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definitions - frog

frog (n.)

1.any of various tailless stout-bodied amphibians with long hind limbs for leaping; semiaquatic and terrestrial species

2.a decorative loop of braid or cord

3.a person of French descent

frog (v.)

1.hunt frogs for food

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Merriam Webster

FrogFrog (frŏg), n. [AS. froggu, frocga a frog (in sensel); akin to D. vorsch, OHG. frosk, G. frosch, Icel. froskr, fraukr, Sw. & Dan. frö.]
1. (Zoöl.) An amphibious animal of the genus Rana and related genera, of many species. Frogs swim rapidly, and take long leaps on land. Many of the species utter loud notes in the springtime.

☞ The edible frog of Europe (Rana esculenta) is extensively used as food; the American bullfrog (R. Catesbiana) is remarkable for its great size and loud voice.

2. [Perh. akin to E. fork, cf. frush frog of a horse.] (Anat.) The triangular prominence of the hoof, in the middle of the sole of the foot of the horse, and other animals; the fourchette.

3. (Railroads) A supporting plate having raised ribs that form continuations of the rails, to guide the wheels where one track branches from another or crosses it.

4. [Cf. fraco of wool or silk, L. floccus, E. frock.] An oblong cloak button, covered with netted thread, and fastening into a loop instead of a button hole.

5. The loop of the scabbard of a bayonet or sword.

Cross frog (Railroads), a frog adapted for tracks that cross at right angles. -- Frog cheese, a popular name for a large puffball. -- Frog eater, one who eats frogs; -- a term of contempt applied to a Frenchman by the vulgar class of English. -- Frog fly. (Zoöl.) See Frog hopper. -- Frog hopper (Zoöl.), a small, leaping, hemipterous insect living on plants. The larvæ are inclosed in a frothy liquid called cuckoo spit or frog spit. -- Frog lily (Bot.), the yellow water lily (Nuphar). -- Frog spit (Zoöl.), the frothy exudation of the frog hopper; -- called also frog spittle. See Cuckoo spit, under Cuckoo.

FrogFrog (?), v. t. To ornament or fasten (a coat, etc.) with trogs. See Frog, n., 4.

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Wikipedia

Frog

                   
Frogs
Temporal range: Early Triassic-Recent, 250–0 Ma
Pg
Australian Green Tree Frog (Litoria caerulea)
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Class: Amphibia
Subclass: Lissamphibia
Order: Anura
Merrem, 1820
Suborders

Archaeobatrachia
Mesobatrachia
Neobatrachia
-
List of Anuran families

Native distribution of frogs (in black)

Frogs are amphibians in the order Anura (meaning "tailless", from the Ancient Greek an-, without + oura, tail), formerly referred to as Salientia (Latin salere (salio), "to jump"). Most frogs are characterized by a short body, webbed digits (fingers or toes), protruding eyes, bifid tongue and the absence of a tail. Frogs are widely known as exceptional jumpers, and many of the anatomical characteristics of frogs, particularly their long, powerful legs, are adaptations to improve jumping performance. Due to their permeable skin, frogs are often semi-aquatic or inhabit humid areas, but move easily on land. They typically lay their eggs in puddles, ponds or lakes, and their larvae, called tadpoles, have gills and tails to develop in water. Adult frogs follow a carnivorous diet, mostly of arthropods, annelids and gastropods. Frogs are most noticeable by their call, which can be widely heard during the night or day, mainly in their mating season.

The distribution of frogs ranges from tropic to subarctic regions, but most species are found in tropical rainforests. Consisting of more than 5,000 species described, they are among the most diverse groups of vertebrates. However, populations of certain frog species are declining significantly.

A popular distinction is often made between frogs and toads on the basis of their appearance, but this has no taxonomic basis. (Members of the anuran family Bufonidae are called true toads, but many species from other families are also called toads.) In addition to their ecological importance, frogs have many cultural roles, such as in literature, symbolism and religion, and they are also valued as food and as pets.

Contents

Etymology and terminology

The name frog derives from Old English frogga, (compare Old Norse frauki, German Frosch, older Dutch spelling kikvorsch), cognate with Sanskrit plava (frog), probably deriving from Proto-Indo-European praw = "to jump".[1]

A distinction is often made between frogs and toads on the basis of their appearance, caused 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

The order Anura contains 4,810 species[2] in 33 families, of which the Leptodactylidae (1100 spp.), Hylidae (800 spp.) and Ranidae (750 spp.) are the richest in species. About 88% of amphibian species are frogs.

  European Fire-bellied Toad (Bombina bombina)
  Young American bullfrog found in a stream in New Jersey

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 and/or moist skins, and the term "toad" generally refers to species that tend to be terrestrial with dry, warty skin.[3] An exception is the fire-bellied toad (Bombina bombina): while its skin is slightly warty, it prefers a watery habitat.[4]

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. Neobatrachia is further divided into the Hyloidea and Ranoidea.[5] 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 anuran families.[6]

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).[7] The fire-bellied toads Bombina bombina and Bombina variegata similarly form hybrids, although these are less fertile, giving rise to a hybrid zone.

