Frog
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/* Distribution and conservation status */
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{{Taxobox
| name = '''Frog'''
| image = Caerulea3 crop.jpg
| image_width = 240px
| image_caption = [[White's Tree Frog]] (''Litoria caerulea'')
| fossil_range = [[Triassic]] - [[Holocene|Recent]]
| regnum = [[Animal]]ia
| phylum = [[Chordate|Chordata]]
| classis = [[Amphibian|Amphibia]]
| ordo = '''Anura'''
| ordo_authority = [[Blasius Merrem|Merrem]], 1820
| range_map = Frog distribution.png
| range_map_width = 240px
| range_map_caption = Distribution of frogs (in black)
| subdivision_ranks = Suborders
| subdivision = [[Archaeobatrachia]]<br />
[[Mesobatrachia]]<br />
[[Neobatrachia]] <br /> - <br />
[[List of Anuran families]]
}}
The '''frog''' is an [[amphibian]] in the order '''Anura''' (meaning "tail-less", from Greek ''an-,'' without + ''oura'', tail), formerly referred to as ''Salientia'' (Latin ''saltare'', to jump). The name frog derives from [[Old English language|Old English]] ''frogga'',<ref>http://en.wiktionary.org/wiki/frog</ref> (compare [[Old Norse]] ''frauki'', [[German language|German]] ''Frosch'', older [[Dutch language|Dutch]] spelling ''kikvorsch''), cognate with [[Sanskrit]] ''plava'' (frog), probably deriving from [[Proto-Indo-European language|Proto-Indo-European]] ''praw'' = "to jump".<ref>[http://ehl.santafe.edu/cgi-bin/response.cgi?root=config&morpho=0&basename=/data/ie/piet&first=2221 Indo-European etymology database]</ref>
Adult frogs are characterised by long hind legs, a short body, webbed digits, protruding [[eye]]s 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 [[Egg (biology)|egg]]s in puddles, [[pond]]s or [[lake]]s, and their [[larva]]e, called [[tadpole]]s, have [[gill]]s and develop in [[water]]. Adult frogs follow a [[carnivore|carnivorous]] diet, mostly of [[arthropod]]s, [[annelid]]s and [[Gastropoda|gastropod]]s. Frogs are most noticeable by their call, which can be widely heard during the night or day, mainly in their [[Estrous cycle|mating season]].
The distribution of frogs ranges from [[tropics|tropic]] to [[subarctic]] regions, but most species are found in [[tropical rainforest]]s. Consisting of more than 5,000 species described, they are among the most diverse groups of [[vertebrate]]s. However, populations of certain frog species are significantly [[Decline in amphibian populations|declining]].
A distinction is often made between frogs and [[toad]]s on the basis of their appearance, caused by the [[convergent evolution|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 ''[[toad|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==
[[Image:Bombina bombina 1 (Marek Szczepanek) tight crop.jpg|thumb|[[European Fire-bellied Toad]] (''Bombina bombina'')]]
{{details|List of Anuran families}}
The order ''Anura'' contains 5,250 species 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.
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. 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. ''Neobatrachia'' is further divided into the ''Hyloidea'' and ''Ranoidea''.<ref>{{cite journal|last=Ford| first=L.S.| coauthors=D.C. Cannatella| year=1993| title=The major clades of frogs| journal=Herpetological Monographs| volume=7| pages=94–117| doi=10.2307/1466954}}</ref> 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.<ref>{{cite journal|last=Faivovich| first=J.| coauthors=C.F.B. Haddad, P.C.A. Garcia, D.R. Frost, J.A. Campbell, and W.C. Wheeler| title=Systematic review of the frog family Hylidae, with special reference to Hylinae: Phylogenetic analysis and taxonomic revision| journal=Bulletin of the American Museum of Natural History| volume=294| pages=1–240| doi=10.1206/0003-0090(2005)294[0001:SROTFF]2.0.CO;2| year=2005}}</ref>
Some species of anurans [[Hybrid (biology)|hybrid]]ise 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==
{{details|Frog zoology}}
[[Image:Rana skeleton.png|thumb|Skeleton of ''[[Rana]]'']]
The [[morphology (biology)|morphology]] of frogs is unique among amphibians. Compared with the other two groups of amphibians, ([[salamander]]s and [[caecilian]]s), 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 [[vertebrate]]s) because oxygen can 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 cause of the [[Decline in amphibian populations|decline in frog populations]].
