Editing Frog (section)

== Morphology and physiology ==
[[File:Frog limbs.jpg|thumb|right|A [[American bullfrog|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.]]

Frogs have no tail, except as larvae. Most frogs have long hind legs, elongated ankle bones, webbed toes, no claws, large eyes, and either smooth or warty skin. They have short vertebral columns, with no more than 10 free vertebrae and fused tailbones (urostyle or coccyx).<ref name=Flam>{{cite web |author=Flam, F. |year=1995 |title=Finding earliest true frog will help paleontologists understand how frog evolved its jumping ability |url=http://www.highbeam.com/doc/1G1-17454183.html |archive-url=https://web.archive.org/web/20130513111921/http://www.highbeam.com/doc/1G1-17454183.html |url-status=dead |archive-date=May 13, 2013 |publisher=Knight Ridder/Tribune News Service |access-date=June 10, 2012}}</ref> Frogs range in size from a [[snout–vent length]] of {{cvt|7.7|mm}} (the ''[[Paedophryne amauensis]]'' of [[Papua New Guinea]])<ref>{{cite news |title=Tiny frog claimed as world's smallest vertebrate |url=https://www.theguardian.com/environment/2012/jan/12/world-smallest-frog |newspaper=The Guardian |date=January 12, 2012 |access-date=September 28, 2012}}</ref>  to about {{cvt|35|cm}} (the [[goliath frog]] (''Conraua goliath'') of central Africa, which is about {{cvt|3.3|kg}}).<ref name=Colgan1982>{{cite book | last=Colgan | first=P.V | year=1982 |title=The Guinness Book of Animal Facts and Feats | url=https://archive.org/details/guinnessbookofan00wood/ | edition=3 | publisher=Guinness Superlatives | pages=118–119 | isbn=978-0851122359 }}</ref> Some extinct prehistoric species were even larger.<ref>{{cite journal |author1=Otero, R.A. |author2=P. Jimenez-Huidobro |author3=S. Soto-Acuña |author4=R.E.Yury-Yáñez |year=2014 |title=Evidence of a giant helmeted frog (Australobatrachia, Calyptocephalellidae) from Eocene levels of the Magallanes Basin, southernmost Chile |journal=Journal of South American Earth Sciences |volume=55 |pages=133–140 |doi=10.1016/j.jsames.2014.06.010 |bibcode=2014JSAES..55..133O }}</ref>

===Feet and legs===
A frog's foot and leg structure is related to its habitat. Across species, these structures vary based on whether the species lives primarily on the ground, in water, in trees, or in burrows. Adult anurans have four fingers on the hands and five toes on the feet,<ref>[https://pubmed.ncbi.nlm.nih.gov/28597591/ Morphological Variation in Anuran Limbs: Constraints and Novelties]</ref> but the smallest species often have hands and feet where some of the digits are vestigial.<ref>[https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0213314 Morphological and ecological convergence at the lower size limit for vertebrates highlighted by five new miniaturised microhylid frog species from three different Madagascan genera]</ref> 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 jumpers or descend from ancestors that were, with much of the [[musculoskeletal]] [[morphology (biology)|morphology]] modified for this purpose. The tibia, fibula, and [[tarsus (skeleton)|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, allowing frogs to push against the ground for a longer period on take-off. The [[ilium (bone)|ilium]] 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 frogs to transfer force from the legs to the body during a leap.<ref name=Flam/>

[[File:Rana temporaria 04 by-dpc.jpg|thumb|alt=Webbed foot|Webbed hind foot of [[common frog]] <br /> (''Rana temporaria'')]]
[[File:Litoria tyleri.jpg|thumb|[[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.<ref name=Frogjump/>

