Snakes
There are over 3,400 species of snake in 500 genera in over 20 families. Snakes live on every continent except Antarctica and in the sea. Unlike lizards, snakes have made their way to many oceanic islands.
Snakes range greatly in size: from 10 cm burrowing thread snakes to 7+ meter pythons and boas.
Specialization is extensive. Arboreal snakes are long and thin. Many vipers, boas and pythons are heavy-set and relatively short. A diverse range between the ends of that spectrum exist. Female snakes are typically larger than males, though sexual dimorphism is not generally pronounced in snakes.
Anatomy
For all their diversity, snakes are a clearly recognizable group. They have a flexible body with no limbs. Their eyes have no eyelids. Snake eyes are protected by the same keratin sheet that covers the body.
Snakes differ from other reptiles by what they lack: sternum (breastbone) and forelimbs, along with shoulder girdle. There are other anatomical anomalies.
Internally, snakes are mostly a digestive tract. The stomach accounts for more than 1/3rd of body length in some snakes.
Like lizards and birds, snakes have no urinary bladder. Nitrogenous wasted is passed as semi-solid uric acid; not urea, as with mammals.
Most snakes have a single lung. In the snakes with 2 lungs, the left lung is invariably smaller: from 85% to as little as 1%.
A snake heart has 3 chambers. The right atria receives blood from the lung(s); the left from the rest of the body. Both atria pass their blood to the single ventricle for recirculation. Lacking a diaphragm, a snake’s heart can move around to accommodate ingestion of large prey.
The snake skull is an impressively flexible construction. With few exceptions, most skull bones are movably connected to each other, and only loosely attached to the braincase. This allows a snake to stretch and distort much of its head in various directions.
Snake jaws are constructed to create a cavernous opening. Snakes easily swallow something wider than they are.
Snake teeth are hinged on flexible ligaments, enabling them to lock their teeth in a backward-pointing position when prey is being swallowed, thus preventing dinner from struggling free.
Otherwise, snake dentition varies. Most snakes have 2 rows of teeth on the upper jaw, and one on the lower jaw(s). Some snakes’ lower jaws are not fused into a single jaw, so that there is a gap between the 2 lower jaws.
A snake doesn’t suffocate while swallowing a huge meal because the end of its windpipe is reinforced with cartilage rings. Further, the windpipe can be extended forward, along the floor of its mouth, so that breathing can continue unimpeded.
Because they have no breastbone, the ends of ribs can widely separate to let large prey pass into the stomach.
Poisonous snakes have serious fangs: either grooved, to guide venom delivery from glands on the top of the head; or hollow, like hypodermic needles, through which venom flows.
Skin
Snakes, like all reptiles, are covered in scales, which protect from abrasion and dehydration. The outermost layer is a continuous sheet of keratin, which is a tough fibrous protein.
Snakeskin, which is very flexible and elastic, is water impermeable. Hence, snakes cannot absorb moisture through their skins. But about 2/3rds of their water loss is through the skin, with the rest by excretion. Snake scales are no better at retaining water than other skin types.
Snakes grow throughout their entire lives. So, to accommodate growth and facilitate repair, snakes periodically shed their skin; a process termed ecdysis.
Snakes are the only vertebrate that completely molt their entire skin. Snakes molt 1 to 3 times a year, depending upon species. A snake typically does not eat for a couple weeks prior to molting.
Temperature Regulation
Snakes are typically ectothermic creatures. Cold-blooded is a hoary misnomer: so-called cold-blooded animals actually have warm blood.
Endothermic animals have internal mechanisms to self-regulate internal body temperature, at the expense of a greater metabolism: requiring roughly double the energy for every 10 °C rise in temperature.
Ectothermic animals need an external assist to regulate their body temperature: typically, sunlight or something warmed by sunlight. Snakes, like all ectotherms, keep warm by behavioral habits.
The long, slender snake body has a high surface-to-weight ratio. Since heat is absorbed over a surface, snakes can increase heat uptake by stretching out. Flattening out is even better, both for catching rays from above and warming the underside by conduction.
