Amphibian (animal) - biology.

Amphibian (animal) - biology. I INTRODUCTION Amphibian (animal), animal with moist, hairless skin through which water can pass in and out. Nearly all amphibians live the first part of their lives in water and the second part on land--a double life reflected in the name amphibian, which comes from the Greek words amphi, meaning "both," and bios, meaning "life." Amphibians were the first animals with backbones to adapt to life on land. They are the ancestors of reptiles, which in turn gave rise to mammals and birds. Scientists recognize more than 5,000 species of amphibians, all of which are members of one of three main groups: frogs and toads, salamanders, or caecilians. Frogs and toads are the most abundant of all amphibians, numbering more than 4,000 species. Frogs have smooth skin and long limbs. Toads, in contrast, have warty skin and short limbs. There are about 400 known species in the salamander group, which also includes newts and mud puppies. Members of this group have long, slender bodies ending in tails. Some salamanders live entirely on land, whereas others never leave the water, and still others spend some time in the water and some on land. Caecilians, with about 160 species, are the rarest of amphibians. They have no limbs and look much like earthworms. Most live underground and spend their time burrowing in the soil, but a few are aquatic. II RANGE AND HABITAT Amphibians inhabit every continent except Antarctica. The most widespread of all amphibians are frogs, which are found just about everywhere except on some islands, the polar regions, and the driest deserts. Salamanders are also widespread. In the western hemisphere, their range extends from North America to the northern part of South America. They also inhabit Europe, the Mediterranean area, Africa, and Asia. Caecilians have a more limited range; they are found in Central and South America, parts of Southeast Asia, and from India and Sri Lanka to the Philippines. Amphibians live in many environments, including grasslands, rain forests, conifer forests, alpine areas, and even deserts, although most species require freshwater habitats such as ponds, swamps, streams, or other wet environments for breeding. Remarkably, many amphibians are adept at finding moisture in seemingly dry environments. For example, a unique group of burrowing frogs survives in Australia's dry outback by breeding and feeding only during the area's infrequent rainstorms. Deep in tropical forests, some frog species rely on pools of water that collect in tree hollows or in the cup-shaped bases of epiphytes--plants that grow on the trunks and branches of trees. III PHYSICAL CHARACTERISTICS Amphibians range in size from the Japanese giant salamander, which can exceed 1.5 m (5 ft) in length, to tiny frogs, such as the gold frog, that reach only 1 cm (about 0.4 in). Most salamanders are between 5 and 20 cm (2 and 8 in) long, and most frogs measure between 2 and 8 cm (0.8 and 3 in). Caecilians are more variable in size--most species are between 10 and 50 cm (4 and 20 in), but some grow as long as 1.5 m (5 ft). Most amphibians begin their lives as a larva, an immature form that has a significantly different body plan than that of an adult of the species. Amphibian larvae have many features in common with fish. Sometimes called pollywogs or tadpoles, these larvae live in water and have gills at the sides of their heads that enable them to breathe underwater. They also have a tail that they use in swimming. Most amphibian larvae have tiny teeth. Adult amphibians typically have body structures that enable them to move about on land as well as in the water. Frogs and toads have hind legs that are longer and stronger than their fore legs for use in jumping, their primary mode of locomotion on land. Zoologists believe that adult frogs and toads do not have tails because they would be a hindrance in jumping. In contrast, most salamanders have four short legs and a long, strong tail, which they use for balance while walking on land and to propel them through the water when swimming. Caecilians have no limbs at all. They burrow in the soil by using their strong skulls as battering rams and swim by moving their muscular bodies back and forth like eels. Most adult amphibians retain their teeth, but in some species, teeth are reduced in size or not present at all. A Internal Anatomy Amphibian internal anatomy is similar to that of other vertebrates. Adult amphibians typically have lungs, rather than gills, for breathing oxygen, but some waterdwelling species have both lungs and gills, and others obtain all the oxygen they need to survive through their permeable skin. A three-chambered heart pumps blood throughout a complex circulatory system, delivering oxygen and nutrients to the cells and carrying away waste products. The amphibian digestive system consists of a mouth, esophagus, stomach, and intestine, which opens into a chamber at their posterior called the cloaca. Digestive wastes are processed by the kidneys, excreted into the cloaca, then expelled from the body through a muscular opening in the cloaca. This opening is also where eggs and sperm exit the body. Amphibians are cold-blooded, or more correctly, ectotherms--that is, they are not able to generate their own body heat. Instead, their body temperature is determined by their surroundings. This means that they cannot control the speed at which their body systems work. In cold weather, they become sluggish, and some enter a state of reduced activity, or torpor, which is similar to hibernation. B Skin Always hairless and rarely scaled, amphibian skin provides the animals with protective coloring, a way to absorb water and oxygen from their environments, and a defense against arid conditions and hungry predators. Amphibians come in every color of the rainbow. Some are brilliantly hued, while others display drab coloring that blends with