Amphibians

Amphibians are cold-blooded animals that possess backbones and display features that lie between those of fish and reptiles. They spend time both in water and on land. Their larvae (not yet fully developed offspring) mature in water and breathe through gills, like fish, while adults breathe air through lungs and skin. Amphibians are in the class Amphibia, which includes over 3,500 species. They are further divided into three orders: Anura (frogs and toads), Urodela (salamanders and newts), and Gymnophiona (caecilians, pronounced sih-SILL-yuhns, which are wormlike in appearance).

History

Amphibians evolved from fish about 400 million years ago, when the amount of dry land on Earth increased greatly. Certain fish adapted to these changing conditions by gradually developing limbs to crawl with and lungs to breathe with. Such organisms, capable of life both in water and on land, came to be called amphibians, a name that means "double life." Amphibians were the first vertebrates (animals with backbones) to live on land. However, they returned to the water to breed. The largest variety of amphibians occurred about 360 to 230 million years ago, when the environment was continually alternating between wet and dry conditions. Many of the species that developed during this period no longer exist. The groups of amphibians that survived to the present day can be traced back no further than 200 million years.

Characteristics

Amphibians are cold-blooded animals, meaning they do not have a constant body temperature but instead take on the temperature of their environment. They have moist, scaleless skin that absorbs water and oxygen, but that also makes them vulnerable to dehydration (loss of bodily fluids). Without moist conditions, their skin dries out and they die. Therefore, amphibians are most often found near ponds, marshlands, swamps, and other areas where freshwater is available. Some amphibians become inactive when conditions are unfavorable for survival. This period of inactivity is called estivation when it occurs during hot, dry weather and hibernation when it occurs in response to cold temperatures. Activity resumes when favorable conditions return.

The thin skin of amphibians contains many glands, among them poison glands that protect certain species against predators. The poison from the glands of the brightly colored poison-dart frog is particularly toxic and is used by South American Indians to coat the tips of their arrows. Some amphibians protect themselves from enemies by changing color to blend in with their surroundings.

Life cycle

The life cycle of most amphibians begins in water when the female lays eggs that are fertilized outside of her body. The eggs then hatch into larvae, or tadpoles, that breathe through external gills. The larvae grow flat tails and feed on vegetation. During a process called metamorphosis, physical changes occur and external gills give way to lungs. The tadpoles also change from plant-eating animals to meat eaters. Amphibians usually reach full adulthood at three to four years.

Words to Know

Estivation: State of inactivity during the hot, dry months of summer.

Gill: A bodily organ capable of obtaining oxygen from water.

Hibernation: State of rest or inactivity during the cold winter months.

Invertebrate: An animal lacking a spinal column.

Larva: An animal in its early form that does not resemble the parent and must go through metamorphosis, or change, to reach its adult stage.

Vertebrate: An animal having a spinal column.

Not all amphibians follow this pattern of reproduction. Some salamanders live out their entire lives on land, where they give birth to fully formed live young. Others lay their eggs in moist places on the forest floor, where they hatch as tiny versions of the adults. Some newts retain their external gills throughout their lives. The red-spotted newt of eastern North American spends its juvenile stage on land as the red eft, returning to water to develop and live as an adult.

Three major groupings

Anurans. Frogs and toads make up the order Anura, the largest group of living amphibians, comprising about 3,000 species. Anurans lack tails and have long hind legs that are well adapted for jumping and swimming. Most anurans live in areas where there is freshwater, although some are well adapted to drier habitats. Some common anurans of North America

include the bullfrog, spring peeper, American toad, and spadefoot toad. Frogs and toads differ in that toads have shorter legs and drier skin that appears warty in comparison to the smooth skin of frogs. Frogs range in size, the smallest measuring about 1 inch (2.5 centimeters) and the largest (the West African Goliath frog) measuring more than 1 foot (about 30 centimeters).

Frogs and toads live mainly on a diet of insects and other invertebrates. The largest frogs and toads also eat small mammals, birds, fish, and other amphibians.

Urodeles. The order Urodela contains about 250 species of newts and salamanders. Urodeles range in size from approximately 4 inches (about 10 centimeters) to the largest of all amphibians, the giant salamander of Japan, which grows to more than 5 feet (about 1.5 meters). Urodeles have long tails and small, underdeveloped legs. They are usually found in or near water and often reside in moist soil under rocks or logs. Adults usually spend most of their time on land and have a diet consisting of insects and worms.

