A Thing
The world slowly begins to collapse around you as everything starts to fall apart, including you. The last thoughts you have before ceasing to exist are of frogs. You think about how weird frogs are and how lovely they are! You begin to think over the entire Wikipedia page on frogs which you just happen to have memorized.
Frog
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This article is about the group of amphibians. For other uses, see Frog (disambiguation).
Frogs
Temporal range:
Early Jurassic - Present, 200–0 Ma
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Anoures.jpg
Various types of frogs.
Scientific classification edit
Kingdom: Animalia
Phylum: Chordata
Clade: Salientia
Order: Anura
Duméril, 1806 (as Anoures)
Suborders
Archaeobatrachia
Mesobatrachia
Neobatrachia
–
List of Anuran families
Distribution.anura.1.png
Native distribution of frogs (in green)
A frog is any member of a diverse and largely carnivorous group of short-bodied, tailless amphibians composing the order Anura (literally without tail in Ancient Greek). 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.[1] 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.
Contents
1 Etymology and taxonomy
2 Evolution
3 Morphology and physiology
3.1 Feet and legs
3.2 Skin
3.3 Respiration and circulation
3.4 Digestion and excretion
3.5 Reproductive system
3.6 Nervous system
3.7 Sight
3.8 Hearing
3.9 Call
3.10 Torpor
4 Locomotion
5 Life history
5.1 Reproduction
5.2 Life cycle
5.2.1 Eggs / frogspawn
5.2.2 Tadpoles
5.2.3 Metamorphosis
5.2.4 Adults
5.3 Parental care
6 Defence
7 Distribution
8 Conservation
9 Uses
9.1 Culinary
9.2 Scientific research
9.3 Pharmaceutical
10 Cultural beliefs
11 References
11.1 Notes
11.2 Bibliography
12 External links
Etymology and taxonomy
The name frog derives from Old English frogga, abbreviated to frox, forsc, and frosc, probably deriving from Proto-Indo-European preu = "to jump".[2] Moreover, the etymology of the order name Anura — and its original spelling Anoures — corresponds to the two ancient greek words ἀν- (an-), expressing the "absence" (this prefix is an alpha privative), and οὐρά (ourá), meaning "animal tail".[3][4] It refers to the tailless character of these amphibians.
About 88% of amphibian species are classified in the order Anura.[5] 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.[5]
Dark-colored toad facing left
European fire-bellied toad (Bombina bombina)
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.[6][7] There are numerous exceptions to this rule. The European fire-bellied toad (Bombina bombina) has a slightly warty skin and prefers a watery habitat[8] whereas the Panamanian golden frog (Atelopus zeteki) is in the toad family Bufonidae and has a smooth skin.[9]
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),[10] an unsupported tongue, lymph spaces underneath the skin, and a muscle, the protractor lentis, attached to the lens of the eye.[11] The anuran larva or tadpole has a single central respiratory spiracle and mouthparts consisting of keratinous beaks and denticles.[11]
Panamanian golden frog
Panamanian golden frog (Atelopus zeteki).
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. The Neobatrachia suborder is further divided into the two superfamilies Hyloidea and Ranoidea.[12] 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.[13]
Some species of anurans hybridize readily. For instance, the edible frog (Pelophylax esculentus) is a hybrid between the pool frog (P. lessonae) and the marsh frog (P. ridibundus).[14] 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.[15]
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 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.[16] 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.[17] 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.[18] 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.[19]
A fossilized frog from the Czech Republic, possibly Palaeobatrachus gigas.
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.[20][21] However, others have suggested that Gerobatrachus hottoni was only a dissorophoid temnospondyl unrelated to extant amphibians.[22]
Salientia (Latin salere (salio), "to jump") is the name of the total group that includes modern frogs in the order Anura as well as their close fossil relatives, the "proto-frogs" or "stem-frogs". The common features possessed by these proto-frogs include 14 presacral vertebrae (modern frogs have eight or 9), a long and forward-sloping ilium in the pelvis, the presence of a frontoparietal bone, and a lower jaw without teeth. The earliest known amphibians that were more closely related to frogs than to salamanders are Triadobatrachus massinoti, from the early Triassic period of Madagascar (about 250 million years ago), and Czatkobatrachus polonicus, from the Early Triassic of Poland (about the same age as Triadobatrachus).[23] The skull of Triadobatrachus is frog-like, being broad with large eye sockets, but the fossil has features diverging from modern frogs. These include a longer body with more vertebrae. The tail has separate vertebrae unlike the fused urostyle or coccyx in modern frogs. The tibia and fibula bones are also separate, making it probable that Triadobatrachus was not an efficient leaper.[23]
The earliest known "true frogs" that fall into the anuran lineage proper all lived in the early Jurassic period.[6][24] One such early frog species, Prosalirus bitis, was discovered in 1995 in the Kayenta Formation of Arizona and dates back to the Early Jurassic epoch (199.6 to 175 million years ago), making Prosalirus somewhat more recent than Triadobatrachus.[25] Like the latter, Prosalirus did not have greatly enlarged legs, but had the typical three-pronged pelvic structure of modern frogs. Unlike Triadobatrachus, Prosalirus had already lost nearly all of its tail[26] and was well adapted for jumping.[27] Another Early Jurassic frog is Vieraella herbsti, which is known only from 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. The main evolutionary changes in this species involved the shortening of the body and the loss of the tail. The evolution of modern Anura likely was complete by the Jurassic period. Since then, evolutionary changes in chromosome numbers have taken place about 20 times faster in mammals than in frogs, which means speciation is occurring more rapidly in mammals.[28]
According to genetic studies, the families Hyloidea, Microhylidae, and the clade Natatanura (comprising about 88% of living frogs) diversified simultaneously some 66 million years ago, soon after the Cretaceous–Paleogene extinction event associated with the Chicxulub impactor. All origins of arboreality (e.g. in Hyloidea and Natatanura) follow from that time and the resurgence of forest that occurred afterwards.[29][30]
Frog fossils have been found on all continents except Antarctica, but biogeographic evidence suggests they also inhabited Antarctica in an earlier era when the climate was warmer.[31]
Frog classification
A cladogram showing the relationships of the different families of frogs in the clade Anura can be seen in the table above. This diagram, in the form of a tree, shows how each frog family is related to other families, with each node representing a point of common ancestry. It is based on Frost et al. (2006),[32] Heinicke et al. (2009)[33] and Pyron and Wiens (2011).[34]
Morphology and physiology
Skeleton of frog
Skeleton of Pelophylax esculentus showing bones of the head, vertebral column, ribs, pectoral and pelvic girdles, and limbs.
