Amphibians are members of the class Amphibia, a group of vertebrates whose living forms include frogs, toads, salamanders, newts and caecilians. They are characterized as non-amniote, ectothermic tetrapods meaning that their eggs are not surrounded by membranes, they are cold-blooded and have four limbs. Most amphibians lay their eggs in water and the larvae undergo metamorphosis from a juvenile form with gills to an adult air-breathing form with lungs. Some however are paedomorphs that retain the juvenile water-breathing form throughout life. Mudpuppies and olms are examples of this, retaining juvenile gills into adulthood. Adult amphibians also use their skins for respiration. The three modern orders of amphibians are placed in the subclass Lissamphibia and are the Anura (frogs and toads), Caudata (salamanders and newts) and Gymnophiona (caecilians, limbless amphibians that resemble snakes). The total number of Amphibian species is approximately 6,500. They are superficially similar to reptiles, but reptiles, along with mammals and birds, are amniotes, having impervious membranes that surround the egg. Amphibians are ecological indicatorsand in recent decades there has been a dramatic decline in amphibian populations around the globe. Many species are now threatened or extinct. The study of amphibians is called batrachology.
The earliest amphibians evolved in the Devonian period from lobe-finned fish that used their strong, bony fins to venture onto dry land. They were the top predators in the Carboniferous and Permian periods, but they later faced competition from their descendants, the reptiles, who were better-adapted to life on dry land. Many lineages came to an end during the Permian–Triassic extinction. One group, the metoposaurs, remained important predators during the Triassic, but as the world became drier during the Early Jurassic they died out, leaving a handful of relict temnospondyls like Koolasuchus and the modern orders of Lissamphibia.
Amphibian is derived from the Ancient Greek term ἀμφίβιος (amphíbios), which means "both kinds of life", amphi meaning "of both kinds" and bio meaning "life". The term was initially used generally for animals that could live on land or in water, including seals and otters. About 1600 it became restricted to what we now call amphibians and the taxon "Amphibia" was first used in scientific classification in about 1819.
The amphibians are a class of vertebrate animals with four limbs. They are non-amniotes which means that their eggs are not surrounded by the several membranes, some impervious, which enable mammals, reptiles and birds to reproduce on land. Amphibians typically reproduce in fresh water and are not found in the sea, with the exception of one or two frogs that live in brackish water in mangrove swamps. Most amphibians lay eggs that have a gelatinous coating which swells when it comes in contact with water. The larvae that hatch from the eggs are mostly quite dissimilar to the adult form. In the case of frogs and toads they have a large head and a dorsally flattened tail and are known as tadpoles. They are vegetarians and breathe with gills. They have no limbs at first, the back limbs thrusting through then skin at a later stage, followed by the fore limbs after which the tail is reabsorbed. After this metamorphosis the juveniles look like miniature versions of the adult. Newt and salamander larvae have long bodies and feathery gills. They are carnivorous and the front legs develop before the back ones. They do not undergo metamorphosis in the same way that frogs and toads do. The caecilians either produce live young or lay eggs in damp positions in their burrows.
Amphibians are cold blooded animals and unable to maintain their body temperature above that of their surroundings. This means that they are only able to be active when the temperature is high enough. There is great variability in the sensitivity of different species to cold. Many species hibernate in winter, going into a state of torpor either in an underground chamber or underwater. In colder climates, they may be in a state of hibernation for more than half the year. In hot weather they may aestivate underground, sometimes buried in the mud of a dried up pond, reviving when cooler weather and rain restores their habitat. An advantage of being cold blooded is that little energy is required to provide body heat. This means that they can go without food for prolonged periods without coming to harm.
The order Anura includes the frogs and toads. Members of this order with smooth skins are commonly referred to as frogs while those with warted skins are known as toads. There are no taxonomic differences between these two groups but members of the family Bufonidae are known as true toads. Frogs and toads have broad heads and plump bodies with short, stout fore limbs and long hind limbs that fold underneath them. Although many species are associated with water and damp habitats, some are specialised to live in trees and in deserts. They are found worldwide.