Snakes must keep themselves between 4–38 °C or risk death. Their body temperatures are typically higher than their surroundings: around 30 °C (versus 23 °C room temperature).
While active, many snakes can maintain their body temperatures within a desired 1 °C range. This is remarkable considering snakes lack feathers, fur, or other insulation, and skin is far from ideal for retaining heat.
Heat can be conserved by coiling. Snakes, even those of different species, will sometimes aggregate to keep warm and conserve moisture.
When prevailing conditions make it impossible to adequately regulate themselves, snakes retreat to dormancy. Adders in northern Europe hibernate 8 months of the year. Conversely, tropic regions, with long, dry, hot summers force snakes to sequester themselves in a cool burrow or beneath the bark of a tree to avoid dehydration.
Water snakes are limited in thermoregulatory opportunities. Hence, aquatic marine snakes are found only in the warm-water regions of the world.
Senses
There’s been this enduring myth that snakes are deaf. ~ American biologist Bruce Young
Snakes lack external ears. They have only an ear bone connected to the mandible with no ear drum.
It was long presumed that snakes could detect only ground-based low frequencies. Instead, snakes hear sounds through the air via skull vibrations. The best reception appears to be between 80–160 hertz: the lowest cello notes.
The sensitivity of snakes to vibration is great. ~ American neurobiologists Peter Hartline & Howard Campbell
Generally, snakes are short-sighted. Most lack keen sight. All snakes lack eyelids. Limited snake vision may be accounted for by their descent from burrowing lizards.
Some snakes see well enough, especially nocturnal hunters, who have quite light-sensitive sight. Snakes generally detect movement better than form. What’s to see? If it moves right and is an edible size, that’s all a snake needs to know.
Little is known about how snakes see. One oddity is that snakes have a greater diversity of photoreceptor cell types than other vertebrates, indicating that they may get cross-referential visual information which compensates for direct lack of sharpness. Another peculiarity is that snakes control blood flow to their eyes to optimize sight when they need it most: during potential emergencies.
Whatever deficiencies in vision and hearing snakes may have are compensated for by adept senses of smell and heat sensitivity.
Smell
Snakes have a tremendous sense of a smell, courtesy of 2 physically independent systems: the nose and a vomeronasal organ (VNO): open sacs lined with sensory cells at the roof of the mouth (palate), near the nostrils. The nasal and VNO systems are somewhat disconnected, using different signaling pathways. A combined sense of smell comes together in the mind.
A snake flicks its forked tongue out, picking up airborne scents. The retreated tongue tips traipse into the VNO sacs, whereupon the scent sensation is passed to the brain via potassium ion channels and then mentally interpreted.
Some lizards, such as monitor lizards, also have this chemosensory system. Other animals also have a usable VNO, including salamanders, and several species of mammals: rodents, dogs, cats, pigs, goats, cattle, and elephants, along with several primates, such as lemurs, lorises, and some New World monkeys. Humans, while apparently having the genes, lack VNO genetic expression.
For many terrestrial vertebrates, VNO plays a key role in identifying potential sexual partners, aggressors, possible prey, or predators. VNO is often a secondary system for chemoreception. The common primary means of smell, and the only one people have, is through the nose.
Infrared Detection
Some snakes, including pit vipers, boas, and pythons, have infrared-sensing pits in the nose area. These facial pits are not directly sight-related, though infrared (IR) is a visible spectrum in some animals. In snakes, infrared is detected through heat sensitivity: incoming radiation warms an ion channel which triggers a nerve impulse. The pit membrane is then rapidly cooled by vascularization.
The portion of the snake brain active while processing IR-pit input is the optic tectum, which receives other sensory information, including optical and auditory stimulations, as well as motor and proprioceptive (body-related) perception. This physical centrality coincides with mental integration.
Locomotion
Lack of limbs is no impediment to snakes getting around. Snakes have distinctly different modes of locomotion for different terrain, though some species are more specialized.
Adjustments in locomotion must be made because snake scales create different amounts of friction depending upon direction. Friction is greatest going forward, which is why snakes undulate.
Serpentine locomotion is the well-known side-to-side wiggling. It happens via large ventral plates linked to independent muscle groups, not by shifting ribs, as popularly believed. This lateral undulation is used for crossing uneven surfaces.
