Snake

Snakes are not a separate branch of reptiles sitting alongside lizards. They are nested inside the lizards — a squamate lineage that lost its limbs and then did extremely well without them, to the tune of around four thousand species on every continent but Antarctica. Some retain vestigial hind limbs: pythons and boas have small claw-like spurs either side of the vent, which are the remains of legs.

Losing limbs happened repeatedly in squamates, and several living lizards have done it independently — which is why "is that a snake or a legless lizard" is a real question rather than a pedantic one. The practical differences are on the head. Legless lizards keep movable eyelids and external ear openings; snakes have neither. A snake's eye is covered by a transparent scale called a spectacle, which is why it never blinks and why the eye goes cloudy before a shed.

• No eyelids — a fixed transparent scale over each eye instead
• No external ear opening, and no eardrum
• Paired organs reduced or lost: most snakes have one functional lung
• Ribs along most of the body, with no breastbone joining them at the front
• A tongue that collects chemicals rather than tasting them

That last skeletal detail matters more than it sounds. Because the ribs are not tied together at the front by a sternum, the body wall can spread — which is what allows a snake to accommodate something wider than itself, and why a snake that has eaten recently is a visibly different shape.

A dislocation is a bone leaving a joint it belongs in. It is an injury, it is painful, and no animal does it routinely as part of eating. What a snake actually has is a jaw that was never assembled the way ours is. In a human, the two halves of the lower jaw are fused at the chin into a single bone. In a snake they are separate, joined at the front only by a stretchy ligament — so they do not come apart, because they were never together. They simply spread.

The back of the jaw adds the rest of the range. The quadrate, the bone suspending the lower jaw from the skull, is itself mobile where it meets the braincase and where it meets the jaw — effectively a double hinge. Combined with a skull whose bones are loosely connected throughout, the result is a mouth that opens around prey much wider than the head, without anything leaving its socket.

That ratchet is the part the myth cannot account for, and it is the reason the real mechanism is better. A snake eating live prey has no hands and no way to hold something down. If its jaw were genuinely disarticulated it could not grip at all. Instead it keeps continuous purchase with one side while the other advances, and the backward-curving teeth make the whole thing a one-way system: the meal can go in and cannot come back out.

Breathing during this is handled by the glottis, a tube in the floor of the mouth that the snake can push forward past the prey — the reason a snake with a mouthful of rat is not suffocating. Swallowing a large meal can take an hour or more.

And then the harder part starts. A Burmese python that has fasted for weeks has shut its digestive system down — the intestine of a fasting python is a fraction of its working size, because maintaining an unused gut is expensive. On feeding, metabolic rate rises around fortyfold, comparable to a mammal at maximum exertion and sustained for days, while the intestine, heart, liver and kidneys all grow substantially and then shrink back once digestion is finished. The swallowing is the easy part. The digestion requires the animal to build a bigger gut first.

The tongue has no taste buds and no chemical receptors of any kind. It is a collector: it flicks out, picks up molecules from air and surfaces, and delivers them to the vomeronasal organs — a pair of chemical sensors in the roof of the mouth, sometimes called Jacobson's organ. So "snakes smell with their tongues" is half right, and the wrong half is the interesting one.

The fork is not there to gather more. The tips are held apart during a flick by roughly the width of a scent trail, and each delivers to its own vomeronasal opening, so two samples arrive simultaneously from slightly different places. The animal is comparing left against right. That converts a concentration — "something is here" — into a direction, and it is why snakes can follow a trail rather than merely detect one. Forked tongues have evolved several times over in lizards that trail-follow, which is the comparative evidence that this is what the fork is for.

Snakes have no external ear and no eardrum, and for a long time that was read as no hearing. The inner ear is there, and it is coupled to the quadrate and the lower jaw — which are, in a snake, usually resting on the ground. Drive vibration and airborne sound separately and the animal responds to both, with vibration producing much the stronger response and the best sensitivity in a low band around 80 to 160 hertz. The practical folk version turns out to be accurate for the wrong reason: a snake usually notices you walking long before it notices you talking.

