Rattlesnake - biology.

Rattlesnake - biology. I INTRODUCTION Rattlesnake, also called rattler, any of 30 species of venomous snakes characterized by a horny rattle at the tip of the tail, which is shaken vigorously to warn off intruders. Rattlesnakes are members of a larger family of poisonous snakes called pit vipers. Pit vipers have heat-sensitive pits on the sides of their heads that help them detect warm-blooded prey. Rattlesnakes are found from southern Canada to northern Argentina and Uruguay. In the United States, they occur in all states except Alaska, Hawaii, Maine, and Delaware. Mexico, in the middle of the rattlesnake's range, is home to 27 of the 30 rattlesnake species. The United States has 15 species of rattlers; a single species lives in Central America; and 3 species occur in South America. As cold-blooded animals whose body temperature depends on their surrounding environment, rattlesnakes prefer mainly hot, dry locations, such as grassy plains, sand hills, deserts, and brushy or rocky hillsides. They are found from sea level to about 4,420 m (14,500 ft). Rattlesnakes are especially common in areas with abundant rodents, their favorite form of prey. II PHYSICAL CHARACTERISTICS Rattlesnakes vary widely in size. The pygmy rattlesnake rarely reaches 0.6 m (2 ft) in length and 113 g (4 oz) in weight. The largest rattlesnake is the eastern diamondback, which can grow to just under 2.5 m (8 ft) and weigh 9 kg (20 lb). Rattlesnakes have stout bodies and flattened, triangular heads. Most rattlesnakes are typically gray or brownish, although some have bright yellow or burnt orange markings. Many have blotched patterns, typically a series of light-bordered dark hexagons down the center of the back and a dark streak running from each eye to the angle of the jaw. Like other snakes, rattlesnakes have a large number of ribs, which are linked by muscles to the large, specialized scales called scutes that cover a snake's belly. Rattlesnakes glide by hooking the ground with their scutes and then contracting their muscles so that the scutes push against the ground. A modified movement is demonstrated by the sidewinder, which moves sidewise along soft desert sands with a series of S-shaped curves resembling a rolling spring. The right lung of a rattlesnake extends almost the full length of its body, and when the snake inhales air, it appears fatter and more threatening to enemies. The defining characteristic of a rattlesnake is its rattle, which is made up of loosely connected, interlocking horny rings. The rattle produces a buzzing sound when shaken. Other snakes may produce similar sounds by twitching their tails against dry leaves, but only rattlesnakes have a true rattle. This unusual body part may have evolved to warn away large grazing animals, such as bison, that lived among rattlesnakes in grassy plains. Rattlesnakes are born with a small rattle called a prebutton, which is shed a few days after birth and replaced with the first rattle segment, called a button. Each time the rattlesnake sheds its skin (one to four times a year), a new segment is added at the base of the rattle. Rattles rarely have more than 12 segments because as they grow longer, they tend to get caught in rocks and brush and break apart. Like other members of the pit viper family, rattlesnakes have deep pits on the sides of the head that are lined with heat-detecting receptors. These pits enable rattlesnakes to sense the extremely small differences in temperature that occur when warm-blooded animals draw near. This ability to sense heat makes it possible for rattlesnakes to find their prey even in darkness. Rattlesnakes have a pair of long, hollow fangs that lie folded back against the roof of their mouth. These fangs are connected to the venom glands by small tubes, or ducts. Rattlesnakes produce venom that may contain two types of poisons. One poison impairs the functioning of the victim's heart and lungs, and the other causes the victim's tissues to begin to disintegrate. When a rattlesnake strikes, it swings its fangs downward so that they are in position for biting. Venom travels from glands on either side of the head to the fangs and into the victim. After striking, the rattlesnake recoils from the prey and waits for it to be immobilized by the venom. Contrary to popular belief, humans are rarely bitten by rattlesnakes--in the United States, rattlesnakes cause fewer than 12 deaths each year. III DIET Rodents, including mice, rats, and chipmunks, make up the bulk of the rattlesnake's diet. Large snakes may also prey on larger animals, including squirrels, prairie dogs, rabbits, and young groundhogs, as well as ground-nesting birds, bird eggs, and occasionally, frogs and toads. Some small rattlesnakes rely on lizards as a dietary staple. Rattlesnakes are sit-and-wait predators. They remain coiled next to a regularly used animal path, often for days at a time, and wait silently for prey to come within striking distance. After stunning their prey with venom, they swallow the animal, and then move to another favored ambush site. Feeding ceases when winter approaches; these cold-blooded animals cannot digest food when it is cold, so rattlesnakes retreat into deep dens to hibernate until the following spring. IV REPRODUCTION Reproduction among rattlesnakes is initiated by the female, whose skin releases a chemical scent called a pheromone when she is ready to mate. The male rattlesnake follows the scent trail to the female and courts her by crawling alongside and over her and nudging her body with his head. Copulation occurs when the male inserts one fork of his double penis, or a hemipenis, into a chamber in the female's body called the cloaca. Rattlesnakes give birth to live young that have hatched inside of the mother's body. Females typically have broods of about 4 to 12 young, although females of some large species may give birth to more than 20 young at a time. The young are usually born between August and October. The mother may remain near her young for several days after they are born, but like other snakes, rattlesnakes must feed and care for themselves from birth. Young rattlesnakes are able to attack prey within minutes after being born. Among most species, rattlesnakes measure 15 to 20 cm (6 to 8 in) at birth and reach maturity after about three years. Most rattlesnakes die within their first year of life, often because they are unable to catch enough food or because they are eaten by hawks, skunks, or other snakes. Those that survive to adulthood may live for as long as 25 years. V SURVIVAL Rattlesnakes are declining in number. The greatest threat to their survival is the destruction of their habitats. Snakes also suffer when their habitats are divided into fragments separated by roads, farms, housing developments, and shopping malls, making it harder for them to find mates, food, and breeding sites. In addition, tens of thousands of rattlesnakes are collected and killed each year by hunters who sell their patterned skin so that it can be made into such products as boots, ties, and belts. The most endangered rattlesnake is the Aruba Island rattlesnake; only about 250 adults of this species are thought to survive in the wild. The ridge-nosed rattlesnake of New Mexico is considered endangered by the United States Fish and Wildlife Service. Another threatened rattlesnake is the timber rattlesnake, which has disappeared from Canada and is considered an endangered species within several individual states of the United States. Scientific classification: Rattlesnakes are members of the subfamily Crotalinae within the family Viperidae. The pygmy rattlesnake is classified as Sistrurus miliarius, and the eastern diamondback is Crotalus adamanteus. The timber rattlesnake is Crotalus horridus, the sidewinder is Crotalus cerastes, the ridge-nosed rattlesnake of New Mexico is Crotalus willardi obscurus, and the Aruba Island rattlesnake is Crotalus unicolor. Contributed By: John L. Behler Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.