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•The order Anura contains 4,810 species in 33families.•About 88% of amphibian species are frogs.• From a taxonomic perspective, all members of theorder Anura are frogs, but only members of the familyBufonidae are considered "true toads". The use ofthe term "frog" in common names usually refers tospecies that are aquatic or semi-aquatic with smoothand/or moist skins, and the term "toad" generallyrefers to species that tend to be terrestrial with dry,warty skin.•Frogs and toads are broadly classified into threesuborders: Archaeobatrachia, which includes fourfamilies of primitive frogs; Mesobatrachia, whichincludes five families of more evolutionaryintermediate frogs; and Neobatrachia, by far thelargest group, which contains the remaining 24
Morphology and PhysiologyThe morphology of frogs is dramatically different from mostother vertebrates. Compared with the other two groups ofamphibians, (salamanders and caecilians), frogs are unusualbecause they lack tails as adults and their legs are moresuited to jumping than walking. The physiology of frogs isgenerally like that of other amphibians (and differs from otherterrestrial vertebrates) because oxygen can pass throughtheir highly permeable skin. This unique feature allows frogsto respire largely through their skins. The ribs are poorlydeveloped so the lungs are filled by buccal pumping. It hasbeen shown that a frog that has been deprived of its lungscan continue living, respiring entirely through itsskin. Because the oxygen is dissolved in an aqueous film onthe skin and passes from there to the blood, the skin mustremain moist at all times; this makes frogs susceptible tomany toxins in the environment, some of which can similarlydissolve in the layer of water and be passed into theirbloodstream. This may be one of the causes of the decline in
While many species of frog show deviations from the "typical"anuran body plan, some general characteristics distinguish themfrom other amphibians. Frogs are usually well suited to jumping, withlong hind legs and elongated ankle bones. They have a shortvertebral column, with no more than ten free vertebrae, followed by afused tailbone (urostyle or coccyx), typically resulting in a taillessphenotype.The skin hangs loosely on the body because of the lack of looseconnective tissue. Skin texture varies: it can be smooth, warty orfolded. Frogs have three eyelid membranes: one is transparent toprotect the eyes underwater, and two vary from translucent toopaque. Frogs have a tympanum on each side of the head, which isinvolved in hearing and, in some species, is covered by skin. Mostfrogs have teeth, specifically pedicellate teeth in which the crown isseparated from the root by fibrous tissue. Most only have teeth onthe edge of the upper jaw (maxillary teeth) as well as vomerineteeth on the roof of their mouth. They do not have any teeth on theirlower jaw, so they usually swallow their food whole. The teeth aremainly used to hold the prey and keep it in place till they can get agood grip on it and swallow their meal, assisted by retracting theireyes into their head.
True toads lack any teeth at all. Some species, like the Africanbull frog, which preys on relatively large organisms including miceand other frogs, have cone shaped projections of bone calledodontoid processes at the front of the lower jaw which functionlike teeth.Feet and legsMany frogs, especially those that live in water, have webbed toes.The degree to which the toes are webbed is directly proportionalto the amount of time the species lives in the water. Arborealfrogs(spends a major portion of its lifespan in trees) have "toepads" to help grip vertical surfaces. These pads, located on theends of the toes, do not work by suction. Rather, the surface ofthe pad consists of interlocking cells, with a small gap betweenadjacent cells. When the frog applies pressure to the toe pads,the interlocking cells grip irregularities on the substrate. The smallgaps between the cells drain away all but a thin layer of moistureon the pad, and maintain a grip throughcapillarity. This allows thefrog to grip smooth surfaces, and does not function when the padsare excessively wet.
In many arboreal frogs, a small "intercalary structure" ineach toe increases the surface area touching thesubstrate. Furthermore, since hopping through trees canbe dangerous, many arboreal frogs have hip joints thatallow both hopping and walking. Some frogs that live highin trees even possess an elaborate degree of webbingbetween their toes, as do aquatic frogs. In these arborealfrogs, the webs allow the frogs to "parachute" or controltheir glide from one position in the canopy to another.Ground-dwelling frogs generally lack the adaptations ofaquatic and arboreal frogs. Most have smaller toe pads, ifany, and little webbing. Some burrowing frogs have a toeextension—a metatarsal tubercle—that helps them toburrow. The hind legs of ground dwellers are moremuscular than those of aqueous and tree-dwelling frogs.
