Snake (reptile). I INTRODUCTION Snake (reptile), legless animal with a long, flexible body covered with overlapping scales. Snakes are reptiles, a diverse group of animals that also includes lizards, turtles, and crocodiles. Snakes are thought to have evolved from lizards and share many characteristics with this group--particularly the so-called legless lizards, which have tiny, almost imperceptible legs. But unlike most lizards, snakes have thin, forked tongues, and they lack external ears. And while most lizards have movable eyelids that periodically close to protect and lubricate the eyes, a snake's eyes are always open, protected by immobile, transparent scales. Like all reptiles, snakes are cold-blooded, or more correctly, ectothermic--that is, they cannot produce their own body heat. Instead, they rely on the sun to heat their bodies and then regulate their temperature with behavior. Because they do not rely on energy from food to generate body heat, snakes can survive on an extremely meager diet. Some wait for months between successive meals, and a few survive by eating a single, large meal just once or twice a year. When they do eat, snakes swallow their prey whole rather than biting off small pieces. Many snakes have specialized jaws that enable them to swallow animals that are far larger than their own heads. Although uncommon, some snakes, such as the African rock python, have been observed eating animals as large as an antelope or a small cow. With over 2,500 species belonging to more than 10 families, snakes are a large and successful group. They owe much of this success to their versatility--snakes occupy habitats ranging from underground burrows to the tops of trees to ocean depths as great as 150 m (490 ft). They are found on every continent except Antarctica, and although they are most abundant in tropical areas, many survive in regions marked by extreme cold. The range of the European adder, for instance, extends north of the Arctic Circle. The only places without snakes are parts of the polar regions and isolated islands, such as Ireland and New Zealand. II PHYSICAL CHARACTERISTICS Although all snakes have a long, cylindrical body, many species boast unique modifications suited to particular habitats and lifestyles. Burrowing snakes, for example, have muscular, stout bodies and solidly built heads that they use to push through soil. Sea snakes have flattened, paddlelike tails for swimming, and the long, thin shape of many arboreal, or tree-dwelling, snakes provides agility when navigating between branches. Some snakes, including pythons, retain characteristics that reflect their evolution from lizards or from lizardlike ancestors. These snakes have traces of hind limbs called spurs, which are usually more prominent in males than in females. The smallest snakes are the blind snakes. One member of this group, the Texas slender blind snake, reaches just 13 cm (slightly more than 5 in) in length when full grown and weighs less than 2 g (less than 0.1 oz). The largest snakes are the anaconda and the reticulated python, both of which grow as long as 10 m (about 33 ft) and can weigh up to 250 kg (about 550 lb). Among most species of snakes, females are larger than males. A Skin Snake bodies are covered in overlapping scales composed of a horny material called keratin. These transparent scales make up the dry, smooth, outer layer of skin, whose primary function is to prevent water loss. Snakes owe their coloration to pigment cells located in the skin layer below the scales. Most snakes display drab earth tone colors to blend with their natural surroundings. Arboreal species, such as the emerald tree boa, are often vibrant shades of green, a coloration that helps them hide among leafy foliage. Some snakes, such as coral snakes, have brilliant yellow and orange stripes that warn predators of their venomous bite. Snakes regularly shed the outer layer of their skin as they grow. Even in snakes that are not growing, the scales become drab and worn over time, and must be periodically replaced by a new, healthy layer. Some species of snakes shed their skin about every 20 days, but other species shed it only once a year. In the shedding process, a new layer develops below the surface of the old one, which gradually separates in preparation for being shed, or sloughed. The snake begins the shedding process by rubbing its nose against rocks or other hard objects to separate the old layer from its lips. After the old layer is loosened, the snake crawls out of its old skin, typically shedding it in a single piece. B Skeleton A snake's skeleton is lightweight and highly flexible. Like other reptiles, as well as fish, amphibians, birds, and mammals, snakes are vertebrates--that is, they have a backbone made of small, interconnecting bones called vertebrae. Snakes have an especially large number of vertebrae--all snakes have at least 100 vertebrae, and some species have more than 400. By comparison, humans have just 32 vertebrae. Unlike other vertebrates, the ribs of snakes are not anchored in place by a breastbone, or sternum. Instead, each rib is connected to vertebrae and other ribs by elastic muscles. This makes it possible for a snake to greatly expand its ribcage to pass whole, large prey animals through its digestive tract. These sinuous connections also enable snakes to coil up into compact balls. Snake skull anatomy shows similar modifications. The bones that make up a snake's jaws are not rigidly attached to its skull. Instead, they are linked to the rest of the skull and to one another by muscles and stretchy tissues called