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.
Frogs range in size from {{convert|10|mm|abbr=on}} (''[[Brachycephalus didactylus]]'' of [[Brazil]] and ''[[Eleutherodactylus iberia]]'' of [[Cuba]]) to {{convert|300|mm|abbr=on}} ([[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 (zoology)|tympanum]] on each side of the head, which is involved in hearing and, in some species, is covered by skin. Most frogs do in fact have teeth of a sort. They have a ridge of very small cone teeth around the upper edge of the jaw. These are called ''maxillary teeth''. Frogs often also have what are called ''vomerine teeth'' on the roof of their mouth. They do not have anything that could be called teeth on their lower jaw, so they usually swallow their food whole. The so-called "teeth" are mainly used to hold the prey and keep it in place till they can get a good grip on it and squash their eyeballs down to swallow their meal. Toads, however, do not have any teeth.
[[Image:L tyleri.jpg|thumb|left|[[Tyler's Tree Frog]] (''Litoria tyleri'') illustrates large toe pads and webbed feet.]]
===Feet and legs===
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. 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.
[[Tree frog|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.<ref>{{cite journal|last=Emerson| first=S.B.| coauthors=Diehl, D.| year=1980| title=Toe pad morphology and mechanisms of sticking in frogs| journal=Biol. J. Linn. Soc.| volume=13| issue=3| pages=199–216| doi=10.1111/j.1095-8312.1980.tb00082.x}}</ref>
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.<ref>{{cite journal|last=Harvey|first=M. B| coauthors=A. J. Pemberton, and E. N. Smith| year=2002| title=New and poorly known parachuting frogs (Rhacophoridae : ''Rhacophorus'') from Sumatra and Java| journal=Herpetological Monographs| volume=16| pages=46–92| doi=10.1655/0733-1347(2002)016[0046:NAPKPF]2.0.CO;2}}</ref>
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.
While frog species can use a variety of locomotor modes ([[running]], [[walking]], [[gliding]], [[swimming]] and [[climbing]]), more are either proficient at [[jumping]] or descended from ancestors who were, with much of the [[musculo-skeletal]] [[morphology (biology)|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 [[Ranidae|Ranids]] or [[Hylidae|Hylids]], functions as an additional limb joint to further power the leaps.
===Skin===
[[Image:Hip-pocket Frog - Assa darlingtoni.jpg|thumb|right|[[Pouched Frog]] (''Assa darlingtoni'') camouflaged against leaf litter.]]
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. Some tree frogs reduce water loss with a waterproof layer of skin. Others have adapted behaviours to 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 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 fold]]s 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.
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[[Image:Dendrobates pumilio.jpg|thumb|left|''[[Oophaga pumilio]]'', a [[poison dart frog]], contains numerous alkaloids which deter predators.]]
===Poison===
Many frogs contain mild [[toxin]]s that make them unpalatable 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 frog]]s, are especially toxic. The chemical makeup of toxins in frogs varies from irritants to [[hallucinogen]]s, [[seizure|convulsant]]s, [[neurotoxin|nerve poisons]], and [[vasoconstrictor]]s. 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;<ref>{{cite journal|last=Saporito| first=R.A.| coauthors= H.M. Garraffo, M.A. Donnelly, A.L. Edwards, J.T. Longino, and J.W. Daly| year=2004| title=Formicine ants: An arthropod source for the pumiliotoxin alkaloids of dendrobatid poison frogs| journal=Proceedings of the National Academy of Science| volume=101| pages=8045–8050| doi=10.1073/pnas.0402365101| pmid=15128938}}</ref> others, such as the Australian [[Corroboree Frog]]s (''Pseudophryne corroboree'' and ''Pseudophryne pengilleyi''), can manufacture an [[alkaloid]] not derived from their diet.