A frog is any member of a diverse and largely carnivorous group of short-bodied, tailless amphibians composing the order Anura (Ancient Greek ἀν-, without + οὐρά, tail). The oldest fossil "proto-frog" appeared in the early Triassic of Madagascar, but molecular clock dating suggests their origins may extend further back to the Permian, 265 million years ago. Frogs are widely distributed, ranging from the tropics to subarctic regions, but the greatest concentration of species diversity is in tropical rainforests. There are approximately 4,800 recorded species, accounting for over 85% of extant amphibian species. They are also one of the five most diverse vertebrate orders. Warty frog species tend to be called toads, but the distinction between frogs and toads is informal, not from taxonomy or evolutionary history.
An adult frog has a stout body, protruding eyes, cleft tongue, limbs folded underneath, and no tail. Frogs have glandular skin, with secretions ranging from distasteful to toxic. Their skins 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. Adult frogs live in fresh water and on dry land; some species are adapted for living underground or in trees.
Frogs typically lay their eggs in water. The eggs hatch into aquatic larvae called tadpoles that have tails and internal gills. They have highly specialized rasping mouth parts suitable for herbivorous, omnivorous or planktivorous diets. The life cycle is completed when they metamorphose into adults. A few species deposit eggs on land or bypass the tadpole stage. Adult frogs generally have a carnivorous diet consisting of small invertebrates, but omnivorous species exist and a few feed on fruit. Frogs are extremely efficient at converting what they eat into body mass. They are an important food source for predators and part of the food web dynamics of many of the world's ecosystems. The skin is semi-permeable, making them susceptible to dehydration, so they either live in moist places or have special adaptations to deal with dry habitats. Frogs produce a wide range of vocalizations, particularly in their breeding season, and exhibit many different kinds of complex behaviours to attract mates, to fend off predators and to generally survive.
Frogs are valued as food by humans and also have many cultural roles in literature, symbolism and religion. Frog populations 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. The number of malformations among frogs is on the rise and an emerging fungal disease, chytridiomycosis, has spread around the world. Conservation biologists are working to understand the causes of these problems and to resolve them.
The name frog derives from Old English frogga, abbreviated to frox, forsc, and frosc, probably deriving from Proto-Indo-European preu = "to jump". About 88% of amphibian species are classified in the order Anura. These include around 4,810 species in 33 families, of which the Leptodactylidae (1,100 spp.), Hylidae (800 spp.) and Ranidae (750 spp.) are the richest in species. 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.
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 breakup 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.
Frogs have no tail, except as larvae, and most have long hind legs, elongated ankle bones, webbed toes, no claws, large eyes, and a smooth or warty skin. They have short vertebral columns, with no more than 10 free vertebrae and fused tailbones (urostyle or coccyx). Like other amphibians, oxygen can pass through their highly permeable skins. This unique feature allows them to remain in places without access to the air, respiring through their skins. The ribs are poorly developed, so the lungs are filled by buccal pumping and a frog deprived of its lungs can maintain its body functions without them. For the skin to serve as a respiratory organ, it must remain moist. This makes frogs susceptible to various substances they may encounter in the environment, some of which may be toxic and can dissolve in the water film and be passed into their bloodstream. This may be one of the causes of the worldwide decline in frog populations.
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 to do so. Most frogs are either proficient at jumping or are descended from ancestors that were, with much of the musculoskeletal 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 fore limbs (which must absorb the impact on landing). The metatarsals have become elongated to add to the leg length and allow frogs to push against the ground for a longer period on take-off. The illium has elongated and formed a mobile joint with the sacrum which, in specialist jumpers such as ranids and hylids, functions as an additional limb joint to further power the leaps. The tail vertebrae have fused into a urostyle which is retracted inside the pelvis. This enables the force to be transferred from the legs to the body during a leap.
A frog's skin is protective, has a respiratory function, can absorb water and helps control body temperature. It has many glands, particularly on the head and back, which often exude distasteful and toxic substances (granular glands). The secretion is often sticky and helps keep the skin moist, protects against the entry of moulds and bacteria, and make the animal slippery and more able to escape from predators. The skin is shed every few weeks. It usually splits down the middle of the back and across the belly, and the frog pulls its arms and legs free. The sloughed skin is then worked towards the head where it is quickly eaten.
The skin of a frog is permeable to oxygen and carbon dioxide, as well as to water. There are blood vessels near the surface of the skin and when a frog is underwater, oxygen diffuses directly into the blood. When not submerged, a frog breathes by a process known as buccal pumping. Its lungs are similar to those of humans, but the chest muscles are not involved in respiration, and no ribs or diaphragm exist to help move air in and out. Instead, it puffs out its throat and draws air in through the nostrils, which in many species can then be closed by valves. When the floor of the mouth is compressed, air is forced into the lungs. The fully aquatic Bornean flat-headed frog (Barbourula kalimantanensis) is the first frog known to lack lungs entirely.
Frogs have maxillary teeth along their upper jaw 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. The tongue normally lies coiled in the mouth, free at the back and attached to the mandible at the front. It can be shot out and retracted at great speed. Some frogs have no tongue and just stuff food into their mouths with their hands. The eyes assist in the swallowing of food as they can be retracted through holes in the skull and help push food down the throat. 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 excess water is removed and the wastes are passed out through the cloaca.
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