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).[35] 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.[36] 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.[36] 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.[37][38][39][40]
Frogs range in size from the recently discovered 7.7-millimetre (0.30 in) Paedophryne amauensis of Papua New Guinea[41] to the 30-centimetre (12 in) goliath frog (Conraua goliath) of Cameroon. The skin hangs loosely on the body because of the lack of loose connective tissue. Frogs have three eyelid membranes: one is transparent to protect the eyes underwater, and two vary from translucent to opaque. They have a tympanum on each side of their heads which is involved in hearing and, in some species, is covered by skin. True toads completely lack teeth, but most frogs have them, specifically pedicellate teeth in which the crown is separated from the root by fibrous tissue. These are on the edge of the upper jaw and vomerine teeth are also on the roof of their mouths. No teeth are in the lower jaw and frogs usually swallow their food whole. The teeth are mainly used to grip the prey and keep it in place till swallowed, a process assisted by retracting the eyes into the head.[42] The African bullfrog (Pyxicephalus), which preys on relatively large animals such as mice and other frogs, has cone shaped bony projections called odontoid processes at the front of the lower jaw which function like teeth.[5]
A bullfrog skeleton, showing elongated limb bones and extra joints. Red marks indicate bones which have been substantially elongated in frogs and joints which have become mobile. Blue indicates joints and bones which have not been modified or only somewhat elongated.
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 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.[35]
Webbed foot
Webbed hind foot of common frog
(Rana temporaria)
Tyler's tree frog (Litoria tyleri) has large toe pads and webbed feet.
The muscular system has been similarly modified. The hind limbs of ancestral 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 main leg muscles accounting for over 17% of the total mass of frogs.[43]
Many frogs have webbed feet and the degree of webbing is directly proportional to the amount of time the species spends in the water.[44] The completely aquatic African dwarf frog (Hymenochirus sp.) has fully webbed toes, whereas those of White's tree frog (Litoria caerulea), an arboreal species, are only a quarter or half webbed.[45]
Arboreal frogs have pads located on the ends of their toes to help grip vertical surfaces. These are not suction pads, the surface consisting instead of columnar cells with flat tops with small gaps between them lubricated by mucous glands. When the frog applies pressure, the cells adhere to irregularities on the surface and the grip is maintained through surface tension. This allows the frog to climb on smooth surfaces, but the system does not function efficiently when the pads are excessively wet.[46]
In many arboreal frogs, a small "intercalary structure" on each toe increases the surface area touching the substrate. Furthermore, since hopping through trees can be dangerous, many arboreal frogs have hip joints to allow both hopping and walking. Some frogs that live high in trees even possess an elaborate degree of webbing between their toes. This allows the frogs to "parachute" or make a controlled glide from one position in the canopy to another.[47]
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 such as Couch's spadefoot (Scaphiopus couchii) have a flap-like toe extension on the hind feet, a keratinised tubercle often referred to as a spade, that helps them to burrow.[48]
Sometimes during the tadpole stage, one of the developing rear legs is eaten by a predator such as a dragonfly nymph. In some cases, the full leg still grows, but in others it does not, although the frog may still live out its normal lifespan with only three limbs. Occasionally, a parasitic flatworm (Ribeiroia ondatrae) digs into the rear of a tadpole, causing a rearrangement of the limb bud cells and the frog develops an extra leg or two.[49]
Northern leopard frog (Rana pipiens) moulting and eating its skin.
Skin
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.[50] 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.[51]
Being cold-blooded, frogs have to adopt suitable behaviour patterns to regulate their temperature. To warm up, they can move into the sun or onto a warm surface; if they overheat, they can move into the shade or adopt a stance that exposes the minimum area of skin to the air. This posture is also used to prevent water loss and involves the frog squatting close to the substrate with its hands and feet tucked under its chin and body.[52] The colour of a frog's skin is used for thermoregulation. In cool damp conditions, the colour will be darker than on a hot dry day. The grey foam-nest tree frog (Chiromantis xerampelina) is even able to turn white to minimize the chance of overheating.[53]
Many frogs are able to absorb water and oxygen directly through the skin, especially around the pelvic area, but the permeability of a frog's skin can also result in water loss. Glands located all over the body exude mucus which helps keep the skin moist and reduces evaporation. Some glands on the hands and chest of males are specialized to produce sticky secretions to aid in amplexus. Similar glands in tree frogs produce a glue-like substance on the adhesive discs of the feet. Some arboreal frogs reduce water loss by having a waterproof layer of skin, and several South American species coat their skin with a waxy secretion. Other frogs have adopted behaviours to conserve water, including becoming nocturnal and resting in a water-conserving position. Some frogs may also rest in large groups with each frog pressed against its neighbours. This reduces the amount of skin exposed to the air or a dry surface, and thus reduces water loss.[52] Woodhouse's toad (Bufo woodhousii), if given access to water after confinement in a dry location, sits in the shallows to rehydrate.[54] The male hairy frog (Trichobatrachus robustus) has dermal papillae projecting from its lower back and thighs, giving it a bristly appearance. They contain blood vessels and are thought to increase the area of the skin available for respiration.[55]
Some species have bony plates embedded in their skin, a trait that appears to have evolved independently several times.[56] In certain other species, the skin at the top of the head is compacted and the connective tissue of the dermis is co-ossified with the bones of the skull (exostosis).[57][58]
Camouflage is a common defensive mechanism in frogs. Most camouflaged frogs are nocturnal; during the day, they seek out a position where they can blend into the background and remain undetected. Some frogs have the ability to change colour, but this is usually restricted to a small range of colours. For example, White's tree frog (Litoria caerulea) varies between pale green and dull brown according to the temperature, and the Pacific tree frog (Pseudacris regilla) has green and brown morphs, plain or spotted, and changes colour depending on the time of year and general background colour.[59] Features such as warts and skin folds are usually on ground-dwelling frogs, for whom smooth skin would not provide such effective camouflage. Certain frogs change colour between night and day, as light and moisture stimulate the pigment cells and cause them to expand or contract.[36]
Frog barely recognisable against brown decaying leaf litter.
