Amphibian

For other uses, see Amphibian (disambiguation).

Amphibians Temporal range:
File:Lissamphibia.jpg
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Batrachomorpha
Class:	Amphibia Linnaeus, 1758
Subclasses and Orders
Subclass Labyrinthodontia—extinct Subclass Lepospondyli—extinct Subclass Lissamphibia Order Anura Order Caudata Order Gymnophiona

Amphibians are members of the class Amphibia, whose living forms include frogs, toads, salamanders, newts and caecilians. They are cold-blooded, tetrapod vertebrates. Most have four limbs and live in fresh water or on land but the caecilians, though included in the group, live in burrows in damp soil and are limbless. Amphibians typically have a larval stage that lives in water but there are many different behavioural adaptations that have developed among species to bypass this necessity. The young undergo metamorphosis from a larval form with gills to an adult air-breathing form with lungs. Amphibians also use their skin for respiration and some small terrestrial salamanders rely entirely on this and have no lungs.

The earliest amphibians evolved in the Devonian Period from sarcopterygian fish with lungs and bony-limbed fins, features that were helpful in adapting to dry land. They diversified and became dominant during the Carboniferous and Permian periods, but were later displaced by reptiles and other vertebrates. Over time, amphibians shrank in size and decreased in diversity, leaving only the modern subclass Lissamphibia.

The three modern orders of amphibians are the Anura (frogs and toads), Caudata (salamanders and newts) and Gymnophiona (caecilians, limbless amphibians that resemble large earthworms with jaws). The total number of known amphibian species is approximately 7,000 of which nearly 90% are frogs. They are superficially similar to reptiles, but reptiles, along with mammals and birds, are amniotes, having impervious membranes that surround the egg. With their often complex reproductive needs and permeable skins, amphibians are often ecological indicators, and in recent decades there has been a dramatic decline in amphibian populations of many species around the globe. The smallest vertebrate in the world is the New Guinea frog, Paedophryne amauensis. The largest amphibian is the Chinese Giant Salamander, Andrias davidianus. The study of amphibians is called batrachology while the study of both reptiles and amphibians is called herpetology.

Etymology

The world's smallest vertebrate, Paedophryne amauensis sitting on a U.S. dime, 17.91mm, for scale

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 as an adjective for animals that could live on land or in water, including seals and otters. The word amphibian became restricted in the taxonomical sense to what we now use around 1600 with the taxon "Amphibia" first published in scientific classification circa 1819.

Evolution

File:Lissamphibian phylogeny.jpg

Possible paths of Lissamphibia evolution.

Main article: Labyrinthodontia

See also: List of prehistoric amphibians

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 modern 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. They had started to develop lungs but still breathed predominantly with gills.

Ichthyostega was one of the first primitive amphibians, with nostrils and more efficient lungs. It had four sturdy limbs, a neck, a tail with fins and a skull very similar to the lobe-finned fish, Eusthenopteron. Amphibians evolved adaptations that allowed them to stay out of the water for longer periods. Their lungs improved and their skeletons became heavier and stronger, better able to cope with the increased gravitational effect of life on land. They developed "hands" and "feet" with five digits. Their skin evolved and became more capable of retaining body fluids and resisting desiccation. The fish's hyomandibula bone in the hyoid region behind the gills diminished in size and became the stapes of the amphibian ear, an adaptation necessary for hearing on dry land. An affinity between the amphibians and the teleost fish is the labyrinthic structure of the teeth and the paired supra-occipital bones at the back of the head, both features not found elsewhere in the animal kingdom.

In the late Devonian and early Carboniferous, the seas, rivers and lakes were teeming with life but the land was the realm of early plants and devoid of animal life. In the early Carboniferous (362 to 290 million years ago) the climate became wet and warm and extensive swamps developed with mosses, ferns, horsetails and calamites. Air breathing arthropods began to evolve and invaded the land where they provided food for the carnivorous amphibians that began to emerge from the waters. There were no other tetrapods on the land and the amphibians were at the top of the food chain and occupying the ecological position currently held by the crocodile. Though equipped with limbs and the ability to breathe air, most still had a long tapering body and strong tail. They were the top land predators, sometimes reaching several meters in length, preying on the large insects of the period and many types of fish in the water. They still needed to return to water to lay their shell-less eggs and even modern amphibia have a fully aquatic larval stage with gills like their ancestral fish. It was the development of the amniotic egg, which prevents the developing embryo from drying out, that enabled the reptiles to reproduce on land and which led to their dominance in the period that followed.

During the Triassic Period, the reptiles began to out-compete the amphibians, leading to a reduction in both their size and their importance in the biosphere. According to the fossil record, Lissamphibia, which includes all modern amphibians and is the only surviving lineage, may have branched off from the extinct groups Temnospondyli and Lepospondyli at some period between the Late Carboniferous and the Early Triassic. The relative scarcity of fossil evidence prevents an exact date being known but the most recent molecular study, based on multi-locus data, suggest a Late Carboniferous – Early Permian origin of extant amphibians.

The origins and evolutionary relationships between the three main groups of amphibians is a matter for debate. A 2005 molecular phylogeny based on rDNA analysis suggests that salamanders and caecilians are more closely related to each other than they are to frogs. It also appears that divergence of the three groups took place in the Paleozoic or early Mesozoic before the breakup of the supercontinent Pangaea and soon after their divergence from the lobe-finned fish. The briefness of this period and the swiftness with which radiation took place would help account for the relative scarcity of primitive amphibian fossils. 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.

