Turtle - biology.

Turtle - biology. I INTRODUCTION Turtle, reptile with a bony or leathery shell. Any shelled reptile can be called a turtle, but in North America people commonly use the word tortoise to designate members of a family of turtles that live entirely on land, reserving the word turtle for species that live in or near water. The word terrapin, of Native American origin, is used in North America only for the diamond-backed terrapin, a turtle that lives in the brackish waters of eastern coastal marshes. Turtles are ancient life forms that first appeared on Earth during the Triassic Period, which extended from about 240 million to 205 million years ago. Turtles survived the disasters that wiped out the dinosaurs and many other creatures at the end of the Mesozoic Era about 65 million years ago. The earliest known fossil turtles were similar to the turtles that live today. The earliest turtles had teeth, unlike today's turtles, which are toothless and use their sharp jaws to bite and handle food. Early turtles also had not yet evolved the ability to pull their heads into their shells. There are about 270 living species of turtles, which are grouped into 12 or 13 families. These families are further classified into two suborders: side-necked turtles and hidden-necked turtles. Side-necked turtles protect their heads by folding their necks sideways under the top edge of the shell. These turtles are found only in South America, Africa, and Australia and nearby islands. Hidden-necked turtles pull their heads directly into the shell, using an up-and-down motion of the neck. Hiddennecked turtles are more widespread than side-necked turtles. They include all of the ocean-living and freshwater turtles of North America, Europe, and Asia, as well as the land-living tortoises. Sea turtles, as well as a few other kinds of turtles, are considered members of the hidden-necked group even though they cannot fully withdraw their heads. Turtles inhabit every continent except Antarctica, in habitats as diverse as ponds, rivers, and oceans; forests and grasslands; and even deserts. Like all living reptiles, turtles are ectotherms--cold-blooded animals whose body temperature is influenced by the outside environment. For this reason, turtles are most abundant in warm tropical and subtropical climates. Eastern North America, however, is home to numerous turtle species, despite its temperate climate. Turtles have long fascinated people of many cultures, and they are often used to symbolize wisdom and long life. In many Native American and Asian cultures, turtles are mentioned in myths that explain the origin of the universe. In China and Southeast Asia, turtles are sometimes venerated in religious ceremonies. Despite the reverence turtles have inspired, these animals have a long history of being exploited by humans. People in many parts of the world eat turtle flesh and eggs, and use turtle parts in traditional medicines. Turtle fat is a source of valuable oils. Some turtles, such as the hawksbill turtle, are killed for their decorative shells, which are the source of tortoiseshell used in jewelry. In addition, many thousands of turtles and tortoises are collected and sold as pets. By killing turtles, removing turtles from their natural habitats, and destroying the environments in which turtles thrive, humans have brought many turtle species to the brink of extinction. II PHYSICAL DESCRIPTION Turtles vary widely in size. The world's largest turtle--the great leatherback sea turtle--reaches a shell length of 240 cm (96 in) and can weigh over 900 kg (2,000 lb). The giant tortoises of the Galápagos Islands, near South America, and the island of Aldabra, off the East African coast, can reach at least 130 cm (50 in) in length. The largest North American freshwater turtle is the alligator snapping turtle, which can reach a shell length of 80 cm (31.5 in). Among the smallest turtles are the American mud turtles and musk turtles, which have a range that extends from southern Canada through much of South America. Many species in these groups are less than 13 cm (5 in) in shell length. Other small turtles are the rare bog turtle of the eastern United States, at 11 cm (4.5 in); and the tiny speckled cape tortoise of South Africa, whose shell is only 10 cm (4 in) long. A Shell The upper shell of the turtle, under which the head, limbs, and tail can be more or less completely withdrawn, is called the carapace. The lower shell, which encases the belly, is called the plastron. The carapace and plastron are joined together on a turtle's left and right sides by bony structures called bridges. The inner layer of a turtle's shell is composed of about 60 bones, including portions of the backbone and the ribs. For this reason, a turtle cannot crawl out of its shell. In most turtles, the outer layer of the shell is covered with large horny scales called scutes. The scutes are part of the turtle's outer layer of skin, or epidermis. Scutes are composed of a fibrous protein called keratin, which also makes up the scales of other reptiles. The scutes overlap the