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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 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 Mate choice. Moor frogs typically mate through Multi-male group 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 Omnivore and will consume anything that it can physically ingest.
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 on their first fingers and paired guttural .
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.
The moor frog can be found surviving at varied levels of ; in the western, more European areas of its range, the frog can be found as high as 900 meters above sea level (nearly 3,000’). Further east in its range, such as in the Altai Mountains, the moor frog is found as high-up as 2 kilometres, or around 2000m (over 6,000’) 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 zone vegetation surrounding it. These water sources are often rich in Decomposition 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.
The species has been successfully bred in captivity in the UK and a reintroduction has been proposed as part of Celtic Reptile & Amphibian's rewilding plans.
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.
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 flogs 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.
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 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.
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 in warm, shallow waters.
Moor frogs can adapt to the various effects of acidification through long-term selection causing genetic change or spontaneous behavioral changes mediated by hormonal responses. Stressors that demand immediate solutions, such as a sudden shift in temperature or appearance of a predator, demand that an individual can respond appropriately, such as moving to a more temperate location or evading or fighting off a predator. The extent to which an individual can adapt to respond to a new situation is referred to as an individual's phenotypic plasticity. These plastic adaptations can be quantitatively analyzed through the measurement of hormones that spike when individuals are under stress, such as cortisol. Moor frog tadpoles use and understand a variety of chemicals that signal stressors, and acidification can chemically disrupt a tadpole's ability to receive and send signals, thus making an individual tadpole unable to respond to environmental stressors. Acid-tolerant Moor frogs are larger and more active than Moor frogs that have not acclimatized to acidification. Acid-tolerant moor frogs also exhibit stronger hormonal responses to immediate dangers like the presence of a predator, which, in turn, creates a stronger behavioral response to evade those predators.
Some acid-tolerant Moor frogs have lower levels of sodium, which may be an adaptation to acidification.
Environmental acidification has various reproductive impacts: decreased maternal investment, selection for investment in larger eggs at a cost to fecundity, hindered reproductive output, altered relationship between female phenotype and maternal investment, and strengthened egg-size-fecundity trade-off. High habitat acidity often imposes great costs to survival, which may lead to the culling of Moor frogs. High acidity imposes stress on eggs; when a habitat is acidic enough, embryos often exhibit developmental defects and become inviable. Egg coats are maternally derived structures that surround Moor frog eggs to protect them. Egg coats can buffer the low pH of the Moor frog's acidic habitats; however, drastic decreases in habitat pH caused by human-made pollution affects an egg coat's function. High habitat acidity causes thinning and a loss in the egg coat's ability to attract water. Thinned egg coats are more tacky and opaque. These eggs are more susceptible to drying out, pathogen infection, UV light degradation, and poor gas exchange. The disabling of the egg coat leaves an embryo defenseless and tremendously susceptible to developmental defects. Moor frogs that are more easily killed by acidic waters are less fit and their genes are lost from the gene pool. Acidification is strong enough to cause rapid adaptation due to the high selection pressure it places on the Moor frog. As a result, certain highly acidic habitats have seen the development of Moor frogs that are less sensitive to the stress of highly acidic waters. Eggs of acid-tolerant frogs have coats with a greater negative charge. This suggests give the egg coat its Hydrophile properties. Acid-tolerant eggs also have egg coats that are more acidic which suggests a greater concentration of negatively charged glycans as compared to typical Moor frogs. High acidity is able to reduce an egg coat's attraction to water because high proton concentration in acidic water is able to Protonation the coat, thus neutralizing a glycan's charge. This is also why high habitat pH causes egg coat to Deprotonation which restores the egg coat's negative charge/attraction to water.
The Supercooling (SCP) is the lowest temperature at which an organism can be cooled to (below freezing) before ice crystals form (cold-tolerant animals often use that decrease the freezing temperature to prevent the formation of ice). Freeze-tolerant frogs may see up to 65% of their body freeze solid during winter. Moor frogs, like many frogs, are particularly susceptible to freezing solid because of their skin which is thin and porous—permeable to the exchange of gases and liquids. Formation of ice crystals externally can act as nucleation sites for the formation of crystals inside the moor frog. When temperatures reach below the SCP a moor frog's skin darkens, muscles become rigid, eyes dull, and solid ice can be readily felt through touch. At temperatures between 0 °C and 1 °C frogs assume normal behavior but still respond to external stimuli i.e. frogs will leap away if disturbed. At temperatures immediately below freezing frogs assume an overwintering posture with their limbs adducted. When touched at below-freezing temperatures, frogs are only capable of slight movements of the limbs and body. Siberian populations exhibit 0% mortality at -8 °C, 25% mortality at -10 °C, and 50% mortality at -12 °C. A few members from a population from Karasuk were able to freeze solid to -16 °C, thaw, and survive. The time a frog spends frozen does not seem to affect mortality rather the absolute minimum temperature they experience has the greatest effect on mortality. Frogs have been recorded to spend around 3 months in this frozen state with the potential to survive thawing.
Lactic acid and ethanol are found in higher concentrations in frozen moor frogs. The moor frog is the only known terrestrial vertebrate to produce ethanol as a product of glycolysis. These two molecules are products of anaerobic processes which is to be expected because breathing/aerobic processes drastically slow down to the point of stopping when the moor frog is in a frozen state. Products of the breakdown of DNA are found in higher concentrations in frozen moor frogs suggesting that freezing is a highly stressful process for the frog. Frozen moor frogs also have greater concentrations of antioxidants; which are presumably made in anticipation of the oxidative stress when aerobic respiration resumes after thawing.
The 2009 IUCN Red List status of the moor frog does not properly reflect the current declining nature of the moor frog. There is a general lack of research on the conservation status of the moor frog in many EU member states and in-range countries. However, a European Habitats Directive performed in 2013 revealed that 19 of the 28 member states of the time reported that the conservation status of the moor frog was unfavorable. 11 of the 19 said that their status was in decline as well. It is known that existing populations in Europe are small in number which indicates a significant loss of genetic diversity. This lack of genetic diversity threatens the current stability of populations and long-term survival because of the increased risk of inbreeding.
12 Parasitic worm and nematode species are known to parasitize the moor frog. Trematode infection can cause the formation of in larvae; particularly at areas undergoing metamorphosis. These cysts can cause the formation of Polymelia, deformation to the vertebral skeleton. Frogs with these deformations are particularly susceptible to predation by the trematode's final and definitive hosts.
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