Pouched frog (Assa darlingtoni) camouflaged against leaf litter
Respiration and circulation
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.[60] The fully aquatic Bornean flat-headed frog (Barbourula kalimantanensis) is the first frog known to lack lungs entirely.[61]
Frogs have three-chambered hearts, a feature they share with lizards.[62] Oxygenated blood from the lungs and de-oxygenated blood from the respiring tissues enter the heart through separate atria. When these chambers contract, the two blood streams pass into a common ventricle before being pumped via a spiral valve to the appropriate vessel, the aorta for oxygenated blood and pulmonary artery for deoxygenated blood. The ventricle is partially divided into narrow cavities which minimizes the mixing of the two types of blood. These features enable frogs to have a higher metabolic rate and be more active than would otherwise be possible.[62]
Some species of frog have adaptations that allow them to survive in oxygen deficient water. The Titicaca water frog (Telmatobius culeus) is one such species and has wrinkly skin that increases its surface area to enhance gas exchange. It normally makes no use of its rudimentary lungs but will sometimes raise and lower its body rhythmically while on the lake bed to increase the flow of water around it.[63]
Dissected frog
Anatomical model of a dissected frog: 1 Right atrium, 2 Lungs, 3 Aorta, 4 Egg mass, 5 Colon, 6 Left atrium, 7 Ventricle, 8 Stomach, 9 Liver, 10 Gallbladder, 11 Small intestine, 12 Cloaca
Digestion and excretion
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.[44] Some frogs have no tongue and just stuff food into their mouths with their hands.[44] 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.[44] 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.[64]
Although adapted to terrestrial life, frogs resemble freshwater fish in their inability to conserve body water effectively. When they are on land, much water is lost by evaporation from the skin. The excretory system is similar to that of mammals and there are two kidneys that remove nitrogenous products from the blood. Frogs produce large quantities of dilute urine in order to flush out toxic products from the kidney tubules.[65] The nitrogen is excreted as ammonia by tadpoles and aquatic frogs but mainly as urea, a less toxic product, by most terrestrial adults. A few species of tree frog with little access to water excrete the even less toxic uric acid.[65] The urine passes along paired ureters to the urinary bladder from which it is vented periodically into the cloaca. All bodily wastes exit the body through the cloaca which terminates in a cloacal vent.[66]
Reproductive system
See also: sexual selection in amphibians
In the male frog, the two testes are attached to the kidneys and semen passes into the kidneys through fine tubes called efferent ducts. It then travels on through the ureters, which are consequently known as urinogenital ducts. There is no penis, and sperm is ejected from the cloaca directly onto the eggs as the female lays them. The ovaries of the female frog are beside the kidneys and the eggs pass down a pair of oviducts and through the cloaca to the exterior.[66]
When frogs mate, the male climbs on the back of the female and wraps his fore limbs round her body, either behind the front legs or just in front of the hind legs. This position is called amplexus and may be held for several days.[67] The male frog has certain hormone-dependent secondary sexual characteristics. These include the development of special pads on his thumbs in the breeding season, to give him a firm hold.[68] The grip of the male frog during amplexus stimulates the female to release eggs, usually wrapped in jelly, as spawn.[66] In many species the male is smaller and slimmer than the female. Males have vocal cords and make a range of croaks, particularly in the breeding season, and in some species they also have vocal sacs to amplify the sound.[66]
Nervous system
Frogs have a highly developed nervous system that consists of a brain, spinal cord and nerves. Many parts of frog brains correspond with those of humans. It consists of two olfactory lobes, two cerebral hemispheres, a pineal body, two optic lobes, a cerebellum and a medulla oblongata. Muscular coordination and posture are controlled by the cerebellum, and the medulla oblongata regulates respiration, digestion and other automatic functions.[66] The relative size of the cerebrum in frogs is much smaller than it is in humans. Frogs have ten pairs of cranial nerves which pass information from the outside directly to the brain, and ten pairs of spinal nerves which pass information from the extremities to the brain through the spinal cord.[66] By contrast, all amniotes (mammals, birds and reptiles) have twelve pairs of cranial nerves.[69]
Close-up of frog's head showing eye, nostril, mouth and tympanum.
Sight
The eyes of most frogs are located on either side of the head near the top and project outwards as hemispherical bulges. They provide binocular vision over a field of 100° to the front and a total visual field of almost 360°.[70] They may be the only part of an otherwise submerged frog to protrude from the water. Each eye has closable upper and lower lids and a nictitating membrane which provides further protection, especially when the frog is swimming.[71] Members of the aquatic family Pipidae have the eyes located at the top of the head, a position better suited for detecting prey in the water above.[70] The irises come in a range of colours and the pupils in a range of shapes. The common toad (Bufo bufo) has golden irises and horizontal slit-like pupils, the red-eyed tree frog (Agalychnis callidryas) has vertical slit pupils, the poison dart frog has dark irises, the fire-bellied toad (Bombina spp.) has triangular pupils and the tomato frog (Dyscophus spp.) has circular ones. The irises of the southern toad (Anaxyrus terrestris) are patterned so as to blend in with the surrounding camouflaged skin.[71]
The distant vision of a frog is better than its near vision. Calling frogs will quickly become silent when they see an intruder or even a moving shadow but the closer an object is, the less well it is seen.[71] When a frog shoots out its tongue to catch an insect it is reacting to a small moving object that it cannot see well and must line it up precisely beforehand because it shuts its eyes as the tongue is extended.[44] Whether a frog sees in colour is debatable but it has been shown that it responds positively to blue light, perhaps because that colour is associated with bodies of water that can provide refuge when the frog feels threatened.[72]
Hearing
File:Synchrotron microtomography of Atelopus franciscus head - pone.0022080.s003.ogv
Surface rendering of the head of the frog Atelopus franciscus, with ear parts highlighted.
Frogs can hear both in the air and below water. They do not have external ears; the eardrums (tympanic membranes) are directly exposed or may be covered by a layer of skin and are visible as a circular area just behind the eye. The size and distance apart of the eardrums is related to the frequency and wavelength at which the frog calls. In some species such as the bullfrog, the size of the tympanum indicates the sex of the frog; males have tympani that are larger than their eyes while in females, the eyes and tympani are much the same size.[73] A noise causes the tympanum to vibrate and the sound is transmitted to the middle and inner ear. The middle ear contains semicircular canals which help control balance and orientation. In the inner ear, the auditory hair cells are arranged in two areas of the cochlea, the basilar papilla and the amphibian papilla. The former detects high frequencies and the latter low frequencies.[74] Because the cochlea is short, frogs use electrical tuning to extend their range of audible frequencies and help discriminate different sounds.[75] This arrangement enables detection of the territorial and breeding calls of their conspecifics. In some species that inhabit arid regions, the sound of thunder or heavy rain may arouse them from a dormant state.[74] A frog may be startled by an unexpected noise but it will not usually take any action until it has located the source of the sound by sight.[73]
Call
See also: Sexual selection in frogs
A male Dendropsophus microcephalus displaying its vocal sac during its call.
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Advertisement call of male Atelopus franciscus
The call or croak of a frog is unique to its species. Frogs create this sound 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. Some frog calls are so loud that they can be heard up to a mile away.[76] Additionally, some species have been found to use man-made structures such as drain pipes for artificial amplification of their call.[77]
Frogs in the genera Heleioporus and Neobatrachus lack vocal sacs but can still produce a loud call. Their buccal cavity is enlarged and dome-shaped, acting as a resonance chamber that amplifies the sound. Species of frog that lack vocal sacs and that do not have a loud call tend to inhabit areas close to constantly noisy, flowing water. They need to use an alternative means to communicate. The coastal tailed frog (Ascaphus truei) lives in mountain streams in North America and does not vocalize.[78]
The main reason for calling is to allow male frogs to attract a mate. Males may call individually or there may be a chorus of sound where numerous males have converged on breeding sites. Females of many frog species, such as the common tree frog (Polypedates leucomystax), reply to the male calls, which acts to reinforce reproductive activity in a breeding colony.[79] Female frogs prefer males that produce sounds of greater intensity and lower frequency, attributes that stand out in a crowd. The rationale for this is thought to be that by demonstrating his prowess, the male shows his fitness to produce superior offspring.[80]
A different call is emitted by a male frog or unreceptive female when mounted by another male. This is a distinct chirruping sound and is accompanied by a vibration of the body.[81] Tree frogs and some non-aquatic species have a rain call that they make on the basis of humidity cues prior to a shower.[81] Many species also have a territorial call that is used to drive away other males. All of these calls are emitted with the mouth of the frog closed.[81] A distress call, emitted by some frogs when they are in danger, is produced with the mouth open resulting in a higher-pitched call. It is typically used when the frog has been grabbed by a predator and may serve to distract or disorientate the attacker so that it releases the frog.[81]
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Distinctive low "jug-o-rum" sound of Banded bullfrog.