<ref>{{cite journal| last=Smith| first=B. P.| coauthors=Tyler M. J., Kaneko T., Garraffo H. M., Spande T. F., Daly J. W.| year=2002| title=Evidence for biosynthesis of pseudophrynamine alkaloids by an Australian myobatrachid frog (pseudophryne) and for sequestration of dietary pumiliotoxins| journal=J Nat Prod| volume=65| issue=4| pages=439–47| doi=10.1021/np010506a}}</ref> Some native people of South America extract poison from the poison dart frogs and apply it to their [[dart]]s for hunting,<ref>{{cite journal| last=Myers| first=C.W.| coauthors= J.W. Daly| year=1983| title=Dart-poison frogs| journal=Scientific American| volume=248| pages=120–133}}</ref> 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]]).<ref>{{cite book| last=Savage| first=J. M.| year=2002| title=The Amphibians and Reptiles of Costa Rica| publisher=University of Chicago Press, Chicago}}</ref><ref>{{cite journal| last=Duellman| first= W. E.| year=1978| title=The Biology of an Equatorial Herpetofauna in Amazonian Ecuador| journal=University of Kansas Museum of Natural History Miscellaneous Publication| volume=65| pages=1–352}}</ref>
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.<ref>{{cite journal|last=VanCompernolle| first=Scott. E.| coauthors=R. J. Taylor, K. Oswald-Richter, J. Jiang, B. E. Youree, J. H. Bowie, M. J. Tyler, M. Conlon, D. Wade, C. Aiken, T. S. Dermody, V. N. KewalRamani, L. A. Rollins-Smith and D. Unutmaz| year=2005| title=Antimicrobial peptides from amphibian skin potently inhibit Human Immunodeficiency Virus infection and transfer of virus from dendritic cells to T cells| journal=Journal of Virology| volume=79| pages=11598–11606| doi=10.1128/JVI.79.18.11598-11606.2005| pmid=16140737}}</ref> Arrow and dart poisons are under active investigation for their potential as therapeutic drugs.<ref>{{cite journal| last=Phillipe| first=G.| coauthors=Angenot L.| year=2005| title=Recent developments in the field of arrow and dart poisons| journal=J Ethnopharmacol| volume=100(1–2)| pages=85–91| doi=10.1016/j.jep.2005.05.022}}</ref>
The skin secretions of some toads, such as the [[Colorado River toad]] and [[cane toad]], contain [[bufotoxin]]s, 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 [[rib]]s or [[diaphragm (anatomy)|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. 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. <ref> [http://news.yahoo.com/s/ap/20080410/ap_on_sc/indonesia_lungless_frog] </ref>
Frogs are known for their three-chambered [[heart]], which they share with all [[tetrapod]]s except [[bird]]s and [[mammal]]s. In the three-chambered heart, oxygenated blood from the lungs and de-oxygenated blood from the [[Respiration (physiology)|respiring]] tissues enter by separate [[atrium (anatomy)|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 to 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.
===Life cycle===
[[Image:Frogspawn closeup.jpg|thumb|Frogspawn]]
[[Image:Frog spawn time-lapse.gif|thumb|right|Frogspawn development]]
[[Image:Haswell's Frog - Paracrinia haswelli tadpole.jpg|thumb|Tadpole of [[Haswell's Froglet]] (''Paracrinia haswelli'']]
[[Image:Green-leopard-frog-in-swamp.jpg||thumb|Adult [[leopard frog]]]]
The life cycle of a frog starts with an egg. A female generally lays frogspawn, or egg masses containing thousands of eggs, in water. The eggs are highly vulnerable to [[predation]], so frogs have evolved many techniques to ensure the survival of the next generation. In colder areas of the first, 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.<ref>{{cite journal| last=Warkentin| first=K.M.| year=1995| title=Adaptive plasticity in hatching age: a response to predation risk trade-offs| journal=Proceedings of the National Academy of Sciences| volume=92| pages=3507–3510| doi=10.1073/pnas.92.8.3507| pmid=11607529}}</ref> 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 goes through their whole larval phase inside the eggs or the mother, or they have direct development.
