Common frog
The common frog or grass frog, also known as the European common frog, European common brown frog, European grass frog, European Holarctic true frog, European pond frog or European brown frog, is a semi-aquatic amphibian of the family Ranidae, found throughout much of Europe as far north as Scandinavia and as far east as the Urals, except for most of the Iberian Peninsula, southern Italy, and the southern Balkans. The farthest west it can be found is Ireland. It is also found in Asia, and eastward to Japan. The nominative, and most common, subspecies Rana temporaria temporaria is a largely terrestrial frog native to Europe. It is distributed throughout northern Europe and can be found in Ireland, the Isle of Lewis and as far east as Japan.
Common frogs undergo metamorphosis through three distinct life stages — aquatic larva, terrestrial juvenile, and adult. They have plump bodies with rounded snouts, webbed feet, and long hind legs adapted for swimming and hopping on land. They are often confused with the common toad, but can be easily distinguished by their longer legs, hopping movements, and moist skin; toads have shorter legs, crawl and have a dry 'warty' skin. The two species also lay their eggs differently, frogs lay their eggs in clumps, whereas toads lay theirs in long strings.
The common frog is very similar to its close relative the moor frog. Where both species occur together, the most reliable way to distinguish them is to look at their metatarsal tubercles, which are small spurs on the hind feet next to the innermost toe. In the common frog, this spur is soft and small, less than a third of the length of the innermost toe, whereas in the moor frog, it is hard and approximately half the length of the innermost toe. The two species also have different calls, and during the breeding season, moor frog males may turn blue, while common frog males at most get a faint blue tint.
There are 3 subspecies of the common frog, R. t. temporaria, R. t. honnorati and R. t. palvipalmata. R. t. temporaria is the most common subspecies of this frog.
Description
The adult common frog has a body length of. In addition, its back and flanks vary in colour from olive green to grey-brown, brown, olive brown, grey, yellowish and rufous. However, it can lighten and darken its skin to match its surroundings. Some individuals have more unusual colouration—both black and red individuals have been found in Scotland, and albino frogs have been found with yellow skin and red eyes. During the mating season the male common frog tends to turn greyish-blue. The average mass is ; the female is usually slightly larger than the male.The flanks, limbs and backs are covered with irregular dark blotches and they usually sport a chevron-shaped spot on the back of their neck and a dark spot behind the eye. Unlike other amphibians, common frogs generally lack a mid-dorsal band but, when they have one, it is comparatively faint. In many countries moor frogs have a light dorsal band which easily distinguishes them from common frogs. The underbelly is white or yellow and can be speckled with brown or orange. The eyes are brown with transparent horizontal pupils, and they have transparent inner eyelids to protect the eyes while underwater, as well as a 'mask' which covers the eyes and eardrums. Although the common frog has long hind legs compared to the common toad, they are shorter than those of the agile frog with which it shares some of its range. The longer hind legs and fainter colouration of the agile frog are the main features that distinguish the two species.
Males can be distinguished from females by their smaller size and the hard swellings, known as nuptial pads, on the first digits of their forelimbs. These are used to grasp females during mating. During the mating season, the throats of males often turn white, and their overall colour is generally light grey, while females are browner or even red.
These smooth-skinned frogs can grow to an average weight of 22.7 grams and length of seven to ten centimetres with colors varying from gray to green, brown, yellow, or red and may be covered in blotches. The underside is white or yellow, often with spots.
Habitat and distribution
Outside the breeding season, common frogs live a solitary life in damp wetland niches near ponds or marshes or among long riparian 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 favorable, 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 or lakes primarily with a current. The oxygen uptake through the skin suffices to sustain the needs of the cold and motionless frogs during hibernation.Common frogs are 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 the 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. It is also found in Asia, and eastward to Japan.
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 colonizers and recent artificial introductions from Western Europe.
Genetic population structure
The common frog is a very widely distributed species, being common all throughout Europe and northwest Asia. The more peripheral subpopulations of common frogs are significantly less in number, as well as less genetically variable. There is a steep genetic decline when approaching the periphery of the common frog's distribution range. Additionally, genetic differentiation of common frog subpopulations tends to decrease in relation to increasing latitude. The colder climates create a strong selective pressure favoring common frog populations able to behaviorally thermoregulate at a high degree.Fossil record
The common frog has a fossil record going back to at least the Early Pleistocene, being found at the Süttő 21 site in Hungary dating back to around the time of the Mid-Pleistocene Transition.Conservation
Long-term impact of diseases
Of the many diseases that affect common frogs, one of the most deadly is Ranavirus, which has been responsible for causing declines in amphibian populations worldwide. Two of the most important and deadly symptoms caused by ranavirus in common frogs are skin ulcerations and hemorrhaging. Mortality rates associated with the disease are very high, in some cases exceeding 90%. Deaths caused by ranavirus occur at all stages of the common frog's lifecycle and are concentrated mostly during the summer months. Overall, populations of common frog affected by ranavirus experience consistent and substantial declines in population size. Recent metagenomic studies of common frogs from the UK have revealed widespread viral infections with Rana tamanavirus, a positive-sense RNA virus that is closely related to Tamana bat virus, with no pathology or effects on life history traits observed to date.Impact of urbanization
Due to the widespread nature of Rana temporaria, common frogs can make their homes in both urban and rural environments. However, many of the populations living in urban areas are subject to the detrimental effects of urbanisation. The construction of roads and buildings – absolute barriers to migration – has prevented gene flow and drift between urban populations of common frogs, resulting in lower levels of genetic diversity in urban populations of common frog compared to their rural counterparts. Urban common frog populations also experience higher levels of mortality and developmental abnormalities, indicating forced inbreeding.However, the common frog is listed as a species of least concern on the IUCN Red List of Threatened Species.