Many frogs have webbed feet and the degree of webbing is directly proportional to the amount of time the species spends in the water.<ref name=Exploratorium>{{cite web |url=http://www.exploratorium.edu/frogs/mainstory/ |title=The amazing adaptable frog |author=Tesler, P. |year=1999 |publisher=Exploratorium:: The museum of science, art and human perception |access-date=June 4, 2012}}</ref> 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.<ref>{{cite web |author=Vincent, L. |year=2001 |publisher=James Cook University |title=''Litoria caerulea'' |url=http://www.jcu.edu.au/school/tbiol/zoology/herp/Litoriacaerulea.PDF |archive-url=https://web.archive.org/web/20040422212337/http://www.jcu.edu.au/school/tbiol/zoology/herp/Litoriacaerulea.PDF |archive-date=April 22, 2004 |access-date=August 3, 2012 }}</ref> Exceptions include [[flying frog]]s in the [[Hylidae]] and [[Rhacophoridae]], which also have fully webbed toes used in gliding.

[[Tree frog|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 [[Capillarity|surface tension]]. This allows the frog to climb on smooth surfaces, but the system does not function efficiently when the pads are excessively wet.<ref>{{cite journal |last=Emerson |first=S. B. |author2=Diehl, D. |year=1980| title=Toe pad morphology and mechanisms of sticking in frogs |journal=Biological Journal of the Linnean Society |volume=13 |issue=3 |pages=199–216 |doi=10.1111/j.1095-8312.1980.tb00082.x}}</ref>

In many arboreal frogs, a small "intercalary structure" on each toe increases the surface area touching the [[Substrate (biology)|substrate]]. Furthermore, many arboreal frogs have hip joints that allow both hopping and walking. Some frogs that live high in trees even possess an elaborate degree of webbing between their toes. This allows the frogs to "parachute" or make a controlled glide from one position in the canopy to another.<ref>{{cite journal |last=Harvey |first=M. B. |author2= Pemberton, A. J. |author3=Smith, E. N. |year=2002 |title=New and poorly known parachuting frogs (Rhacophoridae : ''Rhacophorus'') from Sumatra and Java |journal=Herpetological Monographs |volume=16 |pages=46–92 |doi=10.1655/0733-1347(2002)016[0046:NAPKPF]2.0.CO;2 |s2cid=86616385 }}</ref>

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 Toad|Couch's spadefoot]] (''Scaphiopus couchii'') have a flap-like toe extension on the hind feet, a [[keratin]]ised [[tubercle]] often referred to as a spade, that helps them to burrow.<ref>{{cite web |url=http://www.desertmuseum.org/books/nhsd_spadefoot.php |title=Couch's spadefoot (''Scaphiopus couchi'') |publisher=Arizona-Sonora Desert Museum |access-date=August 3, 2012}}</ref>

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 one or more extra legs.<ref>{{cite news |url=http://news.bbc.co.uk/earth/hi/earth_news/newsid_8116000/8116692.stm |title=Legless frogs mystery solved |work=BBC News |date=June 25, 2009 |first=M. |last=Walker}}</ref>

[[File:Ranapipiensmoulting.jpg|thumb|left|[[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 ([[Skin|granular glands]]). The secretion is often sticky and helps keep the skin moist, protects against the entry of moulds and bacteria, and makes the animal slippery and more able to escape from predators.<ref>{{cite book |title=A Natural History of Amphibians |last=Stebbins |first=Robert C. |author-link1=Robert C. Stebbins |author2=Cohen, Nathan W. |year=1995 |publisher=Princeton University Press |isbn=978-0-691-03281-8 |pages=10–14 }}</ref> 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.<ref>{{cite journal |jstor=2456779 |pages=530–540 |last1=Frost |first1=S. W. |title=Notes on feeding and molting in frogs |volume=66 |issue=707 |journal=The American Naturalist |year=1932 |doi=10.1086/280458 |bibcode=1932ANat...66..530F |s2cid=84796411 }}</ref>

