A frog is any member of a diverse and largely semiaquatic group of short-bodied, tailless amphibian vertebrates composing the order Anura (coming from the Ancient Greek , literally 'without tail'). Frog species with rough skin texture due to wart-like parotoid glands tend to be called toads, but the distinction between frogs and toads is informal and purely cosmetic, and does not reflect taxonomy or evolutionary history.
Frogs are widely distributed, ranging from the tropics to subarctic regions, but the greatest concentration of species diversity is in tropical rainforest and associated wetlands. They account for around 88% of extant amphibian species, and are one of the five most diverse vertebrate orders. The oldest fossil "proto-frog" Triadobatrachus is known from the Early Triassic of Madagascar (250million years ago), but molecular clock dating suggests their divergence from other amphibians may extend further back to the Permian, 265million years ago.
Adult frogs have a stout body, protruding eyes, anteriorly-attached tongue, limbs folded underneath, and no tail (the "tail" of tailed frogs is an extension of the male cloaca). Frogs have glandular skin, with secretions ranging from distasteful to toxic. Their skin varies in colour from well-camouflaged dappled brown, grey and green, to vivid patterns of bright red or yellow and black to show toxicity and ward off predators (aposematism). Adult frogs live in both fresh water and on dry land; some species are adapted for living underground or in trees. As their skin is semi-permeable, making them susceptible to dehydration, they either live in moist niches or have special adaptations to deal with drier habitats. Frogs produce a wide range of vocalisations, particularly in their breeding season, and exhibit many different kinds of complex behaviors to attract mates, to fend off predators and to generally survive.
Frogs typically spawn their eggs in bodies of water. The eggs then hatch into fully aquatic larvae called tadpoles, which have tails and internal gills. A few species lay eggs on land or bypass the tadpole stage altogether. Tadpoles have highly specialised rasping mouth parts suitable for herbivorous, omnivorous or planktivorous diets. The life cycle is completed when they metamorphose into semiaquatic adults capable of terrestrial locomotion and hybrid respiration using both lungs aided by buccal pumping and gas exchange across the skin, and the larval tail regresses into an internal urostyle. Adult frogs generally have a carnivorous diet consisting of small invertebrates, especially insects, but omnivorous species exist and a few feed on plant matter. Frogs generally seize and ingest food by protruding their adhesive tongue and then swallow the item whole, often using their eyeballs and extraocular muscles to help pushing down the throat, and their digestive system is extremely efficient at converting what they eat into body mass. Being low-level consumers, both tadpoles and adult frogs are an important food source for other predators and a vital part of the food web dynamics of many of the world's ecosystems.
Frogs (especially their muscular hindlimbs) are eaten by humans as food in many cuisines, and also have many cultural roles in literature, symbolism and religion. They are environmental bellwethers, with declines in frog populations considered early warning signs of environmental degradation. Global frog populations and diversities have declined significantly since the 1950s. More than one third of species are considered to be threatened with extinction, and over 120 are believed to have become extinct since the 1980s. Frog malformations are on the rise as an emerging fungal disease, chytridiomycosis, has spread around the world. Conservation biologists are working to solve these problems.
Etymology and taxonomy
The use of the common names frog and toad has no taxonomic justification. From a classification 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 and have smooth, moist skins; the term toad generally refers to species that are terrestrial with dry, warty skins. There are numerous exceptions to this rule. The European fire-bellied toad (Bombina bombina) has a slightly warty skin and prefers a watery habitat whereas the Panamanian golden frog (Atelopus zeteki) is in the toad family Bufonidae and has a smooth skin.
Etymology The origin of the order name Anura—and its original spelling Anoures—is the Ancient Greek alpha privative prefix ( from before a vowel) 'without', and () 'animal tail'. meaning "tailless". It refers to the tailless character of these amphibians.
The origins of the word frog are uncertain and debated. The word is first attested in Old English as , but the usual Old English word for the frog was (with variants such as and ), and it is agreed that the word frog is somehow related to this. Old English remained in dialectal use in English as frosh and frosk into the nineteenth century, and is paralleled widely in other Germanic languages, with examples in the modern languages including German , Norwegian , Icelandic , and Dutch . These words allow reconstruction of a Common Germanic ancestor . The third edition of the Oxford English Dictionary finds that the etymology of is uncertain, but agrees with arguments that it could plausibly derive from a Proto-Indo-European base along the lines of , meaning 'jump'.