Phylogeny

Anura

Leiopelmatidae


Pipanura
Xenoanura

Pipidae



Rhinophrynidae



Bombianura
Costata

Alytidae



Bombinatoridae



Acosmanura
Anomocoela


Pelodytidae



Scaphiopodidae





Pelobatidae



Megophryidae




Neobatrachia

Heleophrynidae


Phthanobatrachia
Hyloides
Sooglossoidea

Sooglossidae



Nasikabatrachidae



Notogaeanura
Australobatrachia

Limnodynastidae




Myobatrachidae



Calyptocephalellidae




Nobleoanura

Hemiphractidae


Meridianura
Terrarana

Ceuthomantidae




Eleutherodactylidae




Brachycephalidae




Craugastoridae



Strabomantidae






Cladophrynia

Cryptobatrachidae


Tinctanura

Amphignathodontidae


Athesphatanura

Hylidae


Leptodactyliformes


Centrolenidae




Leptodactylidae



Leiuperidae




Chthonobatrachia

Ceratophryidae


Hesticobatrachia

Cycloramphidae


Agastorophrynia

Bufonidae


Nobleobatia

Hylodidae




Aromobatidae



Dendrobatidae















Ranoides
Allodapanura

Microhylidae


Afrobatrachia
Xenosyneunitanura

Brevicipitidae



Hemisotidae



Laurentobatrachia

Hyperoliidae



Arthroleptidae





Natatanura

Ptychadenidae


Victoranura

Ceratobatrachidae


Telmatobatrachia


?Indirana



Micrixalidae



Ametrobatrachia
Africanura

Phrynobatrachidae




Petropedetidae



Pyxicephalidae




Saukrobatrachia

Dicroglossidae


Aglaioanura


Nyctibatrachidae



Ranidae





Rhacophoridae



Mantellidae
















The cladogram is based on Frost et al. (2006)[8] y Heinicke et al. (2009).[9]

Morphology and physiology

Skeleton of frog
  Skeleton of Rana esculenta showing bones of the head, vertebral column, ribs, pectoral and pelvic girdles and limbs

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 can pass through their highly permeable skin. This unique feature allows frogs to respire largely through their skins.[10] The ribs are poorly developed so the lungs are filled by gulping down air. It has been shown that a frog that has been deprived of its lungs can continue living, respiring entirely through its skin.[10] 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 one of the causes of the decline in frog populations.[11][12][13][14][15][16][17][18][19]

Webbed foot
  Webbed hind foot of common frog
(Rana temporaria)

Many characteristics are not shared by all of the approximately 5,250 described frog species. However, some general characteristics distinguish them from other amphibians. Frogs are usually well suited to jumping, with long hind legs and 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.[20]

Frogs range in size from 10 mm (0.39 in) (Brachycephalus didactylus of Brazil and Eleutherodactylus iberia of Cuba) to 300 mm (12 in) (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. Most frogs have teeth, specifically pedicellate teeth in which the crown is separated from the root by fibrous tissue. Most only have teeth on the edge of the upper jaw (maxillary teeth) as well as vomerine teeth on the roof of their mouth. They do not have any teeth on their lower jaw, so they usually swallow their food whole. The teeth are mainly used to hold the prey and keep it in place till they can get a good grip on it and swallow their meal, assisted by retracting their eyes into their head.[21] True toads lack any teeth at all. Some species, like the African bull frog (Pyxicephalus), which preys on relatively large organisms including mice and other frogs, have cone shaped projections of bone called odontoid processes at the front of the lower jaw which function like teeth.[2]

Feet and legs

  Tyler's Tree Frog (Litoria tyleri) illustrates large toe pads and webbed feet
  A bullfrog skeleton, showing elongate limb bones and extra joints. Red marks indicate bones which have been substantially elongated in frogs and joints which have become mobile. Blue indicates joints and bones which have not been modified or only somewhat elongated.

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, in trees, or in burrows. 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.[22] For example, the completely aquatic African dwarf frog (Hymenochirus sp.) has fully webbed toes, whereas the toes of White's tree frog (Litoria caerulea), an arboreal species, are only a half or a quarter webbed.

  A frog gripping on a palm frond

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.[23]

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. In these arboreal frogs, the webs allow the frogs to "parachute" or control their glide from one position in the canopy to another.[24]

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.

Sometimes during the tadpole stage, one of the animal's rear leg stubs is eaten by a dragonfly nymph. In some of these cases, the full leg grows anyway, and in other cases, it does not, although the frog may still live out its normal lifespan with only three legs. Other times, a parasitic flatworm (Riberoria trematodes) digs into the rear of a tadpole, where it rearranges the limb bud cells, which sometimes causes the frog to have extra legs.[25]

Skin

  Pouched Frog (Assa darlingtoni) camouflaged against leaf litter
  Microscopic view of frog skin

Being cold-blooded, frogs have to adopt behaviour patterns to regulate their temperature. To warm up they can move into the sun or onto a warm surface and to cool down they can move into the shade or adopt a stance that exposes the minimum area of skin to the air. This involves squatting down, tucking the forefeet under the chin and the hind feet under the belly.[26] The colour of a frog's skin is used for thermo-regulation. In cool damp conditions the colour will be darker than on a hot dry day. The grey foam-nest tree frog (Chiromantis xerampelina) is even able to turn white to minimize the chance of overheating.[27]

