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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

Frogspawn
Frogspawn development
Tadpole of Haswell's Froglet (Paracrinia haswelliThe 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.[16] 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. 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 lives among wet rocks,[17][18] 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 dorsal and ventral folds of skin for swimming.[19] 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.[1] 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 England in the species Rana temporaria (common frog).[20]

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.[21]


Adult leopard frogWith 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 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.

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Adventist Youth Honors Answer Book/Nature/Life cycle of a frog
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.[22] 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.[23]

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.

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.[24] 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 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.[25] The call of a frog is unique to its species. Frogs call by passing air through the larynx in the throat. In most calling frogs, the sound is amplified by one or more vocal sacs, membranes of skin under the throat or on the corner of the mouth that distend during the amplification of the call. 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.[26] 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 onomatopoeic spelling is "ribbit". The croak of the American bullfrog (Rana catesbiana) is sometimes spelt "jug o' rum".[27] 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

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.[28][29] 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, whose habitat extends north of the Arctic Circle, buries itself in the ground during winter when much of its body freezes.


Golden toad (Bufo periglenes) - last seen in 1989Frog 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.[30] 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 indicators of broader ecosystem health because of their intermediate position in food webs, permeable skins, and typically biphasic life (aquatic larvae and terrestrial adults).[31] 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 .[32]

A Canadian study conducted in 2006, suggested heavy traffic near frog habitats as a large threat to frog populations.[33] In a few cases, captive breeding programs have been attempted to alleviate the pressure on frog populations, and these have proved successful.[34][35] In 2007, it was reported the application of certain probiotic bacteria could protect amphibians from chytridiomycosis.[36] 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.[37]

Zoos and aquariums around the world named 2008 the Year of the Frog, to draw attention to the conservation issues.[38]

Evolution

A fossilized frog from the Czech Republic, possibly Palaeobatrachus gigasUntil the discovery of the 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.[39] 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,[40] 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.[41]

The earliest 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[42] and had all modern frog features, but bore 9 presacral vertebrae instead of the 8 of modern frogs.[43]

Frog fossils have been found on all continents, including Antarctica.[citation needed]

Uses in agriculture and research
For more details on this topic, see 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 dissections in high school and university anatomy classes, often after being injected with coloured plastics to enhance the contrast between the organs. This practice has declined in recent years with the increasing concerns about animal welfare.

Frogs have served as important model organisms throughout the history of science. Eighteenth-century biologist Luigi Galvani discovered the link between electricity and the nervous system through studying frogs. The African clawed frog or platanna (Xenopus laevis) was first widely used in laboratories in pregnancy assays in the first half of the 20th century. When human chorionic gonadotropin, a hormone found in substantial quantities in the urine of pregnant women, is injected into a female X. laevis, it induces them to lay eggs. In 1952, Robert Briggs and Thomas J. King cloned a frog by somatic cell nuclear transfer, the same technique later used to create Dolly the Sheep, their experiment was the first time successful nuclear transplantation had been accomplished in metazoans.[44]

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),[45] facilitating faster studies across generations. The genome sequence of X. tropicalis will probably be completed by 2015 at the latest.[46]

Cultural beliefs
For more details on this topic, see Frogs in popular culture.

Moche Frog 200 A.D. Larco Museum Collection Lima, PeruFrogs 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 behaviour of more flamboyant characters.

The Moche people of ancient Peru worshipped animals and often depicted frogs in their art.[47]