A different moving of ventral plates affords rectilinear crawling: a slow advance in a straight line; good for sneaky attack approaches.
Concertina locomotion is most often seen in burrowing snakes, but other snakes use it for crawling through a tube or tight space: the head and front of the body is extended, while the back and tail remain curved, providing an anchor to the ground or sides of a burrow. Once fully extended, the front gains purchase, and draws the tail up.
Desert snakes employ sidewinding: the front of the body makes an arc, then the rest of body moves like a spring. Sidewinding leaves parallel tracks in the sand.
Diet
All snakes are carnivorous, with prey preferences that are size-appropriate. Smaller snakes eat smaller prey. Insects and earthworms are frequent fare for many slighter snakes, with fish and frogs a popular step up. Eggs are a rich and defenseless protein source.
Many snakes eat other snakes, even their own species (cannibalism). Some snakes have special tastes, but many are not finicky, eating whatever is alive and available. A few snakes will occasionally eat carrion.
Different species have different feeding strategies. Some snakes patrol for prey. Other take advantage of cryptic coloration to ambush: remaining still, waiting for prey to come to them.
Some species uses a body part as bait. African twig snakes waggle their strangely colored tongues, or sway gently in low tree branches, imitating chameleons. This poses as bait to their prey, which are lizards, frogs, and birds.
Death adders and moccasins shake their colorful tails to lure potential prey. The African vine snake uses its brilliant red tongue to attract edible curiosity seekers.
Some snakes use a venomous bite to kill their prey before consuming it. Snake venoms are complex potions which not only incapacitate prey but also aid digestion.
The Big Squeeze
Boas squeeze the life out of an impending meal (constriction). Squeezing a victim to death takes 7 times the energy of sitting on a rock enjoying the Sun. While constricting the next meal into the afterlife, a boa senses when the prey’s heart stops beating, and so knows when its work is done.
This heart-stopping sense was probably an early adaptation. Some ectothermic animals can survive for hours on little oxygen. If a boa didn’t know its prey’s heart had stopped, faking dead by not breathing might give a would-be victim a last-chance getaway.
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Constriction is an adaptive refinement. Most snakes waste no time with killing technique, simply swallowing prey while it is still alive. Unable to tear their food into pieces, snakes must swallow their meals whole. As such, consumption is such an intense activity that a snake must siesta during digestion.
The organs of a Burmese python grow 30–100% after eating, starting within 12 hours, and reaching their maximum at 3 days. Metabolic rate becomes 45 times normal, accommodated by an enlarged heart and digestive organs.
At 10 days, when digestion is done, the organs are back to between-meal norm. In snakes that feed sporadically, such as the python, the entire intestine becomes inactive between meals to conserve energy.
Some snakes can survive long periods without eating. The longest recorded interval, jointly held by a green anaconda and an African rock python, is 3 years.
Birds are hard to snag, but a welcome meal when caught. Turnabout being fair play, many birds eat snakes.
The secretary bird famously dances snakes to death. This bird walks the grasslands of East Africa with its long legs. When it finds a snake, it stomps it until the snake expires. Then it eats its footwork.
Snake Eaters
A snake makes a meal for many mammals. Foxes, skunks, the European hedgehog, and North American raccoons all enjoy snake occasionally. Mongooses are snake-eating specialists.
Lizards eat snakes, as do other snakes. Even some spiders eat snakes, the black widow included.
Aquatic bugs will eat water snakes, as will fish when they get the chance; not to mention crocodiles and alligators. Perhaps being mistaken for a worm, frogs and toads will eat small snakes.
Tentacled Snakes
The tentacled snake is a unique aquatic snake, endemic to Southeast Asia waters. Adults may grow to 60 cm long.
The tentacled snake has a shtick for catching fish. On the sides of its head are 2 tentacles sensitive to water movement. The snake makes a J shape with its body, the head at the tip of the J. It then sits motionless in the water.
A fish swims up, alongside the snake. Using the tentacles to detect the fish’s progress, the snake twitches its body opposite its mouth; a feint that propagates a slight pressure wave. The wave startles the fish, who, by instinct, swims the other way: right into a trap.