The order Caudata includes the salamanders and one of its constituent families, Salamandridae, includes the true salamanders and the newts. Salamanders and newts have pointed heads, long cylindrical bodies, four similar sized, short legs and long tails. They may be terrestrial or aquatic but many spend part of the year in each habitat. When on land, they mostly spend the day hidden under stones or logs or in dense vegetation, emerging in the evening and night to forage for worms, insects and other invertebrates. They are found in the Holarctic region of the northern hemisphere and in Central and South America north of the Amazon Basin.
The order Gymnophiona includes the caecilians. These are long, cylindrical, limbless animals that resemble snakes or worms. Their skin has circular folds which enhances their similarity to the segments of earthworms. Some are aquatic but most live underground in burrows they hollow out. Many caecilians give birth to live young, and in the animals that do not do this, the eggs may undergo metamorphosis before they hatch. Caecilians are found in tropical Africa, Asia and Central and South America.
The first major groups of amphibians developed in the Devonian period from lobe-finned fish similar to the modern coelacanth and lungfish, which had evolved multi-jointed leg-like fins with digits that enabled them to crawl along the sea bottom. Some fish had developed primitive lungs to help them breathe air when the stagnant pools of the Devonian swamps were lacking in oxygen. They could also use their strong fins to hoist themselves out of the water and onto dry land if circumstances required it. Eventually, their bony fins would evolve into limbs and they would become the ancestors to all tetrapods, including amphibians, reptiles, birds, and mammals. Despite being able to crawl on land, many of these prehistoric tetrapodomorph fish still spent most of their time in the water. Ichthyostega was one of these tetrapods and had four sturdy limbs, a neck, a tail with fins and a skull very similar to the lobe-finned fish, Eusthenopteron. Amphibians evolved adaptations which allowed them to stay out of the water for longer periods. However, they never developed the amniotic egg which prevented the developing embryo from drying out and which allowed early reptiles to move on to the land to reproduce. They still need to return to water or find a damp place to lay their shell-less eggs and most have a fully aquatic larval stage.
There are large gaps in the fossil record but the discovery of a batrachian from the Early Permian in Texas in 2008 provided a missing link with a lot of the characteristics of modern frogs. Molecular analysis suggests that the frog–salamander divergence took place considerably earlier than the palaeontological evidence indicates. However the date of the divergence of the caecilians deduced by molecular phylogenetics agrees with the fossil record.
The first true amphibians appeared in the Carboniferous Period, by which time they were already moving up the food chain and occupying the ecological position currently claimed by such animals as crocodiles. Amphibians were once the top land predators, sometimes reaching several meters in length, preying on the large insects on land and many types of fish in the water. During the Triassic Period, the better-adapted reptiles began to compete with amphibians, leading to the reduction of their size and importance in the biosphere. Lissamphibia, which includes all modern amphibians and is the only surviving lineage of amphibians left, could have branched off from the extinct groups Temnospondyli and/or Lepospondyli at some time between the Late Carboniferous and the Early Triassic according to the fossil record. The relative scarcity of fossil evidence does not permit an exact date, and the most recent molecular clock study based on multi-locus data suggest a Late Carboniferous–Early Permian origin of extant amphibians.
Of these only the last subclass includes recent species.
With the phylogenetic classification, the taxon Labyrinthodontia has been discarded as it is a paraphyletic group without unique defining features apart from shared primitive characteristics. Classification varies according to the preferred phylogeny of the author and whether they use a stem-based or a node-based classification. Traditionally, amphibians as a class are defined as all tetrapods with a larval stage, while the group that includes the common ancestors of all living amphibians (frogs, salamanders and caecilians) and all their descendants is called Lissamphibia. The phylogeny of Paleozoic amphibians is by no means satisfactory understood, and Lissamphibia may possibly include extinct groups like the temnospondyls (traditionally placed in the subclass “Labyrinthodontia”), and the Lepospondyls, and in some analysis even the amniotes. This means that advocates of phylogenetic nomenclature have removed a large number of basal Devonian and Carboniferous amphibian-type tetrapod groups that were formerly placed in Amphibia in Linnaean taxonomy and included them elsewhere under cladistic taxonomy.
All recent amphibians are included in the subclass Lissamphibia, superorder Salientia, which is usually considered a clade (which means that it is thought that they evolved from a common ancestor apart from other extinct groups), although it has also been suggested that salamanders arose separately from a Temnospondyl-like ancestor, and even that caecilians are the sister group of the advanced reptiliomorph amphibians, and thus of amniotes.
