Common frog

Habitat and distribution

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

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 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.

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

Genetic population structure

The common frog is a very widely distributed species, being common all throughout Europe and northwest Asia. The more peripheral subpopulations of common frogs are significantly less in number, as well as less genetically variable. There is a steep genetic decline when approaching the periphery of the common frog's distribution range. Additionally, genetic differentiation of common frog subpopulations tends to decrease in relation to increasing latitude. The colder climates create a strong selective pressure favoring common frog populations able to behaviorally thermoregulate at a high degree.

Fossil record

The common frog has a fossil record going back to at least the Early Pleistocene, being found at the Süttő 21 site in Hungary dating back to around the time of the Mid-Pleistocene Transition.

Conservation

Long-term impact of diseases

Of the many diseases that affect common frogs, one of the most deadly is Ranavirus, which has been responsible for causing declines in amphibian populations worldwide. Two of the most important and deadly symptoms caused by ranavirus in common frogs are skin ulcerations and hemorrhaging. Mortality rates associated with the disease are very high, in some cases exceeding 90%. Deaths caused by ranavirus occur at all stages of the common frog's lifecycle and are concentrated mostly during the summer months. Overall, populations of common frog affected by ranavirus experience consistent and substantial declines in population size. Recent metagenomic studies of common frogs from the UK have revealed widespread viral infections with Rana tamanavirus, a positive-sense RNA virus that is closely related to Tamana bat virus, with no pathology or effects on life history traits observed to date.

Impact of urbanization

Due to the widespread nature of Rana temporaria, common frogs can make their homes in both urban and rural environments. However, many of the populations living in urban areas are subject to the detrimental effects of urbanisation. The construction of roads and buildings – absolute barriers to migration – has prevented gene flow and drift between urban populations of common frogs, resulting in lower levels of genetic diversity in urban populations of common frog compared to their rural counterparts. Urban common frog populations also experience higher levels of mortality and developmental abnormalities, indicating forced inbreeding.

However, the common frog is listed as a species of least concern on the IUCN Red List of Threatened Species.

Diet

Juvenile

During metamorphosis, when the tadpole's front legs have developed, the frog stops eating for a short time. Recently metamorphosed juveniles feed mainly on small arthropods such as Collembola (springtails), Acarina (mites and ticks), and small fly larvae. Rana temporaria tadpoles, however, mostly feed on algae and decomposed plants, but when their hind legs develop, they become carnivorous.

Adults

The common frog takes its place as an unspecialized and opportunistic feeder wherever it is found. In other words, common frogs will consume whatever prey that is most available and easy to catch. This usually means that the common frog feeds by remaining inactive and waiting for a suitable prey to enter the frog's area of capture. This also means that the common frog's diet changes according to the season when its prey is most abundant. In the summer, the common frog's diet mostly consists of adult crane flies and the larvae of butterflies and moths. To a slightly lesser extent, common frogs will feed on woodlice, arachnids, beetles, slugs, snails, and earthworms. In addition, common frogs will typically feed on bigger prey as they become larger. Therefore, newly developed common frogs are limited to smaller insect prey, whereas larger frogs are able to consume a wide range of insects. Common frogs will hide in damp places, such as in the water, during the day, and at night, they will begin searching for food.

Reproduction and mating patterns

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. 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.

Competition among males

Like its close cousin, the moor frog (R. arvalis), R. Temporaria does not exhibit territoriality, resulting in a lack of physical fighting among males. During the breeding season, male common frogs undergo a period of a few days (less than 10 days) during which they display rapid and frenzied breeding behavior, during which the purpose of the male is to quickly find and mate with as many female frogs as possible. Males with higher mating success rates typically have longer thumbs than single males, which gives them a better grip on females.

Mating interactions

About three years after birth, the common frog will return to its original birthplace and emit a mating call. Males will be the first to arrive at the pond and wait for females to enter. During this period of pre-female competition, the pond becomes significantly male-dominated, and there is a substantial degree of intrasexual competition taking place. The shallower portion of the pond, which is more conducive to egg-laying, is more predominantly occupied by the larger males. However, once the females arrive, this territoriality quickly dissipates and male-female amplexed pairs are free to move anywhere in the pond. Additionally, once an amplexus is established, it is rare for single males to attempt to displace or "take over" the paired male.

It is also important to note the effect of size on the mating strategies of a male common frog. Smaller frogs are crowded out of the shallow areas of the pond during the pre-spawning period, a problem which they circumvent by searching for females on the land or in areas of the pond where they first arrive. Meanwhile, the larger frogs occupy the spawning site, where they encounter more amplexed pairs and therefore rely on their ability to displace amplexed males to secure a mate. However, the frequency of these takeovers is not consistent.

Life cycle

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. Additionally, the eggs are typically laid in the shallower regions of the pond to prevent hypoxia-induced fatality of the embryos.

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. By June and July, most tadpoles will have metamorphosized, and the remaining time until winter is used to feed and grow larger. Only the largest frogs will survive the winter, which places a large emphasis on rapid development until then. The development rate of the common frog correlates with temperature. In lower temperature regions, common frogs will hatch earlier and metamorphosize sooner than common frogs living in warmer climate regions. Sexual maturity occurs only after three years, and common frogs will typically live between six and eight years.

Development in the presence of predators

The presence of a predator in the early development of the tadpole affects its metamorphic traits. For example, it can lead to a longer larval period and a smaller size and mass at metamorphosis. Once the predator is removed, the growth rate of the tadpole returns to, or even exceeds, baseline. This influence of predator threat is only significant during early tadpole development.

One of the common frog's most pervasive predators is the red-eared slider (Trachemys scripta elegans), which is a very invasive species of turtle.

Thermoregulation

As an ectotherm, the common frog is highly dependent on temperature as this 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. Due to these adaptations, the common frog's ability to thermoregulate has become so effective that the species has become widespread in a variety of environments and climates. The species has been found living as far north as the Arctic Circle in Scandinavia, which is further north than any other amphibian in the region. 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. 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.

Social behavior

Similar to other anuran species (Anaxyrus 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. Rana temporaria tend to aggregate as the result of environmental pressures, such as temperature or predators.

Predators

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

Interactions with humans and livestock

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.

Farming

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

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. 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.