The moor frog (Rana arvalis) is a slim, reddish-brown, semiaquatic amphibian native to Europe and Asia. Moor frogs are known for their ability to freeze solid and survive thawing. The frog makes use of various cryoprotectants i.e. antifreeze that decrease its internal freezing temperature. The species is distributed over a large range, covering a significant portion of Eurasia. Male moor frogs are known to turn blue temporarily during the height of mating season. This coloration is assumed to signal a mate's fitness. Moor frogs typically mate through multimale amplexus, a form of polyandry.
The moor frog spawns its eggs in large batches in still bodies of acidic waters. Human-caused pollution is causing excessive acidification of habitat which harms egg health. The moor frog's habitat is also under destruction due to a variety of other anthropogenic means. The species has an IUCN listing of Least Concern. However, a majority of European states independently consider the conservation status of the moor frog to be unfavorable. The moor frog, like other members of Rana, is omnivorous and will consume anything that it can physically ingest.
Description The moor frog is a medium to large bog frog, characterized by a mixture of brown (back) and white (front) coloration, with small dark spots on the throat. The species is reddish-brown, but can also be yellow, grey, or light-olive. Common traits include white or yellow pigmentation on its underside and black stripes from its nostrils along the sides of its head. The Moor frog ranges from long, but can reach up to in length. Their heads are more tapered than those of the common frog (Rana temporaria), but the most reliable way to tell the species apart is by looking at their metatarsal tubercle, a small spur on the hind feet next to the innermost toe: in common frog it is soft and small, less than a third of the length of the innermost toe, while in moor frog it is hard and approximately half the length of the innermost toe. Common frog and moor frog also have different calls, and for a short period during the breeding season moor frog males may become blue in color, while common frog males at most get a faint blueish tint.
The skin on their flanks and thighs is smooth, and their tongue is forked and free. Pupils are horizontally oriented, feet are partially webbed, and back legs are shorter than those in the same family of frogs. Males, unlike females, have nuptial pads on their first fingers and paired guttural vocal sacs.
Taxonomy The family the moor frog belongs to, Ranidae, is a broad group containing 605 species. The family contains ranoid frogs that do not belong to any other families and are found on every continent but Antarctica. The moor frog belongs to genus Rana, which includes species found in Europe, Asia, and North America. The moor frog is not found in the Americas.
The moor frog's scientific name, Rana arvalis, means "frog of the fields". It is also called the Altai brown frog because frogs from the Altai Mountains in Asia have been included in the R. arvalis species. The Altai frogs have some different characteristics such as shorter shins, but currently there is no official distinction and all frogs are known as one species—Rana arvalis.
The moor frog was first reported by Nilsson in volume 3 of Skandinavisk fauna with a moderate muzzle and prominent first cuneiform bone.
Distribution and habitat The moor frog can be found over a vast majority of mainland, central Eurasia; its longitudinal range extends from northeastern France and northern Belgium all the way east to the Lena River in Siberia, in and around the city of Novosibirsk. Their latitudinal distribution extends from the 69th parallel in Finland south through the Pannonian Basin and the inland Balkans in Central Europe.
The moor frog can be found surviving at varied levels of altitudes; in the western, more European areas of its range, the frog can be found as high as above sea level. Further east in its range, such as in the Altai Mountains, the moor frog is found as high up as above sea level. Within the higher-elevation ranges, the species is often found near bodies of still (or very slow-moving) water, with adequate riparian or littoral vegetation surrounding it. These water sources are often rich in decomposing organic material, resulting in a considerably acidic pH level, often at, or below, a 6. The diversity of habitats demonstrates the frog's plasticity.
Moor frogs can live in near-tundra conditions, taiga, conifer forest, forest steppe, dry steppe, open forest, glades, chaparral-like (arid) areas, swamp, meadows, fields, bushland, and private farms or water gardens (though they tend to prefer areas away from humans and predators). Nonetheless, they are adaptable, and are often seen in such urban spots as city-adjacent meadows, bogs, pastures, or public parks.
Moor frogs provide a good model for studying local adaptation as they experience a wide range of environments and are relatively limited in their movements. Their restriction in movements implies limited gene flow and facilitates evolution through adaptive genetic differentiation among populations.
Historical distribution The earliest fossil record of the moor frog extends back to between the Pliocene and Early Pleistocene found in Dvorníky-Včeláre, Slovakia. Other fossil records of the moor frog from the early Pleistocene were found on land inside the modern range of the moor frog. Fossil records from the middle Pleistocene demonstrate the range extended as far south as south-central France and as far west as the eastern coast of Great Britain. Records from the late Pleistocene show the range extended as far south as Bosnia and Herzegovina and Azerbaijan. In the Holocene, the moor frog also had a wider distribution until the Early Modern period, when widespread land-drainage became technologically and economically viable. Widespread peat-cutting also impacts the species, especially in its range edge.