Many frogs are able to absorb water and oxygen directly through the skin, especially around the pelvic area. However, the permeability of a frog's skin can also result in water loss. Glands located all over the body exude mucous which helps keep the skin moist and reduces evaporation. Some glands on the hands and chest of males are specialized to produce sticky secretions to aid in amplexus. Similar glands in tree frogs produce a glue-like substance on the adhesive discs of the feet. Some tree frogs reduce water loss by having a waterproof layer of skin and several South American species coat their skin with a waxy secretion. Others frogs have adopted behaviours to conserve water, including becoming nocturnal and resting in a water-conserving position. This posture involves the frog lying with its toes and fingers tucked under its body and chin with no gap between the body and the substrate. Some frog species will also rest in large groups with each frog pressed against its neighbours. This reduces the amount of skin exposed to the air or a dry surface, and thus reduces water loss. The Woodhouse's Toad (Bufo woodhousii), if given access to water after confinement in a dry location, sits in the shallows to rehydrate itself.[28] These water conservation measures only reduce water loss enough for a predominantly 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, but this is usually restricted to shades of one or two colours. For example, White's tree frog 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.[citation needed]

Certain frogs change colour between night and day, as light and moisture stimulate the pigment cells and cause them to expand or contract.[10]

Poison

Many frogs contain mild toxins that make them unpalatable to potential predators. For example, all toads have large poison glands—the parotoid 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. Many predators of frogs have adapted to tolerate high levels of these poisons. Others, including humans, may be severely affected.

  Oophaga pumilio, a poison dart frog, contains numerous alkaloids which deter predators

Some frogs obtain poisons from the ants and other arthropods they eat;[29] others, such as the Australian Corroboree Frogs (Pseudophryne corroboree and Pseudophryne pengilleyi), can manufacture an alkaloid not derived from their diet.[30] Some native people of South America extract poison from the poison dart frogs and apply it to their darts for hunting,[31] although few species are toxic enough to be used for this purpose. It was previously a misconception 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).[32][33]

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.[34] Arrow and dart poisons are under active investigation for their potential as therapeutic drugs.[35]

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 (which often have valves which close when the frog is submerged), causing the throat to puff out, then compressing the floor of the mouth, which forces the air into the lungs. In August 2007 an aquatic frog named Barbourula kalimantanensis was discovered in a remote part of Indonesia. The Bornean Flat-headed Frog (B. kalimantanensis) is the first species of frog known to science without lungs.

Frogs are known for their three-chambered heart, which they share with all tetrapods except birds, crocodilians and mammals.[36] 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 artery 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 to be more active than otherwise.

Some species of frog have remarkable adaptations that allow them to survive in oxygen deficient water. The lake titicaca frog (Telmatobius culeus) is one such species and to survive in the poorly oxygenated waters of Lake Titicaca it has incredibly wrinkly skin that increases its surface area to enhance gas exchange. This frog will also do 'push-ups' on the lake bed to increase the flow of water around its body.[37]

Digestion and excretion

The frog's digestive system begins with the mouth. Frogs have teeth along their upper jaw called maxillary teeth, which are used to hold food before it is swallowed. These teeth are very weak, and cannot be used to chew or catch and harm agile prey. Instead, the frog uses its sticky, cleft tongue to catch flies and other small moving prey. This normally lies coiled in the mouth, free at the back and attached to the mandibles at the front. It can be shot out and retracted at great speed.[22] Some frogs have no tongue and just stuff food into their mouths with their hands.[22] The eyes assist in the swallowing of food as they are able to be retracted through holes in the skull and help push food down the throat.[22] The food then moves through the oesophagus into the stomach where digestive enzymes are added and it is churned up. It then proceeds to the small intestine (duodenum and ileum) where most digestion occurs. Pancreatic juice from the pancreas, and bile, produced by the liver and stored in the gallbladder, are secreted into the small intestine, where the fluids digest the food and the nutrients are absorbed. The food residue passes into the large intestine where water is absorbed and wastes are routed to the cloaca.

The excretory system is similar to that of mammals. There are two kidneys which remove urea from the blood and convert it into urine. This passes along paired ureters to the urinary bladder from which it is vented periodically into the cloaca. All bodily wastes exit the body through the cloaca and the cloacal vent.[38]

Reproductive system

In frogs the sexes are separate. In many species the male is smaller and slimmer than the female. Males have vocal cords and make a range of croaks, particularly in the breeding season, and in some species they also have vocal sacs to amplify the sound. In the male the two testes are attached to the kidneys and sperm passes into the kidneys through fine tubes called efferent ducts. They then travel through the ureters which are consequently called urinogenital ducts in the male frog. There is no penis and sperm are ejected from the cloaca directly onto the eggs that the female is laying during amplexus. The ovaries of the female frog are also beside the kidneys and the eggs pass down a pair of oviducts to the exterior. During amplexus, the firm grip of the male frog stimulates the release of eggs, usually wrapped in jelly, as spawn.[38]

Nervous system

The frog has a highly developed nervous system which consists of a brain, spinal cord and nerves. Many parts of the frog's brain correspond with those of humans. The brain consists of two olfactory lobes, two cerebral hemispheres, a pineal body, two optic lobes, a cerebellum and a medulla oblongata. Muscular coordination and posture are controlled by the cerebellum and the medulla oblongata regulates respiration, digestion and other automatic functions.[38] The relative size of the cerebrum of a frog is much smaller than that of a human. Frogs have ten cranial nerves (nerves which pass information from the outside directly to the brain) and ten pairs of spinal nerves (nerves which pass information from extremities to the brain through the spinal cord).[38] By contrast, all amniotes (mammals, birds and reptiles) have twelve cranial nerves. Frogs do not have external ears; the eardrums (tympanic membranes) are directly exposed. As in all animals, the ear contains semicircular canals which help control balance and orientation. Due to their short cochlea, frogs use electrical tuning to expand their range of audible frequencies.