seams between the shell bones, adding strength to the shell. Some turtles, like the leatherback sea turtle and the soft-shelled turtles, lack horny scutes, and instead have shells covered with leathery skin. Some turtles have shells with flexible hinges that allow the turtles to close off the openings in their shells through which they withdraw their heads, tails, and limbs. These hinges protect against predators and help prevent the turtles from drying out when they are on land. Several types of turtles, including Blanding's turtles, which inhabit wetlands in the northeastern United States and Canada, and the American box turtles, have a single hinge across the plastron. American mud turtles have two hinges on their plastrons. African hinge-backed tortoises have a hinge across the back of the carapace. The shape of a turtle's shell provides clues about its way of life. Most tortoises have high, dome-shaped shells. The shape of these shells makes it difficult for predators to crush these turtles between their jaws. An exception is the African pancake tortoise, which relies on its flat, flexible shell to hide in rock crevices. Most aquatic turtles have flatter, more streamlined shells, which aid in swimming and diving. American musk and snapping turtles have small, cross-shaped plastrons that enable greater leg movement for walking on the bottom of ponds or streams. Tortoises have rather heavy shells. In contrast, sea turtles and soft-shelled turtles that live continuously in water have lighter shells, which help them avoid sinking in water and increase their swimming speed and agility. These lighter shells have large spaces called fontanelles between the bones. The shells of leatherback turtles are extremely light for their size because they contain many fontanelles and lack horny scutes. Turtle shells are commonly brown, black, or olive green. In some species, shells may have red, orange, yellow, or gray markings. These markings commonly appear as spots, lines, or irregular blotches. An especially colorful turtle is the eastern painted turtle, which has a yellow plastron and a black or olive shell with red markings around its rim. B Head Most turtles and tortoises have conspicuous eyes placed well forward on the upper sides of their heads. Turtle species that live most of their lives on land usually focus their eyes downward toward objects in front of them. Some water-living turtles, like snapping turtles and soft-shelled turtles, have eyes nearer the top of the head. These turtles can hide from predators and prey in shallow bodies of water, where they lie nearly entirely submerged except for their eyes and nostrils. Sea turtles have glands near their eyes that produce salty tears, ridding the turtles' bodies of the excess salt that they take in from drinking seawater. When a sea turtle is in the water, its tears are immediately washed away, but when these turtles arrive on land, they look as though they are crying. Turtles use their jaws to cut and handle food. Instead of teeth, a turtle's upper and lower jaws are covered by horny ridges, similar to a bird's beak. Meat-eating turtles commonly have knife-sharp ridges for slicing through their prey. Plant-eating turtles often have ridges with serrated edges that help them cut through tough plants. Turtles use their tongues in swallowing food, but unlike many other reptiles, such as chameleons, they cannot stick out their tongues to capture food. C Limb Structure Turtle limbs, used for locomotion, are adapted to their particular habitat. Land-dwelling tortoises have strong, thick legs to support their heavy shells. They typically move at slow speeds of less than 0.5 km/h (0.3 mph). The gopher tortoise of the American southeast has flattened front limbs that function as scoops for digging the deep burrows in which it lives. Aquatic turtles move either by swimming or by walking on the bottom of a body of water, such as a pond. Many aquatic turtles, including painted turtles, sliders, and soft-shelled turtles, have long toes connected by webbing. These turtles spread out their toes to obtain a large surface area for pushing against the water, which helps them to dive and to swim quickly to escape predators. Soft-shelled turtles are the fastest freshwater turtles and can swim faster than most fish. Sea turtles are the most specialized swimmers of all turtles. Their forelimbs are modified into flipper-shaped blades. These turtles practically fly through the water, using their hind feet primarily as rudders. Despite turtles' reputation for being slow-moving animals, sea turtles can achieve swimming speeds of more than 30 km/h (19 mph), a speed an elite sprinter might reach for a short distance on land. D Tail Most turtles have rather short tails, but the Asian big-headed turtle has an extremely large, muscular tail covered with protective scales that it can use to climb steep rocks and logs in mountain streams. While climbing, these turtles press their tails onto the climbing surface to support their weight. The American snapping turtles also have very long tails. Among most turtle