Eurasian Marsh Frog

Vomerine teeth present. Posterior part of the tongue free and forked. Toes webbed. Omosternum and sternum ossified. Pupil of the eye horizontal. Snout moderately sharp. When the shins are positioned perpendicularly to the body axis, the heels overlap (with the exception of Transcaucasian specimens). Inner metatarsal tubercle low. Dorsal coloration different tints of greyish and green, from entirely grey to green. Large dark dorsal spots vary considerably in size, number and arrangement. Light middorsal line often present. No temporal spot. Belly greyish-white or greyish-yellow with dark spotted or blotched-like pattern, sometimes without this pattern. Males differ from females by having paired grey vocal sacs behind the mouth angles and nuptial pads on the first finger.

The western margin of the range runs by Eastern France (northeastwards from Lyon City and between Mulhouse and Strasbourg). From France, the northern margin of the range generally corresponds to the European coast of the Atlantic Ocean, including the southern shore of the Baltic Sea. The northern margin of the range formerly extended from Estonia (including the islands of Saaremaa and Hiumaa in the Baltic Sea) through the middle reaches of the Zapadnaya Dvina and Velikaya Rivers. At present, the frog is thought to be extinct in Estonia. Therefore, the northern margin of the range runs from Northern Latvia (area of Riga City) through Russia approximately along the line: Pskov City (57º49'N, 28º20'E) - St. Petersburg City (59º55'N, 30º15'E) - Tver City - north of Yaroslavl Province - south of Kostroma Province - Kirov Province - from northwestern to southeastern Udmurtia - the south and east of Perm Province - Bashkiria - Northeastern Kazakhstan - Northwestern China. Within Kazakhstan and Middle Asia, the frog is absent from the dry areas of the Kara-Kum and Kyzyl-um deserts and from Southern Pamir. Its distribution there is attached to river valleys and floodland lakes. Eastwards, the frog is found in isolated populations established by introductions. These are the populations in many sites of Siberia (the cities of Tomsk, Novosibirsk, Eketerinburg, Verkhnii Tagil, Chelyabinsk, Gorno-Altaisk and Barnaul. In the 1970's, the frog was introduced into Yakutsk City in East Siberia, which is the easternmost locality for this species (62º02'N, 129º39'E). In the south, the Marsh Frog uses rivers and channels as pathways for dispersal in arid areas and its range there changes relatively rapidly. This phenomenon is widespread in the European and Asiatic part of the former Soviet Union. Through river valleys, the frog has penetrated lakes Balkhash and Issyk-Kul. In 1960s-1980s, the frog was introduced into the cities of Karaganda, Ust-Kamenogorsk, Semipalatinsk and Pavlodar in Kazakhstan, probably from local universities and institutes. At present, the frog is dispersing along the Irtysh River and associated rivulets. From the south, R. ridibunda is known from the area from France to Switxerland to the Balkan Peninsula to the Western Turkey, Northern and Central Iran, Afghanistan, Central Tajikistan, Kirgizia, Southern Kazakhstan and Northwestern China (Ili River valley in Xinjiang-Uigur Autonomous Region, where it is a dispersing species). The southern margin of the range is insufficiently known because more and more "southern" species are described from different geographic populations of R. ridibunda. Rana ridibunda is a highly opportunistic amphibian. It lives in mixed and deciduous forests, forest steppe, steppe, semidesert and desert zones. Being a semiaquatic species, the frog inhabits a wide variety of flowing and stagnant water habitats, from shallow puddles and ponds to large lakes and rivers, as well as mountain streams. The Marsh Frog is tolerant of high water salinity (it occurs in waters of salinity 0.9-8.3 parts per thousand), and in some places reproduces near the western shore in the Caspian Sea. In general, the Marsh Frog prefers open, well-warmed areas with abundant herbaceous vegetation. The diversity of inhabited water bodies and the extent of terrestrial migrations are higher in wet than in arid areas. Nevertheless, the latter are also successfully colonized by the frog using river valleys and channels.

Rana ridibunda is generally an abundant amphibian. In the rivers of the southern part of Europe, its abundance may reach more than a thousand individuals per kilometer of the riverbank. However, it is relatively rare in swift mountain streams. In general, the species is most abundant in the southern parts of its range. Hibernation occurs from September - October (in northern regions) or November - December (in the south) to the beginning of June or January - February, respectively. In southern regions, the hibernation is frequently interrupted by warm weather. In unfrozen water bodies, the frog remains active throughout the winter: this is normal in the southern parts of its distribution. As a rule, hibernation occurs in water, but in some locations it occurs in rodent burrows and holes in river banks and lake shores. Group hibernation is typical, but the groups usually do not exceed several dozen individuals. Breeding starts from several days to one month after the frogs' spring appearance. The males form loud choruses, which are especially intensive at the peak of the breeding period. Amplexus is pectoral (axillary). The clutch contains about 670-13,000 eggs. The time of metamorphosis depends on weather, peculiarities of habitat and latitude but usually falls in April - November. In some places of Europe and Asia regular hibernation of tadpoles (up to a few thousand individuals in one pond) has been recorded. Such tadpoles attain sometimes exceptionally large size (total length to 186 mm) and undergo metamorphosis in the next spring. Sexual maturity is attained in the 1st-4th year of life, and the maximum life span has been recorded as 5 - 12 years. Tadpoles consume detritus, algae, and higher plants in addition to animals (mainly invertebrates) and their corpses. Benthic objects remain the most important component of their diet. Adults consume mainly terrestrial and aquatic insects. Feeding does not cease during the breeding season. Rana ridibunda is quite voracious and sometimes attacks not only animals but even the branches of riparian vegetation moving in the wind. In fish ponds, it eats small fishes, but the intensity of predation is too low for any significant influence on the fish crop. Marsh Frog adults, being the largest frogs in Europe, often eat conspecific and other amphibians, as well as reptiles and even small birds and rodents. Cannibalism becomes especially severe during periods of low humidity and precipitation, as well as high temperature.

In general, the Marsh Frog seems to be in the safest situation among the amphibians of Eurasia. Its populations are generally large, the range is broad and has been widened due to dispersal and deliberate introductions of the frog by people. There are indications of a negative influence of dispersing R. ridibunda on local amphibian populations (e.g., R. asiatica).

As in other amphibian species, the drainage of water bodies and urbanization causes the decline of some populations R. ridibunda. The building of concreted vertical embankments on the shores of some rivers and lakes has caused extinction of some populations from the northern parts of the species range. On the other hand, in southern regions frog populations successfully survive in such condition if the individuals can also use undisturbed water bodies. Rana ridibunda is one of the amphibian species most resistant to environmental pollution. It lives not only in waters polluted by domestic wastes or fertilizers, but also in the polluted neighborhoods of large metallurgic and chemical enterprises where other amphibians are not able to survive. Many frogs are caught for the purposes of education, medicine and science. Along with the Common Frog (Rana temporaria), the Marsh Frog is the main subject for such collecting. The amount of these two frog species caught in the 1970s - 1980s in the former USSR measured by several tons annually. However, no population declines resulted from this. The Marsh Frog is one of those amphibians which has the best facility for synanthropization. It is numerous in settlements and cities in southern regions and is both expanding its geographic range and increasing in population size.