See also
A Plague of Frogs (book)
See-through frog
Cited references
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^ Myers, C.W.; J.W. Daly (1983). "Dart-poison frogs". Scientific American 248: 120–133.
^ Savage, J. M. (2002). The Amphibians and Reptiles of Costa Rica. University of Chicago Press, Chicago.
^ Duellman, W. E. (197 cool . "The Biology of an Equatorial Herpetofauna in Amazonian Ecuador". University of Kansas Museum of Natural History Miscellaneous Publication 65: 1–352.
^ VanCompernolle, Scott. E.; 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 (2005). "Antimicrobial peptides from amphibian skin potently inhibit Human Immunodeficiency Virus infection and transfer of virus from dendritic cells to T cells". Journal of Virology 79: 11598–11606. doi:10.1128/JVI.79.18.11598-11606.2005. PMID 16140737.
^ Phillipe, G.; Angenot L. (2005). "Recent developments in the field of arrow and dart poisons". J Ethnopharmacol 100(1–2): 85–91. doi:10.1016/j.jep.2005.05.022.
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^ "Ontogenetic Changes in Diet and Intestinal Morphology in Semi-Terrestrial Tadpoles of Nannophrys ceylonensis (Dicroglossidae)". http://www.encyclopedia.com/doc/1P.html.
^ "Nannophrys ceylonensis - Sri Lanka rock frog". Amphibiaweb.org. 2005-05-10. http://amphibiaweb.org/cgi-bin/amphib_query?query_src=aw_lists_genera_&table=amphib&where-genus=Nannophrys&where-species=ceylonensis. Retrieved 2009-03-06.
^ "Anura :: From tadpole to adult - Britannica Online Encyclopedia". Britannica.com. http://www.britannica.com/eb/article-40603/Anura. Retrieved 2008-11-03.
^ Frogs Found in the U.K.. Retrieved 18 July 2007.
^ "AmphibiaWeb - Pipidae". Amphibiaweb.org. http://amphibiaweb.org/lists/Pipidae.shtml. Retrieved 2008-11-03.
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^ Curry-Lindahl (1966). Biegler. see also http://www.pondturtle.com/lfrog.html#Bufo
^ Crump, M.L. (1996). "Parental care among the Amphibia". Advances in the Study of Behavior 25: 109–144. doi:10.1016/S0065-3454(0 cool 60331-9.
^ See, for instance, Ohio's Toads and Frogs by the Ohio Department of Natural Resources. Retrieved 18 July 2007.
^ Roy, Debjani (1997). "Communication signals and sexual selection in amphibians". Current Science 72: 923–927.
^ Hilton, Bill Jr. (1986-06-0 cool , "Jug-o-Rum: Call of the Amorous Bullfrog", The Piedmont Naturalist (Hilton Pond Center for Piedmont Natural History) 1, http://www.hiltonpond.org/PNBullfrog860608.html, retrieved 2008-06-26
^ "Freaky Frogs," at National Geographic Explorer. Retrieved 18 July 2007.
^ Evolution Encyclopedia, Volume 3: Geographical Distribution. Retrieved 18 July 2007.
^ Stuart, S.N.; J.S. Chanson, N.A. Cox, B.E. Young, A.S.L. Rodrigues, D.L. Fischman, and R.W. Waller (2004). "Status and trends of amphibian declines and extinctions worldwide". Science 306: 1783–1786. doi:10.1126/science.1103538. PMID 15486254.
^ Phillips, Kathryn (1994). Tracking the Vanishing Frogs. New York: Penguin Books. ISBN 0-14-024646-0.
^ "Blackwell Synergy - Conservation Biology, Volume 12 Issue 1 Page 106-117, February 1998 (Article Abstract)". Blackwell-synergy.com. http://www.blackwell-synergy.com/doi/abs/10.1046/j.1523-1739.1998.96359.x. Retrieved 2008-11-03.
^ New Scientist (July 7, 2006). "Frog population decrease mostly due to traffic". New Scientist (advance online). http://www.newscientist.com/article/dn9506-frogs-toads-and-automobiles--a-fatal-combination.html.
^ Richard Black (Last Updated smile . "BBC NEWS | Science/Nature | New frog centre for London Zoo". News.bbc.co.uk. http://news.bbc.co.uk/1/hi/sci/tech/4298050.stm. Retrieved 2008-11-03.
^ "National recovery plan for the Southern Corroboree Frog (Pseudophryne corroboree): 5. Previous Recovery Actions". Environment.gov.au. http://www.environment.gov.au/biodiversity/threatened/publications/recovery/p-corroboree/part4.html. Retrieved 2008-11-03.
^ "Bacteria show promise in fending off global amphibian killer". Physorg.com. http://www.physorg.com/news99134333.html. Retrieved 2008-11-03.