Authorities also disagree as to whether Salientia is a superorder that includes the order Anura, or whether Anura is a sub-order of the order Salientia. Practical considerations seem to favor using the former arrangement. The Lissamphibia, superorder Salientia, are traditionally divided into three orders, but an extinct salamander-like family, the Albanerpetontidae, is now considered part of the Lissamphibia, besides the superorder Salientia. Furthermore, Salientia includes all three recent orders plus a single Triassic proto-frog, Triadobatrachus.
The actual number of species partly also depends on the taxonomic classification followed, the two most common classifications being the classification of the website AmphibiaWeb, University of California (Berkeley) and the classification by herpetologist Darrel Frost and The American Museum of Natural History, available as the online reference database Amphibian Species of the World. The numbers of species cited above follow Frost.
Amphibian skin is permeable to water and contains many mucous glands which keep the skin from drying out. Gas exchange can take place through the skin and this allows adult amphibians to hibernate at the bottom of ponds. To compensate for their thin and delicate skin, all amphibians have evolved poison glands as a defense mechanism, although toxicity varies by species. Some amphibian toxins can be lethal to humans while others have little effect. The main poison-producing glands, the paratoids, contain the neurotoxin bufotoxin and are located behind the ears of certain frogs and toads and behind the eyes of salamanders. The integumentary structure contains some typical characteristics common to terrestrial vertebrates, such as the presence of highly cornified outer layers, renewed periodically through a molting process controlled by the pituitary and thyroid glands. Local thickenings (often called warts) are common, such as those found on toads. The outside of the skin is shed periodically more or less in one piece in contrast to mammals and birds where it is shed in flakes. Amphibians often eat the sloughed skin.
The skin color of amphibians is produced by three layers of pigment cells called chromatophores. These three cell layers consist of the melanophores (occupying the deepest layer), the guanophores (forming an intermediate layer and containing many granules, producing a blue-green color) and the lipophores (yellow, the most superficial layer). The color change experienced by many species is caused by secretions from the pituitary gland. Unlike bony fish, there is no direct control by the nervous system of the pigment cells. Therefore, the color change is slower. Bright colors usually indicate that the species produces an exceptionally toxic poison.
The skeletal system of amphibians is structurally homologous to other tetrapods, though with a number of variations. They possess a cranium, spine, rib cage, long bones such as the humerus and femur, and short bones such as the phalanges, metacarpals, and metatarsals. Most have four limbs except for caecilians. Bones in most amphibians are hollow and lightweight.
The shoulder girdle of early amphibians is almost identical to that of their predecessors the osteolepiformes, except for the presence of a new dermal bone, the interclavicular (which has been lost in modern amphibians). The pelvic girdle is much more developed. In all tetrapods it consists of three main bones: the ilium in the dorsal and ventral, the pubis in the anterior and the ischium in a posterior position. The meeting point of these three bones forms the acetabulum which articulates with the femur.
Amphibians have a juvenile stage and an adult stage and the circulatory systems of the two are distinct. In the juvenile (or tadpole) stage, gills are used to oxygenate blood and movement is similar to that of fish. In the adult stage, amphibians (especially frogs) lose their gills and develop lungs. They have a heart that consists of a ventricle and two atria (it may be considered a single atrium, if not totally or partially divided) that pumps oxygenated blood through arteries and deoxygenated blood through veins to the lungs. Since amphibians are cold-blooded, they must find ways to keep their blood at a constant temperature to maintain homeostasis.
The nervous system is basically the same as in other vertebrates, with a central brain, a spinal cord, and nerves throughout the body. The amphibian brain is less developed compared to that of reptiles, birds, and mammals. It consists of a cerebrum, midbrain, and cerebellum of similar sizes. The olfactory lobe is the center of the sense of smell. The cerebrum integrates behavior and learning. The optic lobe processes information from the eyes. The cerebellum is the center of muscular coordination. The medulla oblongata controls some organ functions, such as heart rate and respiration. The brain sends signals through the spinal cord and nerves to regulate activity in the rest of the body. The pineal body, known to regulate sleep patterns in humans, is thought to produce the hormones involved in hibernation and estivation in amphibians.
