| Frogs Fossil range: Triassic–present | ||||||||
|---|---|---|---|---|---|---|---|---|
 White's Tree Frog (Litoria caerulea) | ||||||||
| Scientific classification | ||||||||
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 Native distribution of frogs (in black) | ||||||||
| Suborders | ||||||||
| Archaeobatrachia |
Frogs are amphibians 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 frogga, (compare Old Norse frauki, German Frosch, older Dutch spelling kikvorsch), cognate with Sanskrit plava (frog), probably deriving from Proto-Indo-European praw = "to jump".[1]
Most frogs are characterized by long hind legs, a short body, webbed digits (fingers or toes), protruding eyes 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 eggs in puddles, ponds or lakes, and their larvae, called tadpoles, have gills and develop in water. Adult frogs follow a carnivorous diet, mostly of arthropods, annelids and gastropods. Frogs are most noticeable by their call, which can be widely heard during the night or day, mainly in their mating season.
The distribution of frogs ranges from tropic to subarctic regions, but most species are found in tropical rainforests. Consisting of more than 5,000 species described, they are among the most diverse groups of vertebrates. However, populations of certain frog species are declining significantly.
A distinction is often made between frogs and toads on the basis of their appearance, caused by the convergent adaptation among so-called toads to dry environments; however, this distinction has no taxonomic basis. The only family exclusively given the common name "toad" is Bufonidae, but many species from other families are also called "toads," and the species within the toad genus Atelopus are referred to as "harlequin frogs".
Taxonomy
The order Anura contains 4,810 species[2] in 33 families, of which the Leptodactylidae (1100 spp.), Hylidae (800 spp.) and Ranidae (750 spp.) are the richest in species. About 88% of amphibian species are frogs.
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.[3] This classification is based on such morphological features as the number of vertebrae, the structure of the pectoral girdle, and the morphology of tadpoles. While this classification is largely accepted, relationships among families of frogs are still debated. Future studies of molecular genetics should soon provide further insights to the evolutionary relationships among Anuran families.[4]
Some species of anurans hybridise readily. For instance, the Edible Frog (Rana esculenta) is a hybrid of the Pool Frog (R. lessonae) and the Marsh Frog (R. ridibunda). Bombina bombina and Bombina variegata similarly form hybrids, although these are less fertile, giving rise to a hybrid zone.
Morphology and physiology
The morphology of frogs is unique among amphibians. Compared with the other two groups of amphibians, (salamanders and caecilians), frogs are unusual because they lack tails as adults and their legs are more suited to jumping than walking. The physiology of frogs is generally like that of other amphibians (and differs from other terrestrial vertebrates) because oxygen can pass through their highly permeable skin. This unique feature allows frogs to "breathe" largely through their skin.[citation needed] 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 the cause of the decline in frog populations.[citation needed]
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.[citation needed]
Frogs range in size from 10 mm (0.39 in) (Brachycephalus didactylus of Brazil and Eleutherodactylus iberia of Cuba) to 300 mm (12 in) (goliath frog, Conraua goliath, of Cameroon). The skin hangs loosely on the body because of the lack of loose connective tissue. Skin texture varies: it can be smooth, warty or folded. Frogs have three eyelid membranes: one is transparent to protect the eyes underwater, and two vary from translucent to opaque. Frogs have a tympanum on each side of the head, which is involved in hearing and, in some species, is covered by skin. Most frogs 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.[citation needed]
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.
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.[5]
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.[6]
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.
Jumping
Frogs are generally recognized as exceptional jumpers, and the best jumper of all vertebrates. The Australian rocket frog, Litoria nasuta, can leap over 50 times its body length (5.5 cm), resulting in jumps of over 2 meters. The acceleration of the jump may be up to twice gravity. There are tremendous differences between species in jumping capability, but within a species, jump distance increases with increasing size, but relative jumping distance (body-lengths jumped) decreases.
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 modified for this purpose. The tibia, fibula and tarsals have been fused into a single, strong bone, as have the radius and ulna in the forelimbs (which must absorb the impact of landing). The metatarsals have become elongated to add to the leg length and allow the frog to push against the ground for longer during a jump. The illium has elongated and formed a mobile joint with the sacrum which, in specialist jumpers such as Ranids or Hylids, functions as an additional limb joint to further power the leaps. This elongation of the limbs results in the frog being able to apply force to the ground for longer during a jump, which in turn results in a longer, faster jump.[citation needed]
The muscular system has been similarly modified. The hind limbs of the ancestor of frogs presumably contained pairs of muscles which would act in opposition (one muscle to flex the knee, a different muscle to extend it), as is seen in most other limbed animals. However, in modern frogs, almost all muscles have been modified to contribute to the action of jumping, with only a few small muscles remaining to bring the limb back to the starting position and maintain posture. The muscles have also been greatly enlarged, with the muscles involved in jumping accounting for over 17% of the total mass of the frog.
