like all reptiles, snakes are covered in scales. All snakes are carnivorous and can be distinguished from legless lizards by their lack of eyelids, limbs, external ears, and vestiges of forelimbs. The 2,700+ species of snakes spread across every continent except Antarctica ranging in size from the tiny, 10 cm long thread snake to pythons and anacondas at 9 m (30 ft) long. In order to accommodate snakes' narrow bodies, paired organs (such as kidneys) appear one in front of the other instead of side by side.

While venomous snakes comprise a minority of the species, some possess potent venom capable of causing painful injury or death to humans. However, venom in snakes is primarily for killing and subduing prey rather than for self-defense. Snakes may have evolved from a lizard which adapted to burrowing during the Cretaceous period (c 150 Ma), though some scientists have postulated an aquatic origin. The diversity of modern snakes appeared during the Paleocene period (c 66 to 56 Ma).

Snake eating a rodent

Snake eating a lizard near Ibor river (Extremadura, Spain)

All snakes are strictly carnivorous, eating small animals including lizards, other snakes, small mammals, birds, eggs, fish, snails or insects.^[2][3][4] Because snakes cannot bite or tear their food to pieces, a snake must swallow its 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 the most unique jaw in the animal kingdom. 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,^[3] 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.^[4] This snake has no teeth, but does have bony protrusions on the inside edge of its spine which are used to aid in breaking the shells of the eggs it eats.^[4]

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^[5][6]

Some snakes have a venomous bite, which they use to kill their prey before eating it.^[7][3] Other snakes kill their prey by constriction.^[3] Still others swallow their prey whole and alive.^[3][4]

African Egg-eating snake

After eating, snakes become dormant while the process of digestion takes place.^[8] Digestion is an intense activity, especially after consumption of very large prey. In species that 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. Being cold-blooded (ectothermic), the surrounding temperature plays a large role in a snake's digestion. 30 degrees Celsius is the ideal temperature for snakes to digest their food. So much metabolic energy is involved in a snake's digestion that in Crotalus durissus, the Mexican rattlesnake, an increase of body temperature to as much as 1.2 degrees Celsius above the surrounding environment has been observed.^[9] Because of this, a snake disturbed after having eaten recently will often regurgitate its prey in order 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 and claws, which are excreted along with waste.