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Frog

2007 Schools Wikipedia Selection. Related subjects: Insects, Reptiles and
Fish

   iFrogs

                       Fossil range: Triassic - Recent

   White's Tree Frog (Litoria caerulea)
   White's Tree Frog (Litoria caerulea)
               Scientific classification

   Kingdom: Animalia
   Phylum:  Chordata
   Class:   Amphibia
   Order:   Anura
            Merrem, 1820
   Distribution of frogs (in black)
   Distribution of frogs (in black)

                                  Suborders

   Archaeobatrachia
   Mesobatrachia
   Neobatrachia
   -
   List of Anuran families

   The frog is an amphibian in the order Anura (meaning "tail-less" from
   Greek an-, without + oura, tail). Adult frogs are characterised by long
   hind legs, a short body, webbed digits, protruding eyes and the absence
   of a tail. Most frogs have a semi-aquatic lifestyle, but move easily on
   land by jumping or climbing. They typically lay their eggs in puddles,
   ponds or lakes, and their larvae, called tadpoles, have gills and
   develop in water. Adult frogs follow a carnivorous diet, mostly of
   arthropods, annelids and gastropods. Frogs are most noticeable through
   their call, which can be widely heard during the mating season.

   The distribution of frogs ranges from tropic to subarctic regions, with
   most of the species found in tropical rainforests. With over 5,000
   species described, they are among the most diverse groups of
   vertebrates. However the declining numbers of certain species of frogs
   are increasingly giving cause for concern.

   A distinction is often made between frogs and toads on the basis of
   their appearance, prompted by the convergent adaptation among so-called
   toads to dry environments; however, this distinction has no taxonomic
   basis. The only family exclusively given the common name "toad" is
   Bufonidae, but many species from other families are also called
   "toads," and the species within the toad genus Atelopus are referred to
   as "harlequin frogs."

Taxonomy

   European Fire-bellied Toad (Bombina bombina)
   Enlarge
   European Fire-bellied Toad (Bombina bombina)

   The order Anura contains 5,250 species in 33 families, whereof the
   Leptodactylidae (1100 spp.), Hylidae (800 spp.) and Ranidae (750 spp.)
   are the richest in species. About 88% of amphibian species are frogs.

   The use of the common names "frog" and "toad" has no taxonomic
   justification. From a taxonomic perspective, all members of the order
   Anura are frogs, but only members of the family Bufonidae are
   considered "true toads". The use of the term "frog" in common names
   usually refers to species that are aquatic or semi-aquatic with smooth
   or moist skins, and the term "toad" generally refers to species that
   tend to be terrestrial with dry, warty skin. An exception is the
   Fire-bellied toad (Bombina bombina): while its skin is slightly warty,
   it prefers a watery habitat.

   Frogs and toads are broadly classified into three suborders:
   Archaeobatrachia, which includes four families of primitive frogs;
   Mesobatrachia, which includes five families of more evolutionary
   intermediate frogs; and Neobatrachia, by far the largest group, which
   contains the remaining 24 families of "modern" frogs, including most
   common species throughout the world. The Neobatrachia is further
   divided into the Hyloidea and Ranoidea. This classification is based on
   such morphological features as the number of vertebrae, the structure
   of the pectoral girdle, and the morphology of tadpoles. While this
   classification is largely accepted, relationships among families of
   frogs are still debated. Future studies of molecular genetics should
   soon provide further insights to the evolutionary relationships among
   frog families.

   Some species of anurans hybridise readily. For instance, the Edible
   Frog (Rana esculenta) is a hybrid of the Pool Frog (R. lessonae) and
   the Marsh Frog (R. ridibunda). Bombina bombina and Bombina variegata
   similarly form hybrids, although these are less fertile, giving rise to
   a hybrid zone.