their muddy habitat. Amphibians owe their diverse coloration to both pigment granules in the upper layer of skin and specialized pigment-containing cells called chromatophores in the skin's lower layer. Many amphibians change their skin color by concentrating or dispersing the various pigments in the chromatophores. This behavior helps them to adjust their body temperature because light colors reflect heat more than dark colors, and it also acts as a camouflaging mechanism, helping them to escape notice by predators. Amphibians use their permeable skin to obtain both oxygen and water from their environment. Amphibian skin takes in oxygen through a network of tiny blood vessels called capillaries. Although most adult amphibians have lungs for breathing air, they rely on their skin to garner additional oxygen. Several types of salamander have neither lungs nor gills and obtain all their necessary oxygen through the skin. Water can also permeate this delicate skin, and most amphibians do not drink water because they soak up as much as they need through their skin. Amphibian skin also contains glands that secrete a slimy mucous layer to protect the skin from drying out and help draw in oxygen through the skin. In the water, these protective secretions help amphibians retain a healthy balance of salt and water within their internal tissues. In many amphibian species, mucus-secreting glands in the skin are modified to produce toxins and other substances that will repel predators. One such species is the poison arrow frog of South America, which produces a poison strong enough to kill potential predators. C Hearing, Vision, and Vocalizations Amphibians rely on their senses to find food and evade predators. Amphibians lack external ears but have well-developed internal ears. Hearing is most acute in frogs, which typically have a middle ear cavity for transferring sound vibrations from the eardrum, or tympanum, to the inner ear. Frogs and toads also use their keen hearing in communicating with one another. Using a true voice box, or larynx, and a large, expandable vocal sac attached to the throat, they produce a wide variety of vocalizations, which they use in mating and territorial disputes. Salamanders, caecilians, and some frogs lack an eardrum. These amphibians cannot hear high-frequency airborne sounds, but they are able to detect vibrations that travel through the ground or water. Neither salamanders nor caecilians have a true voice box, but when threatened, some salamanders can produce yelps or barking sounds. Vision is also critical for some amphibians. Frogs have bulging eyes that protrude from either side of their head, enabling them to watch for danger and search for prey in nearly every direction. Caecilians, which live underground, are the only amphibians that are blind. Caecilians are equipped with eyes, but these are covered by skin and sometimes by bone. To gather information about their surroundings, caecilians use two small tentacles--one on each side of the head--to detect chemical changes in the environment. For a sense of smell and taste, amphibians use an organ in the roof of the mouth called Jacobson's organ. This organ probably detects chemical changes inside the mouth. IV BEHAVIOR Much of an amphibian's lifestyle is dictated by the necessity of keeping its skin moist and preventing its body temperature from becoming too hot or too cold. Some species bask in the sun in order to raise their body temperature. In hotter climates, many adult amphibians are active at night rather than in the day to avoid excessive heat and guard against water loss. During the daylight hours, these amphibians shelter in moist sites beneath rocks or logs, or in burrows or cracks in the earth. In cold areas, amphibians become torpid, or inactive, during the cooler months. Some amphibians, such as the water-holding frog, remain dormant deep beneath the ground during times of especially hot weather. To keep from drying out, these frogs secrete a waterproof mucus layer that combines with the smooth clay around them to form a cocoonlike structure, which prevents the escape of water from the frogs' bodies. Nearly all adult amphibians are carnivorous, capturing a wide variety of moving prey, such as insects, spiders, crustaceans, worms, small reptiles, and sometimes, smaller amphibians. Caecilians typically approach their prey slowly and then seize it quickly with their sharp teeth. Salamanders that feed in water draw in their prey by sucking them into their mouths. On land, some salamanders flick out their sticky tongues to capture small prey. Frogs are better equipped to capture prey by this method because a frog's tongue is attached at the front of its mouth, an arrangement that provides additional length and agility for snapping up unsuspecting prey. Their soft skin and lack of claws with which to defend themselves leave amphibians particularly vulnerable to their many predators, which include a host of small mammals, birds, lizards, snakes, turtles, and even larger amphibians. When facing a predator, many amphibians pretend to be dead. Other amphibians rely on toxins in their skin to give them a bad taste or make them poisonous to predators. To warn predators of their poisonous glands, these amphibians often are brightly colored or able to change their color in the face of danger. Salamanders make use of their impressive tails in defense, whipping them about to fend off predators. Many salamanders can break off a part of their tail if it is grasped by a predator. The tail then wriggles on the ground and distracts the attacker while the salamander slips away and later regenerates a new tail. Frogs and toads often respond to threats by puffing up their bodies so that they appear much larger than their real size. V REPRODUCTION Little is known about courtship among the secretive caecilians, but many frogs and salamanders exhibit elaborate mating behaviors. Male