A fossilized frog from the Czech Republic, possibly Palaeobatrachus gigas

Classification

a small specimen of the Chinese giant salamander

Traditionally, amphibians includes all tetrapod vertebrates that are not amniotes. Amphibia in its widest sense (sensu lato) was divided into three subclasses, two of which are extinct:

• Subclass Labyrinthodontia† (diverse Paleozoic and early Mesozoic group)
• Subclass Lepospondyli† (small Paleozoic group, sometimes included in the Labyrinthodontia, which may actually be more closely related to amniotes than Lissamphibia)
• Subclass Lissamphibia (all modern amphibians including frogs, toads, salamanders, newts and caecilians)
  • Family Albanerpetontidae—Jurassic to Miocene (extinct)
  • Superorder Salientia
    • Genus Triadobatrachus—Triassic (extinct)—a stem Anuran
    • Order Anura (frogs and toads): Jurassic to recent—5,602 recent species in 48 families
    • Order Caudata or Urodela (salamanders, newts): Jurassic to recent—571 recent species in 10 families
    • Order Gymnophiona (caecilians): Jurassic to recent—190 recent species in 10 families

The actual number of species in each group depends on the taxonomic classification followed, the two most common systems being the classification adopted by 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.

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 uncertain, and Lissamphibia may possibly fall within 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, a group of species that have evolved from a common ancestor. 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 favour 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 alongside the superorder Salientia. Furthermore, Salientia includes all three recent orders plus the Triassic proto-frog, Triadobatrachus.

Characteristics

The amphibians are tetrapods, 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. Amphibians evolved from lunged fish and had to make certain adaptations for living on land. They needed to develop new means of locomotion to replace the sideways thrust of their tails that had been used for swimming. Their vertebral columns, limbs, limb girdles and musculature needed to be strong enough to raise them off the ground for locomotion and feeding. Terrestrial adults needed to discard their lateral line systems and adapt their sensory systems to receive stimuli via the medium of air. They needed to develop new methods to regulate their body heat in order to cope with fluctuations in ambient temperatures. They needed to develop behaviours suitable for reproduction in a terrestrial environment. Their skins were exposed to harmful ultraviolet rays that had previously been absorbed by the water and needed to become more protective membranes that prevented excessive water loss. Amphibians are restricted to moist habitats because their skin needs to be kept damp and they need to produce copious amounts of dilute urine. They also require water bodies for reproduction, although some species have developed various strategies for protecting or bypassing the vulnerable aquatic larval stage.

Amphibians are cold blooded vertebrates. Their metabolic rate is low and as a result, their food and energy requirements are limited. In the adult state they have tear ducts and movable eyelids and most species have ears that can detect airborne or ground vibrations. They have a muscular tongue which in many species can be protruded. Modern amphibians have fully ossified vertebrae with articular processes. Their ribs are usually short and may be fused to the vertebrae. Their skulls are mostly broad and short and are often incompletely ossified. Their skin contains little keratin and lacks scales, apart from a few fish-like scales in certain caecilians. It contains many mucous glands and in some species, poison glands. The hearts of amphibians have three chambers. The left and right atria receive oxygenated and de-oxygenated blood respectively. On contraction, these pass the blood to a single ventricle which pumps it round the body and to the lungs. The two blood streams remain largely separate in the process. Amphibians have a urinary bladder and nitrogenous waste products are excreted primarily as urea. Most amphibians lay their eggs in water and have aquatic larvae that undergo metamorphosis to become terrestrial adults. Amphibians breathe by means of a bucco-pharyngeal pump but supplement this with gas exchange through the skin.

Anura

The order Anura (from the Ancient Greek an-, without and oura, tail) comprises the frogs and toads. These usually have long hind limbs that fold underneath them, shorter forelimbs, webbed toes with no claws, no tails, large eyes and glandular moist skin. Members of this order with smooth skins are commonly referred to as frogs while those with warted skins are known as toads. The difference is not a formal one taxonomically and there are numerous exceptions to this rule. Members of the family Bufonidae are known as the "true toads". Frogs range in size from the 30 centimetres (12 in) goliath frog (Conraua goliath) of West Africa to the 7.7 millimetres (0.30 in) long Paedophryne amauensis, first described in Papua New Guinea in 2012. This latter is also the smallest known vertebrate. Although most species are associated with water and damp habitats, some are specialised to live in trees and in deserts. They are found worldwide except for polar areas. Anura is divided into three suborders which are broadly accepted by the scientific community but the relationships between some families remains unclear. Future molecular studies should provide further insights into their evolutionary relationships.

The suborder Archaeobatrachia contains four families of primitive frogs. These comprise the families Ascaphidae, Bombinatoridae, Discoglossidae and Leiopelmatidae.

The suborder Mesobatrachia contains six families of more evolutionary advanced frogs. These comprise the families Megophryidae, Pelobatidae, Pelodytidae, Scaphiopodidae, Rhinophrynidae and Pipidae.

The suborder Neobatrachia is by far the largest group and includes the remaining families of modern frogs, including most common species. Ninety six percent of the over 5000 extant species of frog are neobatrachians.

Caudata

The order Caudata (from the Latin cauda meaning "tail") consists of the salamanders, elongated, low-slung animals that mostly resemble lizards in form, though this is a symplesiomorphic trait and the two groups are no more closely related than salamanders are to mammals. Salamanders lack claws, have a scale-free skin, either smooth or covered with tubercles, and a tail that is usually flattened from side to side and often finned. They range in size from the Chinese giant salamander (Andrias davidianus), which can grow to a length of 1.8 metres (5 ft 11 in), to the diminutive (Thorius pennatulus) from Mexico which seldom exceeds 20 millimetres (0.79 in). Salamanders are distributed widely in the Holarctic region of the northern hemisphere. The family Plethodontidae are also found in Central and South America north of the Amazon Basin. Urodela is a name sometimes used for all the extant species of salamander. Members of several families of salamanders have become paedomorphic and either fail to complete their metamorphosis or retain some larval characteristics as adults. Most salamanders are under 15 centimetres (5.9 in) long. 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.