Eggs hatch and continue life as [[tadpole]]s (occasionally known as polliwogs). At least one species (Nannophrys ceylonensis) has tadpoles that are semi-terrestrial and lives among wet rocks<ref>[http://www.encyclopedia.com/doc/1P3-1416700551.html Ontogenetic Changes in Diet and Intestinal Morphology in Semi-Terrestrial Tadpoles of Nannophrys ceylonensis (Dicroglossidae)]</ref><ref>[http://amphibiaweb.org/cgi-bin/amphib_query?query_src=aw_lists_genera_&table=amphib&where-genus=Nannophrys&where-species=ceylonensis Nannophrys ceylonensis - Sri Lanka rock frog]</ref>, but as a general rule, free living larvae are fully aquatic. They lack lungs, eyelids, front and hind legs, and have a cartilaginous skeleton, a [[lateral line system]], gills for respiration (external gills at first, internal gills later) and tails with folds of skin fins for swimming<ref>[http://www.britannica.com/eb/article-40603/Anura Anura :: From tadpole to adult - Britannica Online Encyclopedia<!-- Bot generated title -->]</ref>. Some species who goes through the metapormhosis inside the egg and hatches to small frogs never develops 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 [[keratin]]ized structures called keradonts in the upper jaw while the lower jaw has three rows of keradonts, surrounded by a [[keratin|horny]] beak, but the number of rows can be lower or absent, or much higher[http://www.answers.com/topic/larvae]. Tadpoles are typically [[herbivore|herbivorous]], feeding mostly on [[alga]]e, including [[diatom]]s filtered from the water through the [[gill]]s. Some species are carnivorous at the tadpole stage, eating insects, smaller tadpoles, and fish. Tadpoles are highly vulnerable to predation by fish, [[newt]]s, predatory [[diving beetle]]s and birds such as [[kingfisher]]s. [[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''). In the [[Pipidae]], with the exception for Hymenochirus, the tadpoles have paired anterior barbells which makes the resemple small [[catfish]]<ref>[http://amphibiaweb.org/lists/Pipidae.shtml AmphibiaWeb - Pipidae<!-- Bot generated title -->]</ref>.
With the exception of the base of the tail, where a few vertebral structures develops to give rise to the urostyle later in life, the tails lacks completely solid, segmental skeletal elements of cartilage or bony tissue that is so typical for other vertebrates, althought it does contains a [[notochord]]<ref>[http://findarticles.com/p/articles/mi_qa3746/is_/ai_n8895798 Tadpole locomotion: Axial movement and tail functions in a largely vertebraeless vertebrate | American Zoologist | Find Articles at BNET.com<!-- Bot generated title -->]</ref>
At the end of the tadpole stage, frogs undergo [[Metamorphosis (biology)|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 [[carnivore|carnivorous]] as adults, eating invertebrates such as [[arthropod]]s, [[annelid]]s and [[Gastropoda|gastropod]]s. A few of the larger species may eat prey such as small [[mammal]]s, [[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.<ref>{{cite journal| last=Silva| first=H. R.| coauthors=Britto-Pereira M. C., & Caramaschi U.| year=1989| title=Frugivory and Seed Dispersal by ''Hyla truncata'', a Neotropical Treefrog| journal=Copeia| volume=1989(3)| pages=781–783| doi=10.2307/1445517}}</ref> Adult frogs are themselves preyed upon by [[bird]]s, large [[fish]], [[snake]]s, [[otter]]s, [[fox]]es, [[badger]]s, [[coati]]s, and other animals. Frogs are also eaten by people (see section on [[#Uses in agriculture and research|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 <ref>{{cite journal| last=Curry-Lindahl | year=1966 | title= | journal=Biegler}} see also http://www.pondturtle.com/lfrog.html#Bufo</ref>.
Although it is not common knowledge, some species of frog in their tadpole stage are known to be carnivorous. Early developers who gain legs may be eaten by the others, so the late bloomers survive longer. This has been observed in England in the species ''Rana temporaria'' (common frog).<ref>[http://www.marinebiology.co.uk/frogs.htm Frogs Found in the U.K.]. Retrieved 18 July 2007.</ref>
Unlike salamanders and newts, frogs and toads never become sexually mature while still in their larval stage.
===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.
[[Image:Bufo bufo couple during migration(2005).jpg|thumb|left|190px|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 fertilization|external]]: the [[Ovum|egg]] and [[Spermatozoon|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 [[United Kingdom|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===
[[Image:Haeckel Batrachia.jpg|thumb|right|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.<ref>{{cite journal|last=Crump| first=M.L.| year=1996| title=Parental care among the Amphibia| journal=Advances in the Study of Behavior| volume=25| pages=109–144| doi=10.1016/S0065-3454(08)60331-9}}</ref> 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 toad]]s, ''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 Frog]]s (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. Some species of frog will leave a 'babysitter' to watch over the frogspawn until it hatches.
===Call===
Some frog calls are so loud, they can be heard up to a mile away.<ref>See, for instance, [http://www.dnr.state.oh.us/publications/amphibians/frogs.htm Ohio's Toads and Frogs] by the Ohio Department of Natural Resources. Retrieved 18 July 2007.</ref>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 sac]]s, membranes of skin under the throat or on the corner of the mouth that distend during the amplification of the call. The field of [[neuroethology]] studies the neurocircuitry
that underlies frog audition.