Morphology and physiology

   Skeleton of Rana
   Enlarge
   Skeleton of Rana

   The morphology of frogs is unique among amphibians. Compared with the
   other two groups of amphibians, ( salamanders and caecilians), frogs
   are unusual because they lack tails as adults and their legs are more
   suited to jumping than walking. The physiology of frogs is generally
   like that of other amphibians (and differs from other terrestrial
   vertebrates) because oxygen may pass through their highly permeable
   skin. This unique feature allows frogs to "breathe" largely through
   their skin. Because the oxygen is dissolved in an aqueous film on the
   skin and passes from there to the blood, the skin must remain moist at
   all times; this makes frogs susceptible to many toxins in the
   environment, some of which can similarly dissolve in the layer of water
   and be passed into their bloodstream. This may be a cause of the
   decline in frog populations.

   Many characteristics are not shared by all of the approximately 5,250
   described species of frogs. However, some general characteristics
   distinguish them from other amphibians. Frogs are usually well suited
   to jumping, with long hind legs with elongated ankle bones. They have a
   short vertebral column, with no more than ten free vertebrae, followed
   by a fused tailbone (urostyle or coccyx), typically resulting in a
   tailless phenotype.

   Frogs range in size from 10 mm ( Brachycepahlus didactylus of Brazil
   and Eleutherodactylus iberia of Cuba) to 300 mm ( Goliath frog, Conraua
   goliath, of Cameroon). The skin hangs loosely on the body because of
   the lack of loose connective tissue. Skin texture varies: it can be
   smooth, warty or folded. Frogs have three eyelid membranes: one is
   transparent to protect the eyes underwater, and two vary from
   translucent to opaque. Frogs have a tympanum on each side of the head,
   which is involved in hearing and, in some species, is covered by skin.

Feet and legs

   Tyler's Tree Frog (Litoria tyleri) illustrates large toe pads and
   webbed feet.
   Enlarge
   Tyler's Tree Frog (Litoria tyleri) illustrates large toe pads and
   webbed feet.

   The structure of the feet and legs varies greatly among frog species,
   depending in part on whether they live primarily on the ground, in
   water or in trees. Frogs must be able to move quickly through their
   environment to catch prey and escape predators, and numerous
   adaptations help them do so.

   Many frogs, especially those that live in water, have webbed toes. The
   degree to which the toes are webbed is directly proportional to the
   amount of time the species lives in the water. For example, the
   completely aquatic African dwarf frog (Hymenochirus sp.) has fully
   webbed toes, whereas the toes of White's Tree Frog, an arboreal
   species, are only a half or a quarter webbed.

   Arboreal frogs have "toe pads" to help grip vertical surfaces. These
   pads, located on the ends of the toes, do not work by suction. Rather,
   the surface of the pad consists of interlocking cells, with a small gap
   between adjacent cells. When the frog applies pressure to the toe pads,
   the interlocking cells grip irregularities on the substrate. The small
   gaps between the cells drain away all but a thin layer of moisture on
   the pad, and maintain a grip through capillarity. This allows the frog
   to grip smooth surfaces, and does not function when the pads are
   excessively wet.

   In many arboreal frogs, a small "intercalary structure" in each toe
   increases the surface area touching the substrate. Furthermore, since
   hopping through trees can be dangerous, many arboreal frogs have hip
   joints that allow both hopping and walking. Some frogs that live high
   in trees even possess an elaborate degree of webbing between their
   toes, as do aquatic frogs. But in these arboreal frogs, the webs allow
   the frogs to "parachute" or control their glide from one position in
   the canopy to another.

   Ground-dwelling frogs generally lack the adaptations of aquatic and
   arboreal frogs. Most have smaller toe pads, if any, and little webbing.
   Some burrowing frogs have a toe extension—a metatarsal tubercle—that
   helps them to burrow. The hind legs of ground dwellers are more
   muscular than those of aqueous and tree-dwelling frogs.

Skin

   Common Eastern Froglet (Crinia signifera) camouflaged against leaf
   litter.
   Enlarge
   Common Eastern Froglet (Crinia signifera) camouflaged against leaf
   litter.