The suborder Cryptobranchoidea contains the primitive salamanders. A number of fossil cryptobranchids have been found but there are only three living species, the Chinese giant salamander (Andrias davidianus), the Japanese giant salamander (Andrias japonicus ) and the hellbender (Cryptobranchus alleganiensis) from North America. These large amphibians retain several larval characteristics in their adult state; gills slits are present and the eyes are unlidded. A unique feature is their ability to feed by suction using only one half of the lower jaw at a time. The males excavate nests, persuade females to lay their egg strings inside them and guard them. As well as breathing with lungs, they respire through the many folds in their skin which is thin with the capillaries close to the surface.

The suborder Salamandroidea contains the advanced salamanders. They differ from the cryptobranchids by having the prearticular bones in their lower jaw fused and reproduction is by internal fertilisation. In salamandrids, the male deposits a bundle of sperm, the spermatophore, and the female picks it up and inserts it into her cloaca where the sperm is stored until the eggs are laid. The largest family in this group is Plethodontidae, the lungless salamanders, which includes sixty percent of all salamander species. The family Salamandridae includes the true salamanders and the name "newt" is given to members of its subfamily Pleurodelinae.

The third suborder Sirenoidea contains the sirens, consisting of four species in a single family, Sirenidae. Members of this order are eel-like aquatic salamanders with much reduced forelimbs and no hind limbs. Some of their features are primitive while others are derived. Fertilisation is likely to be external as sirenids lack the cloacal glands used by male salamandrids to produce spermatophores and the females lack spermathecae for sperm storage. Despite this, the eggs are laid singly, a behaviour not conducive for external fertilisation.

Gymnophiona

The order Gymnophiona (from the Greek, gymnos and ophis meaning "naked serpent") comprise the caecilians. These are long, cylindrical, limbless animals with a snake or worm-like form. The adults vary in length from 8 to 75 centimetres (3 to 30 inches) with the exception of Thomson's caecilian (Caecilia thompsoni) which can reach 150 centimetres (5 feet). A caecilian's skin has a large number of transverse folds and in some species contains tiny embedded dermal scales. It has rudimentary eyes covered in skin which are probably limited to discerning differences in light intensity. It also has a pair of short tentacles near the eye that can be extended and which have tactile and olfactory functions. Some caecilians are aquatic but most live underground in burrows in damp soil, in rotten wood and under plant debris. Most species lay their eggs underground and when the larvae hatch, they make their way to adjacent bodies of water. Others brood their eggs and the larvae undergo metamorphosis before the eggs hatch. A few species give birth to live young, nourishing them with glandular secretions while they are in the oviduct. Caecilians are found in tropical Africa, Asia and Central and South America.

Anatomy and physiology

Skin

The fire salamander has brightly colored yellow spots, indicating that it secretes toxins.

The integumentary structure contains some typical characteristics common to terrestrial vertebrates, such as the presence of highly cornified outer layers, renewed periodically through a moulting 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. Caecilians are unique among amphibians in having mineralized dermal scales embedded in the dermis between the furrows in the skin. The similarity of these to the scales of bony fish is largely superficial. Lizards and some frogs have somewhat similar osteoderms forming bony deposits in the dermis but this is an example of convergent evolution with similar structures having arisen independently in diverse vertebrate lineages.

Amphibian skin is permeable to water. Gas exchange can take place through the skin and this allows adult amphibians to respire without rising to the surface of water and to hibernate at the bottom of ponds. To compensate for their thin and delicate skin, amphibians have evolved mucous glands, principally on their heads, backs and tails. The secretions produced by these help keep the skin moist. In addition, most species of amphibian have granular glands that secrete distasteful or poisonous substances. Some amphibian toxins can be lethal to humans while others have little effect. The main poison-producing glands, the paratoids, produce the neurotoxin bufotoxin and are located behind the ears of certain frogs and toads and behind the eyes of salamanders.

The skin colour 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 colour) and the lipophores (yellow, the most superficial layer). The colour change experienced by many species is caused by secretions from the pituitary gland. Unlike bony fish, there is no direct control of the pigment cells by the nervous system which results in the colour change taking place more slowly. Bright colours usually indicate that the species is toxic, a defence against predators.

Skeletal system and locomotion

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. They have four limbs except for the legless caecilians and a few species of salamander. The bones are hollow and lightweight. The skeletomuscular system is strong to enable it to support the head and body. The bones are fully ossified and the vertebrae interlock with each by means of overlapping processes. The pectoral girdle is supported by muscle and the well developed pelvic girdle is attached to the backbone by a pair of sacral ribs. The ilium slopes forward and the body is held closer to the ground than in mammals.

In most amphibians, there are four digits on the fore foot and five on the hind foot, but no claws on either. Some salamanders have fewer digits and the Amphiumas are eel-like in appearance with tiny stubby legs. The sirens are aquatic salamanders with stumpy forelimbs and no hind limbs. The caecilians are limbless. They burrow in the manner of earthworms with zones of muscle contractions moving along the body. On the surface of the ground or in water they move by undulating their body from side to side.

In frogs, the hind legs are larger than the fore legs, especially so in those species that principally move by jumping or swimming. In the walkers and runners the hind limbs are not so large and the burrowers mostly have short limbs and broad bodies. The feet have adaptations for the way of life with webbing between the toes for swimming, broad adhesive toe pads for climbing and keratinised tubercles on the hind feet for digging (frogs usually dig backwards into the soil). In most salamanders, the limbs are short and more or less the same length and project at right angles from the body. Locomotion on land is by walking and the tail often swings from side to side or is used as a prop, particularly when climbing. In their normal gait, only one leg is advanced at a time in the manner adopted by their ancestors, the lobe-finned fish. Some salamanders in the genus Aneides and certain Plethodontids climb trees and have long limbs, large toepads and prehensile tails. In aquatic salamanders, and in frog tadpoles, the tail has dorsal and ventral fins and is moved from side to side as a means of propulsion. Adult frogs do not have tails and caecilians have only very short ones.