^ "Project launched to fight frog-killing fungus". http://www.guardian.co.uk. http://www.guardian.co.uk/world/feedarticle/8502055. Retrieved 2009-07-19.
^ MacNeill, R., Saving Kermit, Canadian Geographic Magazine: April 2008, p. 19
^ Cannatella, David (1995). "Triadobatrachus massinoti". Tree of Life. http://www.tolweb.org/Triadobatrachus_massinoti/16962. Retrieved 2008-06-26.
^ Weishampel, David B; et al (2004). "Dinosaur distribution (Early Jurassic, North America)." In: Weishampel, David B.; Dodson, Peter; and Osmólska, Halszka (eds.): The Dinosauria, 2nd, Berkeley: University of California Press. Pp. 530-532. ISBN 0-520-24209-2.
^ Foster, J. (2007). "Anura (Frogs)." Jurassic West: The Dinosaurs of the Morrison Formation and Their World. Indiana University Press. pp. 135-136.
^ "China Yields East Asia's Earliest Fossilized Frog". People's Daily. 2001-11-20. http://english.peopledaily.com.cn/200111/20/eng20011120_84925.shtml. Retrieved 2008-06-26.
^ "Chinese frog discovery sheds light on amphibians' evolution". The Dhamurian Society (Australian Broadcasting Company). 2001-11-20. http://www.dhamurian.org.au/zoology/chinesefrog.html. Retrieved 2008-06-26.
^ "Robert W. Briggs Biographical Memoir". http://newton.nap.edu/html/biomems/rbriggs.html. Retrieved 2006-04-22.
^ "Developing the potential of Xenopus tropicalis as a genetic model". http://grants.nih.gov/grants/guide/rfa-files/RFA-HD-01-008.html. Retrieved 2006-03-09.
^ "Joint Genome Institute - Xenopus tropicalis Home". http://genome.jgi-psf.org/Xentr4/Xentr4.home.html. Retrieved 2006-03-03.
^ Berrin, Katherine & Larco Museum. The Spirit of Ancient Peru:Treasures from the Museo Arqueológico Rafael Larco Herrera. New York: Thames and Hudson, 1997.
General references
Beltz, Ellin (2005). Frogs: Inside their Remarkable World. Firefly Books. ISBN 1552978699.
Cogger, H.G.; R.G. Zweifel, and D. Kirschner (2004). Encyclopedia of Reptiles & Amphibians Second Edition. Fog City Press. 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.
Gissi, Carmela; Diego San Mauro, Graziano Pesole and Rafael Zardoya (February 2006). "Mitochondrial phylogeny of Anura (Amphibia): A case study of congruent phylogenetic reconstruction using amino acid and nucleotide characters". Gene 366: 228–237. doi:10.1016/j.gene.2005.07.034.
Holman, J. A (2004). Fossil Frogs and Toads of North America. Indiana University Press. ISBN 0-253-34280-5.
San Mauro, Diego; Miguel Vences, Marina Alcobendas, Rafael Zardoya and Axel Meyer (May 2005). "Initial diversification of living amphibians predated the breakup of Pangaea" (http://www.journals.uchicago.edu/doi/abs/10.1086/429523). American Naturalist 165: 590–599. doi:10.1086/429523. http://www.journals.uchicago.edu/AN/journal/issues/v165n5/40546/40546.html.
Tyler, M. J. (1994). Australian Frogs A Natural History. Reed Books. ISBN 0-7301-0468-0.
External links
Wikimedia Commons has media related to: Frog
Wikibooks Cookbook has a recipe/module on
Frog
Wikiquote has a collection of quotations related to: Frogs
Wikispecies has information related to: Anura
AmphibiaWeb
The Whole Frog Project - Virtual frog dissection and anatomy
Disappearance of toads, frogs has some scientists worried - San Francisco Chronicle, April 20, 1992
Recording UK frogspawn sightings - Springwatch 2006
Frogwatch USA - Volunteer frog and toad monitoring program by National Wildlife Federation and USGS, includes links to frog calls of the United States
Amphibian photo gallery by scientific name - Features many unusual frogs
Scientific American: Researchers Pinpoint Source of Poison Frogs' Deadly Defenses
Media
Time-lapse video showing the egg's development until hatching
Frog calls - Short video clips of calling frogs and interviews with scientists about frog issues
Frog vocalisations from around the world - From the British Library Sound Archive
Frog calls - From Manitoba, Canada
Frog calls - From eastern and central North America