White's Tree Frog (Litoria caerulea)
Amphibians (class Amphibia) are cold-blooded tetrapods (four-legged vertebrates) whose eggs lack a tough protective membrane around the embryo. The term "amphibian" comes from the Greek amphi meaning "both" and bios meaning "life," hence, "double life." This reflects the fact that most amphibians are biphasic, having an aquatic stage where they spend part of their time, as well as a terrestrial stage. Many, but by no means all amphibians, undergo a change from an aquatic larval stage in which they acquire oxygen from water and lack limbs, to a four-legged, air-breathing adult form adapted for living on the land. There are about six thousand distinct living species of amphibians. Examples include frogs, toads, salamanders, newts, mudpuppies, and caecilians.
A highly diverse group of animals with only a core percentage exemplifying the most common defining characteristics, amphibians generally have smooth and naked skin. Yet, some have dermal scales. In comparison with fish, aquatic phase amphibians generally respire through the skin and via lungs rather than with gills, and have limbs instead of fins, but some amphibians utilize gills as well.
Occupying habitats in most parts of the world, amphibians play an important role in the balance of nature. They exemplify bi-level functionality as they consume a significant amount of insects and other invertebrates, and are themselves prey for larger animals, making them an integral part of food webs. They are also important in the cycling of nutrients and as harbingers of deleterious environmental change.
Amphibians also play an important role in human society. Both historically and presently, substances produced from amphibian glands offer an important source of medicine for humans. Amphibians also lower the prevalence of insect-borne diseases by reducing the numbers of insects. In religion, amphibians have often been important symbols, whether in Shamanism, early Egyptian religions, or religions in the pre-Colombian Americas.
Since the 1970s, many amphibian populations have been declining, with much of the reduction attributed to anthropomorphic (human-induced) causes. Both for ethical and practical reasons, humans need to invest in the conservation of these valuable animals.
The study of amphibians and reptiles is known as herpetology.
Most amphibians produce eggs without shells or membranes (amamniotic) that are deposited in water and rely on moisture from the surroundings. Adult amphibians have three chambered hearts (larvae have two-chambered hearts) and usually two lungs. Amphibians have two protusions on the back of the skull (occipital condyles) that articulate with the vertebra of the backbone, whereas reptiles have a single occipital condyle.
Although most amphibians lack the adaptations required for an entirely terrestrial existence, some of them in fact are fully terrestrial, even being born on land; others may require only a moist environment. Some are completely aquatic. Being cold-blooded organisms, many amphibians enter a state of dormancy known under unfavorable conditions in the cold environment of winter as hibernation and during drought conditions in the summer as estivation.
All amphibians belong to the class Amphibia of the Subphylum Vertebrata, of the Phylum Chordata or Craniata. All extant amphibians are placed in a single subclass, Lissamphibia. There are two ancient, extinct, subclasses:
Recently there has been a tendency to restrict the class Amphibia to the Lissamphibia, by excluding those tetrapods that are not more closely related to modern forms than they are to living reptiles, birds, and mammals.
Three orders are recognized in the subclass Lissamphibia:
Taxonomists disagree on whether to consider Salientia a superorder that includes the order Anura, or whether Anura is a sub-order of the order Salientia. In effect, Salientia includes all the Anura plus a single, extinct Triassic proto-frog species, Triadobatrachus massinoti.
Frogs and toads belong to the order Anura ("without a tail") or Salientia. About five thousand species of anurans have been identified, and these are classified into about 30 families. Frogs and toads differ from the other amphibian orders by the presence of larger hind limbs among the four limbs. Extant adult anurans lack tails. Frogs and toads are the most numerous and diverse amphibians, being found in nearly all habitats, including aboral, aquatic and terrestrial niches, and every continent except Antarctica. Three species have ranges that extend above the Arctic Circle. The terms frog and toad are imprecise, with "toad" commonly being used for any species that is adapted to a dry environment. Anurans have well developed voices, whereas the other two orders of amphibians are limited to sounds such as coughs and grunts.
Salamanders, newts, waterdogs, mudpuppies, sirens, and amphiuma are members of the order Caudata or Urodela ("visible tail"). Over five hundred species of caudates have been identified, and these are organized into about ten families. All caudates have tails. Generally, caudates have similar-sized limbs, but Amphiuma has reduced limbs, and the sirens lack hind limbs and possess reduced forelimbs. The largest amphibian in the world is a caudate, the Chinese Giant Salamander, Andrias davidanius, which can reach two meters long (six feet), and its close relative, the Japanese Giant Salamander, Andrias japonicus, which grows to 1.6 meters (5 feet 3 inches). Lungless salamanders rely on their skin for gas exchange. Salamanders are most abundant and diverse in temperate zones.