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.<ref name=BandN>{{cite book |title=Frogs |last=Badger |first=D. |author2=Netherton, J. |year=1995 |publisher=Airlife Publishing |isbn=978-1-85310-740-5 |page=27 }}</ref> 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 Treefrog|grey foam-nest tree frog]] (''Chiromantis xerampelina'') is even able to turn white to minimise the chance of overheating.<ref>{{cite book |title=Amphibians and Their Ways |last=Smyth |first=H. R. |year=1962 |publisher=Macmillan |isbn=978-0-02-612190-3 }}</ref>

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 specialised 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.<ref name=BandN/> [[Woodhouse's toad]] (''Bufo woodhousii''), if given access to water after confinement in a dry location, sits in the shallows to rehydrate.<ref>{{cite book |title=The Frog Book: North American Frogs and Toads |last=Dickerson |first=M. C. |year=1969 |publisher=Dover Publications |isbn=978-0-486-21973-8 |url=https://archive.org/details/frogbooknorthame00dick_0 }}</ref> The male [[hairy frog]] (''Trichobatrachus robustus'') has [[dermal papillae]] projecting from its lower back and thighs, giving it a bristly appearance. These contain blood vessels and are thought to increase the area of the skin available for respiration.<ref>{{cite web |url=http://amphibiaweb.org/cgi-bin/amphib_query?where-genus=Trichobatrachus |title=''Trichobatrachus robustus'' |author=Blackburn, D. C. |date=November 14, 2002 |publisher=AmphibiaWeb |access-date=August 18, 2012}}</ref>
[[File:Hip-pocket Frog - Assa darlingtoni.jpg|thumb|alt=Frog barely recognisable against brown decaying leaf litter.|[[Pouched frog]] (''Assa darlingtoni'') camouflaged against leaf litter]]
[[File:WoodFrog DarienLakesStatePark 2020-06-16 (02).jpg|thumb|[[Wood frog]] (''Lithobates sylvaticus'') uses [[disruptive coloration]].]]

Some species have [[Osteoderm|bony plates]] embedded in the skin, a trait that appears to have evolved independently several times.<ref>{{cite journal |author1=Ruibal, Rodolfo |author2=Shoemaker, Vaughan |year=1985 |title=Osteoderms in Anurans |journal=Journal of Herpetology |volume=18 |issue=3 |pages=313–328 |jstor=1564085 |doi=10.2307/1564085}}</ref> 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]]).<ref>{{cite book |title=Herpetology: An Introductory Biology of Amphibians and Reptiles |last1=Vitt |first1=Laurie J. |last2=Caldwell |first2=Janalee P. |year=2013 |publisher=Academic Press |isbn=9780123869203 |page=50  |url={{Google books|Gay9N_ry79kC|page=50|plainurl=yes}} }}</ref><ref>{{cite journal |author1=Jared, C. |author2=Antoniazzi, M. M. |author3=Navas, C. A. |author4=Katchburian, E. |author5=Freymüller, E. |author6=Tambourgi, D. V. |author7=Rodrigues, M. T. |year=2005 |title=Head co-ossification, phragmosis and defence in the casque-headed tree frog ''Corythomantis greeningi'' |journal=Journal of Zoology |volume=265 |issue=1 |pages=1–8 |doi=10.1017/S0952836904005953  |s2cid=59449901 }}</ref>