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.<ref>{{cite journal|last=Roy| first=Debjani| year=1997| title=Communication signals and sexual selection in amphibians| journal=Current Science| volume=72| pages= 923–927}}</ref> 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 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 English [[onomatopoeia|onomatopoeic]] spelling is "ribbit". The croak of the [[bullfrog|American bullfrog]] (''Rana catesbiana'') is sometimes spelt "jug o' rum".<ref>{{citation|last=Hilton|first=Bill Jr.|title=Jug-o-Rum: Call of the Amorous Bullfrog|url=http://www.hiltonpond.org/PNBullfrog860608.html|work=The Piedmont Naturalist|publisher=Hilton Pond Center for Piedmont Natural History|volume=1|date=1986-06-08|accessdate=2008-06-26}}</reF> Other examples are Ancient Greek ''brekekekex koax koax'' for probably ''[[Rana ridibunda]]'', and the description in [[Rigveda]] 7:103.6 ''gómāyur éko ajámāyur ékaħ'' = "one [has] a voice like a cow's, one [has] a voice like a goat's".
==Distribution and conservation status==
[[Image:Bufo periglenes1.jpg|thumb|left|[[Golden toad]] (''Ollotis periglenes'') - last seen in 1989]]
The habitat of frogs extends almost worldwide, but they do not occur in [[Antarctica]] and are not present on many oceanic islands.<ref>"[http://magma.nationalgeographic.com/ngexplorer/0403/articles/mainarticle.html Freaky Frogs]," at ''[[National Geographic]] Explorer. Retrieved 18 July 2007.</ref><ref>[http://evolution-facts.org/Ev-V3/3evlch27.htm ''Evolution Encyclopedia'', Volume 3: Geographical Distribution]. Retrieved 18 July 2007.</ref> 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 [[hibernation|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]], whose habitat extends north of the [[Arctic Circle]], buries itself in the ground during winter when much of its body freezes.
Frog populations have [[Decline in amphibian populations|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.<ref>{{cite journal| last=Stuart| first=S.N.| coauthors= J.S. Chanson, N.A. Cox, B.E. Young, A.S.L. Rodrigues, D.L. Fischman, and R.W. Waller| year=2004| title=Status and trends of amphibian declines and extinctions worldwide| journal=Science| volume=306| pages=1783–1786| doi=10.1126/science.1103538| pmid=15486254}}</ref> 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 believe that amphibians, including frogs, are excellent biological [[indicator species|indicators]] of broader ecosystem health because of their intermediate position in food webs, permeable skins, and typically biphasic life (aquatic larvae and terrestrial adults).<ref>{{cite book| last=Phillips| first=Kathryn| title=Tracking the Vanishing Frogs| location=New York | publisher=Penguin Books| year=1994| id=ISBN 0-14-024646-0}}</ref> 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 [[Resistance|resistant]].<ref>[http://www.blackwell-synergy.com/doi/abs/10.1046/j.1523-1739.1998.96359.x Blackwell Synergy - Conservation Biology, Volume 12 Issue 1 Page 106-117, February 1998 (Article Abstract)<!-- Bot generated title -->]</ref>
A Canadian study conducted in 2006 proposed heavy traffic near frog habitats as a large threat to frog populations.<ref>{{cite journal|title=Frog population decrease mostly due to traffic| author=New Scientist|journal=New Scientist|location=advance online| date=July 7, 2006|url=http://www.newscientist.com/article/dn9506-frogs-toads-and-automobiles--a-fatal-combination.html}}</ref>
In a few cases, captive breeding programs have been attempted to alleviate the pressure on frog populations, and these have proved successful.<ref>[http://www.nationalparks.nsw.gov.au/npws.nsf/Content/dec_media_070109_01 DECC | Future improving for near extinct frog in Kosciuszko National Park<!-- Bot generated title -->]</ref><ref>[http://news.bbc.co.uk/1/hi/sci/tech/4298050.stm BBC NEWS | Science/Nature | New frog centre for London Zoo<!-- Bot generated title -->]</ref><ref>[http://www.environment.gov.au/biodiversity/threatened/publications/recovery/p-corroboree/part4.html National recovery plan for the Southern Corroboree Frog (Pseudophryne corroboree): 5. Previous Recovery Actions<!-- Bot generated title -->]</ref> In May 2007, it was reported the application of certain probiotic bacteria could protect amphibians from chytridiomycosis.<ref>[http://www.physorg.com/news99134333.html Bacteria show promise in fending off global amphibian killer<!-- Bot generated title -->]</ref>
Zoos and aquariums around the world have named 2008 the Year of the Frog, to draw attention to the conservation issues.<ref>MacNeill, R., ''Saving Kermit'', Canadian Geographic Magazine: April 2008, p. 19</ref>
==Evolution==
[[Image:Fossilised frog.jpg|thumb|right|A fossilized frog from the [[Czech Republic]], possibly ''[[Palaeobatrachus gigas]]''.]]