species, males tend to have longer, thicker tails than the females. E Physiology Turtles breathe air with lungs, as do other reptiles and all land-living vertebrates. Since turtle ribs are part of their shell, turtles cannot move their ribs in and out to expand or deflate their lungs. Instead, turtles alternately expand and contract various groups of muscles, including those in their abdomen and above their front and hind legs, to change the amount of space within the shell. When these muscles expand, less space is available and the lungs are compressed, permitting the turtle to exhale. A turtle takes in air as these muscles contract to provide more space into which the lungs can expand. Many aquatic turtles remain submerged in water for periods of several hours to several days--and for many months during winter hibernation. Many turtles are able to take in oxygen from water through the linings of the mouth, throat, and an internal chamber called the cloaca, as well as through the skin. However, when they are active, aquatic turtles need to rise to the surface periodically to breathe air with their lungs. Like most reptiles, except for crocodiles and their close relatives, turtles have a heart with three chambers. A turtle's heart operates almost as if it had four chambers, however, because one of its chambers, called the ventricle, has an incomplete divider, or partition. This divider helps prevent the blood that has received oxygen from the lungs and is ready to circulate through the turtle's body from mixing with blood that is depleted in oxygen and needs to travel to the lungs for a fresh supply. In a turtle's digestive system, food passes from a turtle's mouth through a tubelike esophagus to the stomach, where digestion begins. Food passes from the stomach into the intestine, where nutrients can be absorbed into the bloodstream. Wastes from the intestine are emptied into the cloaca, from which the wastes leave the turtle's body. Turtles also have a urinary system, which filters waste products from the blood and excretes them through the cloaca. Turtles have a central nervous system and a well-developed brain. They have keen senses that they use to interpret their world. Most turtles have sharp vision and can recognize patterns and colors. The eyes of sea turtles are adapted for seeing underwater, but they can see only short distances when they are on land. Most turtles, including sea turtles, have a good sense of smell. Both the shells and the skin of turtles are sensitive to touch. Turtles' ability to hear sounds that travel through the air is limited to low frequencies, but they can perceive vibrations transmitted through the ground or water. III TURTLE BEHAVIOR A Temperature Regulation Like all reptiles, turtles are ectothermic, or cold-blooded, animals that control their body temperature by moving into or out of warm or cool places. Unlike endothermic, or warm-blooded, animals, such as mammals, turtles do not generate heat in their bodies from digesting food. Leatherback sea turtles are an exception, as they can produce internal heat in their muscles while swimming, and their huge size, together with a thick layer of oily fat under their skin, helps them retain this heat. As a result, they can range into such extremely cold areas as the North Sea. Other marine turtles can survive only in warmer waters. Many activities of turtles help regulate their body temperatures. Aquatic turtles often leave the water to bask in the sun on logs or rocks or along the banks of lakes and streams to warm their bodies. In winter, turtles that live in seasonal climates enter a dormant state resembling hibernation. In this state, called torpor, the turtles stop feeding and their oxygen needs become very low. Aquatic turtles usually remain underwater in winter, relying on their ability to obtain oxygen from water through their skin, throat linings, and sacks within their cloaca. In contrast, land-living turtles burrow into the soil. Eastern box turtles may spend the winter in shallow burrows; they are able to survive partial freezing of their body fluids for several days. Young painted turtles often spend their first winter in an underground nest. These infant turtles have the ability to survive sub-freezing temperatures for several days. B Feeding Most turtles are omnivores--animals that eat both plants and other animals. Most of the smaller pond-, marsh-, and stream-dwelling species, including the American painted and slider turtles and the European pond turtle, eat insects, snails, worms, minnows, and tadpoles, as well as aquatic plants. The terrestrial box turtles of North America commonly eat small animals, but when ripe berries are available, they may eat so many that they become too fat to close their hinged shells. Some turtles, such as the South American side-necked turtles and American cooters, are largely herbivorous, eating soft water plants and fruits that fall into the water. Tortoises, which move too slowly to capture most types of animals, are almost entirely plant and fruit eaters, although many species scavenge the