   Many frogs are able to absorb water directly through the skin,
   especially around the pelvic area. However, the permeability of a
   frog's skin can also result in water loss. Some tree frogs reduce water
   loss with a waterproof layer of skin. Others have adapted behaviours
   that conserve water, including engaging in nocturnal activity and
   resting in a water-conserving position. This position involves the frog
   lying with its toes and fingers tucked under its body and chin,
   respectively, with no gap between the body and substrate. Some frog
   species will also rest in large groups, touching the skin of the
   neighbouring frog. This reduces the amount of skin exposed to the air
   or a dry surface, and thus reduces water loss. These adaptations only
   reduce water loss enough for a predominately arboreal existence, and
   are not suitable for arid conditions.

   Camouflage is a common defensive mechanism in frogs. Most camouflaged
   frogs are nocturnal, which adds to their ability to hide. Nocturnal
   frogs usually find the ideal camouflaged position during the day to
   sleep. Some frogs have the ability to change colour. However, this is
   usually restricted to shades of one or two colours. For example,
   White's tree frog (Litoria caerulea) varies in shades of green and
   brown. Features such as warts and skin folds are usually found on
   ground-dwelling frogs, where a smooth skin would not disguise them
   effectively. Arboreal frogs usually have smooth skin, enabling them to
   disguise themselves as leaves.

   Certain frogs change colour between night and day, as light and
   moisture stimulate the pigment cells and cause them to expand or
   contract.

Poison

   Many frogs contain mild toxins that make them distasteful to potential
   predators. For example, all toads have large poison glands—the parotid
   glands—located behind the eyes on the top of the head. Some frogs, such
   as some poison dart frogs, are especially toxic. The chemical makeup of
   toxins in frogs varies from irritants to hallucinogens, convulsants,
   nerve poisons, and vasoconstrictors (which narrow the blood vessels).
   Many predators of frogs have adapted to tolerate high levels of these
   poisons. Others, including humans, may be severely affected.

   Some frogs obtain poisons from the ants and other arthropods they eat;
   others, such as the Australian Corroboree Frogs (Pseudophryne
   corroboree and Pseudophryne pengilleyi), can manufacture an alkaloid
   not derived from their diet. Some native people of South America
   extract poison from the poison dart frogs and apply it to their darts
   for hunting, although few species are toxic enough to be used for this
   purpose. It was previously a misconception that the poison was placed
   on arrows rather than darts. The common name of these frogs was thus
   changed from "Poison Arrow Frog" to "Poison Dart Frog" in the early
   1980s. Poisonous frogs tend to advertise their toxicity with bright
   colours, an adaptive strategy known as aposematism. There are at least
   two non-poisonous species of frogs in tropical America
   (Eleutherodactylus gaigei and Lithodytes lineatus) that mimic the
   colouration of dart poison frogs' coloration for self-protection (
   Batesian mimicry).

   Because frog toxins are extraordinarily diverse, they have raised the
   interest of biochemists as a "natural pharmacy". The alkaloid
   epibatidine, a painkiller 200 times more potent than morphine, is found
   in some species of poison dart frogs. Other chemicals isolated from the
   skin of frogs may offer resistance to HIV infection. Arrow and dart
   poisons are under active investigation for their potential as
   therapeutic drugs.

   The skin secretions of some toads, such as the Colorado River Toad and
   Cane Toad, contain bufotoxins, some of which, such as bufotenin, are
   psychoactive, and have therefore been used as recreational drugs.
   Typically, the skin secretions are dried and smoked. Skin licking is
   especially dangerous, and appears to constitute an urban myth. See
   psychoactive toad.

Respiration and circulation

   The skin of a frog is permeable to oxygen and carbon dioxide, as well
   as to water. There are a number of blood vessels near the surface of
   the skin. When a frog is underwater, oxygen is transmitted through the
   skin directly into the bloodstream. On land, adult frogs use their
   lungs to breathe. Their lungs are similar to those of humans, but the
   chest muscles are not involved in respiration, and there are no ribs or
   diaphragm to support breathing. Frogs breathe by taking air in through
   the nostrils (causing the throat to puff out), and compressing the
   floor of the mouth, which forces the air into the lungs.