Many species of frog have deep calls. The croak of the American bullfrog (Rana catesbiana) is sometimes written as "jug o' rum".[82] The Pacific tree frog (Pseudacris regilla) produces the onomatopoeic "ribbit" often heard in films.[83] Other renderings of frog calls into speech include "brekekekex koax koax", the call of the marsh frog (Pelophylax ridibundus) in The Frogs, an Ancient Greek comic drama by Aristophanes.[84]
Torpor
During extreme conditions, some frogs enter a state of torpor and remain inactive for months. In colder regions, many species of frog hibernate in winter. Those that live on land such as the American toad (Bufo americanus) dig a burrow and make a hibernaculum in which to lie dormant. Others, less proficient at digging, find a crevice or bury themselves in dead leaves. Aquatic species such as the American bullfrog (Rana catesbeiana) normally sink to the bottom of the pond where they lie, semi-immersed in mud but still able to access the oxygen dissolved in the water. Their metabolism slows down and they live on their energy reserves. Some frogs can even survive being frozen. Ice crystals form under the skin and in the body cavity but the essential organs are protected from freezing by a high concentration of glucose. An apparently lifeless, frozen frog can resume respiration and the heart beat can restart when conditions warm up.[85]
At the other extreme, the striped burrowing frog (Cyclorana alboguttata) regularly aestivates during the hot, dry season in Australia, surviving in a dormant state without access to food and water for nine or ten months of the year. It burrows underground and curls up inside a protective cocoon formed by its shed skin. Researchers at the University of Queensland have found that during aestivation, the metabolism of the frog is altered and the operational efficiency of the mitochondria is increased. This means that the limited amount of energy available to the comatose frog is used in a more efficient manner. This survival mechanism is only useful to animals that remain completely unconscious for an extended period of time and whose energy requirements are low because they are cold-blooded and have no need to generate heat.[86] Other research showed that, to provide these energy requirements, muscles atrophy, but hind limb muscles are preferentially unaffected.[87] Frogs have been found to have upper critical temperatures of around 41 degrees Celsius.[88]
Locomotion
Different species of frog use a number of methods of moving around including jumping, running, walking, swimming, burrowing, climbing and gliding.
Rainforest rocket frog jumping.
Jumping
Frogs are generally recognized as exceptional jumpers and, relative to their size, the best jumpers of all vertebrates.[89] The striped rocket frog, Litoria nasuta, can leap over 2 metres (6 ft 7 in), a distance that is more than fifty times its body length of 5.5 centimetres (2.2 in).[90] There are tremendous differences between species in jumping capability. Within a species, jump distance increases with increasing size, but relative jumping distance (body-lengths jumped) decreases. The Indian skipper frog (Euphlyctis cyanophlyctis) has the ability to leap out of the water from a position floating on the surface.[91] The tiny northern cricket frog (Acris crepitans) can "skitter" across the surface of a pond with a series of short rapid jumps.[92]
Slow-motion photography shows that the muscles have passive flexibility. They are first stretched while the frog is still in the crouched position, then they are contracted before being stretched again to launch the frog into the air. The fore legs are folded against the chest and the hind legs remain in the extended, streamlined position for the duration of the jump.[43] In some extremely capable jumpers, such as the Cuban tree frog (Osteopilus septentrionalis) and the northern leopard frog (Rana pipiens), the peak power exerted during a jump can exceed that which the muscle is theoretically capable of producing. When the muscles contract, the energy is first transferred into the stretched tendon which is wrapped around the ankle bone. Then the muscles stretch again at the same time as the tendon releases its energy like a catapult to produce a powerful acceleration beyond the limits of muscle-powered acceleration.[93] A similar mechanism has been documented in locusts and grasshoppers.[94]
Walking and running
Phrynomantis bifasciatus walking on a level surface
Frogs in the families Bufonidae, Rhinophrynidae, and Microhylidae have short back legs and tend to walk rather than jump.[95] When they try to move rapidly, they speed up the rate of movement of their limbs or resort to an ungainly hopping gait. The Great Plains narrow-mouthed toad (Gastrophryne olivacea) has been described as having a gait that is "a combination of running and short hops that are usually only an inch or two in length".[96] In an experiment, Fowler's toad (Bufo fowleri) was placed on a treadmill which was turned at varying speeds. By measuring the toad's uptake of oxygen it was found that hopping was an inefficient use of resources during sustained locomotion but was a useful strategy during short bursts of high-intensity activity.[97]
The red-legged running frog (Kassina maculata) has short, slim hind limbs unsuited to jumping. It can move fast by using a running gait in which the two hind legs are used alternately. Slow-motion photography shows, unlike a horse that can trot or gallop, the frog's gait remained similar at slow, medium, and fast speeds.[98] This species can also climb trees and shrubs, and does so at night to catch insects.[99] The Indian skipper frog (Euphlyctis cyanophlyctis) has broad feet and can run across the surface of the water for several metres (yards).[92]
Swimming
Common toad swimming
Common toad (Bufo bufo) swimming.
Frogs that live in or visit water have adaptations that improve their swimming abilities. The hind limbs are heavily muscled and strong. The webbing between the toes of the hind feet increases the area of the foot and helps propel the frog powerfully through the water. Members of the family Pipidae are wholly aquatic and show the most marked specialization. They have inflexible vertebral columns, flattened, streamlined bodies, lateral line systems, and powerful hind limbs with large webbed feet.[100] Tadpoles mostly have large tail fins which provide thrust when the tail is moved from side to side.[101]
Burrowing
Some frogs have become adapted for burrowing and a life underground. They tend to have rounded bodies, short limbs, small heads with bulging eyes, and hind feet adapted for excavation. An extreme example of this is the purple frog (Nasikabatrachus sahyadrensis) from southern India which feeds on termites and spends almost its whole life underground. It emerges briefly during the monsoon to mate and breed in temporary pools. It has a tiny head with a pointed snout and a plump, rounded body. Because of this fossorial existence, it was first described in 2003, being new to the scientific community at that time, although previously known to local people.[102]
Purple frog
Purple frog (Nasikabatrachus sahyadrensis).
The spadefoot toads of North America are also adapted to underground life. The Plains spadefoot toad (Spea bombifrons) is typical and has a flap of keratinised bone attached to one of the metatarsals of the hind feet which it uses to dig itself backwards into the ground. As it digs, the toad wriggles its hips from side to side to sink into the loose soil. It has a shallow burrow in the summer from which it emerges at night to forage. In winter, it digs much deeper and has been recorded at a depth of 4.5 m (15 ft).[103] The tunnel is filled with soil and the toad hibernates in a small chamber at the end. During this time, urea accumulates in its tissues and water is drawn in from the surrounding damp soil by osmosis to supply the toad's needs.[103] Spadefoot toads are "explosive breeders", all emerging from their burrows at the same time and converging on temporary pools, attracted to one of these by the calling of the first male to find a suitable breeding location.[104]
The burrowing frogs of Australia have a rather different lifestyle. The western spotted frog (Heleioporus albopunctatus) digs a burrow beside a river or in the bed of an ephemeral stream and regularly emerges to forage.