Salamanders use their tails in defence. Some species in the Plethodontidae have a weak zone at the base of the tail and if the salamander is in danger or if the tail is grabbed by a predator, it breaks off, a process known as autotomy. Other salamanders are also prepared to drop their tail to save their lives. The tail may continue to twitch after separation which may distract the attacker and both tails and limbs are able to be regenerated. Adult frogs are unable to regrow limbs but tadpoles can do so.

Circulatory system

The South American caecilian, Siphonops paulensis

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.

Nervous and sensory systems

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 well developed than that of reptiles, birds and mammals but is similar in morphology and function to that of a fish. It consists of a cerebrum,midbrain and cerebellum of approximately equal size. The olfactory lobe is the centre of the sense of smell. The cerebrum integrates behaviour and learning. The optic lobe processes information from the eyes. The cerebellum is the centre of muscular coordination. The medulla oblongata controls some organ functions including the heart 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 aestivation in amphibians.

Tadpoles retain the lateral line system of their ancestral fishes but this is lost in terrestrial adult amphibians. Some caecilians possess electroreceptors which allow them to locate objects around them when submerged in water. The ears are well developed in frogs. There is no external ear but the large circular eardrum lies on the surface of the head just behind the eye. This vibrates and sound is transmitted through a single bone, the stapes, to the inner ear. Only high frequency sounds like mating calls are heard in this way but low frequency noises can be detected through another mechanism. There is a patch of specialized haircells called "papilla amphibiorum" in the inner ear capable of detecting deeper sounds. Another feature unique to amphibians 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 than frogs as they do not normally communicate with each other by sound.

Tadpoles' eyes resemble those of fishes in structure but at metamorphosis the cornea becomes more dome-shaped, the lens becomes flatter and eyelids and associated glands and ducts develop. The adult eyes 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 wavelengths.

Digestive and excretory systems

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 faecal 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 metabolic waste and transport the urine via ureters to the urinary bladder where it is stored before being passed out periodically through the cloacal vent. Larvae and most aquatic adult amphibians excrete the nitrogen as ammonia in large quantities of dilute urine while terrestrial species, with a greater need to conserve water, excrete the less toxic product urea. Some tree frogs with limited access to water excrete most of their metabolic waste as uric acid.

Respiratory system

Leucistic axolotls at Vancouver Aquarium

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 gases 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.

Reproduction

Frogspawn

For the purpose of reproduction most amphibians require fresh water although some lay their eggs on land and have developed various ingenious ways of keeping them moist. 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.

In the tropics, many amphibians breed continuously or at any time of year. In temperate regions, breeding is mostly seasonal, usually in the spring, and is triggered by increasing day length, rising temperatures or rainfall. Experiments have shown the importance of temperature but the trigger event, especially in arid regions, is often rainfall. In anurans, males usually arrive at the breeding sites before females and the vocal chorus they produce may stimulate the endocrine activity of males that are not yet reproductively active and ovulation in females.

In caecilians, fertilisation is internal, the male extruding an intromittent organ, the phallodeum, and inserting it into the female cloaca. The paired Müllerian glands inside the male cloaca secrete a fluid which resembles that produced by mammalian prostate glands and which may transport and nourish the sperm. Fertilisation probably takes place in the oviduct.

The majority of salamanders also engage in internal fertilisation. In most of these, the male deposits a spermatophore, a small packet of sperm on top of a gelatinous cone, on the substrate either on land or in the water. The female takes the sperm packet into her vent by grasping it with the lips of the cloaca. The spermatozoa move to the spermotheca in the roof of the cloaca where they remain until ovulation which may be many months later. Courtship rituals and methods of transfer of the spermatophore vary between species. In some, the male places the spermatophore into the female cloaca, in others he guides her to it or restrains her with an embrace called amplexus. Certain primitive salamanders in the families Sirenidae, Hynobiidae and Cryptobranchidae practice external fertilisation in a similar manner to frogs, with the female laying the eggs in water and the male releasing sperm onto the egg mass.

With a few exceptions, frogs use external fertilisation. The male grasps the female tightly with his forelimbs either behind her arms or in front of her back legs, or in the case of Epipedobates tricolor, round her neck. They remain in amplexus with their cloacae positioned close together while she lays her eggs and he covers them with his sperm. Roughened nuptial pads on his hands help him retain his grip. Often the male collects and retains the egg mass, forming his hind feet into a sort of basket. An exception is the granular poison frog (Oophaga granulifera) where the male and female place their cloacae in close proximity while facing in opposite directions and then release eggs and sperm simultaneously. The tailed frog (Ascaphus truei) exhibits internal fertilisation. The "tail" is only possessed by the male and is an extension of the cloaca and used to inseminate the female. This frog lives in fast flowing streams and internal fertilisation will prevent the sperm being washed away before fertilisation occurs.

Most frogs can be classified as either prolonged or explosive breeders. Typically, prolonged breeders congregate at a breeding site, the males usually arriving first, calling and setting up territories. Other satellite males remain quietly nearby, waiting for their opportunity to take over a territory. The females arrive sporadically, mate selection takes place and eggs are laid. The females depart and territories may change hands and more females appear and in due course, the breeding season comes to an end. Explosive breeders on the other hand are found where temporary pools appear in dry regions after rainfall. These frogs are typically fossorial species that emerge after heavy rains and congregate at a breeding site. They are attracted there by the calling of the first male to find a suitable place, perhaps a pool that forms in the same place each rainy season. The assembled frogs may call in unison and frenzied activity ensues, the males scrambling to mate with the usually smaller number of females.

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. The common tree frog (Polypedates leucomystax) from Asia makes a foam nest on a leaf above an ephemeral pool. When the eggs hatch, the tadpoles work themselves out of the foam and fall into the water below.

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) 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.

Life cycle

Tadpoles

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.