   Frogs are known for their three-chambered heart, which they share with
   all tetrapods except birds and mammals. In the three-chambered heart,
   oxygenated blood from the lungs and de-oxygenated blood from the
   respiring tissues enter by separate atria, and are directed via a
   spiral valve to the appropriate vessel— aorta for oxygenated blood and
   pulmonary vein for deoxygenated blood. This special structure is
   essential to keeping the mixing of the two types of blood to a minimum,
   which enables frogs to have higher metabolic rates, and be more active
   than otherwise.

Natural history

   The life cycle of frogs, like that of other amphibians, consists of
   four main stages: egg, tadpole, metamorphosis and adult. The reliance
   of frogs on an aquatic environment for the egg and tadpole stages gives
   rise to a variety of breeding behaviours that include the well-known
   mating calls used by the males of most species to attract females to
   the bodies of water that they have chosen for breeding. Some frogs also
   look after their eggs—and in some cases even the tadpoles—for some time
   after laying.

From eggs to adults

   Frogspawn
   Enlarge
   Frogspawn
   Froglet
   Enlarge
   Froglet
   Adult leopard frog
   Enlarge
   Adult leopard frog

   The life cycle of a frog starts with an egg. Eggs are generally laid in
   water, and an individual female may lay egg masses containing thousands
   of eggs, known as frogspawn. The eggs are highly vulnerable to
   predation, so frogs have evolved many techniques to ensure the survival
   of the next generation. Most commonly, this involves synchronous
   reproduction. Many individuals will breed at the same time,
   overwhelming the actions of predators; the majority of the offspring
   will still die due to predation, but there is a greater chance that
   some will survive. Another way in which some species avoid the
   predators and pathogens eggs are exposed to in ponds is to lay eggs on
   leaves above the pond, with a gelatinous coating designed to retain
   moisture. In these species the tadpoles drop into the water upon
   hatching. The eggs of some species laid out of water can detect
   vibrations of nearby predatory wasps or snakes, and will hatch early to
   avoid being eaten. Some species, such as the Cane Toad (Bufo marinus),
   lay poisonous eggs to minimise predation. While the length of the egg
   stage depends on the species and environmental conditions, aquatic eggs
   generally hatch within one week.

   Eggs hatch and continue life as tadpoles (occasionally known as
   polliwogs). Tadpoles are aquatic, lack front and hind legs, and have
   gills for respiration and tails with fins for swimming. Tadpoles are
   typically herbivorous, feeding mostly on algae, including diatoms that
   are filtered from the water through the gills. Some species are
   carnivorous at the tadpole stage, eating insects, smaller tadpoles and
   fish. Tadpoles are highly vulnerable to predation by fish, newts,
   predatory diving beetles and birds such as kingfishers. Cannibalism has
   been observed among tadpoles. Poisonous tadpoles are present in many
   species, such as Cane Toads. The tadpole stage may be as short as a
   week, or tadpoles may overwinter and metamorphose the following year in
   some species, such as the Midwife toad (Alytes obstetricans) and the
   Common Spadefoot (Pelobates fuscus).

   At the end of the tadpole stage, frogs undergo metamorphosis, in which
   they transition into adult form. Metamorphosis involves a dramatic
   transformation of morphology and physiology, as tadpoles develop hind
   legs, then front legs, lose their gills and develop lungs. Their
   intestines shorten as they shift from an herbivorous to a carnivorous
   diet. Eyes migrate rostrally and dorsally, allowing for binocular
   vision exhibited by the adult frog. This shift in eye position mirrors
   the shift from prey to predator, as the tadpole develops and depends
   less upon a larger and wider field of vision and more upon depth
   perception. The final stage of development from froglet to adult frog
   involves apoptosis (programmed cell death) and resorption of the tail.

