Common Frog

Rana temporaria

- Common Frog. Photo: Kate Wright.

Description/identification

The Common Frog is probably the best known of the British amphibians, and is widely distributed throughout Britain and Ireland, where it can be found in almost any habitat where suitable breeding ponds are near by.

Garden ponds are extremely important for common frogs, and many populations in suburban areas depend on them. Our Big Pond Dip results indicate that garden ponds alone may be providing up to 1½ million – 2½ million breeding sites for amphibians.

Identification
Common frogs can be distinguished from the similar looking Common Toad by their smooth moist skin, slightly longer back legs and more angular head and body shape.

Adults can grow up to 9cm long. They are usually olive-green or brown in colour, with a dark patch (or ‘mask’) behind the eyes, but have been recorded in shades of yellow, orange, red, green, brown and even blue. Frogs often have spots or other more irregular marks on their backs, and bands of darker striping on their back legs.

Life Cycle

Frogs are often the first species of amphibian to be seen in the New Year, and spawning usually takes place during early spring (starting in the south of Britain from January onwards). Tadpoles usually take from between twelve to sixteen weeks to metamorphose into tiny froglets, ready to leave the water in early summer. ‘Mature’ frog tadpoles can be readily distinguished from toad tadpoles by their faintly speckled gold/brown colouration.

A number of people have contacted us to tell us that tadpoles are over-wintering in their pond, and then developing into frogs the next spring. The reasons for this are not fully understood, although it seems likely that these tadpoles are in some way ahead of the game come the spring, and the decision about whether to follow this strategy appears to be made quite early on, in the year. For more information have a look at this paper by Prof. Patrick Walsh, Prof. Roger Downie and Prof. Pat Monaghan.

Larval over-wintering: plasticity in the timing of life-history events in the common frog P. T. Walsh, J. R. Downie, P. Monaghan, Journal of Zoology Volume 276, Issue 4, pages 394–401, December 2008

Frogs often hibernate at the bottom of ponds in soft mud. In the winter of 2011 a lot of people reported frog deaths as temperatures plunged, and ponds iced over. We responded to this by running the Big Pond Thaw as a way of looking into the factors affecting this. Over 700 people responded to this survey.

Distribution and threats

Many of you have contacted us with concerns about your frogs. Here is some more information about some of the more important frog diseases.

Chytridiomycosis has been associated with a catastrophic declines in some Australian, North American, Central American, South American and Caribbean amphibian species. The situation in Europe is less clear, although some species have seriously declined in upland areas of Spain.

The disease is caused by a fungus (Batrachochytrium dendrobatidis) that infects the skin of adult amphibians and the mouthparts of tadpoles.

The pathogen which causes this disease has already reached the UK. Thanks to a survey conducted in 2008 we know that it can be found widespread in England and also in Scotland and Wales.

Scientists from the Zoological Society of London (ZSL) worked with 400 volunteers recruited by Amphibian and Reptile Groups of the UK (ARG-UK) to swab more than 6,000 amphibians for the presence of chytrid fungus (Batrachochytrium dendrobatidis) in the Big Swab 2011.

Teams of volunteers headed out after dark between April and June to swab amphibians in more than 200 ponds across the UK. The Defra-funded survey included 100 more sites than the last chytrid survey in 2008, with volunteers in action in Northern Ireland for the first time.

In addition to sampling common toads, natterjack toads and the UK’s three species of native newt, volunteers were also swabbing non-native species such as the alpine newt and marsh frog. ZSL scientists were targeting new species and covering more locations in a bid to create a more complete picture of the UK’s chytrid infection.

Frog Ranavirus (or Red Leg) – Mass mortalities caused by a virus or a group of viruses belonging to the genus Ranavirus have occurred in wild common frogs in England since the 1980s. The virus causes internal bleeding and skin ulceration, and affected animals may appear drowsy, and abnormally thin. Where disease outbreaks occur it is common to find large numbers of dead frogs in a comparatively short period of time.

It should be noted that the ranavirus is only active in warm temperatures so dead frogs found outside the summer months are likely to have died from another cause.