How Old English gave rise to is, however, uncertain, as the development does not involve a regular sound-change. Instead, it seems that there was a trend in Old English to coin nicknames for animals ending in -g, with examples—themselves all of uncertain etymology—including dog, hog, pig, stag, and . Frog appears to have been adapted from as part of this trend.
Meanwhile, the word toad, first attested as Old English , is unique to English and is likewise of uncertain etymology. It is the basis for the word tadpole, first attested as Middle English , apparently meaning 'toad-head'.
Taxonomy About 87% of amphibian species are classified in the order Anura. These include over 7,700 species in 59 families, of which the Hylidae (1062 spp.), Strabomantidae (807 spp.), Microhylidae (758 spp.), and Bufonidae (657 spp.) are the richest in species.
The Anura include all modern frogs and any fossil species that fit within the anuran definition. The characteristics of anuran adults include: 9 or fewer presacral vertebrae, the presence of a urostyle formed of fused vertebrae, no tail, a long and forward-sloping ilium, shorter fore limbs than hind limbs, radius and ulna fused, tibia and fibula fused, elongated ankle bones, absence of a prefrontal bone, presence of a hyoid plate, a lower jaw without teeth (with the exception of Gastrotheca guentheri) consisting of three pairs of bones (angulosplenial, dentary, and mentomeckelian, with the last pair being absent in Pipoidea),Duellman, William E. an unsupported tongue, lymph spaces underneath the skin, and a muscle, the protractor lentis, attached to the lens of the eye. The anuran larva or tadpole has a single central respiratory spiracle and mouthparts consisting of keratinous beaks and denticles.
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 families of modern frogs, including most common species throughout the world. The suborder Neobatrachia is further divided into the two superfamilies Hyloidea and Ranoidea. This classification is based on such morphological features as the number of vertebrae, the structure of the pectoral girdle, and the morphology of tadpoles. While this classification is largely accepted, relationships among families of frogs are still debated.
Some species of anurans hybridise readily. For instance, the edible frog (Pelophylax esculentus) is a hybrid between the pool frog (P. lessonae) and the marsh frog (P. ridibundus). The fire-bellied toads Bombina bombina and B. variegata are similar in forming hybrids. These are less fertile than their parents, giving rise to a hybrid zone where the hybrids are prevalent.
Evolution The origins and evolutionary relationships between the three main groups of amphibians are hotly debated. A molecular phylogeny based on rDNA analysis dating from 2005 suggests that salamanders and caecilians are more closely related to each other than they are to frogs and the divergence of the three groups took place in the Paleozoic or early Mesozoic before the break-up of the supercontinent Pangaea and soon after their divergence from the lobe-finned fishes. This would help account for the relative scarcity of amphibian fossils from the period before the groups split. Another molecular phylogenetic analysis conducted about the same time concluded that lissamphibians first appeared about 330 million years ago and that the temnospondyl-origin hypothesis is more credible than other theories. The neobatrachians seemed to have originated in Africa/India, the salamanders in East Asia and the caecilians in tropical Pangaea. Other researchers, while agreeing with the main thrust of this study, questioned the choice of calibration points used to synchronise the data. They proposed that the date of lissamphibian diversification should be placed in the Permian, rather less than 300 million years ago, a date in better agreement with the palaeontological data. A further study in 2011 using both extinct and living taxa sampled for morphological, as well as molecular data, came to the conclusion that Lissamphibia is monophyletic and that it should be nested within Lepospondyli rather than within Temnospondyli. The study postulated that Lissamphibia originated no earlier than the late Carboniferous, some 290 to 305 million years ago. The split between Anura and Caudata was estimated as taking place 292 million years ago, rather later than most molecular studies suggest, with the caecilians splitting off 239 million years ago.
In 2008, Gerobatrachus hottoni, a temnospondyl with many frog- and salamander-like characteristics, was discovered in Texas. It dated back 290 million years and was hailed as a missing link, a stem batrachian close to the common ancestor of frogs and salamanders, consistent with the widely accepted hypothesis that frogs and salamanders are more closely related to each other (forming a clade called Batrachia) than they are to caecilians. However, others have suggested that Gerobatrachus hottoni was only a dissorophoid temnospondyl unrelated to extant amphibians.