Some species of urodeles are aquatic (live in water), including those of the genus Siren. These North American amphibians are shaped like eels, have small forelegs and no hind legs or pelvis. They breathe through external gills as well as lungs and burrow in mud at the bottom of marshes.

Gymnophions. Caecilians of the order Gymnophiona are blind, legless amphibians shaped like worms. They burrow in moist soil in tropical habitats of Africa and South America, feeding on soil invertebrates such as worms. There are at least 160 species of caecilians, ranging in size from 4 inches (about 10 centimeters) to 4.5 feet (about 1 meter) in length, but most are rarely seen despite their size.

Recent decline

In the last half of the twentieth century, scientists noted the alarming decline in the numbers of amphibians and amphibians species around the world. They theorized the decline was due to a number of factors: pollution of freshwater ecosystems, the destruction of amphibian habitats by ever-spreading human populations, and, possibly, increased ultraviolet radiation due to ozone depletion. Amphibians are known as indicator species, or species whose health is an indicator or sign of the health of the ecosystem they inhabit. As their numbers decrease, so do the number of healthy ecosystems around the world, which in turn results in the loss of many other animal and plant species.

Amphibians

Amphibians are creatures that spend part of their life in water and part of their life on land. Two common amphibians are frogs and toads.

The vertebrate class Amphibia, to date, includes about 3, 500 species in three orders: frogs and toads (order Anura), salamanders and newts (order Caudata), and caecilians (order Gymnophiona). The number of amphibian species that once existed but have since become extinct is much greater. Indeed, amphibians are an ancient life form; they were the first vertebrates to begin exploiting terrestrial environments. Fossil amphibians are known from at least the Devonian era, about 400 million years ago. However, this group was most diverse during the late Carboniferous and Triassic eras, about 360-230 million years ago.

None of the surviving groups of amphibians can be traced back farther than about 200 million years. All of the living amphibians are predators as adults, mostly eating a wide variety of invertebrates, although the largest frogs and toads can also eat small mammals, birds, fish, and other amphibians. In contrast with adults, larval frogs and toads (tadpoles) are mostly herbivorous,

feeding on algae, rotting or soft tissues of higher plants, and infusions of microorganisms.

Amphibians are poikilothermic animals—put another way, their body temperature is not regulated, so it conforms to the environmental temperature.

Amphibians have a moist, glandular, scaleless skin, which is poorly waterproofed in most species; this skin allows gas exchange and actively pumps salts. Most amphibians have tails, but the tail in adult frogs and toads is vestigial, and is fused with the pelvis and sacral vertebrae into a specialized structure called a urostyle. Some species of caecilians have lost their limbs and limb-girdles and have a wormlike appearance.

All amphibians have a complex life cycle, which begins with eggs that hatch into larvae and eventually metamorphose into adult animals. Usually, the eggs are laid in water and are externally fertilized. The larvae or tadpoles have gills or gill slits and are aquatic. Adult amphibians may be either terrestrial or aquatic and breathe either through their skin (when in water) or by their simple saclike lungs (when on land). However, these are all generalized characteristics of the amphibian lifestyle; some species have more specialized life histories, and can display attributes that differ substantially from those described above. Rare idiosyncrasies of amphibian life history can include ovoviviparity, in which fully formed, self-nourishing, developing eggs are retained inside the female’s body until they hatch as tadpoles, and even viviparity, in which larvae develop within the female and are nourished by the parent, as well as by their incompletely formed egg, until they are released as miniature frogs.

Frogs and toads lack tails but have greatly enlarged hind legs that are well adapted for jumping and swimming. Most of the living species of amphibians are anurans, comprising about 3, 000 species. Most anurans are aquatic, but some are well adapted to drier habitats. Some common anurans of North America include the bullfrog (Rana catesbeiana, family Ranidae), spring peeper (Hyla crucifer, family Hylidae), and the American toad (Scaphiopus holbrooki, family Pelobatidae). The latter lives in arid regions, estivating (spending the summer in hibernation) during dry periods but emerging after rains to feed and taking advantage of heavy but unpredictable periods of rain to engage in frenzies of breeding. The largest frogs reach 11.8 in (30 cm) in length and weigh several pounds.

