Editing Frog (section)

===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]]