[[Camouflage]] is a common defensive mechanism in frogs. Features such as warts and [[skin fold]]s 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.<ref name=Observer>{{cite book |title=The Observer's Book of British Wild Animals |last=Burton |first=Maurice |year=1972 |publisher=Frederick Warne & Co |isbn=978-0-7232-1503-5 |pages=204–209 }}</ref> Some are even able to control their skin texture.<ref>{{Cite journal|last1=Guayasamin|first1=Juan M.|last2=Krynak|first2=Tim|last3=Krynak|first3=Katherine|last4=Culebras|first4=Jaime|last5=Hutter|first5=Carl R.|date=2015|title=Phenotypic plasticity raises questions for taxonomically important traits: a remarkable new Andean rainfrog ( Pristimantis ) with the ability to change skin texture: Phenotypic plasticity in Andean rainfrog|journal=Zoological Journal of the Linnean Society|language=en|volume=173|issue=4|pages=913–928|doi=10.1111/zoj.12222|doi-access=free}}</ref> 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.<ref>{{cite journal |author1=Wente, W. H. |author2=Phillips, J. B. |year=2003 |title=Fixed green and brown color morphs and a novel color-changing morph of the Pacific tree frog ''Hyla regilla'' |journal=The American Naturalist |volume=162 |issue=4 |pages=461–473 | jstor=10.1086/378253 |doi=10.1086/378253 |pmid=14582008 |bibcode=2003ANat..162..461W |s2cid=25692966 }}</ref> The [[Wood frog]] (''Lithobates sylvaticus'') uses [[disruptive coloration]] including black eye markings similar to voids between leaves, bands of the dorsal skin (dorsolateral dermal plica) similar to a leaf [[midrib]] as well as stains, spots and leg stripes similar to fallen leaf features.

===Respiration and circulation===
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. Ribs are generally absent, so the lungs are filled by [[buccal pumping]] and a frog deprived of its lungs can maintain its body functions without them.<ref name=Observer/> The fully aquatic [[Bornean flat-headed frog]] (''Barbourula kalimantanensis'') is the first frog known to lack lungs entirely.<ref>{{cite web |url=http://amphibiaweb.org/cgi-bin/amphib_query?where-genus=Barbourula&where-species=kalimantanensis |title=''Barbourula kalimantanensis'' |author=Boisvert, Adam |date=October 23, 2007 |work=AmphibiaWeb |publisher=University of California, Berkeley |access-date=July 9, 2012}}</ref>

Frogs have three-chambered [[heart]]s, a feature they share with [[lizard]]s. Oxygenated blood from the lungs and de-oxygenated blood from the [[Respiration (physiology)|respiring]] tissues enter the heart through separate [[atrium (anatomy)|atria]]. When these chambers contract, the two blood streams pass into a common [[Ventricle (heart)|ventricle]] before being pumped via a spiral valve to the appropriate vessel, the [[aorta]] for oxygenated blood and [[pulmonary artery]] for deoxygenated blood.<ref name=Kimball>{{cite web |url=http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/AnimalHearts.html |title=Animal Circulatory Systems: Three Chambers: The Frog and Lizard |author=Kimball, John |year=2010 |work=Kimball's Biology Pages |access-date=July 9, 2012 |archive-url= https://web.archive.org/web/20160511032359/http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/AnimalHearts.html |archive-date= May 11, 2016}}</ref>

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.<ref>{{cite web |url=http://amphibiaweb.org/cgi-bin/amphib_query?where-genus=Telmatobius&where-species=culeus |title=''Telmatobius culeus'' |author=Lee, Deborah |date=April 23, 2010 |work=AmphibiaWeb |publisher=University of California, Berkeley |access-date=July 9, 2012}}</ref>

[[File:Frog anatomy tags.PNG|thumb|right|alt=Dissected frog|Anatomical model of a dissected frog: 1&nbsp;Right&nbsp;atrium, 2&nbsp;Lungs, 3&nbsp;Aorta, 4&nbsp;Egg mass, 5&nbsp;Colon, 6&nbsp;Left&nbsp;atrium, 7&nbsp;Ventricle, 8&nbsp;Stomach, 9&nbsp;Liver, 10&nbsp;Gallbladder, 11&nbsp;Small&nbsp;intestine, 12&nbsp;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 insects 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.<ref name=Exploratorium/> In amphibians there are salvary glands on the tongue, which in frogs produce what is called a two-phase viscoelastic fluid. When exposed to pressure, like when the tongue is wrapping around a prey, it becomes runny and covers the prey's body. As the pressure drops, it returns to a thick and elastic state, which gives the tongue an extra grip.<ref>[https://stao.ca/the-secret-to-the-stickiness-of-frog-spit/ The Secret to the Stickiness of Frog Spit | STAO]</ref> Some frogs have no tongue and just stuff food into their mouths with their hands.<ref name=Exploratorium/> 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.<ref name="Pou92"/> 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.<ref name=Exploratorium/><ref>{{cite journal |last1=Levine |first1=R. P. |last2=Monroy |first2=J. A. |last3=Brainerd|first3=E. L. |title=Contribution of eye retraction to swallowing performance in the northern leopard frog, ''Rana pipiens'' |doi=10.1242/jeb.00885|pmid=15010487 |date=March 15, 2004|pages=1361–1368 |issue=Pt 8|volume=207 |journal=Journal of Experimental Biology |doi-access=free |bibcode=2004JExpB.207.1361L }}</ref>