Sight

The eyes of frogs are located on or near the top of the head and often project outwards as hemispherical bulges. They have a large field of view and may be the only part of an otherwise submerged frog to protrude from the water. Each eye has a closable upper and lower lid and a nictitating membrane which provides further protection, especially when the frog is swimming. The irises come in a range of colours and the pupils in a range of shapes. The common toad (Bufo bufo) has golden irises and horizontal slit-like pupils, the red-eyed tree frog (Agalychnis callidryas) has vertical slit pupils, the poison dart frog has dark irises, the fire-bellied toad (Bombina spp.) has triangular pupils and the tomato frog (Dyscophus spp.) has circular ones. The irises of the southern toad (Anaxyrus terrestris) are patterned so as to blend in with the surrounding camouflaged skin.[39]

The distant vision of a frog is better than its near vision. Calling frogs will quickly become silent when they see an intruder or even his shadow but the closer an object is, the less well it is seen.[39] When a frog shoots out its tongue to catch an insect it is reacting to a small moving object that it cannot see well and must line it up precisely beforehand because it shuts its eyes as the tongue is extended.[22] Whether a frog sees in colour is debatable but it has been shown that it responds positively to blue light, perhaps because that colour is associated with bodies of water that can provide refuge when the frog feels threatened.[40]

Hearing

Frogs can hear both in the air and below water. There is no external ear but the tympanum or ear drum is visible as a circular area just behind the eye. A noise causes the tympanum to vibrate and the sound is transmitted to the middle and inner ear. The size and distance apart of the eardrums is related to the frequency and wavelength at which the frog calls. In some species, such as the bullfrog, the size of the tympanum indicates the sex of the frog; males have tympani that are larger than their eyes while in females, the eyes and tympani are much the same size. A frog may be startled by an unexpected noise but it will not usually take any action until it has located the source of the sound by sight.[41]

Call

  A male Dendropsophus microcephalus displaying its vocal sac during its call.

The call or croak of a frog is unique to its species. Frogs create this sound 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 frog calls are so loud that they can be heard up to a mile away.[42]

Some frogs, such as those from the genera Heleioporus and Neobatrachus, lack vocal sacs but can still produce a loud call. Their buccal cavity is enlarged and dome-shaped, acting as a resonance chamber that amplifies the sound. Species of frog that lack vocal sacs and that do not have a loud call tend to inhabit areas close to constantly noisy, flowing water. They need to use an alternative means to communicate. The coastal tailed frog (Ascaphus truei) lives in mountain streams in North America and does not vocalize.[43]

The main reason for calling is to allow male frogs to attract a mate. Males may call individually or there may be a chorus of sound where numerous males have converged on breeding sites. Females of many frog species, such as the common tree frog (Polypedates leucomystax), reply to the male calls, which acts as a catalyst for the enhancement of reproductive activity in a breeding colony.[44] Female frogs prefer males that produce sounds of greater intensity and lower frequency, attributes that stand out in a crowd, the rationale being that by demonstrating his prowess, the male shows his fitness to produce progeny.[45]

A different call is emitted by a male frog or unreceptive female when mounted by another male. This is distinct chirruping sound and is accompanied by a vibration of the body.[46] Tree frogs and some non-quatic species also have a rain call that they make on the basis of humidity cues prior to a rain shower.[46] 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.[46]

A distress call, emitted by some frogs when they are in danger, is produced with the mouth open, resulting in a higher-pitched call. It is typically used when the frog has been grabbed by a predator and may serve to distract or disorientate the attacker so that it liberates the frog.[46]

Many species of frog have deep calls, or croaks. The English onomatopoeic spelling is "ribbit". The croak of the American bullfrog (Rana catesbiana) is sometimes written as "jug o' rum".[47] Other examples include The Frogs, an Ancient Greek comic drama by Aristophanes, where the voice of the marsh frog (Pelophylax ridibundus), is rendered as brekekekex koax koax,[48] and the description of a frog's voice in Rigveda 7:103.6 gómāyur éko ajámāyur ékaħ meaning "one has a voice like a cow one has a voice like a goat".

Torpor

Some frogs enter a state of torpor during extreme conditions. In colder regions, many species of frog hibernate in winter. Those that live on land such as the American toad (Bufo americanus) dig a burrow and make a hibernaculum in which to lie dormant. Others, less proficient at digging, find a crevice or bury themselves in dead leaves. Aquatic species such as the American bullfrog (Rana catesbeiana) normally sink to the bottom of the pond where they lie, semi-immersed in mud but still able to access the oxygen dissolved in the water. Their metabolism slows down and they live on their energy reserves. Frogs can even survive being frozen. Ice crystals form under the skin and in the body cavity but the essential organs are protected from freezing by a high concentration of glucose. An apparently lifeless, frozen frog can resume respiration and the heart beat can restart when conditions warm up.[49]

At the other extreme, the striped burrowing frog (Cyclorana alboguttata) regularly aestivates during the hot, dry season in Australia, surviving in a dormant state without access to food and water for nine or ten months of the year. It burrows underground and curls up inside a protective cocoon formed by its shed skin. Researchers at the University of Queensland have found that during aestivation, the metabolism of the frog is altered and the operational efficiency of the mitochondria is increased. This means that the limited amount of energy available to the comatose frog is used in a more efficient manner. The researchers considered why such a mechanism was not used more widely in the animal kingdom. They concluded that it would only be useful to an animal that remained completely unconscious for an extended period of time and whose energy requirements were low because, being cold-blooded, it had no need to generate heat.[50] Other research showed that, to provide these limited energy requirements, muscles were atrophied but that the hind limb muscles were preferentially unaffected.[51]

Locomotion

Different species of frog use a number of methods of moving around including jumping, running, walking, swimming, burrowing, climbing and gliding.