ligaments. Ligaments also join the two halves of the lower jaw, enabling them to stretch far apart. This flexibility is what makes it possible for a snake to eat animals that are much larger than its head. When swallowing prey, a snake advances its jaws stepwise over the animal by anchoring the teeth from one jaw segment into the prey and then slowly moving the other parts of the jaws forward along the prey. C Internal Organs Snakes share an internal anatomy similar to that of other reptiles, but modified to fit within an extremely narrow space. The snake's three-chambered heart can move sideways to accommodate large prey animals traveling from the mouth to the stomach. The snake's respiratory system is also compact: Most snakes rely exclusively on the right lung for respiration. In these animals, the left lung is either very small or nonexistent. Snakes have two kidneys, which are positioned so that the left one lies behind the right one rather than beside it. Similarly, the reproductive organs--a pair of testes for males and a pair of ovaries for females--are situated end-to-end. The snake has an extremely muscular and flexible stomach, a narrow liver, and both large and small intestines. Unlike the small intestines of many other vertebrates, those of snakes are stretched out instead of coiled. Like other reptiles, snakes have a cloaca, an internal chamber that receives wastes from the digestive system and eggs or sperm from the reproductive system before they leave the body. Snakes do not have a urinary bladder; instead, they excrete all their wastes through the rectum. D Senses A snake obtains information about its environment primarily through the Jacobson's organ located in the roof of its mouth. The snake continuously flicks out its forked tongue to collect scent particles from the air and the ground. When the tongue draws back into the mouth, the forked tips fit into cell-lined pockets in the Jacobson's organ, which detects the odors of the particles it receives. This system is keenly sensitive, and snakes rely on it to locate both mates and prey. Vision and the ability to detect vibrations are also important to the survival of most snakes. Snakes lack eardrums and external ear openings, but they have small bones in their heads that conduct sound. They are able to hear low-frequency sounds and to sense vibrations that travel through the ground or water. The majority of snakes have good eyesight, especially for detecting moving objects, although most burrowing snakes can only distinguish between light and dark. Pit vipers, boas, and pythons have an unusual adaptation for detecting warm-blooded prey and predators. On the heads of these snakes are small pits lined with cells that are extremely sensitive to heat. These pits enable the snakes to sense the presence of a warm-blooded animal and strike accurately, even in total darkness. E Venom and Fangs Several groups of snakes inject their prey with poisonous secretions called venom. Most venomous snakes strike out and bite their prey, then wait for their toxic venom to immobilize or even kill the animal before swallowing it whole. Venom is produced by modified salivary glands in the upper jaw and contains a mixture of poisons. Some of these poisons include chemicals that disrupt the nervous system. Others are substances that break down the prey's tissues, so that the prey animal is partially digested even before it reaches the snake's stomach. One of the world's most poisonous snakes is the inland taipan (also known as the fierce snake) of central Australia. Members of one taipan species can deliver enough venom in a single bite to kill nearly 100 people or 250,000 mice. Snakes typically deliver venom through two hollow teeth called fangs. In a large group of snakes, the vipers, the fangs can be rotated into different positions. When the snake is ready to strike, it points its fangs outward, but under other circumstances it folds them flat against the roof of its mouth. In other poisonous snakes, such as cobras and coral snakes, the fangs are fixed in position. In these snakes, the fangs must be short enough so that the snake can close its mouth without biting itself. An unusual group of snakes spit or eject venom in a spray, instead of biting. These snakes, which include spitting cobras, use their poison for self-defense rather than for obtaining food. Aiming their spray at the eyes of their enemies, they sometimes reach their target from as far as 2.4 m (8 ft) away. III LOCOMOTION Despite their lack of legs, snakes have several highly efficient methods of locomotion. The most common type of snake movement is called lateral undulation, or serpentine movement. In this style of movement, a snake alternately tightens and relaxes a set of muscles along each side of its body to produce horizontal waves that travel down the body. At the same time, the snake orients itself so that its tail pushes against a resisting force, such as land, water, or tree branches. Moving in this way, some snakes achieve speeds up to 10 km/h (6 mph). Many tree-climbing snakes, such as tropical American tree boas, make use of concertina, or earthworm, movement. These snakes coil their tails around a tree, then hook their necks into a higher part of the tree and pull the rest of their bodies up behind them. Large snakes, such as pythons, use rectilinear locomotion, in which they extend the scales beneath their bellies into the ground and push against them with their ribs. A dramatic form of snake locomotion is sidewinding, in which a snake makes an arc with the front part of its body and hurls its head