Amphibians swallow their prey whole, with some chewing done in the oral cavities of some species, so they possess voluminous stomachs. Sphincters separate the esophagus from both the oral cavity and the stomach. The relatively short esophagus is lined with cilia that help transport food and secretions to the stomach. Mucus and pepsin, a digestive enzyme, are secreted by glands lining the esophagus. The stomach is separated from the intestine by a pyloric sphincter. The duodenum controls the transport of food into the intestine from the stomach.
Amphibians possess a pancreas, liver and gall bladder. Like mammals, the liver functions as the central metabolic organ that regulates blood sugar, and also produces the final metabolic products and transports them through the vascular system to the kidneys, and finally to excretion. The liver in most amphibians is large with two lobes. The size of the liver is determined by its vital function as a glycogen and fat storage unit, and may change proportionally with the seasons with increasing or decreasing activity. In aquatic amphibians, the liver plays only a small role in processing nitrogen for excretion, and ammonia is diffused mainly through the skin and excretion. The liver of terrestrial amphibians converts ammonia to urea, a less toxic, water soluble nitrogenous compound, as a means of water conservation. In some species, urea is further converted into uric acid. The liver secretions from the liver collect in the gall bladder, and flow into the small intestine. Salamanders lack a valve separating the small intestine from the large intestine. In the small intestine, enzymes digest carbohydrates, fats, and proteins. Salt and water absorption occur in the large intestine, as well as mucous secretion to aid in the transport of fecal matter, which is passed out through the cloaca. Amphibians have two kidneys located dorsally, near the roof of the body cavity. Their job is to filter the blood of waste and transport it to the urinary bladder where it passes out of the cloacal vent.
The lungs in amphibians are primitive compared to those of amniotes, possessing few internal septa and large alveoli and consequently having a comparatively slow diffusion rate for oxygen entering the blood. Ventilation is accomplished by buccal pumping. Most amphibians, however, are able to exchange gasses with the water or air via their skin. To enable sufficient cutaneous respiration, the surface of their highly vascularized skin must remain moist in order to allow the oxygen to diffuse at a sufficiently high rate. Because oxygen concentration in the water increases at both low temperatures and high flow rates, aquatic amphibians in these situations can rely primarily on cutaneous respiration, as in the Titicaca water frog and the hellbender salamander. In air, where oxygen is more concentrated, some small species can rely solely on cutaneous gas exchange, most famously the plethodontid salamanders, which have neither lungs nor gills. Many aquatic salamanders and all tadpoles have gills in their larval stage, with some (such as the axolotl) retaining gills as aquatic adults.
The eyes of amphibians have lids and associated glands and ducts. They are an improvement on invertebrate eyes and were a first step in the development of more advanced vertebrate eyes. They allow colour vision and depth of focus. In the retinas are green rods which are receptive to a wide range of wave lengths.
The tympani, or eardrums, of many frogs are external and lie just behind the eyes. There is also a patch of papilla amphibiorum in the ear which is unique to amphibians and which can detect low frequency sounds. Another unique feature is the columella-opercular complex, adjoining the auditory capsule, which is involved in the transmission of both airborne and seismic signals. The ears of salamanders and caecilians are less highly developed as they do not normally communicate with each other by sound.
For the purpose of reproduction most amphibians require fresh water. A few (e.g. Fejervarya raja) can inhabit brackish water and even survive (though not thrive) in seawater, but there are no true marine amphibians. However, there are reports of particular amphibian populations invading marine waters where their species is normally unable to survive. Such is the case with the Black Sea invasion of the natural hybrid Pelophylax esculentus reported in 2010.
Several hundred frog species in adaptive radiations (e.g., Eleutherodactylus, the Pacific Platymantines, the Australo-Papuan microhylids, and many other tropical frogs), however, do not need any water for breeding in the wild. They reproduce via direct development, an ecological and evolutionary adaptation that has allowed them to be completely independent from free-standing water. Almost all of these frogs live in wet tropical rainforests and their eggs hatch directly into miniature versions of the adult, passing through the tadpole stage within the egg. Reproductive success of many amphibians is dependent not only on the quantity of rainfall, but the seasonal timing.