Caecilians belong to the order Gymnophiona or Apoda ("without legs"), and are elongated, segmented amphibians, looking almost wormlike. Caecilians lack external limbs, but like snakes are still considered tetrapods because the lack of limbs is considered a derived, secondary characteristic, with the assumption being that they evolved from forms that did have appendages. Caecilian heads are adapted for burrowing, being strong with highly ossified skulls. Caecilians are also the only amphibians with dermal scales; these scale-like structures are more similar to fish scales than reptile scales. Reptile scales are keratinized folds of skin, whereas caecilian scales are layers of fibers covered by mineralized nodules. Caecilians have a unique sense organ, a retractable tentacle found between the nostril and eye that acts as a chemical sensor. The name caecilian means "blind," but most have small eyes. Living underground, caecilians are poorly known, and many do not even have common names. There are about 200 known caecilian species. These are found only in tropical and subtropical regions of the world.
Amphibians range in size from the tiny Brachycephalus didactylus (Brazilian Gold Frog) and Eleutherodactylus iberia from Cuba, with a total length of 9.6–9.8 millimeters (0.4 inches), to the Chinese Giant Salamander mentioned above. Amphibians have mastered almost every climate on earth from the hottest deserts to the frozen arctic. They are in almost every environment where there is fresh water at some point during the year. Indeed, some toads survive in deserts in underground burrows, emerging only during periodic, heavy rains.
Amphibians are generally considered to be the first terrestrial vertebrates, and are thought to be descendant from fish ancestors. The first record of amphibian-like animals in the fossil record is 360 to 390 million years ago, during the Devonian period. It is widely assumed that amphibians are the first four-legged animals to have lungs and limbs, having originated the ability to walk on land during the Carboniferous period. Such a feature would have allowed them to avoid aquatic competition and predation while permitting travel from water source to water source. As a group, amphibians were the dominant terrestrial animals for nearly 75 million years. The ancient amphibians were considered to be typically larger than modern amphibians, with massive teeth, and some with scaled skin. The earliest fossils considered to be salamanders, caecilians, and frogs date to the Jurassic (190 to 160 million years ago) (Zardoya and Meyer 2001).
The specific relationship between the three orders of extant amphibians (anurans, caudates, and gymnophionas) represents one of the great controversies in vertebrate evolution. There is no generally accepted consensus regarding the phylogenetic relationships between the three orders (Zardoya and Meyer 2001). One hypothesis is that salamanders are the closest living relatives of frogs, and these are less related to caecilians. This is supported by morphological and paleontological studies of living and fossil specimens and some phylogenetic analysis of mitrochondrial rRNA data. The second hypothesis is that salamanders are the sister group of caecilians and these are less related to frogs. This is supported by molecular studies and some morphological evidence. Zardoya and Meyer (2001) analyzed the complete mitrochondrial genomes of a salamander and a caecilian and compared to a known frog genome, and found support for the view of a sister relationship between salamanders and frogs. The sparse fossil record, with the caecilians in particular having few fossil representatives—as well as the high degree of anatomical specialization of amphibian species—contributes to the difficulty in establishing phylogenetic relationships.
Both external and internal reproduction are known in amphibians. Anurans utilize mostly external fertilization, while salamanders and caecilians largely reproduce internally.
For the purpose of reproduction, most amphibians are bound to fresh water. A few tolerate brackish water, but there are no true sea water amphibians. Several hundred frog species (for example, Eleutherodactylus, the Pacific Platymantines, the Australo-Papuan microhylids, and many other tropical frogs), however, do not need any water whatsoever. They reproduce via direct development, an adaptation that has allowed them to be completely independent from freestanding water. Almost all of these frogs live in wet tropical rainforests and their eggs hatch directly into miniature versions of the adult, bypassing the larval tadpole (or "polliwog") stage entirely. Several species have also adapted to arid and semi-arid environments, but most of them still need water to lay their eggs. Symbiosis with single celled algae that live in the jelly-like layer of the eggs is present in a number of species.
Most amphibians go through both an aquatic stage and a terrestrial stage. The amamniotic (shell-less) eggs rely on water in the environment. Upon hatching, amphibian larvae breathe with exterior gills. Many start to transform gradually into the adult's appearance, via a process called metamorphosis. For example, frog larvae (tadpoles) gradually absorb their tail and develop legs for walking on land. The animals then leave the water and become terrestrial adults.
Adult amphibians have three chambered hearts (larvae have two-chambered hearts) and usually two lungs. Amphibians have two protusions on the back of the skull (occipital condyles) that articulate with the vertebra of the backbone, whereas reptiles have a single occipital condyle. If feet are present, they are webbed and the toes lack claws. Being cold-blooded organisms, many amphibians enter a state of dormancy known under unfavorable conditions in the cold environment of winter as hibernation and during drought conditions in the summer as estivation.
While the most obvious part of amphibian metamorphosis is the formation of four legs in order to support the body on land, there are several other major changes:
The ability of many anuran tadpoles to regenerate lost body parts (such as the tail or leg) generally disappears during metamorphosis. However, many salamanders retain the ability throughout their lifetime to regenerate a wide variety of tissues and structures, such as muscle, cartilage, skin, spinal cord, and parts of eyes and jaws (Sobkow et al 2006).