forward, so that it travels through the air before touching the sand. The snake then brings the rear part of its body in line with its head before generating another arc. Sidewinding is typically used by several desert-dwelling vipers, including the aptly named sidewinders. IV BEHAVIOR Unlike mammals and birds, snakes cannot generate body heat through the digestion of food. They must depend on external sources of heat, such as sunlight, to maintain body temperature. Temperature control is particularly important when snakes are digesting a meal or, in the case of females, reproducing. Many snakes increase the amount of time spent basking in the sun after they have eaten a large meal to speed up the digestive process. By seeking out the correct environmental conditions, snakes can keep their body temperatures close to 30° C (86° F) when they are active. To conserve their heat, snakes coil up tightly, so that only a small portion of their skin is exposed to colder air. When conditions become too cold, snakes seek out protected sites, such as underground dens, where they pass the winter in a state of reduced activity similar to hibernation. In areas with freezing temperatures, snakes shelter in crevices that penetrate far enough below the soil surface to avoid freezing. Usually solitary animals, some snakes gather in large numbers in underground dens during the winter, but this is due to a shortage of suitable sites rather than social attraction. In the Canadian province of Manitoba, thousands of red-sided garter snakes gather each fall to spend the seven-month winter together in limestone caverns. Throughout the rest of the year, they disperse many kilometers to their summer home ranges, living solitary lives until the time comes for them to return to their winter den. A Feeding Snakes have a wide range of food preferences. Many snakes eat worms, insects, lizards, small mammals, birds, and frogs. Some snakes, such as the Australian bandybandy, feed only on other snakes. Several groups of snakes, including the egg-eating snakes of Asia, prefer the eggs of other animals; these snakes have modified teeth and vertebrae in the throat for breaking eggshells. These teeth snag the shell as the egg, swallowed whole, starts down the digestive tract; the broken shell is regurgitated. Among some species, males and females eat different types of food. For example, male Arafura filesnakes eat small fish that inhabit shallow water, while females of the same species eat larger fish that live in deeper water. Many snakes change their diet as they grow larger, as in the reticulated pythons. When young, these snakes feed mostly on rats. When they reach about 4 m (13 ft) in length, they switch to larger prey, such as wild pigs, monkeys, and small deer. Snakes use diverse strategies for capturing their prey. Slender and agile snakes actively pursue their prey, but snakes with thicker bodies, such as pythons, are more likely to wait in a coiled position and ambush their prey as it passes by. Many snakes begin to swallow their prey while it is still alive. The teeth of snakes point backward and are not designed for chewing--instead, snakes use their teeth to pin down their prey to prevent its escape. Others kill prey animals before eating them. Snakes that kill their prey use one of two methods: constriction or envenomation--the injection of venom. Constrictors, such as pythons and kingsnakes, wrap their coils around a prey animal, tightening their grip each time the prey exhales. In this way, constrictors gradually suffocate their victims. Several groups of snakes kill their prey with venom. Copperheads, bushmasters, and other vipers inject their venom and then release the prey immediately, later following the scent trail to find the dead animal. Others, such as cobras, simply hang onto the prey they have poisoned and swallow it when its struggles have ceased. B Defense A wide variety of animals feed on snakes, including birds, carnivorous mammals, larger reptiles such as alligators, and even other snakes. When a predator draws near, many snakes remain motionless in an attempt to escape notice. Snakes that use this strategy typically have complex color patterns on their skins that blend well with their natural backgrounds. Some slender snakes, such as the common racer, tend to flee from predators rather than remain in place. Venomous snakes may have distinctive markings that serve to warn would-be predators that they are poisonous. The Arizona coral snake boasts vibrant yellow and orange stripes, or bands, that predators learn to associate with a toxic bite. Some nonvenomous snakes mimic the coloration of poisonous snakes. For example, predators avoid the harmless milk snake because they mistake its bright, yellow and orange banding for that of the coral snake. Once cornered by a predator, snakes make use of additional defense strategies. Many, such as the hognose snake, hiss to frighten off predators, and some make noise by twitching their tails in dry grass. The rattlesnake has perfected this style of defense; the loud buzz made by its rattle can be heard over long distances. The Costa Rican parrot snake attempts to frighten its enemies by opening its brightly colored mouth. The Arizona coral snake deals with enemies by hiding its head within its coils and displaying its less vulnerable tail. Some snakes strike at attackers, and mambas and cottonmouths display their fangs. V REPRODUCTION AND LIFE CYCLE In climates with cool winters, snakes typically begin mating in the spring; in tropical regions, they mate year-round. Females signal their readiness