There are about 250 species of newts and salamanders, ranging in size from approximately 6 in (15 cm) to more than 5 ft (1.5 m). These amphibians have a tail and similarly-sized legs well adapted to walking, but are usually found in or near water. Most species lay their eggs in water, however adults usually spend most of their time in moist habitats on land. An exception is the red-spotted newt (Notophthalmus viridescens, family Salamandridae) of eastern North America, which in its juvenile stage (the red eft) wanders in moist terrestrial habitats for several years before returning to water to develop into its aquatic adult stage. Some species, such as the lungless red-backed salamander (Plethodon cinereus, family Plethodontidae) of North America, are fully terrestrial. This species lays its eggs in moist places on the forest floor, where the animals develop through embryonic and larval stages and hatch directly as tiny versions of the adult stage.

Caecilians are legless, almost tailless, wormlike, burrowing amphibians found in moist, tropical habitats. They feed on soil invertebrates. There are at least 160 species of caecilians, reaching 5 ft (1.5 m) in length, but most are rarely seen despite their size.

Since the 1950s, an alarming global decline in the gross number of amphibians and amphibian species has been documented. Because of their dependence on fresh water supplies for reproduction, and since almost all species spend their adult lives in moist environments, it is hypothesized that widespread water pollution is catalyzing the decline of this class of organisms. In other cases, the extinction and decline of species is known to be the direct consequence of human intervention (habitat destruction). However, scientists have noted that in untouched, pristine environments where pollution and human encroachment are minimal or nonexistent, amphibian populations also are declining. As such, it is speculated that other changes, such as increased UV radiation due to ozone depletion, or the introduction of competing exotic species is responsible for their collapse. The decline is unfortunate because amphibians are important indicators of the health of ecosystems, sources of potent and medically useful chemicals, stabilizers of ecological balance in areas where they live, and contributors to the aesthetic beauty of the natural world.

Amphibians

The vertebrate class Amphibia, to date, includes about 3,500 species in three orders: frogs and toads (order Anura), salamanders and newts (order Caudata), and caecilians (order Gymnophiona). There is, however, a much larger number of extinct species, because this ancient group of animals were the first vertebrates to begin exploiting terrestrial environments. Fossil amphibians are known from at least the Devonian era, about 400 million years ago. However, this group was most diverse during the late Carboniferous and Triassic eras, about 360-230 million years ago.

None of the surviving groups of amphibians can be traced back farther than about 200 million years. All of the living amphibians are predators as adults, mostly eating a wide variety of invertebrates , although the largest frogs and toads can also eat small mammals , birds , fish , and other amphibians. In contrast with adults, larval frogs and toads (tadpoles) are mostly herbivorous, feeding on algae , rotting or soft tissues of higher plants, and infusions of microorganisms .

Amphibians are poikilothermic animals—their body temperature is not regulated, so it conforms to the environmental temperature. Amphibians have a moist, glandular, scaleless skin, which is poorly waterproofed in most species; this skin allows gaseous exchange and actively pumps salts. Most amphibians have tails, but the tail in adult frogs and toads is vestigial, and is fused with the pelvis and sacral vertebrae into a specialized structure called a urostyle. Some species of caecilians have lost their limbs and limb-girdles, and have a wormlike appearance.

All amphibians have a complex life cycle, which begins with eggs that hatch into larvae, and eventually metamorphose into adult animals. Usually, the eggs are laid into water and are externally fertilized. The larvae or tadpoles have gills or gill slits and are aquatic. Adult amphibians may be either terrestrial or aquatic, and breathe either through their skin (when in water) or by their simple saclike lungs (when on land). However, these are all generalized characteristics of the amphibian lifestyle; some species have more specialized life histories, and can display attributes that differ substantially from those described above. Rare idiosyncrasies of amphibian life history can include ovoviviparity, in which fully formed, self-nourishing, developing eggs are retained inside the female's body until they hatch as tadpoles, and even viviparity , in which larvae develop within the female but are nourished by the parent, as well as by their incompletely formed egg, until they are released as miniature frogs.

