Common frog

The adult common frog has a body length of 6 to 9 centimetres (2.4 to 3.5 in).^[3] In addition, its back and flanks vary in colour from olive green^[2] to grey-brown, brown, olive brown, grey, yellowish and rufous.^[4] However, it can lighten and darken its skin to match its surroundings.^[2] 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 (see video below). The average mass is 22.7 g (0.80 oz); the female is usually slightly larger than the male.^[2]

The flanks, limbs and backs are covered with irregular dark blotches^[2] and they usually sport a chevron-shaped spot on the back of their neck and a dark spot behind the eye.^[4] Unlike other amphibians, common frogs generally lack a mid-dorsal band but, when they have one, it is comparatively faint.^[4] In many countries moor frogs have a light dorsal band which easily distinguishes them from common frogs. The underbelly is white or yellow (occasionally more orange in females) and can be speckled with brown or orange.^[2] 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.^[2] 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 are distinguished from females by being smaller and having hard swellings known as nuptial pads, on the first digits of their forelimbs, which are used to grasp females during mating.^[3][2] During the mating season males' throats often turn white, and their overall colour is generally light and greyish, while the female is browner, or even red.^[4]

These smooth-skinned frogs can grow to an average weight of 22.7 grams and length of seven to ten centimeters (2.8-3.9 in) with colors varying from gray to green, brown, yellow, or red and may be covered in blotches.^[5] The underside is white or yellow, often with spots.^[2]

Outside the breeding season, common frogs live a solitary life in damp wetland niches near ponds or marshes or among long riparian grass.^[6] They are normally active for much of the year, only hibernating in the coldest months.^[4] 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.^[6] 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.^[7] 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.^[2][4][8]

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.^[2][9]

The common frog has long been thought to be an entirely introduced species in Ireland,^[7] however, genetic analyses suggest that particular populations in the south west of Ireland are indeed indigenous to the country.^[10] 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.^[10][11]

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.^[23] 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.^[4]

Clutch sizes of female common frogs range from a few hundred to 5,000 eggs. Many of these eggs form large aggregations that serve to thermoregulate and protect the developing embryo from potential predators. The aggregation of eggs raises the temperature of the embryo relative to the surrounding water, which is important because the rate of tadpole development is faster at higher temperatures.^[26] Additionally, the eggs are typically laid in the shallower regions of the pond to prevent hypoxia-induced fatality of the embryos.^[26]

It usually takes 2-3 weeks for the eggs to hatch. After that, the frog larvae group together in schools, where they help each other to feed on algae and larger plants, and to avoid predators.^[27] By June and July, most tadpoles will have metamorphosized, and the remaining time until winter is used to feed and grow larger.^[27] Only the largest frogs will survive the winter, which places a large emphasis on rapid development until then. In fact, a common frog's rate of development correlates with temperature. In lower temperature regions, common frogs will hatch earlier and metamorphosize sooner than common frogs living in warmer climate regions.^[27] Sexual maturity occurs only after three years, and common frogs will typically live between six and eight years.^[26]

As an ectotherm, the common frog is highly dependent on temperature as it 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. In fact, it is due to the common frog's ability to thermoregulate so effectively that the species has been able to become so widespread in a variety of environments and climates, living as far north as the Arctic Circle in Scandinavia, which is further north than any other amphibian in the region.^[13] Contrary to Lithobates sylvaticus (wood frogs), common frogs do not have the ability to freeze protect themselves by increasing their levels of blood glucose to serve as a cryoprotectant.^[13] 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.^[13]

Similar to other anuran species (Bufo americanus and Rana sylvatica), Rana temporaria are able to naturally discriminate others of its kind. Post-embryonic interaction with conspecifics is not necessary to induce associative behavior for common frogs as an adult. Rather, once common frog tadpoles have reached a certain age, they gain a strong innate associative tendency.^[29] Rana temporaria tend to aggregate as the result of environmental pressures, such as temperature or predators.^[30]

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.^[31] Some frogs are killed, but rarely eaten, by domestic cats, and large numbers are killed on the roads by motor vehicles.^[32]

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.^[31]

R. temporaria are farmed.^[33] Miles et al. 2004 provide improved ingredients for manufacturers of pellet food for farmed common frogs.^[33]

Due to the spread of diseases such as ranavirus, the UK-based amphibian charity Froglife advised the public to avoid moving frogspawn, tadpoles or frogs from one pond to another, even if they are nearby.^[34] 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.