Marsh Frog

The marsh frog is the largest type of frog in most of its range, capable of reaching a snout–vent length of up to 15 cm (5.9 in). Females typically grow slightly larger than males, and anthropogenic activity can also affect the size of individuals in a population; for example, commercial harvesting likely caused average body sizes in a Turkish population to decrease between 1999 and 2008. The skin of a marsh frog is relatively rough as it bears warts and ridges, and forms distinctive folds on the sides of its back. There is a large variation in colour and pattern, ranging from dark green or even black to lighter green, brown or grey, often with a bright green line running down the back. Dark roundish or square patterns are typically present (but sometimes absent) on the back and sides, which may be dense or scant, and the legs usually have barred patterning of a similar colour. The arms and legs can be partly brown, while the thighs and underside are usually white or cream, but may also be yellow or green. Dense black or grey spots are typically present on the underside of the lower body. Adult males have a pair of vocal sacs, one under each tympanum, which are often folded internally but inflate and become visible during vocalisation. …

A widespread animal, the marsh frog occurs in large parts of Europe and is also present in Asia as far east as Kazakhstan, Iran and Afghanistan. Additionally, an isolated relict population is present in southern Hadhramaut, Saudi Arabia. Reports of marsh frogs in China and Iraq are now believed to represent the Terentjev's frog and Levant water frog respectively, which may be subspecies of the marsh frog, though some consider them to be separate species instead. Though present in much of Europe, being native to the central, eastern and southeastern parts, it is notably absent from Scandinavia. It is known to have been introduced to many areas outside of its native range, though the extent of its native range is not entirely certain; for example, it remains unclear if it is native to parts of western Europe such as France, or whether isolated colonies in Bahrain were introduced. Known introduced populations are present in Switzerland, Belgium, Spain, Sardinia, the Kamchatka Peninsula in Far Eastern Russia, the United Kingdom and the Al-Kharj region of Saudi Arabia. The marsh frog typically inhabits large bodies of water such as deep ponds and active rivers, but is highly adaptable and can occur in a wide range of environments. …

Marsh frogs typically breed during late spring or early summer, but may do so throughout the year if conditions are suitable. The breeding period depends on various abiotic factors such as precipitation and temperature, and generally begins only when the water temperature exceeds 18 °C (64 °F). Breeding occurs in ponds or flooded land within a few hundred meters of their hibernation sites, and the frogs of both sexes are able to use the magnetic field of the Earth to locate breeding sites, arriving at about the same time (in contrast to some other amphibians in which one sex arrives at breeding sites before the other). Records have revealed that the breeding period of at least some populations now begins earlier in the year than before, likely due to global warming; a phenological study on frogs in Saratov Oblast, Russia revealed that while the date of the beginning of spawning migrations was relatively stable from 1892 to 1960, this date then shifted earlier by five days from 1961 to 2020. By 2025, this process accelerated suddenly and the date shifted earlier by an extra ten days.

During the breeding season, adult male marsh frogs produce mating calls to attract females. When one approaches, the male uses his forelimbs to grasp onto the area behind her forelimbs, a behaviour known as amplexus which continues while the female lays her eggs. …

The first scientific description of the marsh frog was published in 1771 and written by Prussian zoologist Peter Simon Pallas, who gave it the scientific name Rana ridibunda after collecting specimens of it during an expedition through the Russian Empire. Pallas did not precisely specify the type locality (where he found the frogs), only noting it as "versus mare Caspium; Volgae et Iaico" (opposite to the Caspian Sea, Volga, and on the Yaik River). A more precise location of "Gurjew" on the north coast of the Caspian Sea (now the city of Atyrau in Kazakhstan) was given in a 1928 publication written by German herpetologists Robert Mertens and Lorenz Müller. However, a 1994 paper by herpetologists Alain Dubois and Annemarie Ohler noted that because Pallas never designated a holotype specimen, a neotype needs to be selected for the species. The restriction of the type locality done by Mertens and Müller is therefore invalid unless a specimen from Atyrau is designated as the neotype, which Dubois and Ohler suggest should be done.

Though Pallas initially considered the marsh frog to be a species in the genus Rana, later authors would move it into other genera or treat it as a variant of other species. …