Until the discovery of the [[Permian|Early Permian]] ''[[Gerobatrachus hottoni]]'', 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]].<ref>{{cite web|last=Cannatella|first=David|title=Triadobatrachus massinoti|url=http://www.tolweb.org/Triadobatrachus_massinoti/16962|work=Tree of Life|date=1995|accessdate=2008-06-26}}</ref> 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 (bone)|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, 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 million years ago). It is known only from the [[Dorsum (biology)|dorsal]] and [[ventral]] impressions of a single animal and was estimated to be {{convert|33|mm|abbr=on}} 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<ref>{{cite news|title=China Yields East Asia's Earliest Fossilized Frog|url=http://english.peopledaily.com.cn/200111/20/eng20011120_84925.shtml|work=People's Daily|date=2001-11-20|accessdate=2008-06-26}}</ref> and had all modern frog features, but bore 9 presacral vertebrae instead of the 8 of modern frogs.<ref>{{cite news|title=Chinese frog discovery sheds light on amphibians' evolution|url=http://www.dhamurian.org.au/zoology/chinesefrog.html|work=The Dhamurian Society|publisher=Australian Broadcasting Company|date=2001-11-20|accessdate=2008-06-26}}</ref>
Frog fossils have been found on all continents, including [[Antarctica]].
==Uses in agriculture and research==
{{details|animal testing on frogs}}
Frogs are raised commercially for several purposes. Frogs are used as a food source; [[frog legs]] are a delicacy in [[China]], [[France]], the [[Philippines]], the north of [[Greece]] and in many parts of the [[American South]], especially [[Louisiana]]. Dead frogs are sometimes used for [[dissection]]s in high school and university anatomy classes, often after being injected with coloured plastics to enhance the contrast between the [[organ (anatomy)|organ]]s. 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 [[Egg (biology)|egg]]s. In 1952, [[Robert Briggs]] and [[Thomas J. King]] cloned a frog by [[somatic cell nuclear transfer]], the same technique later used to create [[Dolly the Sheep]], their experiment was the first time successful nuclear transplantation had been accomplished in metazoans.<ref>{{cite web | url=http://newton.nap.edu/html/biomems/rbriggs.html | title=Robert W. Briggs Biographical Memoir | accessdate=2006-04-22}}</ref>
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''),<ref>{{cite web | url=http://grants.nih.gov/grants/guide/rfa-files/RFA-HD-01-008.html | title=Developing the potential of ''Xenopus tropicalis'' as a genetic model | accessdate=2006-03-09 }}</ref> facilitating faster studies across generations. The [[genome project|genome sequence]] of ''X. tropicalis'' will probably be completed by 2015 at the latest.<ref>{{cite web | title=Joint Genome Institute - ''Xenopus tropicalis'' Home | url=http://genome.jgi-psf.org/Xentr4/Xentr4.home.html | accessdate=2006-03-03 }}</ref>
==Cultural beliefs==
[[Image:Frog1larcomuseum.jpg|thumb|right|Moche Frog 200 A.D. [[Larco Museum|Larco Museum Collection]] Lima, Peru.]]
{{details|Frogs in popular culture}}
Frogs feature prominently in [[folklore]], [[fairy tale]]s and popular culture. They tend to be portrayed as benign, ugly, clumsy, but with hidden talents. Examples include [[Michigan J. Frog]], [[The Frog Prince (story)|''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 [[kiss]]ed. 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.