   After metamorphosis, young adults may leave the water and disperse into
   terrestrial habitats, or continue to live in the aquatic habitat as
   adults. Almost all species of frogs are carnivorous as adults, eating
   invertebrates such as arthropods, annelids and gastropods. A few of the
   larger species may eat prey such as small mammals, fish and smaller
   frogs. Some frogs use their sticky tongues to catch fast-moving prey,
   while others capture their prey and force it into their mouths with
   their hands. However, there are a very few species of frogs that
   primarily eat plants. Adult frogs are themselves preyed upon by birds,
   large fish, snakes, otters, foxes, badgers, coatis, and other animals.
   Frogs are also eaten by people (see section on agriculture, below).

Reproduction

   Once adult frogs reach maturity, they will assemble at a water source
   such as a pond or stream to breed. Many frogs return to the bodies of
   water where they were born, often resulting in annual migrations
   involving thousands of frogs. In continental Europe, a large proportion
   of migrating frogs used to die on roads, before special fences and
   tunnels were built for them.
   Male and female Common toad (Bufo bufo) in amplexus
   Enlarge
   Male and female Common toad (Bufo bufo) in amplexus

   Once at the breeding ground, male frogs call to attract a mate,
   collectively becoming a chorus of frogs. The call is unique to the
   species, and will attract females of that species. Some species have
   satellite males who do not call, but intercept females that are
   approaching a calling male.

   The male and female frogs then undergo amplexus. This involves the male
   mounting the female and gripping her tightly. Fertilization is
   external: the egg and sperm meet outside of the body. The female
   releases her eggs, which the male frog covers with a sperm solution.
   The eggs then swell and develop a protective coating. The eggs are
   typically brown or black, with a clear, gelatin-like covering.

   Most temperate species of frogs reproduce between late autumn and early
   spring. In the UK, most common frog populations produce frogspawn in
   February, although there is wide variation in timing. Water
   temperatures at this time of year are relatively low, typically between
   four and 10 degrees Celsius. Reproducing in these conditions helps the
   developing tadpoles because dissolved oxygen concentrations in the
   water are highest at cold temperatures. More importantly, reproducing
   early in the season ensures that appropriate food is available to the
   developing frogs at the right time.

Parental care

   Colour plate from Ernst Haeckel's 1904 Kunstformen der Natur, depicting
   frog species that include two examples of parental care.
   Enlarge
   Colour plate from Ernst Haeckel's 1904 Kunstformen der Natur, depicting
   frog species that include two examples of parental care.

   Although care of offspring is poorly understood in frogs, it is
   estimated that up to 20% of amphibian species may care for their young
   in one way or another, and there is a great diversity of parental
   behaviours. Some species of poison dart frog lay eggs on the forest
   floor and protect them, guarding the eggs from predation and keeping
   them moist. The frog will urinate on them if they become too dry. After
   hatching, a parent (the gender depends upon the species) will move
   them, on its back, to a water-holding bromeliad. The parent then feeds
   them by laying unfertilized eggs in the bromeliad until the young have
   metamorphosed. Other frogs carry the eggs and tadpoles on their hind
   legs or back (e.g. the midwife toads, Alytes spp.). Some frogs even
   protect their offspring inside their own bodies. The male Australian
   Pouched Frog (Assa darlingtoni) has pouches along its side in which the
   tadpoles reside until metamorphosis. The female Gastric-brooding Frogs
   (genus Rheobatrachus) from Australia, now probably extinct, swallows
   its tadpoles, which then develop in the stomach. To do this, the
   Gastric-brooding Frog must stop secreting stomach acid and suppress
   peristalsis (contractions of the stomach). Darwin's Frog (Rhinoderma
   darwinii) from Chile puts the tadpoles in its vocal sac for
   development.

Call

   The call of a frog is unique to its species. Frogs call by passing air
   through the larynx in the throat. In most calling frogs, the sound is
   amplified by one or more vocal sacs, membranes of skin under the throat
   or on the corner of the mouth that distend during the amplification of
   the call.

   Some frogs lack vocal sacs, such as those from the genera Heleioporus
   and Neobatrachus, but these species can still produce a loud call.
   Their buccal cavity is enlarged and dome-shaped, acting as a resonance
   chamber that amplifies their call. Species of frog without vocal sacs
   and that do not have a loud call tend to inhabit areas close to flowing
   water. The noise of flowing water overpowers any call, so they must
   communicate by other means.

