Snake (reptile)

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 bandy-bandy, 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.

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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.

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