Eggs

The egg of an amphibian is typically surrounded by a transparent gelatinous covering secreted by the oviducts and containing mucoproteins and mucopolysaccharides. This capsule is permeable to water and gases and swells considerably as it absorbs water. The ovum is at first rigidly held but in fertilised eggs, the innermost layer liquefies and allows the embryo to move freely and this also happens in salamander eggs even when they are unfertilised. Eggs of some salamanders and frogs contain unicellular green algae. These penetrate the jelly envelope after the eggs are laid and may increase the supply of oxygen to the embryo through photosynthesis. They seem to both speed up the development of the larvae and reduce mortality. Most eggs contain the pigment melanin which raises their temperature and protects them against ultraviolet radiation. Species such as caecilians, plethodontid salamanders and certain frogs that lay eggs underground or in dark locations usually have unpigmented eggs. In the wood frog (Rana sylvatica), the interior of the globular egg cluster has been found to be up to 6°C (11°F) warmer than its surroundings which is an advantage in its cool northern habitat.

The eggs or spawn may be deposited singly or in small groups or may take the form of spherical egg masses, rafts or long strings. In terrestrial caecilians, the eggs are laid in grape-like clusters in burrows near streams. The amphibious salamander Ensatina attaches its similar clusters by stalks to underwater stems and roots. The greenhouse frog (Eleutherodactylus planirostris) lays eggs in small groups in the soil where they develop in about two weeks directly into juvenile frogs without an intervening larval stage. The tungara frog (Physalaemus pustulosus) builds a floating nest from foam to protect its eggs. A raft is built first and then eggs are laid in the centre and there is a foam capping. The foam has anti-microbial properties. It contains no detergents but is created by whipping up proteins and lectins secreted by the female.

Larvae

The eggs of amphibians are typically laid in water and hatch into free-living larvae that complete their development in water and later transform into either aquatic or terrestrial adults. In many species of frog and in most lungless salamanders (Plethodontidae), direct development takes place, the larvae growing within the eggs and emerging as miniature adults. Many caecilians and some other amphibians lay their eggs on land and the newly hatched larvae wriggle or are transported to water bodies. Some caecilians, the Alpine salamander (Salamandra atra) and some of the African live-bearing toads (Nectophrynoides spp.) are viviparous, the larvae feeding on glandular secretions and developing within the female's oviduct, often for long periods. Other amphibians, but not as far as it is known caecilians, are ovoviviparous. The eggs are retained in or on the parent's body but the larvae subsist on the yolks of their eggs and receive no nourishment from the adult. The larvae emerge at varying stages of their growth, either before or after metamorphosis, according to their species. The toad genus Nectophrynoides exhibits all these developmental patterns among its dozen or so members.

Frogs

Frogspawn development

Life cycle of a typical frog

Frog larvae are known as tadpoles and typically have oval bodies and long, vertically flattened tails with fins. The free living larvae are normally fully aquatic but the tadpoles of some species such as (Nannophrys ceylonensis) are semi-terrestrial and live among wet rocks. Tadpoles have cartilaginous skeletons, gills for respiration (external gills at first, internal gills later), lateral line systems and large tails which they use for swimming. Newly hatched tadpoles soon develop gill pouches which cover the gills and their lungs develop early and are used as accessory breathing organs, the tadpoles rising to the water surface to gulp air. Some species complete their development inside the egg and hatch directly into small frogs. These larvae do not have gills but instead have specialised areas of skin through which respiration takes place. Tadpoles do not have true teeth but, in most species, the jaws have long, parallel rows of small keratinized structures called keradonts surrounded by a horny beak. Front legs are formed under the gill sac and hind legs become visible a few days later. Tadpoles are typically herbivorous, feeding mostly on algae, including diatoms filtered from the water through the gills. They are also detrivores, stirring up the sediment at the pond bottom and ingesting edible fragments. They have a relatively long, spiral-shaped gut to enable them to digest this diet. Some species are carnivorous at the tadpole stage, eating insects, smaller tadpoles and fish. Young of the Cuban tree frog (Osteopilus septentrionalis) can occasionally be cannibalistic, the younger tadpoles attacking a larger, more developed tadpole when it is undergoing metamorphosis.

At metamorphosis, rapid changes in the body take place 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.

Salamanders

At hatching, a typical salamander larva has eyes without lids, teeth in both upper and lower jaws, three pairs of feathery external gills and a long, somewhat laterally flattened body and tail with dorsal and ventral fins. The forelimbs may be partially developed and the hind limbs are rudimentary in pond-living species but may be rather more developed in species that reproduce in moving water. Pond-type larvae often have a pair of balancers, rod-like structures on either side of the head which perhaps prevent the gills from getting clogged up with bottom sediment. Some members of the genera Ambystoma and Dicamptodon have larvae that never fully develop into the adult form but this varies with species and with populations. The northwestern salamander (Ambystoma gracile) is one of these and, depending on environmental factors, either remains permanently in the larval state, a condition known as neoteny, or transforms into an adult. Neoteny occurs when the animal's growth rate is very low. This is usually because of adverse conditions such as low water temperatures which may inhibit the production of thyroxin or prevent the larva from reacting to the hormone in the normal way. Other factors that may inhibit metamorphosis include lack of food, lack of trace elements and competition from conspecifics. The tiger salamander (Ambystoma tigrinum) also sometimes behaves in this way and may grow particularly large in the process. The adult tiger salamander is terrestrial but the larva is aquatic and able to breed while still in the larval state. When conditions are particularly inhospitable on land, larval breeding may allow continuation of a population that would otherwise die out. There are fifteen species of obligate neotenic salamanders including Necturus, Proteus and Amphiuma, and many examples of facultative ones that adopt this strategy under appropriate environmental circumstances.

Lungless salamanders in the family Plethodontidae are terrestrial and lay a small number of unpigmented eggs in a cluster among damp leaf litter. Each egg has a large yolk sac and the larva feeds on this while it develops inside the egg, emerging fully formed as a juvenile salamander. The female salamander often broods the eggs and in the genus Ensatinas, she has been observed to coil around them and press her throat area against them, effectively massaging them with a mucous secretion.