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]].<ref>{{cite web |url=http://www.tutorvista.com/biology/frog-digestive-system-diagram |title=Frog Digestive System |year=2010 |publisher=TutorVista.com |access-date=August 4, 2012|archive-url=https://web.archive.org/web/20100603080405/http://www.tutorvista.com/biology/frog-digestive-system-diagram|archive-date=June 3, 2010|url-status=dead}}</ref>

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 [[kidney]]s 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.<ref name=Doritexcretion>{{cite book |title=Zoology |url=https://archive.org/details/zoology0000dori |url-access=registration |last=Dorit |first=R. L. |author2=Walker, W. F.|author3=Barnes, R. D. |year=1991 |publisher=Saunders College Publishing  |isbn=978-0-03-030504-7 |page=[https://archive.org/details/zoology0000dori/page/849 849] }}</ref> 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]].<ref name=Doritexcretion/> The urine passes along paired [[ureter]]s 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.<ref name=TutorVista>{{cite web |url=http://www.tutorvista.com/content/biology/biology-iii/animal-morphology/respiratory-excretory-nervous-reproductive-system-frog.php |title=Frog's internal systems |year=2010 |publisher=TutorVista.com |access-date=June 4, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20080121101351/http://www.tutorvista.com/content/biology/biology-iii/animal-morphology/respiratory-excretory-nervous-reproductive-system-frog.php |archive-date=January 21, 2008}}</ref>

===Reproductive system===
{{See also|Sexual selection in amphibians}}
In the male frog, the two [[Testis|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.<ref name=TutorVista/>

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.<ref>{{cite book | last1=Duellman |first1=William E. |last2=Trueb |first2=Linda |author-link2=Linda Trueb | date = 1986 | title = Biology of Amphibians | location = New York | publisher = McGraw-Hill Publishing Company }}</ref> The male frog has certain hormone-dependent [[secondary sexual characteristic]]s. These include  the development of special pads on his thumbs in the breeding season, to give him a firm hold.<ref>{{cite web |url=http://www2.southeastern.edu/Academics/Faculty/dsever/SeverandStaub.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www2.southeastern.edu/Academics/Faculty/dsever/SeverandStaub.pdf |archive-date=October 9, 2022 |url-status=live |title=Hormones, sex accessory structures, and secondary sexual characteristics in amphibians |author1=Sever, David M. |author2=Staub, Nancy L. |pages=83–98 |work=Hormones and Reproduction of Vertebrates&nbsp;– Vol 2: Amphibians |access-date=August 4, 2012}}</ref> The grip of the male frog during amplexus stimulates the female to release eggs, usually wrapped in jelly, as spawn. 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 sac]]s to amplify the sound.<ref name=TutorVista/>

===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. 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.<ref name=TutorVista/>  By contrast, all [[amniotes]] (mammals, birds and reptiles) have twelve pairs of cranial nerves.<ref>{{cite web |url=http://tolweb.org/Amniota |title=Amniota |author1=Laurin, Michel |author2=Gauthier, Jacques A. |year=2012 |publisher=Tree of Life Web Project |access-date=August 4, 2012}}</ref>