Before you can recite the rest of the wiki page on frogs you finally complete ceasing to exist.
You cease to exist.
You died.
Frog
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This article is about the group of amphibians. For other uses, see Frog (disambiguation).
Frogs
Temporal range:
Early Jurassic - Present, 200–0 Ma
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Anoures.jpg
Various types of frogs.
Scientific classification edit
Kingdom: Animalia
Phylum: Chordata
Clade: Salientia
Order: Anura
Duméril, 1806 (as Anoures)
Suborders
Archaeobatrachia
Mesobatrachia
Neobatrachia
–
List of Anuran families
Distribution.anura.1.png
Native distribution of frogs (in green)
A frog is any member of a diverse and largely carnivorous group of short-bodied, tailless amphibians composing the order Anura (literally without tail in Ancient Greek). 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.[1] 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.
Contents
1 Etymology and taxonomy
2 Evolution
3 Morphology and physiology
3.1 Feet and legs
3.2 Skin
3.3 Respiration and circulation
3.4 Digestion and excretion
3.5 Reproductive system
3.6 Nervous system
3.7 Sight
3.8 Hearing
3.9 Call
3.10 Torpor
4 Locomotion
5 Life history
5.1 Reproduction
5.2 Life cycle
5.2.1 Eggs / frogspawn
5.2.2 Tadpoles
5.2.3 Metamorphosis
5.2.4 Adults
5.3 Parental care
6 Defence
7 Distribution
8 Conservation
9 Uses
9.1 Culinary
9.2 Scientific research
9.3 Pharmaceutical
10 Cultural beliefs
11 References
11.1 Notes
11.2 Bibliography
12 External links
Etymology and taxonomy
The name frog derives from Old English frogga, abbreviated to frox, forsc, and frosc, probably deriving from Proto-Indo-European preu = "to jump".[2] Moreover, the etymology of the order name Anura — and its original spelling Anoures — corresponds to the two ancient greek words ἀν- (an-), expressing the "absence" (this prefix is an alpha privative), and οὐρά (ourá), meaning "animal tail".[3][4] It refers to the tailless character of these amphibians.
About 88% of amphibian species are classified in the order Anura.[5] 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.[5]
Dark-colored toad facing left
European fire-bellied toad (Bombina bombina)
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.[6][7] There are numerous exceptions to this rule. The European fire-bellied toad (Bombina bombina) has a slightly warty skin and prefers a watery habitat[8] whereas the Panamanian golden frog (Atelopus zeteki) is in the toad family Bufonidae and has a smooth skin.[9]
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),[10] an unsupported tongue, lymph spaces underneath the skin, and a muscle, the protractor lentis, attached to the lens of the eye.[11] The anuran larva or tadpole has a single central respiratory spiracle and mouthparts consisting of keratinous beaks and denticles.[11]
Panamanian golden frog
Panamanian golden frog (Atelopus zeteki).
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. The Neobatrachia suborder is further divided into the two superfamilies Hyloidea and Ranoidea.[12] 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.[13]
Some species of anurans hybridize readily. For instance, the edible frog (Pelophylax esculentus) is a hybrid between the pool frog (P. lessonae) and the marsh frog (P. ridibundus).[14] 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.[15]
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 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.[16] 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.[17] 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.[18] 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.[19]
A fossilized frog from the Czech Republic, possibly Palaeobatrachus gigas.
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.[20][21] However, others have suggested that Gerobatrachus hottoni was only a dissorophoid temnospondyl unrelated to extant amphibians.[22]
Salientia (Latin salere (salio), "to jump") is the name of the total group that includes modern frogs in the order Anura as well as their close fossil relatives, the "proto-frogs" or "stem-frogs". The common features possessed by these proto-frogs include 14 presacral vertebrae (modern frogs have eight or 9), a long and forward-sloping ilium in the pelvis, the presence of a frontoparietal bone, and a lower jaw without teeth. The earliest known amphibians that were more closely related to frogs than to salamanders are Triadobatrachus massinoti, from the early Triassic period of Madagascar (about 250 million years ago), and Czatkobatrachus polonicus, from the Early Triassic of Poland (about the same age as Triadobatrachus).[23] The skull of Triadobatrachus is frog-like, being broad with large eye sockets, but the fossil has features diverging from modern frogs. These include a longer body with more vertebrae. The tail has separate vertebrae unlike the fused urostyle or coccyx in modern frogs. The tibia and fibula bones are also separate, making it probable that Triadobatrachus was not an efficient leaper.[23]
The earliest known "true frogs" that fall into the anuran lineage proper all lived in the early Jurassic period.[6][24] One such early frog species, Prosalirus bitis, was discovered in 1995 in the Kayenta Formation of Arizona and dates back to the Early Jurassic epoch (199.6 to 175 million years ago), making Prosalirus somewhat more recent than Triadobatrachus.[25] Like the latter, Prosalirus did not have greatly enlarged legs, but had the typical three-pronged pelvic structure of modern frogs. Unlike Triadobatrachus, Prosalirus had already lost nearly all of its tail[26] and was well adapted for jumping.[27] Another Early Jurassic frog is Vieraella herbsti, which is known only from 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. The main evolutionary changes in this species involved the shortening of the body and the loss of the tail. The evolution of modern Anura likely was complete by the Jurassic period. Since then, evolutionary changes in chromosome numbers have taken place about 20 times faster in mammals than in frogs, which means speciation is occurring more rapidly in mammals.[28]
According to genetic studies, the families Hyloidea, Microhylidae, and the clade Natatanura (comprising about 88% of living frogs) diversified simultaneously some 66 million years ago, soon after the Cretaceous–Paleogene extinction event associated with the Chicxulub impactor. All origins of arboreality (e.g. in Hyloidea and Natatanura) follow from that time and the resurgence of forest that occurred afterwards.[29][30]
Frog fossils have been found on all continents except Antarctica, but biogeographic evidence suggests they also inhabited Antarctica in an earlier era when the climate was warmer.[31]
Frog classification
A cladogram showing the relationships of the different families of frogs in the clade Anura can be seen in the table above. This diagram, in the form of a tree, shows how each frog family is related to other families, with each node representing a point of common ancestry. It is based on Frost et al. (2006),[32] Heinicke et al. (2009)[33] and Pyron and Wiens (2011).[34]
Morphology and physiology
Skeleton of frog
Skeleton of Pelophylax esculentus showing bones of the head, vertebral column, ribs, pectoral and pelvic girdles, and limbs.
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).[35] 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.[36] 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.[36] 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.[37][38][39][40]
Frogs range in size from the recently discovered 7.7-millimetre (0.30 in) Paedophryne amauensis of Papua New Guinea[41] to the 30-centimetre (12 in) goliath frog (Conraua goliath) of Cameroon. The skin hangs loosely on the body because of the lack of loose connective tissue. Frogs have three eyelid membranes: one is transparent to protect the eyes underwater, and two vary from translucent to opaque. They have a tympanum on each side of their heads which is involved in hearing and, in some species, is covered by skin. True toads completely lack teeth, but most frogs have them, specifically pedicellate teeth in which the crown is separated from the root by fibrous tissue. These are on the edge of the upper jaw and vomerine teeth are also on the roof of their mouths. No teeth are in the lower jaw and frogs usually swallow their food whole. The teeth are mainly used to grip the prey and keep it in place till swallowed, a process assisted by retracting the eyes into the head.[42] The African bullfrog (Pyxicephalus), which preys on relatively large animals such as mice and other frogs, has cone shaped bony projections called odontoid processes at the front of the lower jaw which function like teeth.[5]
A bullfrog skeleton, showing elongated limb bones and extra joints. Red marks indicate bones which have been substantially elongated in frogs and joints which have become mobile. Blue indicates joints and bones which have not been modified or only somewhat elongated.