Distribution in Romania The moor frog is found in three regions in Romania. The first is the Transylvanian region which includes the Western Plains (with the largest Romanian population of moor frogs), the Transylvanian Plateau, and the Eastern Carpathians. The second region is the northern part of Romanian Moldavia. The third and smallest region is the Tisa River Basin—north of Maramureș. Most populations of moor frog in Romania are isolated and not contiguous due to the edge effects of human developments. Each population typically has 200–400 adults; however, exceptional populations of 2000 adults have been found as well. Most Romanian populations of moor frog can be found between above sea level; exceptional populations have been found to exist at above sea level.
In Romania, the moor frog is known to live in humid habitats that border land with human activity, such as flooded agricultural fields, ditches on the side of roads, small canals and streams, and human settlements. The moor frog is sparingly found in habitats with little human activity. Swamps are one of the few habitats with little human activity that host moor frogs.
Diet An adult moor frog's diet consists of any mobile and terrestrial animals that they can physically ingest. Moor frogs most commonly consume beetles; however, other insects from the orders hemiptera (true bugs), hymenoptera, and diptera (flies) are consumed as well. Moor frogs also consume non-insect invertebrates from the orders gastropoda (snails and slugs), arachnida, and myriapoda (centipedes and millipedes). Beetles make up the majority of the moor frog's diet due to their abundance. Large moor frogs do appear to have a preference for beetles because they are larger than most other insect prey.
Large moor frogs tend to consume large prey and small moor frogs consume small prey. This behavior is assumed to have evolved to reduce competition between moor frogs or to maximize net energy gained from feeding, as large moor frogs consuming both large and small prey would leave little food for smaller moor frogs. Aside from size preferences, individual moor frogs do not appear to prefer more energetically favorable prey over less energetically favorable prey of equal size. The moor frog will ingest any animal that it is able to swallow.
Moor frogs are opportunistic predators that wait for prey to appear before consuming them, as opposed to intentional predators that actively hunt for prey. More mobile prey are more often consumed by the moor frog because of their opportunistic nature.
Plant matter and inedible objects such as pebbles are also found to be consumed by the moor frog. Plant matter is found to be consumed in greater quantities when more prey has been consumed, which suggests that plant matter is consumed accidentally during the capture of prey. The moor frog's shed skin is also consumed; however, it is unknown whether consumption of shed skin is accidental or intentional in nature.
Mating
Multimale amplexus, in which multiple males mate with a single female, is the predominant method of mating that the moor frog performs. The sperm of male moor frogs compete in the female reproductive tract for fertilization of the female's egg.
Female frogs do not appear to prefer males of a particular size. Instead, they tend to prefer to mate with males that have successfully helped produce offspring with them in the past.
Long thumb length is correlated with poor sperm quality, and short thumb length is correlated with higher sperm quality. Males with higher quality sperm breed progeny with greater chances of survival. Despite this correlation, female individuals do not appear to prefer thumb length or be able to detect variation in thumb length.
Blue coloration
Male moor frogs turn a conspicuous blue during the mating season, but only for a few days during peak reproductive activity. Females remain brown during this time. While the blue is conspicuous to human vision, the greatest color change in male moor frogs occurs in the ultraviolet region from 350 to 450 nm, invisible to human vision. Males who have mated appear bluer and have been recorded as having higher body temperatures.
Blue reflectance may be a form of intersexual communication. It is hypothesized that males with brighter blue coloration may signal greater sexual and genetic fitness; however, studies have only revealed tadpoles fathered by bright blue individuals had greater chances of survival when pitted against large beetle larvae than when fathered by dull individuals.
Ecology Hibernation
Moor frogs hibernate sometime between September and June, depending on their latitude. Frogs in southwestern plain habitats will hibernate (around November or December) and wake earlier (February). However, frogs in cold, polar areas will hibernate sooner (in September) and wake later (in June).
Breeding The mating season takes place between March and June, right after the end of hibernation. Males form breeding choruses that may sound similar to air escaping from a submerged empty bottle, similar to those of the agile frog Rana dalmatina. Males can also develop bright-blue coloration for a few days during the season.
Spawning happens quickly and is completed in 3 to 28 days. The spawn of each frog is laid in one or two clusters of 500–3000 eggs in warm, shallow waters.
Effects of acidification on population Environmental plasticity