The common frog or grass frog (Rana temporaria), 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 (Bufo bufo), 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 (Rana arvalis). 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 The underside is white or yellow, often with spots. They are normally active for much of the year, only hibernating in the coldest months. 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,

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.

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%.

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. but when their hind legs develop, they become carnivorous.

Adults

The common frog takes its place as an unspecialized and opportunistic feeder wherever it is found. In other words, common frogs will consume whatever prey that is most available and easy to catch. This usually means that the common frog feeds by remaining inactive and waiting for a suitable prey to enter the frog's area of capture. This also means that the common frog's diet changes according to the season when its prey is most abundant. In the summer, the common frog's diet mostly consists of adult crane flies and the larvae of butterflies and moths. To a slightly lesser extent, common frogs will feed on woodlice, arachnids, beetles, slugs, snails, and earthworms. In addition, common frogs will typically feed on bigger prey as they become larger. Therefore, newly developed common frogs are limited to smaller insect prey, whereas larger frogs are able to consume a wide range of insects. Common frogs will hide in damp places, such as in the water, during the day, and at night, they will begin searching for food.

Reproduction and mating patterns

thumb|right|Choir of greyish males and a few brownish females still present in a small pond

During the spring the frog's pituitary gland is stimulated by changes in external factors, such as rainfall, day length and temperature, to produce hormones which, in turn, stimulate the production of sex cells – eggs in the females and sperm in the male. The male's nuptial pad also swells and becomes more heavily pigmented. Common frogs breed in shallow, still, fresh water such as ponds, with spawning commencing sometime between late February and late June, but generally in April over the main part of their range. during which the purpose of the male is to quickly find and mate with as many female frogs as possible. Males with higher mating success rates typically have longer thumbs than single males, which gives them a better grip on females.

Mating interactions

About three years after birth, the common frog will return to its original birthplace and emit a mating call. Males will be the first to arrive at the pond and wait for females to enter. During this period of pre-female competition, the pond becomes significantly male-dominated, and there is a substantial degree of intrasexual competition taking place. Additionally, the eggs are typically laid in the shallower regions of the pond to prevent hypoxia-induced fatality of the embryos. Only the largest frogs will survive the winter, which places a large emphasis on rapid development until then. The development rate of the common frog correlates with temperature. In lower temperature regions, common frogs will hatch earlier and metamorphosize sooner than common frogs living in warmer climate regions.

One of the common frog's most pervasive predators is the red-eared slider ('Trachemys scripta elegans'), which is a very invasive species of turtle.

Thermoregulation

As an ectotherm, the common frog is highly dependent on temperature as this directly affects its metabolism, development, reproduction, muscular strength, and respiration. As such, common frogs at medium and high elevations have developed a unique set of strategies to survive in cold climates. Due to these adaptations, the common frog's ability to thermoregulate has become so effective that the species has become widespread in a variety of environments and climates. The species has been found living as far north as the Arctic Circle in Scandinavia, which is further north than any other amphibian in the region. As a result, common frogs must rely on behavioral thermoregulation by seeking out warm microhabitats (such as in the soil or between rocks) during wintertime. In addition, common frogs often hibernate in groups during the winter season in order to maintain body heat. Rana temporaria tend to aggregate as the result of environmental pressures, such as temperature or predators.

thumb|[[Grey heron feeding on a common frog]]

Predators

Tadpoles are eaten by fish, diving beetles, dragonfly larvae and birds. Adult frogs have numerous predators including storks, birds of prey, crows, gulls, ducks, terns, herons, pine martens, stoats, weasels, polecats, badgers, otters and snakes. Some frogs are killed, but rarely eaten, by domestic cats, and large numbers are killed on the roads by motor vehicles.

Interactions with humans and livestock

Common frogs have an important place in human ecology by controlling the insect populations. In particular, their consumption of mosquitos and other crop-damaging insects has been especially valuable. In addition, Rana temporaria, due to their ecological pervasiveness and relative abundance, have become a common laboratory specimen. Miles et al. 2004 provide improved ingredients for manufacturers of pellet food for farmed common frogs. It has also been recommended not to place goldfish or exotic frog species in outdoor ponds as this could have a negative effect on the frog population.

==References==<!-- Molecular Phylogenetics and Evolution 46 (2008) 49–60 -->

  • Amphibians of Europe
  • FrogsWatch.com Web page developed around photographs of the common frog taken in the same suburban garden over a period of 10 years.