[[File:Groene kikker achter Bekaert-draad-detail oog.jpg|thumb|left|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°.<ref name="Howard">{{cite book |last1=Howard |first1=Ian P. |url={{Google books|I8vqITdETe0C|page=651|plainurl=yes}} |title=Binocular Vision and Stereopsis |last2=Rogers |first2=Brian J. |publisher=Oxford University Press |year=1995 |isbn=978-0195084764 |page=651}}</ref> 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.<ref name=Badger>{{cite book |title=Frogs |last=Badger |first=David |author2=Netherton, John |year=1995 |publisher=Airlife Publishing Ltd |isbn=978-1-85310-740-5 |pages=31–35 }}</ref> 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.<ref name=Howard/> The [[iris (anatomy)|iris]]es 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 [[Agalychnis callidryas|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 [[Dyscophus antongilii|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.<ref name=Badger/>

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.<ref name=Badger/> 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.<ref name=Exploratorium/> Although it was formerly debated,<ref>{{cite journal|title=Vision in frogs|last=Muntz|first=W. R. A.|author2=Scientific American Offprints|journal=Scientific American|publisher=W. H. Freeman|year=1964|volume=210|issue=3|pages=110–9|doi=10.1038/scientificamerican0364-110|pmid=14133069|bibcode=1964SciAm.210c.110M|asin=B0006RENBO|oclc=15304238}}</ref> more recent research has shown that frogs can see in colour, even in very low light.<ref>{{Cite journal|last1=Kelber|first1=Almut|last2=Yovanovich|first2=Carola|last3=Olsson|first3=Peter|date=April 5, 2017|title=Thresholds and noise limitations of colour vision in dim light|journal=Philosophical Transactions of the Royal Society B: Biological Sciences|language=en|volume=372|issue=1717|pages=20160065|doi=10.1098/rstb.2016.0065|issn=0962-8436|pmc=5312015|pmid=28193810}}</ref>
{{Clear}}

===Hearing===
[[File:Synchrotron microtomography of Atelopus franciscus head - pone.0022080.s003.ogv|thumb|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 [[Pinna (anatomy)|external ears]]; the eardrums ([[Tympanum (anatomy)|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.<ref name=Neth38>{{cite book |title=Frogs |last=Badger |first=David |author2=Netherton, John |year=1995 |publisher=Airlife Publishing |isbn=978-1-85310-740-5 |page=38 }}</ref> 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.<ref name=Cohen>{{cite book |title=A Natural History of Amphibians |last1=Stebbins |first1=Robert C.|author-link1=Robert C. Stebbins| last2=Cohen|first2= Nathan W. |year=1995 |publisher=Princeton University Press |isbn=978-0-691-03281-8 |pages=67–69 }}</ref> Because the cochlea is short, frogs use [[electrical tuning]] to extend their range of audible frequencies and help discriminate different sounds.<ref>{{cite journal |author1=Armstrong, Cecilia E. |author2=Roberts, William M. |year=1998 |title=Electrical properties of frog saccular hair cells: distortion by enzymatic dissociation |journal=Journal of Neuroscience |volume=18 |issue=8 |pages= 2962–2973 |pmid=9526013 |doi=10.1523/JNEUROSCI.18-08-02962.1998 |pmc=6792591 |doi-access=free }}</ref> This arrangement enables detection of the territorial and breeding calls of their [[Conspecificity|conspecifics]]. In some species that inhabit arid regions, the sound of thunder or heavy rain may arouse them from a dormant state.<ref name=Cohen/> 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.<ref name=Neth38/>

