The common frog, Rana temporaria, also known as the European common frog or European common brown frog, is found throughout much of Europe as far north as Scandinavia and as far east as the Urals, except for most of Iberia, southern Italy, and the southern Balkans. The farthest west it can be found is Ireland, where it has long been thought erroneously to be an entirely introduced species.
Adult common frogs have a body length of 6 to 9 centimetres (2.4 to 3.5 in) and their backs and flanks vary in colour, with olive green grey-brown, brown, olive-brown, grey, yellowish or rufous possible. However, common frogs are known to be able to lighten and darken their skin in order to match their surroundings. It is also not unknown for more unusual colouration- both black and red individuals have been found in Scotland, and male common frogs have been known to turn greyish blue in the mating season (video on page). Additionally, albino common frogs have been found with yellow skin and red eyes. Their average weight is 22.7 g (0.80 oz). Females are usually slightly larger than males.
Common frogs' flanks, limbs and backs are covered with irregular dark blotches and they usually sport a chevron-shaped spot on the back of their neck. Unlike other amphibians, common frogs generally lack a middorsal band, and when they have one, it is comparatively faint. In many countries the moor frogs do have a light dorsal band which easily distinguishes them from common frogs. The frogs' underbellies are white or yellow (occasionally more orange in females) and can be speckled with brown or orange. Common frogs have relatively short hind legs and possess webbed feet. The legs of the agile frog are much longer which distinguishes them from common frogs along with the agile frog's fainter colouration. Their eyes are brown with transparent horizontal pupils, and they have transparent inner eyelids to protect their eyes while underwater, as well as a 'mask' which covers their eyes and eardrums.
Males are distinguishable from females due to hard swellings (called nuptial pads) on their first finger. These are used for gripping females during mating. Also, during the mating season, males' throats often turn white. A final differentiation can be the colour—during the mating season, males are generally light and greyish in colour, whereas the female is browner, or even red.
The mating season is short, just a week in March after which the frogs move back to their terrestrial habitat.
The common frog is found throughout much of Europe as far north as northern Scandinavia inside the Arctic Circle and as far east as the Urals, except for most of Iberia, southern Italy, and the southern Balkans. Other areas where the common frog has been introduced include the Isle of Lewis, Shetland, Orkney and the Faroe Islands.
The common frog has long been thought to be an entirely introduced species in Ireland, however, genetic analyses suggest that particular populations in the south west of Ireland are indeed indigenous to the country. The authors propose that the Irish frog population is a mixed group that includes native frogs that survived the last glacial period in ice free refugia, natural post-glacial colonisers and recent artificial introductions from Western Europe.
Adult common frogs will feed on any invertebrate of a suitable size, although they do not feed at all during the breeding season. Favourite foods include insects (especially flies), snails, slugs and worms. The frogs catch their prey on their long, sticky tongues. Their feeding habits change significantly throughout their lives; whereas older frogs will feed only on land, younger frogs will also feed in the water. Tadpoles are mostly herbivores, feeding on algae, detritus and some plants, although they will also eat other animals in small amounts.
Habitat and habits
Outside the breeding season, common frogs live a solitary life in damp places near ponds or marshes or in long grass. They are normally active for much of the year, only hibernating in the coldest months. In the most northern extremities of their range they may be trapped under ice for up to nine months of the year, but recent studies have shown that in these conditions they may be relatively active at temperatures close to freezing. In the British Isles, common frogs typically hibernate from late October to January. They will re-emerge as early as February if conditions are favourable, and migrate to bodies of water such as garden ponds to spawn. Where conditions are harsher, such as in the Alps, they emerge as late as early June. Common frogs hibernate in running waters, muddy burrows, or in layers of decaying leaves and mud at the bottom of ponds. The oxygen uptake through the skin suffices to sustain the needs of the cold and motionless frogs during hibernation.
Common frogs breed in shallow, still, fresh water such as ponds, with breeding commencing in March. The adults congregate in the ponds, where the males compete for females. The courtship ritual involves croaking, and a successful male grasps the female under the forelegs. During the mating season the males can be recognised by a darkened swelling, the nuptial pad on their 'thumbs'. The actual spawning typically takes place at night but the courtship rituals are also at daytime. The females, which are generally larger than the males, lay between 1,000 and 2,000 eggs which float in large clusters.
Common frogs are susceptible to a number of diseases, including Ranavirus and the parasitic fungus Batrachochytrium dendrobatidis which has been implicated in extinctions of amphibian species around the world. Loss of habitat and the effect of these diseases has caused the decline of populations across Europe in recent years. It is thought that the spread of the Chytridiomycota fungus has been facilitated by the effects of global warming. The common frog is listed as a species of Least Concern on the IUCN Redlist of Threatened Species.
Tadpoles are eaten by fish, beetles, dragonfly larvae and birds. Adult frogs have many predators including storks, birds of prey, crows, gulls, ducks, terns, herons, pine martens, stoats, weasels, polecats, badgers, otters and snakes.
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- Common frog, grass frog
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- FrogsWatch.com Web page developed around photographs of the common frog taken in the same suburban garden over a period of 10 years.