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 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.[35]
Webbed foot
Webbed hind foot of common frog
(Rana temporaria)
Tyler's tree frog (Litoria tyleri) has large toe pads and webbed feet.
The muscular system has been similarly modified. The hind limbs of ancestral 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 main leg muscles accounting for over 17% of the total mass of frogs.[43]
Many frogs have webbed feet and the degree of webbing is directly proportional to the amount of time the species spends in the water.[44] The completely aquatic African dwarf frog (Hymenochirus sp.) has fully webbed toes, whereas those of White's tree frog (Litoria caerulea), an arboreal species, are only a quarter or half webbed.[45]
Arboreal frogs have pads located on the ends of their toes to help grip vertical surfaces. These are not suction pads, the surface consisting instead of columnar cells with flat tops with small gaps between them lubricated by mucous glands. When the frog applies pressure, the cells adhere to irregularities on the surface and the grip is maintained through surface tension. This allows the frog to climb on smooth surfaces, but the system does not function efficiently when the pads are excessively wet.[46]
In many arboreal frogs, a small "intercalary structure" on each toe increases the surface area touching the substrate. Furthermore, since hopping through trees can be dangerous, many arboreal frogs have hip joints to allow both hopping and walking. Some frogs that live high in trees even possess an elaborate degree of webbing between their toes. This allows the frogs to "parachute" or make a controlled glide from one position in the canopy to another.[47]
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 such as Couch's spadefoot (Scaphiopus couchii) have a flap-like toe extension on the hind feet, a keratinised tubercle often referred to as a spade, that helps them to burrow.[48]
Sometimes during the tadpole stage, one of the developing rear legs is eaten by a predator such as a dragonfly nymph. In some cases, the full leg still grows, but in others it does not, although the frog may still live out its normal lifespan with only three limbs. Occasionally, a parasitic flatworm (Ribeiroia ondatrae) digs into the rear of a tadpole, causing a rearrangement of the limb bud cells and the frog develops an extra leg or two.[49]
Northern leopard frog (Rana pipiens) moulting and eating its skin.
Skin
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.[50] 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.[51]
Being cold-blooded, frogs have to adopt suitable behaviour patterns to regulate their temperature. To warm up, they can move into the sun or onto a warm surface; if they overheat, they can move into the shade or adopt a stance that exposes the minimum area of skin to the air. This posture is also used to prevent water loss and involves the frog squatting close to the substrate with its hands and feet tucked under its chin and body.[52] The colour of a frog's skin is used for thermoregulation. In cool damp conditions, the colour will be darker than on a hot dry day. The grey foam-nest tree frog (Chiromantis xerampelina) is even able to turn white to minimize the chance of overheating.[53]
Many frogs are able to absorb water and oxygen directly through the skin, especially around the pelvic area, but the permeability of a frog's skin can also result in water loss. Glands located all over the body exude mucus which helps keep the skin moist and reduces evaporation. Some glands on the hands and chest of males are specialized to produce sticky secretions to aid in amplexus. Similar glands in tree frogs produce a glue-like substance on the adhesive discs of the feet. Some arboreal frogs reduce water loss by having a waterproof layer of skin, and several South American species coat their skin with a waxy secretion. Other frogs have adopted behaviours to conserve water, including becoming nocturnal and resting in a water-conserving position. Some frogs may also rest in large groups with each frog pressed against its neighbours. This reduces the amount of skin exposed to the air or a dry surface, and thus reduces water loss.[52] Woodhouse's toad (Bufo woodhousii), if given access to water after confinement in a dry location, sits in the shallows to rehydrate.[54] The male hairy frog (Trichobatrachus robustus) has dermal papillae projecting from its lower back and thighs, giving it a bristly appearance. They contain blood vessels and are thought to increase the area of the skin available for respiration.[55]
Some species have bony plates embedded in their skin, a trait that appears to have evolved independently several times.[56] In certain other species, the skin at the top of the head is compacted and the connective tissue of the dermis is co-ossified with the bones of the skull (exostosis).[57][58]
Camouflage is a common defensive mechanism in frogs. Most camouflaged frogs are nocturnal; during the day, they seek out a position where they can blend into the background and remain undetected. Some frogs have the ability to change colour, but this is usually restricted to a small range of colours. For example, White's tree frog (Litoria caerulea) varies between pale green and dull brown according to the temperature, and the Pacific tree frog (Pseudacris regilla) has green and brown morphs, plain or spotted, and changes colour depending on the time of year and general background colour.[59] Features such as warts and skin folds are usually on ground-dwelling frogs, for whom smooth skin would not provide such effective camouflage. Certain frogs change colour between night and day, as light and moisture stimulate the pigment cells and cause them to expand or contract.[36]
Frog barely recognisable against brown decaying leaf litter.