Heat sensing is the fact most often stated about snakes in general and it belongs to a minority. Pit organs are found in pit vipers, in many pythons and in some boas — three lineages whose pits are anatomically different from each other and evolved separately. Remarkably, all three converged on the same molecule: TRPA1, a channel that in other vertebrates reports chemical irritation, repurposed as a heat sensor. Most snakes, including all the colubrids that make up the bulk of species, detect no heat at all.

It is also not vision. No lens, no retina, no image formed from photons — a membrane reading radiated warmth, which the brain then combines with what the eyes report. Detection rather than sight, in a minority of snakes.

Vision itself varies enormously and has an unusual history. Snakes have lost several of the visual pigments other reptiles retain, which suggests their common ancestor lived somewhere dark — burrowing, or nocturnal. What snakes have done since is rebuild vision repeatedly and independently from that reduced starting point, so a diurnal tree snake hunting by sight and a burrowing blindsnake with eyes under its scales are not two points on one scale. They are separate reconstructions.

There is no such thing as "how a snake moves". A body without limbs turns out not to be one problem with one workaround but a platform with several distinct solutions, and an individual snake switches between them according to the surface it is on.

Sidewinding looks strange because it is solving a problem the others do not face. Lateral undulation needs the ground to push back, and dry sand simply gives way. So the animal stops pushing sideways and starts placing itself instead, lifting sections clear and setting them down, which leaves the parallel J-shaped tracks that make sidewinder trails recognisable.

Snake scales are folds of keratinised epidermis — the same material as a fingernail — with no mucus glands anywhere. A snake is dry to the touch, and usually smooth, though many species have keeled scales with a ridge down each one. The slimy impression comes from unrelated animals of similar shape: worms and amphibians, neither of which are relatives.

Shedding is not a matter of outgrowing a skin that stayed the same size. A whole new epidermal layer is grown underneath the old one first, and the two are then separated by a fluid layer — which is what turns the eye cap milky and the animal dull for a few days before a shed. The old layer is loosened at the snout and the snake crawls out of it, inverting the whole thing like a sock. Frequency tracks growth, so young snakes shed often and old ones rarely, and a snake that has not fed is not growing and does not shed.

Everyone is taught that a constrictor suffocates its prey by squeezing until it cannot breathe. When somebody finally instrumented the inside of a constriction — recording blood pressure, cardiac activity and blood chemistry through the whole event — the sequence turned out to be different and much faster. Arterial pressure halves within about six seconds; venous pressure rises sharply; the heart becomes electrically disordered within a minute. The animal is dead from circulatory failure long before lack of air could account for it.

That is a mechanism assumed for a century because the outcome was obvious and nobody had measured the middle. It also changes what the behaviour is. A constrictor is not waiting for breathing to stop; it is applying a pressure the circulatory system cannot work against, which is why the process is quick.

Venom is older and more widespread than the fangs-and-antivenom picture suggests. Toxin gene families turn out to be ancestral to a large group of squamates, and are expressed in the oral secretions of many snakes conventionally called non-venomous. That is a fact about evolutionary history, and it has been badly over-read: the presence of a toxin gene establishes nothing about delivery, dose, or whether anything could be harmed. A great many snakes with such genes have no apparatus to inject anything and no capacity to hurt a person.

The medically significant snakes number in the low hundreds out of roughly four thousand species, and they are not evenly distributed. Snakebite kills on the order of a hundred thousand people a year and permanently disables several hundred thousand more, almost entirely among agricultural workers in rural South Asia, sub-Saharan Africa and Latin America. The World Health Organization classifies it as a neglected tropical disease, and the word neglected is the operative one: this is a supply-and-access problem more than a biological one.

Venoms are usually described as neurotoxic or haemotoxic, which is a useful first cut and a considerable simplification — most are complex mixtures doing several things at once, and the balance differs between and within species. Some elapids also spit, projecting venom towards the eyes as a defence rather than to subdue prey.

This is one of the most-asked things about snakes, and NatureHQ declines it on purpose. Not out of caution — declining is the accurate answer, and explaining why is more useful than a rule would be.