Frogs and toads lack tails but have greatly enlarged hind legs that are well adapted for jumping and swimming. Most of the living species of amphibians are anurans, comprising about 3,000 species. Most anurans are aquatic, but some are well adapted to drier habitats. Some common anurans of North America include the bullfrog (Rana catesbeiana, family Ranidae), spring peeper (Hyla crucifer, family Hylidae), and the American toad (Scaphiopus holbrooki, family Pelobatidae). The latter species lives in arid regions, estivating (spending the summer in hibernation ) during dry periods but emerging after rains to feed, and taking advantage of heavy but unpredictable periods of rain to engage in frenzies of breeding. The largest frogs reach 11.8 in (30 cm) in length and weigh several pounds.

There are about 250 species of newts and salamanders, ranging in size from approximately 6 in (15 cm) to more than 5 ft (1.5 m). These amphibians have a tail and similarly sized legs well adapted to walking, but are usually found in or near water. Most species lay their eggs in water, however, adults usually spend most of their time in moist habitats on land. An exception is the red-spotted newt (Notophthalmus viridescens, family Salamandridae) of eastern North America, which in its juvenile stage (the red eft) wanders in moist terrestrial habitats for several years before returning to water to develop into its aquatic adult stage. Some species, such as the

lungless red-backed salamander (Plethodon cinereus, family Plethodontidae) of North America, are fully terrestrial. This species lays its eggs in moist places on the forest floor, where the animals develop through embryonic and larval stages and hatch directly as tiny versions of the adult stage.

Caecilians are legless, almost tailless, wormlike, burrowing amphibians found in moist, tropical habitats. They feed on soil invertebrates. There are at least 160 species of caecilians, reaching 5 ft (1.5 m) in length, but most are rarely seen despite their size.

Recently, within the last 50 years, an alarming global decline in the gross number of amphibians and amphibian species has been documented. Because of their dependence on fresh water supplies for reproduction, and since almost all species spend their adult lives in moist environments, it is hypothesized that widespread water pollution is catalyzing the decline of this class of organisms. In other cases, the extinction and decline of species is known to be the direct consequence of human intervention (habitat destruction). However, scientists have noted that in untouched, pristine environments where pollution and human encroachment are minimal or nonexistent, amphibian populations also are declining. As such, it is speculated that other changes, such as increased UV radiation due to ozone depletion, or the introduction of competing exotic species is responsible for their collapse. The decline is unfortunate because amphibians are important indicators of the health of ecosystems, sources of potent and medically useful chemicals, stabilizers of ecological balance in areas where they live, and contributors to the aesthetic beauty of the natural world.

See also Chordates.

Amphibian

An amphibian is a cold-blooded vertebrate (an animal with a backbone) animal that spends part of its life in the water and part on land. After hatching from an egg, an amphibian usually lives in water and breathes through gills. As it grows, it undergoes a metamorphosis, growing legs and developing air-breathing lungs. At home in both water and land, the amphibian lives in damp places where its thin skin will not dry out.

THE LIFE CYCLE OF AN AMPHIBIAN

The name amphibian comes from Greek words meaning "having two lives," and it is this unique life cycle that most characterizes an amphibian. Frogs, toads, newts, and salamanders are the best-known members of this smallest class of vertebrates. They all share several other characteristics. Most have thin, moist skin that is smooth and soft. They do not have scales or claws, although the skin of a toad is dry and covered with bumps. They have a three-chambered heart (a fish has a two-chambered heart and birds and mammals have four-chambers) and are ectothermic or cold-blooded. This does not mean that they are always cold, but rather that their body temperature matches that of their surroundings. Because of this, they become sluggish or inactive in the cold and usually hibernate (an inactive state resembling deep sleep) during extreme cold. Most adult amphibians have four limbs and are carnivores (meat-eaters) who will eat almost anything they can catch. Although adult amphibians can live on land, they must return to water to reproduce. There they lay their eggs, which are fertilized by the male outside of the female's body. The jelly-coated eggs remain in water until they hatch into larvae (the early stage of an organism's development, which changes structurally as it becomes an adult).