Pouched frog (Assa darlingtoni) camouflaged against leaf litter
Respiration and circulation
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.[60] The fully aquatic Bornean flat-headed frog (Barbourula kalimantanensis) is the first frog known to lack lungs entirely.[61]
Frogs have three-chambered hearts, a feature they share with lizards.[62] Oxygenated blood from the lungs and de-oxygenated blood from the respiring tissues enter the heart through separate atria. When these chambers contract, the two blood streams pass into a common ventricle before being pumped via a spiral valve to the appropriate vessel, the aorta for oxygenated blood and pulmonary artery for deoxygenated blood. The ventricle is partially divided into narrow cavities which minimizes the mixing of the two types of blood. These features enable frogs to have a higher metabolic rate and be more active than would otherwise be possible.[62]
Some species of frog have adaptations that allow them to survive in oxygen deficient water. The Titicaca water frog (Telmatobius culeus) is one such species and has wrinkly skin that increases its surface area to enhance gas exchange. It normally makes no use of its rudimentary lungs but will sometimes raise and lower its body rhythmically while on the lake bed to increase the flow of water around it.[63]
Dissected frog
Anatomical model of a dissected frog: 1 Right atrium, 2 Lungs, 3 Aorta, 4 Egg mass, 5 Colon, 6 Left atrium, 7 Ventricle, 8 Stomach, 9 Liver, 10 Gallbladder, 11 Small intestine, 12 Cloaca
Digestion and excretion
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.[44] Some frogs have no tongue and just stuff food into their mouths with their hands.[44] 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.[44] 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.[64]
Although adapted to terrestrial life, frogs resemble freshwater fish in their inability to conserve body water effectively. When they are on land, much water is lost by evaporation from the skin. The excretory system is similar to that of mammals and there are two kidneys that remove nitrogenous products from the blood. Frogs produce large quantities of dilute urine in order to flush out toxic products from the kidney tubules.[65] The nitrogen is excreted as ammonia by tadpoles and aquatic frogs but mainly as urea, a less toxic product, by most terrestrial adults. A few species of tree frog with little access to water excrete the even less toxic uric acid.[65] The urine passes along paired ureters to the urinary bladder from which it is vented periodically into the cloaca. All bodily wastes exit the body through the cloaca which terminates in a cloacal vent.[66]
Reproductive system
See also: sexual selection in amphibians
In the male frog, the two testes are attached to the kidneys and semen passes into the kidneys through fine tubes called efferent ducts. It then travels on through the ureters, which are consequently known as urinogenital ducts. There is no penis, and sperm is ejected from the cloaca directly onto the eggs as the female lays them. The ovaries of the female frog are beside the kidneys and the eggs pass down a pair of oviducts and through the cloaca to the exterior.[66]
When frogs mate, the male climbs on the back of the female and wraps his fore limbs round her body, either behind the front legs or just in front of the hind legs. This position is called amplexus and may be held for several days.[67] The male frog has certain hormone-dependent secondary sexual characteristics. These include the development of special pads on his thumbs in the breeding season, to give him a firm hold.[68] The grip of the male frog during amplexus stimulates the female to release eggs, usually wrapped in jelly, as spawn.[66] In many species the male is smaller and slimmer than the female. Males have vocal cords and make a range of croaks, particularly in the breeding season, and in some species they also have vocal sacs to amplify the sound.[66]
Nervous system
Frogs have a highly developed nervous system that consists of a brain, spinal cord and nerves. Many parts of frog brains correspond with those of humans. It consists of two olfactory lobes, two cerebral hemispheres, a pineal body, two optic lobes, a cerebellum and a medulla oblongata. Muscular coordination and posture are controlled by the cerebellum, and the medulla oblongata regulates respiration, digestion and other automatic functions.[66] The relative size of the cerebrum in frogs is much smaller than it is in humans. Frogs have ten pairs of cranial nerves which pass information from the outside directly to the brain, and ten pairs of spinal nerves which pass information from the extremities to the brain through the spinal cord.[66] By contrast, all amniotes (mammals, birds and reptiles) have twelve pairs of cranial nerves.[69]
Close-up of frog's head showing eye, nostril, mouth and tympanum.
Sight
The eyes of most frogs are located on either side of the head near the top and project outwards as hemispherical bulges. They provide binocular vision over a field of 100° to the front and a total visual field of almost 360°.[70] They may be the only part of an otherwise submerged frog to protrude from the water. Each eye has closable upper and lower lids and a nictitating membrane which provides further protection, especially when the frog is swimming.[71] Members of the aquatic family Pipidae have the eyes located at the top of the head, a position better suited for detecting prey in the water above.[70] The irises come in a range of colours and the pupils in a range of shapes. The common toad (Bufo bufo) has golden irises and horizontal slit-like pupils, the red-eyed tree frog (Agalychnis callidryas) has vertical slit pupils, the poison dart frog has dark irises, the fire-bellied toad (Bombina spp.) has triangular pupils and the tomato frog (Dyscophus spp.) has circular ones. The irises of the southern toad (Anaxyrus terrestris) are patterned so as to blend in with the surrounding camouflaged skin.[71]
The distant vision of a frog is better than its near vision. Calling frogs will quickly become silent when they see an intruder or even a moving shadow but the closer an object is, the less well it is seen.[71] When a frog shoots out its tongue to catch an insect it is reacting to a small moving object that it cannot see well and must line it up precisely beforehand because it shuts its eyes as the tongue is extended.[44] Whether a frog sees in colour is debatable but it has been shown that it responds positively to blue light, perhaps because that colour is associated with bodies of water that can provide refuge when the frog feels threatened.[72]
Hearing
File:Synchrotron microtomography of Atelopus franciscus head - pone.0022080.s003.ogv
Surface rendering of the head of the frog Atelopus franciscus, with ear parts highlighted.
Frogs can hear both in the air and below water. They do not have external ears; the eardrums (tympanic membranes) are directly exposed or may be covered by a layer of skin and are visible as a circular area just behind the eye. The size and distance apart of the eardrums is related to the frequency and wavelength at which the frog calls. In some species such as the bullfrog, the size of the tympanum indicates the sex of the frog; males have tympani that are larger than their eyes while in females, the eyes and tympani are much the same size.[73] A noise causes the tympanum to vibrate and the sound is transmitted to the middle and inner ear. The middle ear contains semicircular canals which help control balance and orientation. In the inner ear, the auditory hair cells are arranged in two areas of the cochlea, the basilar papilla and the amphibian papilla. The former detects high frequencies and the latter low frequencies.[74] Because the cochlea is short, frogs use electrical tuning to extend their range of audible frequencies and help discriminate different sounds.[75] This arrangement enables detection of the territorial and breeding calls of their conspecifics. In some species that inhabit arid regions, the sound of thunder or heavy rain may arouse them from a dormant state.[74] A frog may be startled by an unexpected noise but it will not usually take any action until it has located the source of the sound by sight.[73]
Call
See also: Sexual selection in frogs
A male Dendropsophus microcephalus displaying its vocal sac during its call.
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Advertisement call of male Atelopus franciscus
The call or croak of a frog is unique to its species. Frogs create this sound 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. Some frog calls are so loud that they can be heard up to a mile away.[76] Additionally, some species have been found to use man-made structures such as drain pipes for artificial amplification of their call.[77]
Frogs in the genera Heleioporus and Neobatrachus lack vocal sacs but can still produce a loud call. Their buccal cavity is enlarged and dome-shaped, acting as a resonance chamber that amplifies the sound. Species of frog that lack vocal sacs and that do not have a loud call tend to inhabit areas close to constantly noisy, flowing water. They need to use an alternative means to communicate. The coastal tailed frog (Ascaphus truei) lives in mountain streams in North America and does not vocalize.[78]
The main reason for calling is to allow male frogs to attract a mate. Males may call individually or there may be a chorus of sound where numerous males have converged on breeding sites. Females of many frog species, such as the common tree frog (Polypedates leucomystax), reply to the male calls, which acts to reinforce reproductive activity in a breeding colony.[79] Female frogs prefer males that produce sounds of greater intensity and lower frequency, attributes that stand out in a crowd. The rationale for this is thought to be that by demonstrating his prowess, the male shows his fitness to produce superior offspring.[80]
A different call is emitted by a male frog or unreceptive female when mounted by another male. This is a distinct chirruping sound and is accompanied by a vibration of the body.[81] Tree frogs and some non-aquatic species have a rain call that they make on the basis of humidity cues prior to a shower.[81] Many species also have a territorial call that is used to drive away other males. All of these calls are emitted with the mouth of the frog closed.[81] A distress call, emitted by some frogs when they are in danger, is produced with the mouth open resulting in a higher-pitched call. It is typically used when the frog has been grabbed by a predator and may serve to distract or disorientate the attacker so that it releases the frog.[81]
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Distinctive low "jug-o-rum" sound of Banded bullfrog.