If the fish is not on a presumed escape trajectory into the snake’s mouth, the snake anticipates the fish’s direction and timing, and takes a bite, more often making a meal than a water sandwich. The snake snap is not reaction, which would be too slow, but instinctual anticipation.
The snake’s sensitive tentacles let it detect and catch fish in murky water or at night.
The tentacled snake fishing technique is innate, not learned. The tentacles and the behavior to take advantage of them are bundled; a morphological-behavioral nexus of form and function.
Fish have many predators. Their best bet, most of the time, is to swim away. The tentacled snake outsmarts a fish’s natural reflex with its own.
Eggs Worth Fighting For
Territorial defense of an area with valued resources, such as food or shelter, is widespread in lizards but not snakes.
Kukri snakes, which live in tropical Asia, are egg eaters. Though not venomous, they have impressive weaponry: massive teeth that are used to slit eggshells.
Upon finding a clutch of turtle eggs, a female kukri snake may defend it for weeks, until all the eggs are eaten or the turtles hatch.
When threatened, a kukri snake waves its tail toward the aggressor. This head-tail mimicry lessens the risk of loss resulting in death. Nonetheless, being bitten in the tail is bad news for male kukri snakes, as it risks losing the ability to breed: its hemipenes are in the tail.
Kukri snakes are exemplary of resource defense spurring evolution of territoriality.
Defenses
Most snakes avoid confrontations whenever possible. The first overriding impulse of an imperiled snake is to flee.
Snakes that consider their current cover blown will hightail for the next nearest cover. Many snakes do not venture far from the relative safety of crevices.
Crypsis is the ability to avoid detection or observation. Various snake crypsis techniques include camouflage, nocturnality, and a subterranean lifestyle.
Snake camouflage by coloration and pattern is well known. Many snakes have patterns or lines that pass through their eyes, thereby disturbing being able to easily discern the outline of their heads.
Some snakes will freeze when disturbed, in the hope that the camouflage works. Other snakes have complex camouflage patterns: disruptive coloration that masks movement over native terrain, allowing a snake to slither away. Conversely, aposematic coloration is the opposite of crypsis: conspicuous contrast to surroundings, to act as a warning.
Predators have a mental picture of their prey. Snakes adapt to confound expectation via polymorphism: different patterns in snakes of the same population. Hence, various snakes of the same breed are banded, spotted, ringed, and/or striped.
Color polymorphism conveys similar advantage. Non-poisonous snake species sometimes mimic their venomous brethren. Harmless kingsnakes and milk snakes mimic the venomous Texas coral snake.
Imitation extends beyond coloration. Some harmless snakes bluff with mimetic behavior: imitating the intimidating behavior of the truly dangerous. The nonvenomous and naturally nervous Dasypeltis will rub its scales like the venomous viper echis when disturbed.
The grass snake imitates the hisses and aggressive postures of a viper. That’s often just the warm-up act to other feints.
Arboreal snakes are often gracile. Their long, thin bodies are colored to resemble local vegetation. Vine snakes have the habit of irregular swaying, thus mimicking a vine or branch in the breeze.
Patterning can confuse a predator as to a snake’s speed: longitudinal stripes appear to be in the same place even as a snake has started to move away; a predator may think it has more time than it does. Traverse bands and saddles can create something of an optical illusion in movement, making estimation of direction and speed difficult.
Different snakes have different temperaments, but when cornered, snakes typically resort to intimidation. A common stratagem is to appear larger, usually in the head and neck.
The Australian bandy-bandy has a starkly striking black-and-white banding. Its escape tactic is to raise loops in its body, to confuse an onlooker as to its shape and size.
Numerous snakes can inflate their bodies with air. Cobras can spread the ribs of their neck to form a hood.
Some snakes open their mouths widely, exposing a brightly colored interior. The darkly colored cottonmouth is named for its highly contrasting white mouth.
Such displays are not mere bluff, but a warning, followed by a strike if not heeded. So goes the cottonmouth.