Jumping

Frogs are generally recognized as exceptional jumpers, and the best jumper of all vertebrates.[52] The Australian rocket frog, Litoria nasuta, can leap over 50 times its body length (5.5 cm), resulting in jumps of over 2 meters.[53] The acceleration of the jump may be up to twice gravity. There are tremendous differences between species in jumping capability, but within a species, jump distance increases with increasing size, but relative jumping distance (body-lengths jumped) decreases. The Indian skipper frog (Euphlyctis cyanophlyctis) has the ability to leap from the water from a position floating on the surface.[54] The tiny northern cricket frog (Acris crepitans) can "skitter" across the surface of a pond with a series of short rapid jumps.[55]

Most frogs are either proficient at jumping or descended from ancestors who were, with much of the musculo-skeletal morphology modified for this purpose. The tibia, fibula and tarsals have been fused into a single, strong bone, as have the radius and ulna in the forelimbs (which must absorb the impact of landing). The metatarsals have become elongated to add to the leg length and allow the frog to push against the ground for longer during a jump. The illium has elongated and formed a mobile joint with the sacrum which, in specialist jumpers such as Ranids or Hylids, functions as an additional limb joint to further power the leaps. The tail vertebrae fused into a urostyle which retracted inside the pelvis. This enabled the force to be transferred from the legs to the body during a leap.[20]

The muscular system has been similarly modified. The hind limbs of the ancestor of frogs presumably contained pairs of muscles which would act in opposition (one muscle to flex the knee, a different muscle to extend it), as is seen in most other limbed animals. However, in modern frogs, almost all muscles have been modified to contribute to the action of jumping, with only a few small muscles remaining to bring the limb back to the starting position and maintain posture. The muscles have also been greatly enlarged, with the main leg muscles accounting for over 17% of the total mass of the frog. Slow motion photography shows that the muscles have passive flexibility. They are first stretched while the frog is still in the crouched position, then they are contracted before being stretched again to launch the frog into the air. The forelegs are folded against the chest and the hind legs remain in the extended, streamlined position for the duration of the jump.[56] In some extremely capable jumpers, such as the Cuban tree frog (Osteopilus septentrionalis) and the northern leopard frog (Rana pipiens), the peak power exerted during a jump can exceed that which the muscle is theoretically capable of producing. When the muscles contract, the energy is transferred into the stretched tendon which is wrapped around the ankle bone. Finally the muscles stretch again at the same time as the tendon releases its energy like a catapult to produce a powerful acceleration beyond the limits of muscle-powered acceleration.[57] A similar mechanism has been documented in locusts and grasshoppers.[58]

Walking and running

Frogs in the families Bufonidae, Rhinophrynidae and Microhylidae have short back legs and tend to walk rather than jump.[59] When they try to move rapidly, they speed up the rate of movement of their limbs or resort to an ungainly hopping gait. The western narrow-mouthed toad (Gastrophryne olivacea) has been described as having a gait that is "a combination of running and short hops that are usually only an inch or two in length".[60] In an experiment, Fowler's toad (Bufo fowleri) was placed on a treadmill which was turned at varying speeds. By measuring the toad's uptake of oxygen it was found that hopping was not a good use of resources during sustained locomotion but was a useful strategy during short bursts of high-intensity activity.[61]

The red-legged running frog (Kassina maculata) has short, slim hind limbs unsuited to jumping. It can move fast by using a running gait in which the two hind legs are used alternately. Slow motion photography shows that, unlike a horse that can trot or gallop, the frog's gait remained similar at slow, medium and fast speeds.[62] This species can also climb trees and shrubs and does so at night to catch insects.[63] The Indian skipper frog (Euphlyctis cyanophlyctis) has broad feet and can run across the surface of the water for several metres (yards).[55]

Swimming

Common toad swimming
  Common toad (Bufo bufo) swimming

Frogs that live in or visit water have adaptations that improve their swimming abilities. The hind limbs are heavily muscled and strong. The webbing between the toes of the hind feet increases the area of the foot and helps propel the frog powerfully through the water. Members of the family Pipidae are wholly aquatic and show the most marked specialization. They have little flexibility in their vertebral column, a flattened, streamlined body, a lateral line system and powerful hind limbs with large webbed feet.[64] Tadpoles mostly have large tailfins which provide thrust when the tail is moved from side to side. Frogs are at their most vulnerable to predators when they are undergoing metamorphosis. At this time the tail is being lost and locomotion by means of limbs is only just becoming established.[59]

Burrowing

Purple frog
  Purple frog (Nasikabatrachus sahyadrensis)

