|Common name:||Edible frog|
|Taxonomy:||Pelophylax kl. esculentus [Linnaeus, 1758].|
(Previously classified as Rana kl. esculenta).
The hybrid edible frog is a cross between the marsh frog and the pool frog. Very variable in appearance and many morphological characteristics are midway between those of both parent species. Often has a dorsal stripe, is rarely warty and has longer legs than the pool frog.
This very aquatic species will inhabit all types of water bodies where pool frogs and marsh frogs are found (ponds, lakes, streams, ditches and canals).
Introduced in 19th and 20th Centuries to a few UK sites - small increase in range from sites of introduction.
Recorded in a few southern counties. Rarely present in pure populations; usually present as a mixed population with either pool frogs or marsh frogs.
Diploid forms of this fairly large frog can be up to 12cms long. It has a sharp snout and light grey vocal sacs on each side of the mouth; heel of hind legs reach between the eye and the nares; metatarsal tubercle is large, nearly half as long as the first toe. Variable in colouration but typically marbled on backs of thighs yellow/orange and brown/black. Ventral surface usually white, light grey or dark grey with darker spots. Dorsal surface commonly green (but may become increasingly bronze towards posterior end) and scattered with black spots.
Edible frogs have a complex hybridogenetic form of reproduction and can exist in diploid and triploid forms. In order to reproduce they must backcross with one of the parent species [marsh or pool frogs]. Triploid animals will show closer characteristics to either the pool frog or the marsh frog from which they have received an extra set of chromosomes.
Females do not have vocal sacs nor develop metatarsal tubercles.
Edible frogs are most commonly mistaken for Marsh frogs (Pelophylax ridibundus) and Pool frogs (Pelophylax lessonae). Marsh frogs are warty, robust and have back legs which extend beyond the head. Pool frogs are small, compact, and possess white vocal sacs. The marsh frog has a harsh mating (advertisement) call composed of 6-8 repetitions ‘bre-ke-ke-ke-ke-ke-keh’ whereas the call of the edible frog normally has 12-20 repetitions more rapidly delivered.
Edible frogs have a similar range to the pool frog. They range westwards from the Channel across Europe and Russia; south to the north Balkans and also Italy and Sicily [where a sub-species may occur Pelophylax bergeri] and south Sweden. In Denmark only edible frog populations are found [both diploid and triploid].
There is no evidence that edible frogs naturally colonised the UK after the last glaciation. Only relatively few introduced populations occur in southern England.
A mixture of water frog species were collected from several European countries and introduced to a site at Newdigate at the beginning of the last Century. This 100-year-old population is still extant and edible frogs are only one of several water frog species and hybrids present. These frogs have extended their range since the early 1980s and using the local brooks and ditches have reached Capel, Brockham and the River Mole. Rapidly colonise new ponds in those areas. Also found in the East Surrey area as frogs escaped from a holding pond from a South Nutfield site in the 1960s.
The edible frog inhabits all types of water bodies where pool frogs and marsh frogs are found (ponds, lakes, streams, ditches and canals). In general it can be found on or near the margins of the water body, sheltered by a mosaic of vegetation, in a sunny position. Usually found in mixed populations with either pool frogs or marsh frogs but rarely both.
Edible frogs are opportunistic feeders and feed on seasonally abundant invertebrate species. Usually triggered by movement of the prey item the feeding activities usually take place on the pond margins. The edible frog also readily leaps for passing insects such as hoverflies, wasps and dragonflies.
Predation is mainly by species of heron (including the Bittern) and the Grass snake.
Edible frogs are extremely sensitive to movement and when predators are nearby will stop calling, flatten bodies to ground relying on camouflage, slip underwater or dive into the water from their sunning areas with a mighty leap.
Either hibernate on land [usually in frost-free holes and cavities amongst rocks and tree and hedge roots] or, less commonly, underwater.
Often found at the base of bank-side vegetation facing south or amongst a mosaic of marshy vegetation.
Edible frogs are territorial and communicate by means of a variety of calls produced by light grey vocal sacs on either side of the mouth. Initial declaration of territory is by a series of simple ‘query’ sounds “qwark” with a rising cadence. This may receive a response from male competitor as a “qwark” with a descending cadence. The full mating call is composed of a series of sharp ‘bre-ke-ke-ke-ke-ke-keh’ similar to the marsh frog but the call of the edible frog normally has 12-20 repetitions lasting 1.5-2 seconds. The pitch rises throughout the call which is often delivered as a repetitive series.
A single male may initiate the calling and is rapidly joined by others to form a chorus. Transition calls between the two call types also occur. A low pitch rhythmic release call is heard when one male mistakes another for a female or when an unresponsive female is approached.
Males often gather in high densities aggressively swimming towards each other and sometime liquid jets are ejected from the rear of the frog at the same time.
Female edible frogs only approach the breeding pond in response to the production of mating calls by the pool and edible frog males. They tend to be attracted to the calls of dominant pool frog males and after fertilisation and egg-laying leave the breeding pond. Between 3,000 and 10,000 eggs are laid during the season. Egg size is very variable [1-2.5mm] as larger triploid eggs may be present. Tadpoles bask in shallow waters during the day but retreat to deeper water at night.
As an introduced species, the Edible frog is not covered by UK conservation legislation. Priority for conservation resources are targeted at endangered native animals.
|Species of conservation concern:||No|
|Bonn Convention (1979):||No|
|Wildlife & Countryside act (1981):||No|
|Bern Convention (1982):||No|
|Conservation Regulations (1994/2007):||No|
|EC Habitats & Species Directive:||No|
|Principal Author:||Main Illustrator:||Review:||Updated:|
|Dr Julia Wycherley||Various||Reviewer||28 Jan 2008|