Take the two most widely shared rules. A triangular head is supposed to mean venomous: in fact a great many harmless snakes flatten their heads into a triangle when threatened, because looking like a viper is a good defence and that is precisely what the display is for. Slit pupils are supposed to mean venomous: pupil shape tracks when an animal is active, not what is in its glands. Harmless nocturnal snakes have slit pupils. Cobras, mambas and taipans have round ones.

That second failure is the dangerous kind. A rule that produces false negatives on some of the most dangerous snakes alive is worse than no rule at all, because it manufactures confidence. And every one of these heuristics is regional: the rhymes about red and yellow bands describe the coral snakes of one part of one continent and are actively wrong elsewhere.

One thing worth saying plainly, because it drives a great deal of fear: no snake hunts people. Every snake is limited to prey it can swallow whole, and a person is not on that list for any species alive. The experience of being chased is real and the interpretation is not — a frightened snake heads for cover, and if you are standing between it and the cover, it comes at you. Bite statistics agree: bites happen overwhelmingly when somebody treads on or reaches towards an animal they had not seen.

Most snakes lay eggs, and roughly one species in five does not — and the exceptions are not scattered at random. Live birth turns up again and again in cold places, at higher latitudes and altitudes, and the reason is thermal: a mother carrying developing young can move them into the sun, and an egg buried in cold ground cannot be moved at all. The same pressure has produced the same answer independently many times over within squamates.

Parental care beyond laying is uncommon and not absent. Several pythons coil around the clutch and shiver, generating heat by muscle contraction to keep the eggs warm — an ectotherm deliberately making heat for its offspring. Some vipers and cobras remain with the eggs or with newly born young for a period. King cobras build a nest, which almost no other snake does.

Lifespans vary widely with size and are much longer than most people assume: small colubrids often live a decade or more, and large pythons and boas can reach thirty or forty years in captivity. Growth continues throughout life, slowing rather than stopping, which is why very old individuals of the large species are the ones that produce the famous measurements.

In temperate climates snakes spend the cold months in brumation — the reptile counterpart of hibernation, and a genuinely different thing rather than a synonym. Body temperature simply follows the surroundings down, activity ceases, and the animal may emerge to drink or to bask on a warm day. Many species overwinter communally in the same den year after year, which is where some of the largest snake aggregations on record occur.

Snakes were described as solitary for two centuries, and the description was never really tested. When somebody did test it, the design had to remove two boring explanations first: that some spots are simply better, and that snakes follow the scent of whoever got there first. Cleaning the arena between trials and shuffling the animals to new starting positions removes both — and the same individuals still ended up together.

The right description is non-random, repeatable association with individual preference, plus consistent differences between individuals in how sociable they are. The wrong description is "snakes have friends", which imports recognition, memory of a relationship and some benefit from it, none of which was tested. The finding is surprising enough as it stands: an animal characterised as asocial for two hundred years turns out to have preferences about company, largely because nobody had looked.

It is worth keeping the scope in view. This is one garter snake subspecies, in captivity, in an arena with a fixed number of shelters. Almost none of the four thousand snake species have been examined this way, and aggregation can serve warmth or safety without implying anything about relationships between the animals doing it.

• What sets the upper limit on the size of prey a snake can take?

  Why it matters: It is usually assumed to be gape, but skin elasticity and the cost of digestion may bind first — and which one it is determines what a snake's body size actually buys it.

  What would settle it: Measurements of maximum prey mass against gape, skin extensibility and post-feeding metabolic cost in the same individuals.

• Do snakes use airborne hearing for anything, or is it an incidental by-product?

  Why it matters: An ear that transmits a signal is not the same as an animal that acts on it, and the difference decides whether snakes have one sensory channel here or two.

  What would settle it: Behavioural threshold experiments with airborne sound and substrate vibration independently controlled.

• How widespread is preferential association across snakes, and does it persist in the wild?

  Why it matters: A single captive study in one garter snake subspecies is a thin basis for revising the social biology of an entire suborder.

  What would settle it: Field studies of marked individuals in populations where animals are free to disperse.

• How much detail does a heat-sensing pit actually resolve?

  Why it matters: Whether the pit gives a vague warm direction or something closer to a coarse thermal image changes how it is combined with vision in the brain.

  What would settle it: Behavioural discrimination tests of thermal targets with the eyes occluded.