Many amphibians exhibit different kinds of parenting behaviour. After their hatching, the tadpoles of different species of poison dart frogs (family Dendrobatidae) are carried by the adults to a suitable place where they can pass metamorphosis. Such places are the rosettes of many bromeliads in which water is gathered and used by the plant. The Surinam toad raises its young in pores at its back and after enough time they appear out of these pores fully developed. The ringed caecilian (Siphonops annulatus) has developed a unique adaptation for the purposes of reproduction. The progeny feeds on a skin layer that is specially developed by the adult. This phenomenon is known as maternal dermatophagy.
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 lives in the jelly-like layer of the eggs has evolved several times. The larvae of frogs (tadpoles or polliwogs) breathe with exterior gills at the start, but soon a pouch is formed that covers the gills and the front legs. Lungs are also formed quite early to assist in breathing. Newt larvae have large external gills that gradually disappear and the larvae of newts are quite similar to the adult form from an early age.
Most amphibians go through metamorphosis, a process of significant morphological change after birth. In typical amphibian development, eggs are laid in water and larvae are adapted to an aquatic lifestyle. Frogs, toads and salamanders all hatch from the egg as larvae with external gills. Metamorphosis in amphibians is regulated by thyroxin concentration in the blood, which stimulates metamorphosis, and prolactin, which counteracts its effect. Specific events are dependent on threshold values for different tissues. Because most embryonic development is outside the parental body, development is subject to many adaptations due to specific ecological circumstances. For this reason tadpoles can have horny ridges for teeth, whiskers and fins. They also make use of the lateral line organ. After metamorphosis, these organs become redundant and will be reabsorbed by controlled cell death, called apoptosis. The extent of adaptations to specific ecological circumstances among amphibians is remarkable, with many discoveries still being made.
With frogs and toads, the external gills of the newly hatched tadpole are covered with a gill sac after a few days, and lungs are quickly formed. Front legs are formed under the gill sac, and hind legs are visible a few days later. Following that there is usually a longer stage during which the tadpole has a vegetarian diet, scraping algae off plant surfaces with their horny tooth ridges. Tadpoles have a relatively long, spiral-shaped gut to enable them to digest this diet.
Rapid changes in the body can then be observed as the lifestyle of the frog changes completely. The spiral‐shaped mouth with horny tooth ridges is reabsorbed together with the spiral gut. The animal develops a big jaw, and its gills disappear along with its gill sac. Eyes and legs grow quickly, a tongue is formed, and all this is accompanied by associated changes in the neural networks (development of stereoscopic vision, loss of the lateral line system, etc.) All this can happen in about a day, so it is truly a metamorphosis. It isn't until a few days later that the tail is reabsorbed, due to the higher thyroxin concentrations required for tail resorption.
In newts and salamanders, there is no true metamorphosis because the larvae already feed as predators and continue doing so as adults. Their gills are never covered by a gill sac and will be reabsorbed only just before the animal leaves the water. Just as in tadpoles, their lungs are functional early, but the larvae don't make as much use of them as tadpoles do. They often have an aquatic phase in spring and summer, and a land phase in winter. For adaptation to a water phase, prolactin is the required hormone, and for adaptation to the land phase, thyroxin. External gills do not return in subsequent aquatic phases because these are completely absorbed upon leaving the water for the first time.
Most caecilians lay eggs in burrows formed in moist soil close to water. The eggs are watched over by the female and, on hatching, the young make their way into the adjoining pond or stream. They have gill slits but no external gills. In some species, the young are produced by vivipary. Up to four eggs begin their development in the oviduct. When their yolk has been consumed, the developing larvae feed on maternal secretions and on the cells lining the oviduct, scraping them off with their deciduous teeth.
Adult amphibians are predators, feeding mainly on live invertebrates. The diet mostly consists of small items of prey that do not move too fast such as beetles, caterpillars, earthworms and spiders. Many amphibians have extensible tongues with sticky tips which can be flicked forward to catch the prey. They usually swallow the food whole but may chew it lightly first in order to subdue it. The larvae of frogs and toads have a mostly vegetarian diet, using their specialised mouthparts, consisting of a horny beak edged by several rows of labial teeth, to feed on aquatic plants.