In some extremely capable jumpers, such as the cuban tree frog, the peak power exerted during a jump can exceed what muscle is capable of producing. Currently, it is hypothesized that frogs are storing muscular energy by stretching their tendons like springs, then triggering the release all at once, allowing the frog to increase the energy of its jump beyond the limits of muscle-powered acceleration. A similar mechanism has already been documented in locusts and grasshoppers.[citation needed]
Skin
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 folds are usually found on ground-dwelling frogs, where a smooth skin would not disguise them effectively. Arboreal frogs usually have smooth skin, enabling them to disguise themselves as leaves.[citation needed]
Certain frogs change colour between night and day, as light and moisture stimulate the pigment cells and cause them to expand or contract.
Poison
Many frogs contain mild toxins that make them unpalatable to potential predators. For example, all toads have large poison glands—the parotoid glands—located behind the eyes on the top of the head. Some frogs, such as some poison dart frogs, are especially toxic. The chemical makeup of toxins in frogs varies from irritants to hallucinogens, convulsants, nerve poisons, and vasoconstrictors. Many predators of frogs have adapted to tolerate high levels of these poisons. Others, including humans, may be severely affected.
Some frogs obtain poisons from the ants and other arthropods they eat;[7] others, such as the Australian Corroboree Frogs (Pseudophryne corroboree and Pseudophryne pengilleyi), can manufacture an alkaloid not derived from their diet.[8] Some native people of South America extract poison from the poison dart frogs and apply it to their darts for hunting,[9] 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).[10][11]
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.[12] Arrow and dart poisons are under active investigation for their potential as therapeutic drugs.[13]
The skin secretions of some toads, such as the Colorado River toad and cane toad, contain bufotoxins, some of which, such as bufotenin, are psychoactive, and have therefore been used as recreational drugs. Typically, the skin secretions are dried and smoked. Skin licking is especially dangerous, and appears to constitute an urban myth. See psychoactive toad.
Respiration and circulation
The skin of a frog is permeable to oxygen and carbon dioxide, as well as to water. There are a number of blood vessels near the surface of the skin. When a frog is underwater, oxygen is transmitted through the skin directly into the bloodstream. On land, adult frogs use their lungs to breathe. Their lungs are similar to those of humans, but the chest muscles are not involved in respiration, and there are no ribs or diaphragm to support breathing. Frogs breathe by taking air in through the nostrils (which often have valves which close when the frog is submerged), causing the throat to puff out, then compressing the floor of the mouth, which forces the air into the lungs. In August 2007 an aquatic frog named Barbourula kalimantanensis was discovered in a remote part of Indonesia. The Bornean Flat-headed Frog (B. kalimantanensis) is the first species of frog known to science without lungs.
Frogs are known for their three-chambered heart, which they share with all tetrapods except birds and mammals. In the three-chambered heart, oxygenated blood from the lungs and de-oxygenated blood from the respiring tissues enter by separate atria, and are directed via a spiral valve to the appropriate vessel—aorta for oxygenated blood and pulmonary 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.
Some species of frog have remarkable adaptations that allow them to survive in oxygen deficient water. The lake titicaca frog (Telmatobius culeus) is one such species and to survive in the poorly oxygenated waters of Lake Titicaca it has incredibly wrinkly skin that increases its surface area to enhance gas exchange. This frog will also do 'push-ups' on the lake bed to increase the flow of water around its body.[14]
Digestion and excretion
The frog's digestive system begins with the mouth. Frogs have teeth along their upper jaw called the maxillary teeth, which are used to grind food before swallowing. These teeth are very weak, and cannot be used to catch or harm agile prey. Instead, the frog uses its sticky tongue to catch food (such as flies or other insects). The food then moves through the esophagus into the stomach. The food then proceeds to the small intestine (duodenum and ileum) where most digestion occurs. Frogs carry pancreatic juice from the pancreas, and bile (produced by the liver) through the gallbladder from the liver to the small intestine, where the fluids digest the food and extract the nutrients. When the food passes into the large intestine, the water is reabsorbed and wastes are routed to the cloaca. All wastes exit the body through the cloaca and the cloacal vent.