   The main reason for calling is to allow males to attract a mate. Males
   call either individually or in a group called a chorus. Females of many
   frog species, for example Polypedates leucomystax, produce calls
   reciprocal to the males', which act as the catalyst for the enhancement
   of reproductive activity in a breeding colony. A male frog emits a
   release call when mounted by another male. Tropical species also have a
   rain call that they make on the basis of humidity cues prior to a rain
   shower. Many species also have a territorial call that is used to chase
   away other males. All of these calls are emitted with the mouth of the
   frog closed.

   A distress call, emitted by some frogs when they are in danger, is
   produced with the mouth open, resulting in a higher-pitched call. The
   effectiveness of the call is unknown; however, it is suspected that the
   call intrigues the predator until another animal is attracted,
   distracting them enough for its escape.

   Many species of frog have deep calls, or croaks. The onomatopoeic
   spelling is often "crrrrk" in Britain and "ribbit" in the US. This
   difference is due to the different species within each region (e.g.,
   Common frog (Rana temporaria) in Britain and Leopard frog (Rana
   pipiens) in the US). The croak of the American bullfrog (Rana
   catesbiana) is sometimes spelt "jug o' rum".

Distribution and conservation status

   Golden toad (Ollotis periglenes) - last seen in 1989
   Enlarge
   Golden toad (Ollotis periglenes) - last seen in 1989

   Frogs are found nearly worldwide, but they do not occur in Antarctica
   and are not present on many oceanic islands. The greatest diversity of
   frogs occurs in the tropical areas of the world. This is because water
   is readily available, which suits frogs' requirements due to their
   skin. Some frogs inhabit arid areas such as deserts, where water may
   not be easily accessible, and rely on specific adaptations to survive.
   The Australian genus Cyclorana and the American genus Pternohyla will
   bury themselves underground, create a water-impervious cocoon and
   hibernate during dry periods. Once it rains, they emerge, find a
   temporary pond and breed. Egg and tadpole development is very fast in
   comparison to most other frogs so that breeding is complete before the
   pond dries up. Some frog species are adapted to cold, like the Wood
   Frog which lives in the Arctic Circle, the species buries itself in the
   ground during winter and much of its body freezes.

   Frog populations have declined dramatically since the 1950s, with more
   than one third of species believed to be threatened with extinction and
   more than 120 species suspected to have become extinct since the 1980s.
   Among these species are the Golden toad of Costa Rica, and the
   Gastric-brooding Frogs of Australia. Habitat loss is a significant
   cause of frog population decline, as are pollutants, climate change,
   the introduction of non-indigenous predators/competitors, and emerging
   infectious diseases including chytridiomycosis. Many environmental
   scientists feel that amphibians, including frogs, are excellent
   biological indicators of broader ecosystem health because of their
   intermediate position in food webs, permeable skins, and typically
   biphasic life (aquatic larvae and terrestrial adults).

   A Canadian study conducted in 2006 proposed that heavy traffic near
   frog habitats is a large threat to frog populations.

Evolution

   A fossilized frog from the Czech Republic, possibly Palaeobatrachus
   gigas.
   Enlarge
   A fossilized frog from the Czech Republic, possibly Palaeobatrachus
   gigas.

   The earliest known (proto) frog is Triadobatrachus massinoti, from the
   early Triassic of Madagascar. It is about 250 million years old, and
   had not yet evolved the full combination of features currently
   associated with frogs. The skull is frog-like, being broad with large
   eye sockets, but the fossil has a number of other features diverging
   from modern amphibia. These include a different ilium, a longer body
   with more vertebrae, and separate vertebrae in its tail (whereas in
   modern frogs, the tail vertebrae are fused, and known as the urostyle
   or coccyx). The tibia and fibula bones are unfused and separate, making
   it probable that Triadobatrachus was not an efficient leaper.