Although snakes have poor airborne hearing, their predators don’t. So, when threatened, most snakes will hiss, a few can rattle their tails, and even fewer rub their scales together to make a rasping sound (a few species in Africa and the Middle East, including the saw-scaled viper).
Some snakes defensively ball: make a tightly coiled mass, protecting the head in the center. Others display the tail as the head, while hiding the head in a coil. Some snakes have tail markings that make the tail look like a head.
While many lizards are can lose their tail with little loss, caudal autotomy (voluntary tail loss) is practically non-existent in snakes. Lost snake tails do not regenerate like lizards. A snake may escape with a lost tail only once without endangering its life.
Most snakes have musk glands at the base of their tails. The rather unpleasant musk is a hormonal production, letting it leave chemical communication trails. A snake roughly handled might muster a musk defense: excreting a generous portion as a deterrent to an attacker. The European grass snake is a real stinker, emitting a garlic-smelling fluid from its anal glands.
A very few snakes practice autohaemorrhagy: voluntary bleeding (from small blood vessels). Dwarf boas of the West Indies, and 3 species in North America, are known to have this defense, though how it works is not understood; the presumption is that it creates an unpleasant taste, prompting a predator to drop the little bleeder.
Only spitting cobras can project their venom. All other snakes have to bite.
Playing Dead
Pretending to be dead is something of a strange strategy, as many predators are perfectly happy to eat carrion. That said, it must work sometimes, because the adaptation evolved in several snake species. The hognose snake, African rinkhals, Egyptian cobra, and a couple of small western hemisphere snakes – the brown snake and the redbelly snake – feign death.
The European grass snake is a notably convincing actor: turning over onto its back, opening its mouth, and hanging its tongue out. They may also optionally bleed from the nose and mouth while playing dead.
Playing dead is a personal predilection; not all snakes of a population perform the ritual. Some individuals won’t stoop to it. Even among expiration actors, feigning death is a last resort, after intimidation and putting a stink on have failed.
Snake Venom
Snake venom is a vastly modified saliva with immobilizing zootoxins. Comprising from 20 to over 100 different compounds, certain proteins do the dirty work.
The bite of a pit viper induces hemorrhages in many animals, but not in the opossums that eat them. Viper venom targets a certain blood-clotting protein that is chemically modified in opossums to evade the toxin’s molecular clutches.
Other venomous snake eaters, such as hedgehogs, honey badgers, and mongooses, disarm toxins differently, indicating that anti-venom techniques independently evolved several times.
To ensure efficacy, the toxins of venomous snakes tend to evolve rapidly. Cycling predator-prey reciprocal adaptations are a common evolutionary dynamic.
Snake-eating opossums show rapid evolutionary adaptation in the specific amino acids that interact with snake toxin proteins, thus keeping snake on the menu.
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Squamates (lizards and snakes) share many of the same defenses, notably coloration adaptations and behav-iors, such as playing dead and autohaemorrhaging, includ-ing spitting blood as a repulsion maneuver.
Reproduction
Snakes often appear indifferent to their kind, but mating season brings a different temperament. Almost all snakes have courtship behaviors that precede mating.
A male searches out a receptive female by following her scent trail, left by secretions from glands in her tail as she moves. All snakes leave a scent trail from a pair of these anal glands.
On approach, a male works his way forward over her body with quick, quivering massage movements, while rubbing his chin along her back. Boas and pythons use their vestige hindlimb claws to caress. The male is constantly flicking his tongue in and out, savoring her scent.
When he reaches the nape of her neck, he maneuvers into mating position by looping his body over the lower part of her back and entwining his tail about the opening of her cloaca. The cloaca is the shared portal for the tail end of the digestive tract and the reproductive tract.
If the female is willing, she responds by raising her tail slightly and opening her cloaca. The male everts one of his hemipenes and copulates with her.
A hemipenis is a male snake’s penis, which is usually tucked inside the body. Males have a pair of hemipenes. They often alternate between the two for successive copulations.
Mating may be a prolonged affair, lasting hours. In species where a female is likely to mate with multiple males, a male often extends his copulation time. After sperm transfer, he may insert a plug that reduces the likelihood that other males can inseminate her.