In newts and salamanders, metamorphosis is less dramatic than in frogs. This is because the larvae are already carnivorous and continue to feed as predators when they are adults so few changes are needed to their digestive systems. Their lungs are functional early but the larvae don't make as much use of them as do tadpoles. Their gills are never covered by gill sacs and are reabsorbed just before the animals leave the water. Other changes include the reduction in size or loss of tail fins, the closure of gill slits, thickening of the skin, the development of eyelids, and certain changes in dentition and tongue structure. Salamanders are at their most vulnerable at metamorphosis as swimming speeds are reduced and transforming tails are encumbrances on land. Adult salamanders 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.

Caecilians

Most terrestrial caecilians that lay eggs do so in burrows or moist places on land near bodies of water. The development of the young of Ichthyophis glutinosus, a species from Sri Lanka, has been much studied. The eel-like larvae hatch out of the eggs and make their way to water. They have three pairs of external red feathery gills, a blunt head with two rudimentary eyes, a lateral line system and a short tail with fins. They swim by undulating their body from side to side. They are mostly active at night, soon lose their gills and make sorties onto land. Metamorphosis is gradual but by the age of about ten months they have developed a pointed head with sensory tentacles near the mouth and lost their eyes, lateral line systems and tails. The skin has becomes thicker, embedded scales have developed and the body has become divided into segments. By this time the caecilian has constructed a burrow and is living on land.

In the majority of species of caecilians, the young are produced by vivipary. Typhlonectes compressicauda, a species from South America, is typical of these. Up to nine larvae can develop in the oviduct at any one time. They are elongated and have paired sac-like gills, small eyes and specialised scraping teeth. At first they feed on the yolks of the eggs but as this source of nourishment declines they begin to rasp at the ciliated epithelial cells that line the oviduct. This stimulates the secretion of fluids rich in lipids and mucoproteins on which they feed along with scrapings from the oviduct wall. They may increase their length sixfold and be two fifths as long as their mother before being born. By this time they have undergone metamorphosis, lost their eyes and gills, developed a thicker skin and mouth tentacles and reabsorbed their teeth. A permanent set of teeth grow through soon after birth.

Parental care

The care of offspring among amphibians has been little studied but in general, the larger the number of eggs in a batch, the less likely it is that any degree of parental care takes place. Nevertheless, it is estimated that up to twenty percent of amphibian adults play some role in the care of the young. Those species that breed in smaller water bodies or other specialised habitats tend to have more complex patterns of behaviour in the care of the young than do others.

Many woodland salamanders lay clutches of eggs under dead logs or stones on land. The black mountain salamander does this, the mother brooding the eggs and guarding them from predation as the larvae feed on the yolks of their eggs. They eventually break their way out of the egg capsules and disperse. The male hellbender, a primitive salamander, excavates an underwater nest and encourages females to lay there. He then guards the site for the two to three months before the eggs hatch, undulating his body and fanning them with his skin folds to increase their supply of oxygen.

The tiny male frog Colostethus subpunctatus protects his egg cluster which is hidden under a stone or log. When the eggs hatch, he transports the tadpoles on his back, stuck there by a mucous secretion, to a temporary pool where he dips himself into the water and the tadpoles drop off. The male midwife toad (Alytes obstetricans) winds egg strings round his thighs and carries the eggs around for up to eight weeks. He keeps them moist and when they are ready to hatch, visits a pond or ditch and releases the tadpoles. The female gastric-brooding frog (Rheobatrachus sp.), believed to be extinct, swallows her eggs and rears her larvae in her stomach. She stops feeding while they are there and they consume their egg yolks. When they are ready to undergo metamorphosis, she regurgitates them and they hop away from her mouth. The pouched frog (Assa darlingtoni) lays eggs on the ground and, when they hatch, the male carries the tadpoles around in brood pouches on his hind legs. The aquatic Surinam toad (Pipa pipa) raises its young in pores on its back where they remain until metamorphosis. The granular poison frog (Oophaga granulifera) is typical of a number of tree frogs in the poison dart frog family Dendrobatidae. Its eggs are laid on the forest floor and when they hatch, the tadpoles are carried one by one on the back of an adult to a suitable water filled crevice such as the axil of a leaf or the rosette of a bromeliad. The female visits the nursery sites regularly and deposits one or more unfertilised eggs in the water which is consumed by the tadpole.

Feeding and diet

With a few exceptions, adult amphibians are predators, feeding on virtually anything that moves that they can swallow. The diet mostly consists of small items of prey that do not move too fast such as beetles, caterpillars, earthworms and spiders. The sirens (Siren spp.) often ingest aquatic plant material with the invertebrates on which they feed and a Brazilian tree frog Xenohyla truncata includes a large quantity of fruit in its diet. The Mexican burrowing toad (Rhinophrynus dorsalis) has a specially adapted tongue for picking up ants and termites. It projects it with the tip foremost whereas other frogs flick out the rear part first, their tongues being hinged at the front.

Food is mostly selected by sight, even in conditions of dim light. Movement of the prey triggers a feeding response. Frogs have been caught on fish hooks baited with red flannel and green frogs (Rana clamitans) have been found to have stomachs full of elm seeds that they had seen floating past. Toads, salamanders and caecilians also use smell to detect prey. This response is mostly secondary because salamanders have been observed to remain stationary near odoriferous prey but only feed if it moves. Cave dwelling amphibians normally hunt by smell and some salamanders seem to have learned to recognize immobile prey even when it has no smell.

Amphibians usually swallow food whole but may chew it lightly first in order to subdue it. They typically have small hinged pediceled teeth. These have bicuspid conical crowns attached by a pedicel to the jaw and are replaced at intervals. Salamanders, caecilians and some frogs have one or two rows of teeth in both jaws but some frogs (Rana spp.) lack teeth in the lower jaw and toads (Bufo spp.) have no teeth. In many amphibians there are also vomerine teeth attached to a facial bone in the roof of the mouth.