   Another fossil frog, discovered in Arizona and called Prosalirus bitis,
   was uncovered in 1985, and dates from roughly the same time as
   Triadobatrachus. Like Triadobatrachus, Prosalirus did not have greatly
   enlarged legs, but had the typical three-pronged pelvic structure.
   Unlike Triadobatrachus, Prosalirus had already lost nearly all of its
   tail.

   The earliest true frog is Vieraella herbsti, from the early Jurassic
   (188–213 Mya). It is known only from the dorsal and ventral impressions
   of a single animal and was estimated to be 33 mm in snout-vent length.
   Notobatrachus degiustoi from the middle Jurassic is slightly younger,
   about 155–170 million years old. It is likely that the evolution of
   modern Anura was completed by the Jurassic period. The main
   evolutionary changes involved the shortening of the body and the loss
   of the tail.

   The earliest full fossil record of a modern frog is that of
   sanyanlichan, which lived 125 million years ago and had all modern frog
   features, but bore 9 presacral vertebrae instead of the 8 of modern
   frogs, apparently still being a transitional species.

   Frog fossils have been found on all continents, including Antarctica.

   Frogs have also been subject to convergent evolution. In one case,
   frogs in totally different areas learned to eat certain arthropods
   containing certains poisons that the frogs incorporated into their own
   defenses. In another case, some frogs in South America have evolved
   dorsal brood pouches analogous to those of marsupials.

Uses in agriculture and research

   Frogs are raised commercially for several purposes. Frogs are used as a
   food source; frog legs are a delicacy in China, France, and in many
   parts of the American South, especially Louisiana. Dead frogs are
   sometimes used for dissections in high school and university anatomy
   classes, often after being injected with coloured plastics to enhance
   the contrast between the organs. This practice has declined in recent
   years with the increasing concerns about animal welfare.

   Frogs have served as important model organisms throughout the history
   of science. Eighteenth-century biologist Luigi Galvani discovered the
   link between electricity and the nervous system through studying frogs.
   The African clawed frog or platanna, Xenopus laevis, was first widely
   used in laboratories in pregnancy assays in the first half of the 20th
   century. When human chorionic gonadotropin, a hormone found in
   substantial quantities in the urine of pregnant women, is injected into
   a female X. laevis, it induces them to lay eggs. In 1952 Robert Briggs
   and Thomas J. King cloned a frog by somatic cell nuclear transfer, the
   same technique that was later used to create Dolly the Sheep, their
   experiment was the first time successful nuclear transplantation had
   been accomplished in metazoans.

   Frogs are used in cloning research and other branches of embryology
   because frogs are among the closest living relatives of man to lack egg
   shells characteristic of most other vertebrates, and therefore
   facilitate observations of early development. Although alternative
   pregnancy assays have been developed, biologists continue to use
   Xenopus as a model organism in developmental biology because it is easy
   to raise in captivity and has a large and easily manipulatable embryo.
   Recently, X. laevis is increasingly being displaced by its smaller
   relative X. tropicalis, which reaches its reproductive age in five
   months rather than one to two years (as in X. laevis), facilitating
   faster studies across generations. The genome sequence of X. tropicalis
   will probably be completed by 2015 at the latest.

Frogs in popular culture

   Frogs feature prominently in folklore, fairy tales and popular culture.
   They tend to be portrayed as benign, ugly, clumsy, but with hidden
   talents. Examples include Michigan J. Frog, The Frog Prince, and Kermit
   the Frog. Michigan J. Frog, featured in a Warner Brothers cartoon, only
   performs his singing and dancing routine for his owner. Once another
   person looks at him, he will return to a frog-like pose. "The Frog
   Prince" is a fairy tale of a frog who turns into a handsome prince once
   kissed. Kermit the Frog, on the other hand, is a conscientious and
   disciplined character of Sesame Street and The Muppet Show; while
   openly friendly and greatly talented, he is often portrayed as cringing
   at the fanciful behaviour of more flamboyant characters.
   Retrieved from " http://en.wikipedia.org/wiki/Frog"
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