In some snake species, females may be surrounded by several suitors. The rivals passionately try to mate with her while squeezing out the competition. The result is often a mass of entwined snakes; a behavior termed balling.
Aquatic snakes are prone to balling. A breeding ball of anacondas may contain 10 or more males, all coiled around a single female. They may stay knotted together for up to 4 weeks.
Aside from balling, nuptial combat among males is common in rattlesnakes, vipers, mambas, pythons, and some colubrids. (Colubrids comprise the most specious snake family, with over 2/3rds of all kinds of snakes.) These are typically bloodless rituals: determinations of strength and stamina. One eventually concedes and slithers away. Male adders get so wrapped up in their ritualistic combat dance that they may continue even after the rival has been replaced with a stick.
Fertilization does not necessarily occur immediately after copulation. A female has control over her fertilization.
Sperm can survive in the female reproductive tract for a considerable duration. Females of some species in captivity, having been kept away from males for several years, may birth healthy offspring. (Some snakes lay eggs (oviparity), while others give birth to live young (viviparity).)
Snake parental care is generally quite limited. A female may guard a clutch of eggs for a few days. Some adaptations are more elaborate. The Indian python uses its body heat to warm eggs through muscular contractions.
The king cobra uniquely builds a complex nest, with eggs deposited in the center, in a cavity covered with earth, leaves, and grass. A mother king cobra remains nearby, defending the eggs from intruders.
Rattlesnakes stay with their newborn young for a week or more; a rare instance in neonatal maternal care in snakes.
Cognition
We have here an instance not only of cunning, but of a very excellent memory. ~ American physician Charles Abbott
Given their descent from lizards, it would be unparalleled that snakes are stupider than the animals from which they evolved, and inexplicable, given their adaptations to so many different environments.
Lizards were long thought to be relatively thoughtless. This turned out to be a gross underestimation.
The same is probably true of snakes, who more likely than not to have quiet title to contemplation. Early humans, through sheer observation, may have properly pegged snakes are silently smart, keepers of secrets.
Snakes have an excellent sense of direction and navigation capability. Their mental spatial maps are detailed.
Snakes exhibit spatial learning that rivals the abilities of birds and rodents. ~ American neurobiologist David Holtzman
Snakes are presumed to be non-social animals, which would make them unique among vertebrates in not having group behaviors or territorial inclinations. As much as possible, snakes live furtive existences, which is how an animal with such a mighty mythological presence in human history retains its mystique.
Snakes & Humans
Rodents have been a main source of plague since antiquity. Snakes’ voluminous consumption of rodents earned admiration from primitive agrarian societies.
In many ancient societies, snakes were sacred animals. Their power to ameliorate the rodent scourge, their magical “regeneration” by molting, the ability to enter holes and access hidden places, and thus to appear and disappear unpredictably, produced wonderment.
Ancestral wisdom was thought to come from the depths of mother Earth’s bosom, where only snakes seemed to be able to reach. Snakes were seen as a repository of that secret knowledge.
Human instinctual fear and awe of snakes has led to an outsized fascination: the subject of symbolism and mythology, beginning with ancient Egypt, and then in Greece, India, Peru, and China, among others.
The fascination is rewarded. Considering their minimalist form, snakes exhibit an astonishing variety in look, habitat, habits, and behaviors.
Snakes are prolific. Their reproductive capacity fascinated ancient humans, which is why the Judeo-Christian tradition associates snakes with the sin of lust.
The snake stood up for evil in the Garden. ~ American poet Robert Frost
Christian mythology has a serpent tempting Eve into the sin of eating a forbidden apple, leading God to expel Adam and Eve from paradise in the Garden of Eden. According to the Bible, man’s descent began with corruption, from a woman tempted by a snake.
Snake charming is an ancient tradition, with the earliest evidence from ancient Egypt, where practitioners were healers and magicians. Snake charming as is now known probably originated in India. Its practice spread throughout Southeast Asia, the Middle East, and North Africa.
Snake charmers typically play a flute to have the snake rhythmically weave, having emerged from its basket to be charmed. While snakes do have internal ears, the charming is not from the sound, but from the flowing movement of the flute.