Nervous system
The frog has a highly developed nervous system which consists of a brain, spinal cord and nerves. Many parts of the frog's brain correspond with those of humans. The medulla oblongata regulates respiration, digestion, and other automatic functions. Muscular coordination and posture are controlled by the cerebellum. The relative size of the cerebrum of a frog is much smaller than that of a human. Frogs have ten cranial nerves (nerves which pass information from the outside directly to the brain) and ten pairs of spinal nerves (nerves which pass information from extremities to the brain through the spinal cord). By contrast, all amniotes (mammals, birds and reptiles) have twelve cranial nerves. Frogs do not have external ears; the eardrums (tympanic membranes) are directly exposed. As in all animals, the ear contains semicircular canals which help control balance and orientation.
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
The life cycle of a frog starts with an egg. A female generally lays gelatinous egg masses containing thousands of eggs, in water. Each anuran species lays eggs in a distinctive, identifiable manner. An example are the long strings of eggs laid by the common American toad. The eggs are highly vulnerable to predation, so frogs have evolved many techniques to ensure the survival of the next generation. In colder areas the embryo is black to absorb more heat from the sun, which speeds up the development. Most commonly, this involves synchronous reproduction. Many individuals will breed at the same time, overwhelming the actions of predators; the majority of the offspring will still die due to predation, but there is a greater chance some will survive. Another way in which some species avoid the predators and pathogens eggs are exposed to in ponds is to lay eggs on leaves above the pond, with a gelatinous coating designed to retain moisture. In these species the tadpoles drop into the water upon hatching. The eggs of some species laid out of water can detect vibrations of nearby predatory wasps or snakes, and will hatch early to avoid being eaten.[15] 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,[16][17] 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.[18] 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).[19]
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.[20]
With the exception of the base of the tail, where a few vertebral structures develop to give rise to the urostyle later in life, the tail lacks the completely solid, segmental, skeletal elements of cartilage or bony tissue that are so typical for other vertebrates, although it does contain a notochord
At the end of the tadpole stage, frogs undergo metamorphosis, in which they 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 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.[21] 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.[22]
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.
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
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.[23] 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.[24] 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.[25] 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".[26] 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 habitat of frogs extends almost worldwide, but they do not occur in Antarctica and are not present on many oceanic islands.[27][28] 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.
Frog populations have declined dramatically since the 1950s: more than one third of species are believed to be threatened with extinction and more than 120 species are suspected to be extinct since the 1980s.[29] 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).[30] 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 .[31]
A Canadian study conducted in 2006, suggested heavy traffic near frog habitats as a large threat to frog populations.[32] In a few cases, captive breeding programs have been attempted to alleviate the pressure on frog populations, and these have proved successful.[33][34] In 2007, it was reported the application of certain probiotic bacteria could protect amphibians from chytridiomycosis.[35]
Zoos and aquariums around the world named 2008 the Year of the Frog, to draw attention to the conservation issues.[36]
Evolution
Until 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.[37] 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,[38] 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.[39]
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[40] and had all modern frog features, but bore 9 presacral vertebrae instead of the 8 of modern frogs.[41]
Frog fossils have been found on all continents, including Antarctica.[citation needed]
Uses in agriculture and research
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.[42]
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),[43] facilitating faster studies across generations. The genome sequence of X. tropicalis will probably be completed by 2015 at the latest.[44]
Cultural beliefs
Frogs feature prominently in folklore, fairy tales and popular culture. They tend to be portrayed as benign, ugly, clumsy, but with hidden talents. Examples include Michigan J. Frog, The Frog Prince, and Kermit the Frog. Michigan J. Frog, featured in the Warner Brothers cartoon One Froggy Evening, only performs his singing and dancing routine for his owner. Once another person looks at him, he will return to a frog-like pose. "The Frog Prince" is a fairy tale of a frog who turns into a handsome prince once kissed. Kermit the Frog, on the other hand, is a conscientious and disciplined character of The Muppet Show and Sesame Street; while openly friendly and greatly talented, he is often portrayed as cringing at the fanciful behaviour of more flamboyant characters.
The Moche people of ancient Peru worshipped animals and often depicted frogs in their art.[45]
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