Some frogs have become adapted for burrowing and a life underground. They tend to have rounded bodies, short limbs, small heads with bulging eyes and hind feet adapted for excavation. An extreme example of this is the Purple frog (Nasikabatrachus sahyadrensis) from southern India which feeds on termites and spends almost its whole life underground. It emerges briefly during the monsoon to mate and breed in temporary pools. It has a tiny head with a pointed snout and a plump rounded body. Because of this fossorial existence it was first described in 2003, being new to science before that date although previously known to local people.[65]

The spadefoot toads of North America are also adapted to underground life. The plains spadefoot toad (Spea bombifrons) is typical and has a flap of keratinised bone attached to one of the metatarsals of the hind foot which it uses to dig itself backwards into the ground. As it digs, the toad wriggles its hips from side to side in order to sink itself into the loose soil. It has a shallow burrow in the summer from which it emerges at night to forage. In winter it digs much deeper and has been recoded at a depth of 15 feet (4.6 m).[66] The tunnel is filled with soil and the toad hibernates in a small chamber at the end. During this time, urea accumulates in the tissues and water is drawn in from the surrounding damp soil by osmosis to supply the toad's needs.[66] Spadefoot toads are "explosive breeders", all emerging from their burrows at one time and converging on temporary pools, attracted by the calling of the first male to find a suitable breeding location.[67]

The burrowing frogs of Australia have a rather different lifestyle. The western spotted frog (Heleioporus albopunctatus) digs a burrow beside a river or in the bed of an ephemeral stream and regularly emerges to forage. Mating takes place and eggs are laid in a foam nest inside the burrow. They partially develop there but do not hatch until they are submerged following heavy rainfall. The tadpoles then swim out into the open water and rapidly complete their development.[68] The Madagascan burrowing frogs mostly burrow in leaf litter. The green burrowing frog (Scaphiophryne marmorata) has greatly enlarged terminal discs on its fore feet which help it to clamber around in bushes. It has a flattened head with a short snout and well developed metatarsal tubercles on its hind feet to help with excavation.[69] It breeds in temporary pools that have formed after rains.[70]

Climbing

Tree frogs are found high in the canopy where they scramble around on the branches, twigs and leaves, sometimes never coming down to earth. The "true" tree frogs belong to the family Hylidae but members of other frog families have independently adopted an arboreal habit, a case of convergent evolution. These include the glass frogs (Centrolenidae), the bush frogs (Hyperoliidae), some of the narrow-mouthed frogs (Microhylidae) and the shrub frogs (Rhacophoridae).[59] Most tree frogs are under 10 cm (4 in) in length, with long legs and long toes with adhesive pads on the tips. The surface of the toe pads are formed from a closely packed layer of flat-topped, hexagonal epidermal cells separated by grooves into which glands secrete mucous. The surface of the pads is formed from closely packed peg-like projections, each with a dimple at the end. It is these, moistened by the mucous, which provide the grip on any wet or dry surface, including glass. The forces involved include surface tension and viscosity but mostly involve boundary friction of the toe pad epidermis on the surface.[71] Tree frogs are very acrobatic and can catch insects while hanging by one toe from a twig or clutching onto the blade of a windswept reed.[72] Some members of the subfamily Phyllomedusinae have opposable toes on their feet. The reticulate leaf frog (Phyllomedusa ayeaye) has a single opposed digit on each fore foot and two opposed digits on its hind feet. This means that it can grasp the stems of bushes as it clambers around in its riverside habitat.[73]

Gliding

During the evolutionary history of the frog, several different groups have independently taken to the air.[74] Some frogs in the tropical rainforest are specially adapted for gliding from tree to tree or parachuting to the forest floor. Typical of them is Wallace's flying frog (Rhacophorus nigropalmatus) from Malaysia and Borneo. It has large feet with the fingertips expanded into flat adhesive discs and the digits fully webbed. There are flaps of skin on the lateral margins of the limbs and across the tail region. With the digits splayed, the limbs outstretched and these flaps spread, it can glide considerable distances but is unable to undertake powered flight.[75] It can alter its direction of travel and navigate distances of up to 15 metres (49 ft) between trees.[76]

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 they are laid.

Life cycle

  Frogspawn
  Frogspawn development
  Tadpole of Haswell's Froglet (Paracrinia haswelli

The life cycle of a frog starts with an egg. A female generally lays gelatinous egg masses containing thousands of eggs, in water. Each anuran species lays eggs in a distinctive, identifiable manner. An example are the long strings of eggs laid by the common American toad. The eggs are highly vulnerable to predation, so frogs have evolved many techniques to ensure the survival of the next generation. In colder areas the embryo is black to absorb more heat from the sun, which speeds up the development. 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 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.[77] 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. Other species go through their whole larval phase inside the eggs or the mother, or they have direct development. Unlike salamanders and newts, frogs and toads never become sexually mature while still in their larval stage.