The tiger salamander (Ambystoma tigrinum) is typical of the frogs and salamanders that hide under cover ready to ambush unwary invertebrates. Others amphibians such as the Bufo spp. toads actively search for prey, while the Argentine horned frog (Ceratophrys ornata) lures inquisitive prey closer by raising its hind feet over its back and vibrating its yellow toes. Among leaf litter frogs in Panama, frogs that actively hunted prey had narrow mouths and were slim, often brightly coloured and toxic, while ambushers had wide mouths and were broad and well camouflaged. Caecilians do not flick their tongues but catch their prey by grabbing it with their slightly backward-pointing teeth. The struggles of the prey and further jaw movements work it inwards and the caecilian usually retreats into its burrow. The subdued prey is gulped down whole.

When they are newly hatched, frog larvae feed on the yolk of the egg. When this is exhausted some move on to feed on bacteria, algal crusts, detritus and raspings from submerged plants. Water is drawn in through their mouths, which are usually at the bottom of their heads, and passes through branchial food traps between their mouths and their gills where fine particles are trapped in mucus and filtered out. Others have specialised mouthparts consisting of a horny beak edged by several rows of labial teeth. They scrape and bite food of many kinds as well as stirring up the bottom sediment, filtering out larger particles with the papillae round their mouths. Some, such as the spadefoot toads, have strong biting jaws and are carnivorous or even cannibalistic.

Vocalization

The calls made by caecilians and salamanders are limited to occasional soft squeaks, grunts or hisses and have not been much studied. A clicking sound sometimes produced by caecilians may be a means of orientation, as in bats, or a form of communication. Most salamanders are considered voiceless but the California giant salamander (Dicamptodon ensatus) has vocal cords and can produce a rattling or barking sound. Some species of salamander emit a quiet squeak or yelp if attacked.

Frogs are much more vocal, especially during the breeding season when they use their voices 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 most species, the sound is produced by expelling air from the lungs over the vocal cords into an air sac or sacs in the throat or at the corner of the mouth. This may distend like a balloon and acts as a resonator, helping to transfer the sound to the atmosphere, or the water at times when the animal is submerged. The main vocalisation is the male's loud advertisement call which seeks to both encourage a female to approach and discourage other males from intruding on its territory. This call is modified to a quieter courtship call on the approach of a female or to a more aggressive version if a male intruder draws near. Calling bears the risk of attracting predators and involves the expendirure of much energy. Other calls include those given by a female in response to the advertisement call and a release call given by a male or female during unwanted attempts at amplexus. When a frog is attacked, a distress or fright call is emitted, often resembling a scream. The usually nocturnal Cuban tree frog (Osteopilus septentrionalis) produces a rain call when there is rainfall during daylight hours.

Territorial behaviour

Little is known of the territorial behaviour of caecilians but some frogs and salamanders defend home ranges. These are usually feeding, breeding or sheltering sites and it is the males that normally exhibit such behaviour though in some species, females and even juveniles are also involved. Although in many frog species, females are larger than males, this is not the case in most species where males are actively involved in territorial defence. Some of these have specific adaptations such as enlarged teeth for biting or spines on the chest, arms or thumbs.

In salamanders, defence of a territory involves adopting an aggressive posture and if necessary attacking the intruder. This may involve snapping, chasing and sometimes biting, occasionally causing the loss of a tail. The behaviour of red-backed salamanders (Plethodon cinereus) has been much studied. It was found that 91% of marked individuals which were later recaptured were within a metre (yard) of their original location (foraging behaviour would have given a different result at night). The salamanders left odour marks which deterred the intrusion of others and delineated the boundaries of their territory. A lot of the behaviour seemed stereotyped and did not involve any actual contact between individuals. An aggressive posture involved raising the body off the ground and glaring at the opponent who often turned away submissively. If the intruder persisted, a biting lunge was usually launched at either the tail region or the naso-labial grooves. Damage to either of these areas can reduce the fitness of the rival, either because of the need to regenerate tissue or because it impairs their ability to detect food.

In frogs, male territorial behaviour is often observed at breeding locations and calling is both an announcement of ownership of part of this resource and an advertisement call to potential mates. In general, a deeper voice represents a heavier and more powerful individual, and this may be sufficient to prevent intrusion by smaller males. Much energy is used in the vocalization and it takes a toll on the territory holder who may be displaced by a fitter individual if he loses too much condition. There is a tendency for males to tolerate the holders of neighbouring territories while vigorously attacking unknown intruders. Holders of territories have a “home” advantage and usually come off better in an encounter between two similar sized frogs. If threats are insufficient, chest to chest tussles may take place. Other fighting methods include pushing and shoving, deflating the opponent’s vocal sac, jumping on his back, biting, chasing, splashing and ducking him and seizing him by the head.

Defence mechanisms

Rough-skinned newt (Taricha granulosa)

Common reed frog (Hyperolius viridiflavus)