SkinBeing cold-blooded, frogs have to adopt behaviour patterns toregulate their temperature. To warm up they can move into thesun or onto a warm surface and to cool down they can moveinto the shade or adopt a stance that exposes the minimumarea of skin to the air. This involves squatting down, tucking theforefeet under the chin and the hind feet under the belly. Thecolour of a frogs skin is used for thermo-regulation. In cooldamp conditions the colour will be darker than on a hot dry day.Many frogs are able to absorb water and oxygen directlythrough the skin, especially around the pelvic area. However,the permeability of a frogs skin can also result in water loss.Glands located all over the body exude mucus which helpskeep the skin moist and reduces evaporation. Some glands onthe hands and chest of males are specialized to produce stickysecretions to aid in amplexus. Similar glands in tree frogsproduce a glue-like substance on the adhesive discs of thefeet.
American GreenTree Frog (Hylacinerea)Cane Toad(Bufomarinus)
Some tree frogs reduce water loss by having a waterproof layer ofskin and several South American species coat their skin with awaxy secretion. Others frogs have adopted behaviours to conservewater, including becoming nocturnal and resting in a water-conserving position. This posture involves the frog lying with itstoes and fingers tucked under its body and chin with no gapbetween the body and the substrate. Some frog species will alsorest 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. Camouflage is a common defensivemechanism in frogs. Most camouflaged frogs are nocturnal, whichadds to their ability to hide. Nocturnal frogs usually find the idealcamouflaged position during the day to sleep. Some frogs havethe ability to change colour, but this is usually restricted to shadesof one or two colours. Features such as warts and skin folds areusually found on ground-dwelling frogs, where a smooth skin wouldnot disguise them effectively. Arboreal frogs usually have smoothskin, enabling them to disguise themselves as leaves. Certain frogschange colour between night and day, as light and moisturestimulate the pigment cells and cause them to expand or contract.
PoisonMany frogs contain mild toxins that make them unpalatable topotential predators. For example, all toads have large poisonglands—the parotoid glands—located behind the eyes, on the topof the head. Some frogs, such as some poison dart frogs, areespecially toxic. The chemical makeup of toxins in frogs varies fromirritants to hallucinogens, convulsants, nerve poisons,and vasoconstrictors. Many predators of frogs have adapted totolerate high levels of these poisons. Others, including humans,may be severely affected.Some frogs obtain poisons from the ants and other arthropods theyeat. Poisonous frogs tend to advertise their toxicity with brightcolours, an adaptive strategy known as aposematism. There are atleast two non-poisonous species of frogs in tropical America(Eleutherodactylus gaigei and Lithodytes lineatus) that mimic thecolouration of dart poison frogs coloration for self-protection(Batesian mimicry).
Respiration and circulationThe skin of a frog is permeable to oxygen and carbon dioxide, aswell as to water. There are a number of blood vessels near thesurface of the skin. When a frog is underwater, oxygen istransmitted through the skin directly into the bloodstream. Onland, adult frogs use their lungs to breathe. Their lungs aresimilar to those of humans, but the chest muscles are notinvolved in respiration, and there are no ribs or diaphragm tosupport breathing. Frogs breathe by taking air in through thenostrils (which often have valves which close when the frog issubmerged), causing the throat to puff out, then compressing thefloor of the mouth, which forces the air into the lungs.The Bornean Flat-headed Frog (B. kalimantanensis) is the firstspecies of frog known to science without lungs.
Frogs are known for their three-chambered heart, which theyshare with all tetrapods except birds, crocodilians and mammals.In the three-chambered heart, oxygenated blood from the lungsand de-oxygenated blood from the respiring tissues enter byseparate atria, and are directed via a spiral valve to theappropriate vessel—aorta for oxygenated blood and pulmonaryartery for deoxygenated blood. This special structure is essentialto keeping the mixing of the two types of blood to a minimum,which enables frogs to have higher metabolic rates, and to bemore active than otherwise.Digestion and excretionFrogs have teeth along their upper jaw called maxillary teeth,which are used to hold food before it is swallowed. These teethare very weak, and cannot be used to chew or catch and harmagile prey. Instead, the frog uses its sticky, cleft tongue to catchflies and other small moving prey.
The eyes assist in the swallowing of food as they areable to be retracted through holes in the skull andhelp push food down the throat. The food then movesthrough the esophagus into the stomach wheredigestive enzymes are added and it is churned up. Itthen proceeds to the small intestine (duodenum andileum) where most digestion occurs. Pancreatic juicefrom the pancreas, and bile, produced by the liverand stored in the gallbladder, are secreted into thesmall intestine, where the fluids digest the food andthe nutrients are absorbed. The food residue passesinto the large intestine where water is absorbed andwastes are routed to the cloaca.The excretory system is similar to that of mammals.There are two kidneys which remove urea from theblood and convert it into urine. This passes alongpaired ureters to the urinary bladder from which it isvented periodically into the cloaca. All bodily wastes
Reproductive systemIn many species the male is smaller and slimmer than the female. Maleshave vocal cords and make a range of croaks, particularly in the breedingseason, and in some species they also have vocal sacs to amplify thesound. In the male the two testes are attached to the kidneys and spermpasses into the kidneys through fine tubes called efferent ducts. They thentravel through the ureters which are consequently called urinogenital ductsin the male frog. There is no penis and sperm are ejected from the cloacadirectly onto the eggs that the female is laying during amplexus. Theovaries of the female frog are also beside the kidneys and the eggs passdown a pair of oviducts to the exterior. During amplexus, the firm grip of themale frog stimulates the release of eggs, usually wrapped in jelly, asspawn.