Many species of frog have deep calls. The croak of the American bullfrog (Rana catesbiana) is sometimes written as "jug o' rum".[82] The Pacific tree frog (Pseudacris regilla) produces the onomatopoeic "ribbit" often heard in films.[83] Other renderings of frog calls into speech include "brekekekex koax koax", the call of the marsh frog (Pelophylax ridibundus) in The Frogs, an Ancient Greek comic drama by Aristophanes.[84]
Torpor
During extreme conditions, some frogs enter a state of torpor and remain inactive for months. In colder regions, many species of frog hibernate in winter. Those that live on land such as the American toad (Bufo americanus) dig a burrow and make a hibernaculum in which to lie dormant. Others, less proficient at digging, find a crevice or bury themselves in dead leaves. Aquatic species such as the American bullfrog (Rana catesbeiana) normally sink to the bottom of the pond where they lie, semi-immersed in mud but still able to access the oxygen dissolved in the water. Their metabolism slows down and they live on their energy reserves. Some frogs can even survive being frozen. Ice crystals form under the skin and in the body cavity but the essential organs are protected from freezing by a high concentration of glucose. An apparently lifeless, frozen frog can resume respiration and the heart beat can restart when conditions warm up.[85]
At the other extreme, the striped burrowing frog (Cyclorana alboguttata) regularly aestivates during the hot, dry season in Australia, surviving in a dormant state without access to food and water for nine or ten months of the year. It burrows underground and curls up inside a protective cocoon formed by its shed skin. Researchers at the University of Queensland have found that during aestivation, the metabolism of the frog is altered and the operational efficiency of the mitochondria is increased. This means that the limited amount of energy available to the comatose frog is used in a more efficient manner. This survival mechanism is only useful to animals that remain completely unconscious for an extended period of time and whose energy requirements are low because they are cold-blooded and have no need to generate heat.[86] Other research showed that, to provide these energy requirements, muscles atrophy, but hind limb muscles are preferentially unaffected.[87] Frogs have been found to have upper critical temperatures of around 41 degrees Celsius.[88]
Locomotion
Different species of frog use a number of methods of moving around including jumping, running, walking, swimming, burrowing, climbing and gliding.
Rainforest rocket frog jumping.
Jumping
Frogs are generally recognized as exceptional jumpers and, relative to their size, the best jumpers of all vertebrates.[89] The striped rocket frog, Litoria nasuta, can leap over 2 metres (6 ft 7 in), a distance that is more than fifty times its body length of 5.5 centimetres (2.2 in).[90] There are tremendous differences between species in jumping capability. Within a species, jump distance increases with increasing size, but relative jumping distance (body-lengths jumped) decreases. The Indian skipper frog (Euphlyctis cyanophlyctis) has the ability to leap out of the water from a position floating on the surface.[91] The tiny northern cricket frog (Acris crepitans) can "skitter" across the surface of a pond with a series of short rapid jumps.[92]
Slow-motion photography shows that the muscles have passive flexibility. They are first stretched while the frog is still in the crouched position, then they are contracted before being stretched again to launch the frog into the air. The fore legs are folded against the chest and the hind legs remain in the extended, streamlined position for the duration of the jump.[43] In some extremely capable jumpers, such as the Cuban tree frog (Osteopilus septentrionalis) and the northern leopard frog (Rana pipiens), the peak power exerted during a jump can exceed that which the muscle is theoretically capable of producing. When the muscles contract, the energy is first transferred into the stretched tendon which is wrapped around the ankle bone. Then the muscles stretch again at the same time as the tendon releases its energy like a catapult to produce a powerful acceleration beyond the limits of muscle-powered acceleration.[93] A similar mechanism has been documented in locusts and grasshoppers.[94]
Walking and running
Phrynomantis bifasciatus walking on a level surface
Frogs in the families Bufonidae, Rhinophrynidae, and Microhylidae have short back legs and tend to walk rather than jump.[95] When they try to move rapidly, they speed up the rate of movement of their limbs or resort to an ungainly hopping gait. The Great Plains narrow-mouthed toad (Gastrophryne olivacea) has been described as having a gait that is "a combination of running and short hops that are usually only an inch or two in length".[96] In an experiment, Fowler's toad (Bufo fowleri) was placed on a treadmill which was turned at varying speeds. By measuring the toad's uptake of oxygen it was found that hopping was an inefficient use of resources during sustained locomotion but was a useful strategy during short bursts of high-intensity activity.[97]
The red-legged running frog (Kassina maculata) has short, slim hind limbs unsuited to jumping. It can move fast by using a running gait in which the two hind legs are used alternately. Slow-motion photography shows, unlike a horse that can trot or gallop, the frog's gait remained similar at slow, medium, and fast speeds.[98] This species can also climb trees and shrubs, and does so at night to catch insects.[99] The Indian skipper frog (Euphlyctis cyanophlyctis) has broad feet and can run across the surface of the water for several metres (yards).[92]
Swimming
Common toad swimming
Common toad (Bufo bufo) swimming.
Frogs that live in or visit water have adaptations that improve their swimming abilities. The hind limbs are heavily muscled and strong. The webbing between the toes of the hind feet increases the area of the foot and helps propel the frog powerfully through the water. Members of the family Pipidae are wholly aquatic and show the most marked specialization. They have inflexible vertebral columns, flattened, streamlined bodies, lateral line systems, and powerful hind limbs with large webbed feet.[100] Tadpoles mostly have large tail fins which provide thrust when the tail is moved from side to side.[101]
Burrowing
Some frogs have become adapted for burrowing and a life underground. They tend to have rounded bodies, short limbs, small heads with bulging eyes, and hind feet adapted for excavation. An extreme example of this is the purple frog (Nasikabatrachus sahyadrensis) from southern India which feeds on termites and spends almost its whole life underground. It emerges briefly during the monsoon to mate and breed in temporary pools. It has a tiny head with a pointed snout and a plump, rounded body. Because of this fossorial existence, it was first described in 2003, being new to the scientific community at that time, although previously known to local people.[102]
Purple frog
Purple frog (Nasikabatrachus sahyadrensis).
The spadefoot toads of North America are also adapted to underground life. The Plains spadefoot toad (Spea bombifrons) is typical and has a flap of keratinised bone attached to one of the metatarsals of the hind feet which it uses to dig itself backwards into the ground. As it digs, the toad wriggles its hips from side to side to sink into the loose soil. It has a shallow burrow in the summer from which it emerges at night to forage. In winter, it digs much deeper and has been recorded at a depth of 4.5 m (15 ft).[103] The tunnel is filled with soil and the toad hibernates in a small chamber at the end. During this time, urea accumulates in its tissues and water is drawn in from the surrounding damp soil by osmosis to supply the toad's needs.[103] Spadefoot toads are "explosive breeders", all emerging from their burrows at the same time and converging on temporary pools, attracted to one of these by the calling of the first male to find a suitable breeding location.[104]
The burrowing frogs of Australia have a rather different lifestyle. The western spotted frog (Heleioporus albopunctatus) digs a burrow beside a river or in the bed of an ephemeral stream and regularly emerges to forage.
Before you can recite the rest of the wiki page on frogs you finally complete ceasing to exist.
You cease to exist.
You died.