Eggs hatch and continue life as tadpoles (occasionally known as polliwogs), which typically have oval bodies and long, vertically flattened tails. At least one species (Nannophrys ceylonensis) has tadpoles that are semi-terrestrial and live among wet rocks,[78][79] but as a general rule, free living larvae are fully aquatic. They lack eyelids and have a cartilaginous skeleton, a lateral line system, gills for respiration (external gills at first, internal gills later) and tails with dorsal and ventral folds of skin for swimming.[80] From pretty early onward they develop a gill pouch that covers the gills and the front legs and also the lungs are developed in an early stage as an accessory breathing organ. Some species which go through the metamorphosis inside the egg and hatch to small frogs never develop gills, instead there are specialised areas of skin that takes care of the respiration. Tadpoles also lack true teeth, but the jaws in most species usually have two elongate, parallel rows of small keratinized structures called keradonts in the upper jaw while the lower jaw has three rows of keradonts, surrounded by a horny beak, but the number of rows can be lower or absent, or much higher.[81] Tadpoles are typically herbivorous, feeding mostly on algae, including diatoms filtered from the water through the gills. Some species are carnivorous at the tadpole stage, eating insects, smaller tadpoles, and fish. Cannibalism has been observed among tadpoles. Early developers who gain legs may be eaten by the others, so the late bloomers survive longer. This has been observed in the Cuban tree frog (Osteopilus septentrionalis).[82]

Tadpoles are highly vulnerable to predation by fish, newts, predatory diving beetles and birds such as kingfishers. 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). In the Pipidae, with the exception for Hymenochirus, the tadpoles have paired anterior barbels which make them resemble small catfish.[83]

With the exception of the base of the tail, where a few vertebral structures develop to give rise to the urostyle later in life, the tail lacks the completely solid, segmental, skeletal elements of cartilage or bony tissue that are so typical for other vertebrates, although it does contain a notochord

At the end of the tadpole stage, frogs undergo metamorphosis, in which they undergo a transition into the adult form. This metamorphosis last typically only 24 hours and consists of:

  Larva of the common frog Rana temporaria a day before metamorphisis
  Common frog - Metamorphosis stage. Notice the deformed jaws, large eyes and the remains of the gill pouch.
  Young frog with tail remains after metamorphosis
  Adult leopard frog
  • The disappearance of the gill pouch, making the front legs visible.
  • The transformation of the jaws into the big jaws of predatory frogs (most tadpoles are scrapers of algae or are filter feeders)
  • The transformation of the digestive system: the long spiral gut of the larva is being replaced by the typical short gut of a predator.
  • An adaptation of the nervous system for stereoscopic vision, locomotion and feeding
  • A quick growth and movement of the eyes to higher up the skull and the formation of eyelids.
  • Formation of skin glands, thickening of the skin and loss of the lateral line system
  • An eardrum is developed to lock the middle ear.

The disappearance of the tail is somewhat later (occurs at higher thyroxin levels) and after the tail has been resorbed the animals are ready to leave the water. The material of the tail is being used for a quick growth of the legs. The disappearing of the larval structures is a regulated process called apoptosis.

  Incident of frog cannibalism

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.[84] 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 uses in agriculture and research, below).

Frogs and toads can live for many years; though little is known about their life span in the wild, captive frogs and toads are recorded living up to 40 years.[85]

Frogs from temperate climates hibernate through the winter, and 4 species are known to freeze during this time, most notably Rana sylvatica.[86]

Reproduction of frogs

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
  A Male and Female common toad in amplexus. The black strands are eggs released into open water minutes after birth.

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 (sometimes with special nuptial pads) 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. Members of the West African genus Nimbaphrynoides are the only species known to be viviparous and the Tanzanian genus Nectophrynoides are the only known to be ovoviviparous: In both cases, fertilization is internal and females give birth to fully developed, small toadlets.[87][88]

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.

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.[89] 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 sex 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 (see next section) for development. Some species of frog will leave a 'babysitter' to watch over the frogspawn until it hatches.

The evolution of parental care in frogs is driven primarily by the size of the water body in which they breed. There is an inverse relationship between the level of parental care in a frog species and the size of the body of water—frogs that breed in smaller water bodies tend to have more complex parental care behaviors.[90] Water body size shows this strong relationship with parental care because it encompasses several important variables that interact to select for parental care: predation, desiccation, competition, and resource limitation. Because predation of eggs and larvae is high in large water bodies, a number of frog species evolved terrestrial oviposition. Once eggs are deposited on land, the desiccating terrestrial environment demands uniparental care in the form of egg hydration to ensure egg survival.[91] The subsequent need to transport hatched tadpoles to a water source requires an even more intense form of uniparental care. In small water bodies where predators are mostly absent, such as phytotelmata (water-filled leaf axils or small woody cavities), inter-tadpole competition becomes the variable that constrains tadpole survival. Certain frogs species avoid this competition by evolving the use of smaller phytotelmata as tadpole deposition sites.[92] However, while these smaller tadpole rearing sites are free of competition, they also lack nutrients. Because they do not have sufficient nutrients to support a tadpole without parental provisioning behavior, frog species that transitioned from the use of larger to smaller phytotelmata have evolved trophic (unfertilized) egg laying. In this complex form of biparental care, the female provides her offspring with nutritive eggs. While each of these variables select for different behaviors, they correlate with the size of a species' tadpole-rearing site and influence the degree of parental care displayed by a species.

Distribution and conservation status

  The Red-eyed Tree Frog (Litoria chloris) is a species of tree frog native to eastern Australia.