While in many species of amphibians the newly hatched aquatic larvae undergo metamorphosis into the adult form or terrestrial juveniles, there are many exceptions to this way of development. Many salamander larvae look similar to juveniles and adults, with the exception of aquatic features such as gills. Some amphibians develop without a larval form, with juveniles hatching directly from the egg. Furthermore, while many species develop adult features quickly, some larvae remain aquatic for months, even years, until the proper conditions occur. Neoteny (or paedomorphism) is the retention of larval characteristics in sexually mature animals, and it is common in many caudate species.
When the typical biphasic species returns to the water to breed, some caudates that spend a great deal of time in the water undergo a second metamorphosis whereby adaptations to an aquatic lifestyle manifest, such as thinner skin to absorb more oxygen and modified vision to see underwater.
In particular, adult amphibians are significant predators of insects, as well as other invertebrates and some vertebrates. Larval amphibians are also consumers of insects, algae, and zooplankton in the aquatic environment. On the other hand, amphibians are also a food source for fish, birds, mammals, reptiles, and other amphibians. As such, they play a critical role in food webs. The loss of amphibians have often correlated with population increases of insects, while the loss of amphibian larvae can also lead to algae blooms, low oxygen, and fish kills. By controlling insects, amphibians also help in reducing the threat to humans of insect-borne diseases.
Amphibians are also important sources of medicine. The lower skin layer (dermis) of almost all amphibians has mucous glands, to provide moisture, and poison glands, to produce toxins. These toxins, which range from mildly noxious to deadly, are generally toxic to natural enemies, such as birds and some mammals—but often are harmless to humans. Amphibian toxins, which aid in defense from predators and in prevention of bacterial and fungal growth on their skin, can serve as a medicinal drug for human use, when used in the correct dose. Indeed, dilute amphibian toxins have been used for thousands of years, including to treat edema, leprosy, and tumors.
Medicine produced from amphibians now is used to treat heart aliments, bacterial infections, skin and colon cancers, depression, and chronic pain, among others. The fact that many amphibian toxins are similar to those that regulate human muscles and nerves contributes to their utility. Amphibian toxins continue to be studied by scientists for possible applications. For example, a poison frog from South America (Epipedobates tricolor) secretes a non-addictive painkiller 200 times more potent than morphine, offering promise in this area if the toxicity can be neutralized.
Amphibians play an important role in nutrient cycles and as environmental indicators. Nutrients that have washed from the land into bodies of water via erosion can be recycled by amphibians as they enter the land after metamorphosis. And as organisms generally highly susceptible to pollutants because of their permeable skin, amphibians serve as indicators of environmental health.
Amphibians also play an important role in human culture and religion. Besides their historical use in folk medicine, amphibians have been prominently featured either as evil entities (probably somewhat a function of their often nocturnal nature) or as agents of good luck, fertility, and rain. Shamans, the spiritual leaders in the religion of Shamanism, have used them as religious symbols and in creating hallucinogenic drugs. In some cultures, including early Asiatic cultures and pre-Columbian American civilizations, the toad was considered a divinity, and the source and end of all life. In Egypt, the goddess of childbirth, Hequet, is pictured with a frog's head, and items with frog shapes were placed in Egyptian tombs to repel demons from the underworld. In some other cultures, frogs and toad have had less than positive connotations, being correlated with witches and their brews, or as plagues as seen in the biblical book of Exodus. In Guatemala, fanciful myths exist of nocturnal salamanders that climb into babies' beds and cause their sudden death, and of caecilians that jump into bodily orifices.
British scholar and novelist C.S. Lewis used the biphasic nature of amphibians as a metaphor for the human state: "Humans are amphibians: Half spirit and half animal. As spirits they belong to the eternal world, but as animals they inhabit time."
Since the 1970s, dramatic declines in amphibian populations in many parts of the world have been reported, including population crashes and mass localized extinctions. Such amphibian declines are often perceived as one of the more critical threats to global biodiversity. A number of causes are believed to be involved, including habitat destruction and modification; pollution; introduced species (including other amphibians); traffic mortality; over-exploitation and human collections (for food, medicines, bait, pets, and even for teaching biology); acid rain; agricultural chemicals; ultraviolet radiation increases due to decreased stratospheric ozone; and disease. Habitat destruction particularly has been a significant factor. An estimated 50 percent of the world's original wetlands have been lost, and 54 percent of wetlands in the United States. Wetlands in France, Germany, Italy, and Spain experienced losses of 57-66 percent during the twentieth century (Barbier et al 1997). In some places, such as southern Ontario, 90 percent of the wetlands have disappeared. Direct removal of amphibians likewise has resulted in intense pressure on populations. There have been reports of up to one million Leopard Frogs having been collected in one province in Canada (Manitoba) in some years. Fish stocking has also harmed amphibian populations, as many amphibians cannot survive under such circumstances.
Most the above-mentioned causes have human origins. Clearly, beyond the practical need to preserve amphibian populations, humans have an ethical and moral responsibility to care for amphibians, as well as all species.
In balance, it should also be noted that only about one percent of amphibian species have experienced global declines (Beebee 1995). Many of the causes of amphibian declines remain poorly understood, and amphibian declines are currently a topic of much ongoing research.
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