salamanders often emit odors to attract a female's attention. If a female looks their way, these males may display bright colors and complex postures. Frogs gather in huge numbers--sometimes thousands of individuals--when conditions are right for breeding. Male frogs rely on their calls--which are familiar to humans as ribbets and croaks but may also consist of clicks, whistles, or trills--to attract females and keep other males away. During mating, most amphibians make use of the cloaca, the chamber that opens into the animal's digestive, urinary, and reproductive tracts. Among frogs, fertilization is typically external, meaning that the sperm and egg join outside of the female's body. To mate, a male frog climbs onto the female's back, enabling him to position his cloaca to shed sperm onto the eggs as she lays them. Mating pairs of salamanders may position themselves with the muscular openings of their cloacae touching, so that the male can directly transfer the sperm to the female. Some male salamanders deposit packages of sperm onto the ground, which the female then picks up with her cloaca and uses to fertilize her eggs. Amphibian eggs are not protected by a waterproof shell like those of birds or reptiles; instead each egg is surrounded by a clear, protective, jellylike substance. The eggs need to be placed in water, or in a damp place, to prevent the developing embryo from drying out. Many amphibians lay their eggs directly in water, but some frogs and salamanders, and nearly all caecilians, lay their eggs on land in moist places such as leaf litter, burrows or cracks in the ground, and beneath logs or rocks. Some South American frogs lay their eggs on tree branches above water. When the tadpoles hatch from the eggs, they drop into the water. Most amphibians that lay their eggs in water leave them unattended, but in species that deposit their eggs on land, a parent commonly guards the eggs to prevent hungry predators from stealing them. Many species of frogs show remarkable forms of parental care. For instance, the male Darwin's frog in Chile picks up the eggs deposited on the ground by its partner and carries them in his vocal sac until they develop into adults. The male Surinam toad presses fertilized eggs onto the back of the female, and the skin on the female's back forms a pocket around each egg. The female carries the eggs until they hatch and tiny young emerge from the pockets on her back. VI LIFE CYCLE After hatching into a larval form, most amphibians undergo a dramatic change in anatomy, diet, and lifestyle known as metamorphosis. During this time, amphibian larvae slowly change from fishlike, water-dwelling animals to animals better suited for life on land. In addition to developing lungs, salamanders and frogs grow limbs during metamorphosis. Most amphibians lose their gills, and the tails of frogs and toads disappear. The length of time required for metamorphosis varies widely among different species. For tadpoles that hatch in short-lived puddles of rain on the desert sand, such as the flat-headed frog of Australia, it may be as brief as 8 days. In the European common frog, metamorphosis takes about 16 weeks, and in the marbled salamander, it is completed within 6 months. Metamorphosis in the European common frog is a typical example of amphibian metamorphosis. Within 3 days of hatching, the tadpole has acquired gills for obtaining oxygen and has an open mouth that it uses to feed on microscopic plants. Within 12 weeks of hatching, the tadpole has hind legs that have developed from nubs along the sides of its body, and lungs, which it uses to take in air at the water's surface. At about the same time, a wall of tissue divides the atrium, one of the components of the tadpole's heart, so that the heart now has three chambers rather than two. This change makes it easier for blood to move between the heart and the tadpole's newly formed lungs. At about 16 weeks, the tadpole has four legs and bulging eyes, and its tail has begun to be reabsorbed into its body. The tadpole loses its many tiny teeth, and its mouth grows larger while its intestines grow smaller, in preparation for a diet consisting largely of insects. At this point the frog has achieved its adult form, and it climbs out of the water to make its home on land. Little is known about the longevity of most amphibians in the wild, but studies of captive animals show that some can be extremely long-lived. Some captive salamanders have survived for 20 to 25 years, and a few have lived for more than 50 years. Captive frogs have shorter life spans, typically living for 1 to 10 years. Some toads have survived in captivity for more than 30 years. VII EVOLUTION Amphibians are a living reminder of the progression by which animals with backbones first emerged from the water to conquer life on land. Amphibians are believed to have evolved from air-breathing freshwater fish during the mid-Devonian Period, a stage in the Earth's geologic history that occurred between 416 million to 359 million years ago. Fossils of an ancient fish found on Ellesmere Island in the Canadian territory of Nunavut provide important clues about how this transition happened. Scientists named the creature Tiktaalik in 2006. It had some anatomical features similar to those of land animals, such as wrist and elbow bones, and parts of a primitive hand embedded in the pectoral fins. The freshwater fish that gave rise to amphibians are thought to have had primitive lungs as well as gills, similar to modern lungfish. Their fins were supported by muscular structures called lobes, which enabled them to move on land. Fossil evidence suggests that one of the earliest amphibians was a creature called Ichthyostega, which lived about 360 million years ago. Ichthyostega had a fishlike tail, but it also possessed strong limbs. It apparently did not walk the way later land animals did but moved more like a seal by hunching