The habitat of frogs extends almost worldwide, but they do not occur in Antarctica and are not present on many oceanic islands.[93][94] The greatest diversity of frogs occurs in the tropical areas of the world, where water is readily available, suiting 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 a cold environment; for instance the wood frog (Rana sylvatica), whose habitat extends north of the Arctic Circle, buries itself in the ground during winter when much of its body freezes.[22]

  Golden toad (Bufo periglenes) – last seen in 1989

Frog populations have declined dramatically since the 1950s: more than one third of species are believed to be threatened with extinction and more than 120 species are suspected to be extinct since the 1980s.[95] 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 [96] and ranavirus.[97]

Many environmental scientists believe 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).[98] It appears that it is the species with both aquatic eggs and aquatic larvae that are most affected by the decline, while those with direct development are the most resistant.[99]

A Canadian study conducted in 2006, suggested heavy traffic near frog habitats as a large threat to frog populations.[100] In a few cases, captive breeding programs have been attempted to alleviate the pressure on frog populations, and these have proved successful.[101][102] In 2007, it was reported the application of certain probiotic bacteria could protect amphibians from chytridiomycosis.[103] One current project, The Panama Amphibian Rescue and Conservation Project, has subsequently been developed in order to rescue species at risk of chytridiomycosis in eastern Panama, and to develop field applications of this probiotic cure.[104]

Zoos and aquariums around the world named 2008 the Year of the Frog, to draw attention to the conservation issues.[105]

Evolution

  A fossilized frog from the Czech Republic, possibly Palaeobatrachus gigas

Until the discovery of the Early Permian Gerobatrachus hottoni in 2008, a stem-batrachian with many salamander-like characteristics, the earliest known proto-frog was Triadobatrachus massinoti, from the 250 million year old early Triassic of Madagascar.[106] The skull is frog-like, being broad with large eye sockets, but the fossil has 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 Triadobatrachus was not an efficient leaper.

Another fossil frog, Prosalirus bitis, was discovered in 1995. The remains were recovered from Arizona's Kayenta Formation, which dates back to the Early Jurassic epoch,[107] somewhat younger than 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[citation needed] and was well adapted for jumping.[108]

The earliest known "true frog" is Vieraella herbsti, from the early Jurassic (188–213 million years ago). It is known only from the dorsal and ventral impressions of a single animal and was estimated to be 33 mm (1.3 in) from snout to vent. Notobatrachus degiustoi from the middle Jurassic is slightly younger, about 155–170 million years old. It is likely 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 of sanyanlichan, which lived 125 million years ago[109] and had all modern frog features, but bore 9 presacral vertebrae instead of the 8 of modern frogs.[110]

Frog fossils have been found on all continents except Antarctica, but biogeographic evidence suggests they inhabited Antarctica when it was warmer.[111]

Uses in agriculture and research

  Cooked frog. Jianyang, Fujian, China
  Frog meat less than one minute old in a market in Haikou City, Hainan Province, China

Frogs are raised commercially for several purposes. Frogs are used as a food source; frog legs are a delicacy in China, Indonesia, the Philippines, France, the north of Greece and many parts of the American South, especially Louisiana. Dead frogs are sometimes used for dissections in high school and university anatomy classes, often first being injected with coloured substances to enhance the contrast between the organs. This practice is declining with increasing concerns about animal welfare and "digital frogs" are now available for virtual dissection.[112]

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.[113] 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 frog, it is induced to lay eggs, a discovery made by the English zoologist, Lancelot Hogben.[114] In 1952, Robert Briggs and Thomas J. King cloned a frog by somatic cell nuclear transfer. This was the same technique later used to create Dolly the sheep and their experiment was the first time that successful nuclear transplantation had been accomplished in higher animals.[115]

Frogs are used in cloning research and other branches of embryology. Although alternative pregnancy assays have been developed, biologists continue to use Xenopus as a model organism in developmental biology because their embryos are large and easy to manipulate, they are readily obtainable and can easily be kept in the laboratory.[116] Xenopus laevis is increasingly being displaced by its smaller relative, Xenopus tropicalis, which reaches its reproductive age in five months rather than the one to two years taken by X. laevis,[117] thus facilitating faster studies across generations. The genome sequence of X. tropicalis will probably be completed by 2015.[118]

Cultural beliefs

  Moche Frog 200 A.D. Larco Museum Collection Lima, Peru

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 the Warner Brothers cartoon One Froggy Evening, 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 The Muppet Show and Sesame Street; while openly friendly and greatly talented, he is often portrayed as cringing at the fanciful behavior of more flamboyant characters.

The Moche people of ancient Peru worshipped animals and often depicted frogs in their art.[119] In Panama local legend promised luck to anyone who spotted a golden frog in the wild and some believed that when Panamanian Golden Frogs died, they would turn into a gold talisman, known as a huaca. Today, despite being extinct in the wild, Panamanian Golden Frogs remain an important cultural symbol and can be found on decorative cloth molas made by the Kuna Indians, on T-shirts, as inlaid design on a new overpass in Panama City and even on lottery tickets.[120]

See also

References

Notes

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  9. ^ Heinicke M. P. et al. (2009). "A new frog family (Anura: Terrarana) from South America and an expanded direct-developing clade revealed by molecular phylogeny" (PDF). Zootaxa 2211: 1–35. http://evo.bio.psu.edu/hedgeslab/Publications/PDF-files/220.pdf. 
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  115. ^ "Robert W. Briggs Biographical Memoir". http://newton.nap.edu/html/biomems/rbriggs.html. Retrieved 2006-04-22. 
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