remains of dead animals on occasion. Many species of aquatic turtles are strictly meat-eaters, or carnivores, and some of them specialize in eating certain types of prey. Malaysian snail-eating turtles and American map turtles, especially the larger females, eat mostly snails and clams. These turtles have bony ridges in their mouths that help them crush the shells of their prey. The alligator snapping turtle attracts fish to its open mouth by wriggling an appendage on its tongue that looks like a worm. The matamata, a side-necked turtle from South America, has an unusual strategy for catching fish. Attached to the matamata's head and neck are numerous flaps that move in the water, possibly causing fish to mistake them for worms or other food. These flaps contain abundant nerve endings that alert the matamata to disturbances in the water, including those caused by the approach of prey. When a fish comes within range, the matamata lunges with its jaws, sucking the fish into its mouth in a rush of water. IV LIFE CYCLES AND REPRODUCTION Female turtles have two ovaries--organs that produce eggs--and male turtles each have a pair of sperm-producing organs called testes. Male turtles, like male crocodiles, have a penis that is located on the lower wall of the cloaca. The penis has a groove on its upper side. During mating, sperm travels along this groove and passes into the cloaca of the female. Mating among turtles often begins with ritualized courtship behaviors by males. Depending on the species, some male turtles nip at the female's shell or neck, or bob or swing their heads over the female's head. Male painted and slider turtles vibrate their very long foreclaws on the side of the female's head during courtship. Female turtles that are not ready to mate may pull into their shells or try to escape. Female green sea turtles sometimes avoid a male's advances by swimming away or by folding their hind flippers together. During mating, the male climbs on top of the female and inserts his penis into the female's cloaca to release his sperm, which will later fertilize eggs from the female's two ovaries. In many species, the male's plastron has an inward curve that helps him balance on the female's shell. Female turtles of some species, including American box turtles and diamond-backed terrapins, can store sperm and lay fertile eggs for up to four years after a single mating. Turtles may mate either on land or in water, but all turtles lay their eggs on land. Turtle eggs are either oval or round and are white or nearly white, and they are covered with either thin and flexible or fairly thick and rigid eggshells that protect the eggs from drying out. Most female turtles bury their eggs in a nest hole that they dig in the ground with their hind feet. This nest site is chosen so that it provides proper warmth and moisture for the eggs, as well as safety from flooding. The number of eggs in a clutch--those eggs laid by a female during one nesting--varies from 1 to about 200, depending on the species. Most small species have clutches of between 1 and 6 eggs, but large species may have clutches that consist of dozens of eggs. Sea turtles, such as the green turtle, may migrate 2,000 km (1,242 mi) to find suitable nesting beaches. These turtles must drag themselves along the beach because their paddle-shaped forelimbs are not suitable for walking. After covering the nest with dirt or sand, a female turtle provides no further care for her eggs or young. Many turtle nests are destroyed by predators, such as raccoons, foxes, and skunks, before the eggs can hatch. If they escape being eaten, the eggs of most turtle species incubate for two or three months before hatching, though a few species, including some of the larger tortoises, require a year or more. For reasons not well understood, for most turtle species, the temperature of the egg about midway through incubation determines the sex of the hatchling. For example, red-eared slider turtle eggs kept below 28° C (about 82° F) will produce nearly all male hatchlings; if kept above 30° C (86° F), they hatch into females. At 29° C (about 84° F), both sexes may result. In the American soft-shelled turtles and a few other species, temperature has no effect on a hatchling's sex. In these species, hereditary units called chromosomes may determine sex, as is the case with birds and mammals. Most baby turtles grow quickly during their first years of life, but growth soon slows. Depending on the species, it may take from 5 to 20 years or more before they are mature and able to reproduce. Turtles are known for their long life spans. Many of the smaller pond and river species can live over 30 years. American wood turtles, European pond turtles, and snapping turtles may surpass 60 years. American box turtles and European tortoises sometimes reach the century mark, and captive giant tortoises are reported to have lived for more than 200 years. V HUMAN IMPACT Turtles have been a successful group of animals for over 225 million years, but today the future of many species is in doubt. Though well adapted to