The noises made by newts, salamanders and caecilians are limited to occasional soft squeaks, grunts or hisses. Frogs and toads however use their voice in the breeding season to attract mates. Each call is characteristic of the species, the presence of which in an area may be easier to detect by its voice than by a fleeting glimpse of the animal itself. In ponds where more than one species breed, it is important for the female to be attracted to the correct partner as male frogs, in their ardour, have been known to attempt to mate with other males, females of the wrong species or inanimate objects. Besides the attracting call, another vocalisation is a release call, used by a frog suffering in this way from advances by the wrong frog. Calling bears the risk of attracting predators. Some males do not call at all but station themselves near others that are calling, with the intention of intercepting a female attracted to the sound. Although calling mainly occurs in the breeding season, sporadic calls may be heard at other times of year in some species.
The calls are made by vibrations of the larynx but are often amplified by vocal sacs which act as resonators. Sometimes these are fairly small air-filled pouches inside the mouth but in other species they are much larger and cause a ballooning of the elastic skin of the throat and are known as external vocal sacs. Many species have single sacs but members of the family Ranidae have two, one on each side of the mouth.
Amphibians have soft bodies and are relatively helpless. They are eaten by birds and mammals when on land and by fish when in the water. Many are nocturnal and hide during the day, thereby avoiding predators that hunt by sight, apart from owls which are also nocturnal. Other amphibians use camouflage to avoid being detected. They have various colorings such as mottled browns, greys and olives to make themselves inconspicuous so that they blend into the background. Other species contain poison glands and use bright colours to warn potential predators of their toxicity. These are mostly black and yellow or black and red, like the fire salamander (Salamandra salamandra). Once a predator has tried eating one of these, it is likely to remember the coloration next time it encounters one. In some species, such as the fire-bellied toad (Bombina spp.), the warning coloration is on the belly and these animals adopt a defensive pose when attacked to exhibit the bright colors to the attacker. A few salamanders will autotomise their tails when attacked, sacrificing this part of their body to enable the main part to escape. The tail is regenerated later. Some frogs and toads inflate themselves to make themselves look large and fierce, and some spadefoot toads (Pelobates spp) scream and leap towards the attacker.
Dramatic declines in amphibian populations, including population crashes and mass localized extinction, have been noted in the past two decades from locations all over the world, and amphibian declines are thus perceived as one of the most critical threats to global biodiversity. A number of causes are believed to be involved, including habitat destruction and modification, over-exploitation, pollution, introduced species, climate change, endocrine-disrupting pollutants, destruction of the ozone layer (ultraviolet radiation has shown to be especially damaging to the skin, eyes, and eggs of amphibians), and diseases like chytridiomycosis. However, many of the causes of amphibian declines are still poorly understood, and are a topic of ongoing discussion.
A global strategy to stem the crisis has been released in the form of the Amphibian Conservation Action Plan. Developed by over 80 leading experts in the field, this call to action details what would be required to curtail amphibian declines and extinctions over the next 5 years—and how much this would cost. The Amphibian Specialist Group of the World Conservation Union (IUCN) is spearheading efforts to implement a comprehensive global strategy for amphibian conservation. Amphibian Ark is an organization that was formed to implement the ex-situ conservation recommendations of this plan, and they have been working with zoos and aquaria around the world encouraging them to create assurance colonies of threatened amphibians. One such project is the Panama Amphibian Rescue and Conservation Project that built on existing conservation efforts in Panama to create a country-wide response to the threat of chytridiomycosis rapidly spreading into eastern Panama.
On January 21, 2008, Evolutionarily Distinct and Globally Endangered (EDGE), in a statement made by chief Helen Meredith, identified nature's most endangered species: "The EDGE amphibians are amongst the most remarkable and unusual species on the planet and yet an alarming 85% of the top 100 are receiving little or no conservation attention." The top 10 EDGE listed species include the Chinese giant salamander, a distant relative of the newt, the tiny Gardiner's Seychelles, the limbless Sagalla caecilian, South African ghost frogs, lungless Mexican salamanders, the Malagasy rainbow frog, Chile's Darwin frog (Rhinoderma rufum) and the Betic Midwife Toad.