===Call===
{{See also|Sexual selection in frogs}}
[[File:Dendropsophus microcephalus - calling male (Cope, 1886).jpg|thumb|A male ''[[Dendropsophus microcephalus]]'' displaying its vocal sac during its call]]
[[File:Atelopus franciscus male territorial call - pone.0022080.s002.oga|thumb|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 (1.6{{nbsp}}km) away.<ref>{{cite web |url=http://www.dnr.state.oh.us/Home/species_a_to_z/SpeciesGuideIndex/bullfrog/tabid/6576/Default.aspx |title=Bullfrog |publisher=Ohio Department of Natural Resources |access-date=June 19, 2012 |archive-date=August 18, 2012 |archive-url=https://web.archive.org/web/20120818105255/http://www.dnr.state.oh.us/Home/species_a_to_z/SpeciesGuideIndex/bullfrog/tabid/6576/Default.aspx |url-status=dead }}</ref> Additionally, some species have been found to use man-made structures such as drain pipes for artificial amplification of their call.<ref>{{Cite journal|last1=Tan|first1=W.-H.|last2=Tsai|first2=C.-G.|last3=Lin|first3=C.|last4=Lin|first4=Y. K.|date=June 5, 2014|title=Urban canyon effect: storm drains enhance call characteristics of the Mientien tree frog|journal=Journal of Zoology|language=en|volume=294|issue=2|pages=77–84|doi=10.1111/jzo.12154|issn=0952-8369|url=http://ntur.lib.ntu.edu.tw//handle/246246/260607}}</ref> The [[Ascaphus truei|coastal tailed frog]] (''Ascaphus truei'') lives in mountain streams in North America and does not vocalise.<ref>{{cite web |url=http://www.californiaherps.com/frogs/pages/a.truei.html |title=''Ascaphus truei'': Coastal Tailed Frog |author=Nafis, Gary |year=2012  |publisher=California Herps |access-date=June 19, 2012}}</ref>

The main function of calling is for male frogs to attract mates. Males may call individually or there may be a chorus of sound where numerous males have converged on breeding sites. In many frog species, such as the [[Common Tree Frog|common tree frog]] (''Polypedates leucomystax''), females reply to males' calls, which acts to reinforce reproductive activity in a breeding colony.<ref>{{cite journal| last=Roy| first=Debjani| year=1997| title=Communication signals and sexual selection in amphibians| journal=Current Science| volume=72| pages=923–927| url=http://www.ias.ac.in/jarch/currsci/72/00000944.pdf| url-status=dead| archive-url=https://web.archive.org/web/20120923071112/http://www.ias.ac.in/jarch/currsci/72/00000944.pdf| archive-date=September 23, 2012}}</ref> 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.<ref>{{cite journal |author=Gerhardt, H. C. |year=1994 |title=The evolution of vocalization in frogs and toads |journal=Annual Review of Ecology and Systematics |volume=25 |issue=1 |pages=293–324 |doi=10.1146/annurev.es.25.110194.001453 |bibcode=1994AnRES..25..293G }}</ref>

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.<ref name=Badger3>{{cite book |title=Frogs |last=Badger |first=David |author2=Netherton, John |year=1995 |publisher=Airlife Publishing Ltd |isbn=978-1-85310-740-5 |pages=39–44 }}</ref> Tree frogs and some non-aquatic species have a rain call that they make on the basis of humidity cues prior to a shower.<ref name=Badger3/> 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.<ref name=Badger3/> 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 disorient the attacker so that it releases the frog.<ref name=Badger3/>