survive natural threats, the massive environmental changes caused by human activities are proving to be too much for these long-lived creatures. Some scientists fear that human actions, including destroying turtles' natural habitats, directly killing turtles, and even removing them from the wilderness areas that remain, may result in the extinction of many turtle species in the next few decades. Turtles and tortoises are losing vast portions of their original habitats as humans convert wetlands, forests, and grasslands to agricultural fields, grazing lands, and villages and cities. Pollutants from farms and urban areas have degraded many turtle habitats. Turtles are also harmed when humans alter rivers and streams by creating dams and channels, or build sea walls or jetties on the beaches where sea turtles lay their eggs. Tortoises, which are especially slow moving and easily captured, have long been hunted by humans for food. During the 18th and 19th centuries, whalers captured vast numbers of giant tortoises from the Galápagos Islands to feed to their crews. These ships also harmed tortoises by releasing rats and cats that ate tortoise eggs and young tortoises. By 1900, few tortoises remained, and several forms were already extinct. Efforts by conservationists to breed Galápagos tortoises in protected facilities probably saved them from extinction. Today many smaller mainland tortoise species are also being killed in large numbers and could soon disappear unless efforts are made to save them. Freshwater and marine turtles may be scattered over their habitats much of the year, but all must return to specific shoreline sites to nest, giving humans the opportunity to take both the female turtles and their eggs for food. These nesting habits leave the turtles vulnerable and nearly all sea turtles and many species of freshwater turtles are now greatly reduced in numbers or even threatened with extinction. Many sea turtles die in fishing nets, and others are killed when they eat garbage that has been thrown into the ocean. Hawksbill sea turtles are killed for their thick carapace scutes, which provide the tortoiseshell used to make combs, curios, and small art objects. Sea turtles are also killed to make leather products. In Southeast Asia and China, turtles are both eaten and used in traditional medicine. A greatly increased Asian turtle trade in recent years has brought many formerly common species to the brink of extinction in this region. The pet trade, which affects mainly small terrestrial and semi-aquatic species, is another threat to turtles. North American box and pond turtles, as well as the small tortoises found in the Mediterranean region, are particularly valuable to pet trade collectors. In 1975, the United States Food and Drug Agency banned the sale of turtles with shells measuring less than 10 cm (4 in) because many turtles carry a type of bacteria called Salmonella that can infect humans, causing severe gastrointestinal distress. This ban reflects the concern that young children might put these small turtles in their mouths. Larger turtles are still sold as pets in the United States, and turtle farmers can legally sell small hatchling turtles from the United States to overseas markets. Due to the pet trade, slider turtles that are native to the southeastern United States have been spread to many parts of the world. In many instances these pets have escaped human homes and started populations in the wild. In some areas, these feral turtles have taken over habitats once used by native turtles. Compared to other wildlife, turtles are particularly vulnerable to harm from direct exploitation. Turtles' slow pace of reproduction is normally compensated by the long lives of mature adults. Thus, it is difficult for populations to recover after numerous mature adults have been killed or removed. In such cases, the turtle population inevitably declines and may eventually disappear. Many states and countries have passed laws to protect rare turtle species. More than 130 nations have signed a treaty called the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), which regulates the international transport of endangered forms of wildlife, including many turtles and tortoises. Some countries, including the United States, require large-scale shrimp fisheries to use nets that contain turtle excluder devices. These devices help turtles escape from fishing nets. Unfortunately, many wildlife laws are difficult to enforce, and developing nations in particular may lack the necessary resources for preventing the exploitation of turtles. Unless people around the world take steps to preserve turtles and tortoises within their natural environments, many species of these intriguing and ancient animals will probably become extinct during the 21st century. Scientific classification: Turtles make up the order Testudines in the class Reptilia. Hidden-necked turtles make up the suborder Cryptodira, and side-necked turtles comprise the suborder Pleurodira. Contributed By: James H. Harding Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.