Reptiles
Snake
Serpentes · also called serpent, ophidian
A snake is a lizard that gave up its legs. Somewhere over 100 million years ago a lineage of lizards took to burrowing or hunting in leaf litter, where limbs got in the way, and lost them — along with the eardrums and eyelids. What was left became one of the most successful predator designs on Earth: a flexible tube that smells with its tongue, hears through its jaw, and can swallow a meal wider than its own skull.
Overview
Snakes are the suborder Serpentes, around 4,000 species, on every continent except Antarctica. All are legless carnivores that eat live prey whole. They range from thread snakes the size of an earthworm to the reticulated python at over 6 m and the green anaconda at over 100 kg. Only about 600 species are venomous and roughly 200 dangerous to people.
Tongue, jaw, and ear
The forked tongue samples scent from two points at once, so the snake smells in stereo. The lower jaw splits at the chin, joined by stretchy ligament, and the skull bones are loosely hinged, letting the snake walk its head over prey several times wider than itself. With no eardrum, it hears through its jawbone, picking up ground vibrations and low sound.
Killing methods
Snakes subdue prey by venom, by constriction — tightening coils with each of the prey’s exhalations until blood flow stops — or, for many small species, simply by swallowing it alive.
Growth and shedding
A snake grows for its whole life and periodically sheds its entire outer skin in one piece, including the clear scale over each eye, starting the peel by rubbing its nose on something rough.
Status
Snakes as a group are Not Evaluated, but long-term study sites on several continents show a broad decline in snake numbers, and about a fifth of assessed species are threatened by habitat loss, roads, deliberate killing, the skin and pet trades, and snake fungal disease. Snakebite still kills an estimated 80,000 to 140,000 people a year, almost all for want of antivenom.
Snake: common questions
How did snakes lose their legs?
Snakes are descended from lizards, and their limblessness evolved gradually as an adaptation to a particular way of moving and hunting. The two main hypotheses are that early snakes were burrowers, for which legs are an obstacle in a tunnel, or that they were swimmers in shallow water; genetic and fossil evidence currently leans toward a burrowing or leaf-litter origin. Over time the limb girdles and then the limbs themselves were reduced and lost, along with the external ears and movable eyelids, while the body elongated and gained many extra vertebrae. Pythons, boas, and a few other snakes still have vestigial hip bones and tiny external claw-like spurs, the leftovers of hind legs.
Can snakes hear?
Yes, but not the way we do. Snakes have no eardrum and no ear opening, so they cannot pick up airborne sound the way a mammal does, and for a long time they were thought to be deaf. In fact a snake hears through its jaw. The lower jawbone rests on the ground and is connected by a chain of small bones to a functioning inner ear, so vibrations travelling through the ground, and to a lesser extent low-frequency sound in the air, reach the hearing organ. This lets a snake sense the footfalls of an approaching animal quite well, while leaving it largely unresponsive to a high-pitched voice or a snake charmer's flute.
How do snakes eat animals bigger than their heads?
Their skulls are built to spread apart. The two sides of the lower jaw are not fused at the chin, as ours are, but joined only by an elastic ligament that lets them separate widely, and the jaw is attached to the skull through a loose, double-jointed linkage. Many of the skull bones can also move independently. The snake hooks its backward-curved teeth into the prey, then advances one side of its jaw at a time, effectively walking its head over the meal while the skin of the neck and body stretches enormously. Slow waves of muscle then push the food down to the stomach, where powerful acid digests even bone.
What should you do if you see a snake, or are bitten by one?
If you see a snake, stop, give it space, and let it move away; most snakes flee if they can, and the majority of bites happen when people try to catch or kill them. In snake country, watch where you place hands and feet, wear boots, and step onto logs rather than over them. If someone is bitten by a potentially venomous snake, keep them calm and still, remove rings and watches, immobilise the limb at roughly heart level, note the time, and get to a hospital as fast as possible. Do not cut the wound, suck it, apply ice, use a tight tourniquet, or try to catch the snake.