[[file:Banded_Bull_Frog_Call.ogg|left|thumb|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".<ref>{{cite book|last=Hilton|first=Bill Jr.|chapter=9. 'Jug-o-Rum': Call of the Amorous Bullfrog |title=The Piedmont Naturalist, Volume 1 |publisher=Hilton Pond Center for Piedmont Natural History |location=York, SC |year=1986 |isbn=978-0-9832151-0-3}}</ref> The [[Pacific Tree Frog|Pacific tree frog]] (''Pseudacris regilla'') produces the [[onomatopoeia|onomatopoeic]] "ribbit" often heard in films.<ref>{{cite web |url=http://www.beachwatchers.wsu.edu/island/essays/TreeFrogs.htm |title=The RRRRRRRRiveting Life of Tree Frogs |last=Nash |first=Pat |date=February 2005 |access-date=August 4, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120309105032/http://www.beachwatchers.wsu.edu/island/essays/TreeFrogs.htm |archive-date=March 9, 2012 }}</ref> 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]].<ref>{{cite web |url=http://records.viu.ca/~johnstoi/aristophanes/frogs.htm |title=The Frogs |author=Aristophanes |access-date=June 19, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120513143510/http://records.viu.ca/~johnstoi/aristophanes/frogs.htm |archive-date=May 13, 2012 }}</ref> The calls of the [[Concave-eared torrent frog]] (''Amolops tormotus'') are unusual in many aspects. The males are notable for their varieties of calls where upward and downward frequency modulations take place. When they communicate, they produce calls that fall in the [[ultrasound]] frequency range. The last aspect that makes this species of frog's calls unusual is that nonlinear acoustic phenomena are important components in their acoustic signals.<ref name="ultrasonic">{{cite journal|author1=Suthers, R.A.|author2=Narins, P.M.|author3=Lin, W|author4=Schnitzler, H|author5=Denzinger, A|author6=Xu, C|author7=Feng, A.S.|year=2006|title= Voices of the dead: complex nonlinear vocal signals from the larynx of an ultrasonic frog|journal=Journal of Experimental Biology|volume=209|issue=24|pages=4984–4993| doi= 10.1242/jeb.02594|pmid=17142687|doi-access=free|bibcode=2006JExpB.209.4984S }}</ref>

===Torpor===
During extreme conditions, some frogs enter a state of [[torpor]] and remain inactive for months. In colder regions, many species of frog [[Hibernation|hibernate]] in winter. Those that live on land such as the [[American toad]] (''Bufo americanus'') dig a burrow and make a [[Hibernaculum (zoology)|hibernaculum]] in which to lie [[Dormancy|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 such as the [[wood frog]], [[moor frog]], or [[spring peeper]] 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 its heartbeat can restart when conditions warm up.<ref>{{cite magazine |url=http://www.scientificamerican.com/article.cfm?id=how-do-frogs-survive-wint |title=How do frogs survive winter? Why don't they freeze to death? |author=Emmer, Rick |date=November 24, 1997 |magazine=Scientific American |access-date=June 15, 2012}}</ref>

At the other extreme, the [[striped burrowing frog]] (''Cyclorana alboguttata'') regularly [[Aestivation|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 (silk)|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.<ref>{{cite journal |author1=Kayes, Sara M. |author2=Cramp, Rebecca L. |author3=Franklin, Craig E. |year=2009 |title=Metabolic depression during aestivation in ''Cyclorana alboguttata'' |journal=Comparative Biochemistry and Physiology&nbsp;– Part A: Molecular & Integrative Physiology |volume=154 |issue=4 |pages=557–563 |doi=10.1016/j.cbpa.2009.09.001 |pmid=19737622 }}</ref> Other research showed that, to provide these energy requirements, muscles atrophy, but hind limb muscles are preferentially unaffected.<ref>{{cite journal |author1=Hudson, N. J. |author2=Lehnert, S. A. |author3=Ingham, A. B. |author4=Symonds, B. |author5=Franklin, C. E. |author6=Harper, G. S. |year=2005 |title=Lessons from an estivating frog: sparing muscle protein despite starvation and disuse |journal=[[American Journal of Physiology|AJP: Regulatory, Integrative and Comparative Physiology]] |volume=290 |issue=3 |pages = R836–R843 | doi = 10.1152/ajpregu.00380.2005 | pmid = 16239372 |s2cid=8395980 }}</ref> Frogs have been found to have upper critical temperatures of around 41 degrees Celsius.<ref>{{Cite book|title=Environmental Physiology of Animals|last=Wilmer|first=Pat|publisher=Wiley|year=2009|isbn=9781405107242|pages=[https://archive.org/details/environmentalphy00will/page/188 188]|url=https://archive.org/details/environmentalphy00will/page/188}}</ref>