for mating by producing pheromones, chemicals with a smell that attracts adult males of the same species. Females leave behind pheromones on the ground as they move about, and males follow the pheromone trail to its source. A male snake may track down a female only to discover that other males are already nearby. Among most species, males ignore rivals and continue with their courtship efforts. In some species, however, competing males engage in ritual combat dances, in which they intertwine their bodies and try to force down one another's heads. The larger male usually wins the fight. Biting is generally not involved, although in a few species--typically ones that do not produce venom--males may engage in fierce biting battles. Courtship follows a similar pattern among all snakes. The male rubs the underside of his head along the female's back, stimulating her and orienting his body against hers. To enable copulation to occur, the male may lift the female's tail with his own tail. Pythons use their spurs for this purpose. Male snakes have two penises or hemipenes, one on either side of the body, which are usually contained within small sacs at the base of the tail. During mating, males insert one of their hemipenes into the cloaca of the female. In many snake species, hemipenes are covered with small spines to hold them in place during copulation. Mating lasts from several minutes to several hours. Each female typically mates with several males during her reproductive season, and some females mate with the same male more than once. After mating, sperm is stored in the female's tubelike oviduct. One to two months later, the female's ovary releases large eggs, each with an abundant yolk, which are fertilized by the sperm waiting in her oviduct. The fertilized eggs are funneled to the cloaca, from where they are released and deposited in a shallow hole in the ground or an area under a rock or log. Among a few types of snakes, including the king cobra and the diamond python, the female constructs a nest for her eggs out of vegetation. In these and some other snake species, the female remains with the eggs and guards them against predators until they hatch. Female pythons warm their developing eggs by twitching their muscles in a way that resembles human shivering. In about 20 percent of all snake species, females give birth to live young. This way of reproducing is most common in cold areas, possibly because it helps females to control the temperature of the developing eggs. By basking in sunlight, a pregnant snake can keep her eggs warm and speed up the rate at which they develop so that the young will be born before the onset of cold winter weather. Live births also benefit some snakes that do not live in cold climates. For example, most sea snakes give birth to live young. This frees them from the need to lay eggs on land, where they are less agile and more vulnerable to predators. One snake species--the flowerpot blind snake--reproduces without mating. This species consists entirely of females who reproduce by a process called parthenogenesis. In this type of reproduction, the chromosomes within an unfertilized egg replicate, and the embryo develops as if the egg had been fertilized. One advantage of parthenogenesis is that females can reproduce whenever environmental conditions are optimal, without waiting to make contact with a mate. Most snakes reach maturity at two to four years of age, and those that survive to old age live for about 20 to 30 years. However, the odds are against snake survival. Most succumb to predation or disease long before they reach the end of their natural life span. VI CONSERVATION STATUS The biggest threat to snakes is a problem they share with most animals--the destruction of their natural habitats for agriculture and urban development. Paved roads are a particular danger to snakes, because snakes are drawn to the warm surfaces and then are run over by cars. Snake populations are also threatened by animals that humans have transported to areas where they are not native. The poisonous cane toad, for instance, was introduced to Australia to control an agricultural insect pest. This species, by all external appearances a tasty treat for many Australian snake species, has caused significant population declines because its skin contains poisons that kill the unsuspecting snakes who prey upon it. Human hunting also poses threats to snake populations. Pythons and boas, which move slowly and are easy to kill, are hunted in many parts of their range both for their skins and for their meat. In addition, growing numbers of snakes are removed from the wild to be sold as pets. Snakes play an important role in their ecosystems, both as predators and as prey, yet conservationists have only recently recognized the need for preserving snake populations. Recent conservation legislation provides hope for the preservation of snakes. The Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), an international agreement aimed at plant and animal conservation, protects many species of snakes from being sold for their skins or for the pet trade. In addition, local and national laws protect many types of snakes within individual countries. In North America, for instance, the timber rattlesnake, the indigo snake, the western ribbon snake, and the red-sided garter snake are protected in all or parts of their ranges. Scientific classification: Snakes make up the suborder Serpentes in the lizard and snake order, Squamata, in the class Reptilia. Contributed By: Richard Shine Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.