Nervous systemThe frog has a highly developed nervous system which consists of abrain, spinal cord and nerves. Many parts of the frogs braincorrespond with those of humans. The brain consists of twoolfactory lobes, two cerebral hemispheres, a pineal body, two opticlobes, a cerebellum and a medulla oblongata. Muscularcoordination and posture are controlled by the cerebellum and themedulla oblongata regulates respiration, digestion and otherautomatic functions. The relative size of the cerebrum of a frog ismuch smaller than that of a human. Frogs have ten cranialnerves(nerves which pass information from the outside directly tothe brain) and ten pairs of spinal nerves (nerves which passinformation from extremities to the brain through the spinal cord). Bycontrast, all amniotes (mammals, birds and reptiles) have twelvecranial nerves. Frogs do not have external ears; the eardrums(tympanic membranes) are directly exposed. As in all animals, theear contains semicircular canals which help control balance andorientation. Due to their short cochlea, frogs use electrical tuning toexpand their range of audible frequencies.
SightThe eyes of frogs are located on or near the top of the head and oftenproject outwards as hemispherical bulges. They have a large field of viewand may be the only part of an otherwise submerged frog to protrude fromthe water. Each eye has a closable upper and lower lid and a nictitatingmembrane which provides further protection, especially when the frog isswimming. Theirises come in a range of colours and the pupils in a range ofshapes.
Life CycleThe life cycle of frogs, like that of otheramphibians, consists of four main stages:egg, tadpole, metamorphosis and adult.The reliance of frogs on an aquaticenvironment for the egg and tadpolestages gives rise to a variety of breedingbehaviours that include the well-knownmating calls used by the males of mostspecies to attract females to the bodies ofwater that they have chosen for breeding.Some frogs also look after their eggs—and in some cases even the tadpoles—
SummaryMajor Characteristics of Lissamphibians •Double or paired occipital condyle •Two types of skin glands (mucous & granular) •Fat bodies associated with gonads •Double-channeled sensory papillae in the inner ear •Green rods (a special type of visual cell, unknown in caecilians) •Ribs do not encircle body •Ability to elevate the eye (with levator bulbi muscle) •Forced pump respiratory mechanism •Cylindrical centra (the main body of the vertebra; cylindrical centra are also found in several groups of early tetrapods)
•Pedicellate teeth (the crown of the teeth is separatedfrom the root by a zone of fibrous tissue; also found insome Dissorophoidea; the teeth of somefossil salamanders are not pedicellate)•Bicuspid teeth (two cusps per tooth, also found injuvenile dissorophoids)•Operculum (small bone in the skull, linked to shouldergirdle by the opercularis muscle; perhaps involved inhearing and balance; absent in caecilians and somesalamanders, fused to the stapes (ear bones) in mostanurans)•Loss of posterior skull bones (alsoin Microsauria and Dissorophoidea)•Small, widely separated pterygoids (also foundin Temnospondyli and Nectridea)•Wide cultriform process of the parasphenoid (alsofound in some Microsauria (Rhynchonchos)
Summary: Anurans Major Characteristics:When only living taxa are considered, a list ofsynapomorphies of Anura would include many of theobvious distinctive features of frogs:•Shortened vertebral column (nine or fewer presacralvertebrae)•Presence of a urostyle formed from developing tailvertebrae•Absence of tail in adultsHindlimb longer than forelimb•Fusion of radius and ulna into a single elementFusion of tibia and fibula into a single elementElongate ankle bones (tibiale and fibulare = astragalusand calcaneum)
•Absence of a prefrontal boneFusion of separate hyobranchial elementsinto a hyobranchial (=hyoid) plate•A tongue that lacks intrinsic skeletalsupport from the hyobranchial plateA tadpole, with keratinous beaks anddenticles as larval mouthparts•A single median spiracle in the larva(characteristic of Ortons Type 3 tadpole)•Large subcutaneous lymph spacesbetween the skin and muscle layerTwo protractor lentis muscles attached tolens