Metamorphosis. Upon hatching, a process called metamorphosis (a series of distinct changes in form through which an organism passes as it develops from and egg to an adult) begins that is unique among vertebrates. After the eggs hatch in the spring, legless tadpoles emerge. These animals look like tiny fish with long tails, and they breathe as fish do, using their external gills. Soon, however, the tadpoles begin to change gradually as their tails shrink and the beginnings of legs start to form. Covers over their gills start to grow as their lungs begin to take shape. It may take as long as two years for a frog larva or tadpole to change completely into an adult, but when it does, it looks entirely different from when it was hatched. As an adult, the frog has a tail-less, squat, compact body with four legs, the back two of which have powerful muscles for jumping. Upon maturity, the frog will mate and the cycle will begin all over again with its offspring.

HIBERNATION

During the winter months, frogs and toads become inactive since their body temperature decreases. It is at this time that frogs bury themselves in the mud at the bottom of lakes or ponds and hibernate. Toads do the same only in soft, moist soil. During hibernation, their body processes slow down considerably, and they are able to exchange gases (breathe) through their damp skin. Besides frogs and toads, the other major group of amphibians is made up of the tailed amphibians like the salamander and the newt. Both have long bodies, four short, thin legs, and a tail. Salamanders spend most of their adult lives on land, while newts live in the water. As amphibians, they both begin life with gills living underwater, and soon develop lungs as they mature and change into adults.

AMPHIBIANS OFFER GREAT VARIATION

Even in a small class like the amphibians, there is amazing variation. For example, some frogs and toads are poisonous. Tree frogs have toes with sticky pads for gripping, while other frogs are able to jump and then glide using their webbed feet as a parachute. The pink salamander is able to reproduce before it even matures, and the Caecilian is an amphibian without any legs at all, having a wormlike body and scales. All, however, have the double life of an amphibian. Recently, biologists have noticed the disappearance of some species and an overall decrease in the amphibian population. Some believe that the health of amphibians may serve as an early warning system for the overall health of the environment. Since amphibians metabolize toxic substances in much the same way that humans

do, whenever any serious changes are noticed, such as a sharp drop in population or mutant frogs being born, it suggests that their environment might be equally hazardous to humans.

[See alsoAnimals; Metamorphosis; Vertebrates ]

Amphibian

The class Amphibia consists worldwide of nearly 4,700 species, contained in three major orders: Caudata (salamanders), Gymnophiona (caecilians), and Anura (frogs and toads). Salamanders are composed of about 415 species worldwide, and are typically characterized by their long tails and four limbs of nearly equal size. They first appeared in the fossil record over 190 million years ago in the late Triassic.

The caecilians consist of about 165 species. They have a mostly pantropical distribution, and are characterized by their elongated, annulated (ringed) bodies and lack of legs, resembling worms. These amphibians first appeared in the fossil record nearly 190 million years ago in the early Jurassic.

By far the most successful of the three orders with about 4,100 species worldwide, frogs and toads are characterized by lack of a true tail and by generally having comparatively enlarged hind limbs. The order Anura first surfaced in the fossil record about 230 million years ago in the early Triassic.

Amphibians have relatively moist, scaleless skin and rely heavily on cutaneous respiration and/or the presence of a buccopharyngeal pump (a muscular pump in the throat) to force air into their mouth and lungs, features not found in other classes of terrestrial vertebrates. In addition, most amphibians produce eggs that develop and hatch outside their bodies laying gelatinous, unshelled eggs in water or moist places. Many undergo a larval aquatic existence before metamorphosis into adults (unlike other classes of terrestrial vertebrates). In a few species, the female retains the eggs in her body where they are nourished directly by her before she gives birth to her young, or they develop by absorbing their own yolk (a phenomenon also known to occur in at least one species of sea snake, class Reptilia).

Some populations of amphibians have disappeared or begun to decline, and this has raised concern among biologists worldwide. It is unknown if this phenomenon is uniformly widespread across all continents, or is occurring only in selected areas.

see also Crocodilians; Reptile; Tuatara; Turtle

Joseph T. Collins

Bibliography

Halliday, Tim R., and Kraig Adler. The Encyclopedia of Reptiles and Amphibians. Facts on File, Inc., 1986.

Pough, F. Harvey, R. M. Andrews, J. E. Cadle, M. L. Crump, A. H. Savitzky, and K. D. Wells. Herpetology. Upper Saddle River, NJ: Prentice Hall, 1998.

Stebbins, Robert C., and Nathan W. Cohen. A Natural History of Amphibians. Princeton, NJ: Princeton University Press, 1995.