Amphibians have soft bodies and thin skins, lack claws, defensive armour or spines and seem relatively helpless. Nevertheless they have evolved various defence mechanisms to keep themselves alive. The first line of defence in salamanders and frogs is the mucous secretions that they produce to keep their skin moist and which make them slippery and difficult to grip. The secretion is often sticky and distasteful or toxic. Snakes have been observed yawning and gaping when trying to swallow an African clawed frog (Xenopus spp.) which gives the frog an opportunity to escape. Caecilians have been little studied in this respect but the aquatic (Typhlonectes compressicauda) produces toxic mucous which has caused the death of predatory fish in a feeding experiment in Brazil. In salamanders, the skin is toxic and in frogs, the main poison glands are at the side of the neck and under the warts on the back and these regions are presented to the attacking animal. The skin of the rough-skinned newt (Taricha granulosa) from North America contains the neurotoxin tetrodotoxin (TTX), the most toxic non-protein substance known and almost identical to that produced by pufferfish. Handling the newts does not cause harm but ingestion of even the most minute amounts of the skin is deadly. In feeding trials, fish, frogs, reptiles, birds and mammals were all found to be susceptible. A dose of 0.0002cc of skin was sufficient to kill a mouse in ten minutes. The only predators with some tolerance to the poison are certain populations of common garter snake (Thamnophis sirtalis). In locations where both snake and salamander co-exist, these seems to have developed some immunity through genetic changes and the snakes feed on the newt with impunity. Coevolution occurs with the newt increasing its toxic capabilities at the same rate as the snake further develops its immunity. A number of different animal groups have independently evolved to use TTX as a defensive agent. They may obtain it through the food chain from the bacterium Vibrio alginolyticus which is known to synthesize the substance. The link is not firmly established and there remains doubt as to whether the bacteria are truly the source of TTX in animals.

Poisonous species often use bright colouring to warn potential predators of their toxicity. These warning colours tend to be red or yellow combined with black. The fire salamander (Salamandra salamandra) being an example. Once a predator has sampled one of these, it is likely to remember the colouration next time it encounters a similar animal. In some species, such as the fire-bellied toad (Bombina spp.), the warning colouration is on the belly and these animals adopt a defensive pose when attacked, exhibiting their bright colours to the predator. The frog Allobates zaparo is not poisonous but mimics the appearance of other toxic species in its locality, a strategy that may deceive predators.

Many amphibians are nocturnal and hide during the day, thereby avoiding diurnal predators that hunt by sight. Other amphibians use camouflage to avoid being detected. They have various colourings such as mottled browns, greys and olives to make themselves inconspicuous so that they blend into the background. Some salamanders adopt defensive poses when faced by a potential predator such as the North American northern short-tailed shrew (Blarina brevicauda). Their bodies writhe and they raise and lash their tails which makes it difficult for the predator to avoid contact with their granular glands. A few salamanders will autotomise their tails when attacked, sacrificing this part of their anatomy to enable them to escape with their lives. There may have a constricted tail base to allow this to happen easily. The tail is regenerated later but the cost to the animal of replacing it is significant.

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. Giant Salamanders of the genus Andrias, as well as Ceratophrine and Pyxicephalus frogs possess sharp teeth and are capable of drawing blood with a defensive bite. the Blackbelly Salamander bites an attacking common garter snake (Thamnophis sirtalis) two or three times its size on the head and often manages to escape.

Conservation

Main article: Decline in amphibian populations

The golden toad of Monteverde, Costa Rica, was among the first casualties of amphibian declines. Formerly abundant, it was last seen in 1989.

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.

In many terrestrial ecosystems, amphibians constitute one of the largest parts of the vertebrate biomass. Any decline in amphibian numbers will have an impact on the patterns of predation. The loss of carnivorous species near the top of the food chain will upset the delicate ecosystem balance and may cause dramatic increases in opportunistic species. In the Middle East, a growing appetite for eating frog legs and the consequent gathering of them for food was linked to an increase in mosquitoes. Predators that feed on amphibians are affected by their decline. The western terrestrial garter snake (Thamnophis elegans) in California is largely aquatic and depends heavily on two species of frog that are diminishing in numbers, the Yosemite toad (Bufo canorus) and the mountain yellow-legged frog (Rana muscosa), putting the snake's future at risk. If it were to become scarce, this would affect birds of prey and other predators that feed on it. Meanwhile, in the ponds and lakes, fewer frogs means fewer tadpoles. These normally play an important role in controlling the growth of algae and also forage on detritus that accumulates as sediment on the bottom. A reduction in the number of tadpoles or their absence might lead to algal overgrowth and possibly eutrophication, resulting in the water becoming depleted of oxygen when the algae die and decompose. Aquatic invertebrates and fish might then die and there would be unpredictable ecological consequences.

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 frog, the limbless Sagalla caecilian from Kenya, South African ghost frogs, lungless salamanders from Mexoco, the Malagasy rainbow frog, Chile's Darwin's frog (Rhinoderma rufum) and the Betic midwife toad from Spain.

See also

• Vegetal rotation
• List of amphibians

Further reading

• Carroll, Robert L. (1988). Vertebrate Paleontology and Evolution. New York: W. H. Freeman & Co.
• Carroll, Robert L. (2009). The Rise of Amphibians: 365 Million Years of Evolution. Baltimore: The Johns Hopkins University Press. ISBN 978-0-8018-9140-3.
• Duellman, William E. (1994). Biology of Amphibians. Johns Hopkins University Press. ISBN 978-0-8018-4780-6. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
• Frost, Darrel R. (2006). "The Amphibian Tree of Life". Bulletin of the American Museum of Natural History. 297: 1–291. doi:10.1206/0003-0090(2006)297[0001:TATOL]2.0.CO;2. hdl:2246/5781. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
• Pounds, J. Alan (2006). "Widespread amphibian extinctions from epidemic disease driven by global warming". Nature. 439 (7073): 161–167. doi:10.1038/nature04246. PMID 16407945. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
• Stuart, Simon N. (2004). "Status and trends of amphibian declines and extinctions worldwide". Science. 306 (5702): 1783–1786. doi:10.1126/science.1103538. PMID 15486254. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
• S.N.Stuart, M.Hoffmann, J.S.Chanson, N.A.Cox, R.J.Berridge, P.Ramani, B.E. Young (editors), Collective work (2008). Threatened Amphibians of the World. Published by Lynx Edicions, in association with IUCN-The World Conservation Union, Conservation International and NatureServe. ISBN 978-84-96553-41-5. {{cite book}}: |last= has generic name (help); Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link) 776 pages

External links

• Amphibian Specialist Group
• Amphibian Ark
• AmphibiaWeb
• Global Amphibian Assessment
• Amphibian vocalisations on Archival Sound Recordings