its body. After their successful invasion of land, amphibians diversified into many forms. Unlike modern forms, many of these ancient amphibians had thick skulls and heavy bones. Some even had protective armor and scales on their bodies. The first reptiles evolved from amphibians some time during the Carboniferous Period, which lasted from 359 million to 299 million years before present. The largest known of the early amphibians was Mastodonsaurus, which was similar in appearance to today's crocodile and probably measured up to 6 m (20 ft) in length. It lived during the Triassic Period, which lasted from 251 to 200 million years ago. The types of amphibians that exist today--frogs and toads, salamanders, and caecilians--were not among the earliest amphibians. The earliest known ancestors of these groups date to the Triassic Period. The earliest true frogs, salamanders, and caecilians appeared during the Jurassic Period, which lasted from 200 to 145 million years ago. VIII ENDANGERED AMPHIBIANS Amphibians have survived more than 300 million years through drastic environmental changes that led to the demise of dinosaurs and many other species. Yet scientists are alarmed by the recent rapid decline of amphibians in many parts of the world. One study of more than 900 amphibian populations monitored from 1950 to 1997 found that amphibian populations have been declining at an average rate of 4 percent per year for the past 40 years. These declines are evident in places as widely separated as North America, South America, and Australia, and have prompted international concern. Their two-stage life cycle and permeable skin make amphibians particularly sensitive to environmental disruptions such as drought and pollutants. This sensitivity makes them excellent bioindicators--life forms whose well-being provides clues to the health of an ecosystem. Declines in amphibian populations may be due in part to natural fluctuations, but they more likely suggest that humans are changing the environment more rapidly than amphibians can adapt. One such change is the destruction and modification of amphibian habitats, such as the cutting of forests and the draining of wetlands. In North America, for instance, huge numbers of salamanders are being lost as a result of forest clear-cuts. Mysteriously, many amphibian groups are dwindling--or even disappearing--in areas where their habitat is not being destroyed. In Australia, two species of gastric brooding frogs have not been detected since the early 1980s, yet their habitat remains relatively pristine. The golden toad of Costa Rica has not been seen since 1989, even though its high-elevation rain forest habitat has been protected as a national reserve since the 1970s. Similarly, the red-legged frog has vanished from large areas of the North American Pacific Coast, where it was once abundant. Scientists now think that the rapid spread of the chytrid fungus (Batrachochytrium dendrobatidis), which affects the skin of amphibians, is responsible for many of the rapid declines among frogs and other types of amphibians recorded in the Americas, Europe, Africa, Australia, New Zealand, and the Pacific Islands. The fungus causes the disease chytridiomycosis, which damages the skin of amphibians. The fungus can be highly contagious and lethal in many species. Another disturbing development is that large numbers of amphibians, especially frogs, are being found with misshapen, extra, or missing limbs. A study published in 2007 indicated excess nutrients from farm runoff was a major source of the problem. The runoff stimulates growth of algae, which allows populations of snails to increase. The snails then release parasitic nematodes into bodies of water where frogs live. The nematodes infect frogs and cause cysts to form in their limbs, resulting in malformations as the limbs develop. A variety of other factors may also be responsible for amphibian declines. These may include disease from viral, bacterial, or fungal pathogens; global warming; and increased levels of the ultraviolet-B component of sunlight hitting the Earth as a result of depletion of the protective ozone layer. Global warming and climate change can affect the moisture and temperature conditions where amphibians live. Ultraviolet-B light is particularly suspect in the decline of those amphibians that lay their eggs in shallow water, because such eggs are exposed to sunlight for long periods. At a more local level, chemical pollutants, such as acid rain, pesticides, herbicides, and fertilizers, may be harming amphibians. In some regions, the introduction of nonnative competitors and predators has contributed to amphibian population declines. Habitat loss, particularly loss of wetlands, also has a major impact on amphibians. It is likely that an interaction of some or all of these factors may be exacerbating conditions for amphibians. The Global Amphibian Assessment (GAA) is a collaboration among more than 500 scientists from 60 countries to monitor the status of amphibians worldwide. After a GAA study published in 2004, the World Conservation Union (also known as IUCN) listed 32 percent of amphibian species worldwide as threatened with extinction. More than 20 amphibian species found in the United States are now listed as threatened or endangered under the Endangered Species Act. The growing global extinction crisis for amphibians led the conservation organization Amphibian Ark to declare 2008 the "Year of the Frog" to raise public awareness and promote conservation efforts to save amphibians. Scientific classification: Amphibians are the only members of the class Amphibia, which contains three living orders. Salamanders, a group that includes newts and mud puppies, make up the order Caudata; frogs and toads make up the order Anura, and caecilians make up the order Gymnophiona. Contributed By: William S. Osborne Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.