The [[Moche]] people of ancient [[Peru]] worshipped animals and often depicted frogs in their art. <ref>Berrin, Katherine & Larco Museum. ''The Spirit of Ancient Peru:Treasures from the [[Larco Museum|Museo Arqueológico Rafael Larco Herrera]].'' New York: [[Thames and Hudson]], 1997.</ref>
==Cited references==
{{reflist|2}}
==General references==
<div class="references-small">
* {{cite book|last=Beltz|first=Ellin| year=2005| title=Frogs: Inside their Remarkable World| publisher=Firefly Books| id=ISBN 1552978699}}
* {{cite book|last=Cogger|first=H.G.|coauthors=R.G. Zweifel, and D. Kirschner| year=2004| title=Encyclopedia of Reptiles & Amphibians Second Edition| publisher=Fog City Press| id=ISBN 1-877019-69-0}}
* Estes, R., and O. A. Reig. (1973). "The early fossil record of frogs: a review of the evidence." pp. 11–63 In J. L. Vial (Ed.), ''Evolutionary Biology of the Anurans: Contemporary Research on Major Problems''. University of Missouri Press, Columbia.
*{{cite journal | last = Gissi | first = Carmela | title = Mitochondrial phylogeny of Anura (Amphibia): A case study of congruent phylogenetic reconstruction using amino acid and nucleotide characters | doi = 10.1016/j.gene.2005.07.034 | journal = Gene | volume = 366 | pages = 228–237 | year = 2006 | month = February | coauthors = Diego San Mauro, Graziano Pesole and Rafael Zardoya}}
* {{cite book|last=Holman|first=J. A| year=2004| title=Fossil Frogs and Toads of North America| publisher=Indiana University Press| id=ISBN 0-253-34280-5}}
*{{cite journal | last = San Mauro | first = Diego | title = Initial diversification of living amphibians predated the breakup of Pangaea | url = http://www.journals.uchicago.edu/AN/journal/issues/v165n5/40546/40546.html | journal = American Naturalist | volume = 165 | pages = 590–599 | year = 2005 | month = May| coauthors = Miguel Vences, Marina Alcobendas, Rafael Zardoya and Axel Meyer | doi = 10.1086/429523 | format = {{dead link|date=June 2008}} – <sup>[http://scholar.google.co.uk/scholar?hl=en&lr=&q=author%3ASan+Mauro+intitle%3AInitial+diversification+of+living+amphibians+predated+the+breakup+of+Pangaea&as_publication=American+Naturalist&as_ylo=2005&as_yhi=2005&btnG=Search Scholar search]</sup>}}
*{{cite book| last=Tyler| first=M. J.| year=1994| title=Australian Frogs A Natural History| publisher=Reed Books| id=ISBN 0-7301-0468-0}}
</div>
==External links==
{{commons|Frog}}
{{cookbook}}
{{wikispecies|Anura}}
* [http://www-itg.lbl.gov/ITG.hm.pg.docs/Whole.Frog/Whole.Frog.html The Whole Frog Project] - Virtual frog dissection and anatomy
* ''[http://raysweb.net/specialplaces/pages/frogsdecline.html Disappearance of toads, frogs has some scientists worried]'' - ''San Francisco Chronicle'', April 20, 1992
* [http://www.xenbase.org/ Xenbase] - A ''Xenopus laevis'' and ''tropicalis'' web resource
* [http://amphibiaweb.org Amphibia web]
* [http://markus.nolf.org/blog.php?p=160 Time-lapse video showing the egg's development until hatching]
* [http://www.midwestfrogs.com Frog calls] - Short video clips of calling frogs and interviews with scientists about frog issues
* [http://www.naturenorth.com/spring/sound/shfr2snd.html Frog calls - Canada]
* [http://www.naturesound.com/frogs/frogs.html eastern United States frog calls - eastern United States]
* [http://www.bbc.co.uk/nature/animals/wildbritain/springwatch/ Recording UK frogspawn sightings] - Springwatch 2006
* [http://www.nwf.org/frogwatchUSA/ Frogwatch USA] Volunteer frog and toad monitoring program by National Wildlife Federation and USGS, includes links to frog calls of the United States
* [http://calphotos.berkeley.edu/browse_imgs/amphibian_sci_1.html Amphibian photo gallery by scientific name] - features many unusual frogs
* [http://www.sciam.com/article.cfm?articleID=000A2086-B7D6-12F7-B7D683414B7F0000&ref=sciam Scientific American: Researchers Pinpoint Source of Poison Frogs' Deadly Defenses]''
{{featured article}}
{{Amphibians}}
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