The Common Frog - (Rana temporaria)
The Amphibians were the first group of vertebrate animals to make a serious attempt at a life on land. Their history is long and complex. It is thought that the amphibians arose over 350 million years ago from a fish-like ancestor. They can, in general, move, feed and breathe equally well on land and in fresh water, but nearly all amphibians return to water to breed.
The Amphibians are divided into three groups: the Urodela (newts and salamanders), the Apoda (worm-like caecilians), and the Anura (frogs and toads; BISACRE ET AL 1979). The Anura are the largest and most widely distributed amphibian group with over 3,500 species world-wide (CIHAR ET AL 1994).
There are three species if amphibian found in Ireland - the Natterjack Toad (Bufo calamita), the Smooth Newt (Triturus vulgaris) and the Common Frog (Rana temporaria). The Natterjack toad is extremely rare, and is confined to a few areas in County Kerry. The smooth Newt is fairly widespread in Ireland, although it may be very local in distribution in the north-west and south-west. The Common Frog is the only species of frog found in Ireland and is listed as an internationally important species (MARNELL, 1997, WHILDE 1993). The skin colour and markings of the Common Frog vary enormously. The basic colour ranges from a pale green-grey through yellow to a dark olive-coloured brown. The only regular markings are the dark bars across the limbs, and streaks behind and in front of the eyes.
Slugs, worms, flies and other insects
Damp vegetation, camouflaged ponds, hedgerows
Breed around February and spawn around March, Tadpoles hatch and grow from April to May, Tadpoles metamorphose into froglets, and leave the pond in June/July.
Frog Life Cycle
When the adults emerge from hibernation they migrate to congregate at various breeding sites. They may travel up to half a mile to find a site where they gather in large numbers. The males always arrive first and strike up a chorus of loud croaking to attract females. Frogs do not have any elegant courtship rituals; the eager male simply grabs the nearest female as she arrives at the spawning site. Jumping onto the female's back, the male wraps his fore limbs around her body and grips using nuptial pads, on the fore limbs - a position called amplexus.
Spawning itself can take place any time during amplexus and lasts only a few seconds. The female lays over 2,000 black eggs while the male releases sperm. The eggs are fertilised immediately and before their gelatinous capsules absorb water, swell and rise to the surface. After spawning the female usually leaves the pond, while the male often goes on to search for another mate.
Both male and female frogs return to the same pond year after year, probably recognising it from the smell of the water and algae.
Eggs & Frog Spawn
Each frog egg is 2-3mm is a diameter and is enclosed in an envelope of jelly. When the egg is deposited in the water the jelly swells to a diameter of 8-10mm insulating the eggs from the water. The egg develops into a tadpole in 10-21 days (the higher the temperature the shorter the development time).
The tadpole digests the spawn jelly using a special secretion and hatches. Specific adhesive organs fasten the newly hatched tadpole to other spawn or plants in the pool. At this early stage tadpoles have no mouth, and until its mouth organs form it feeds on an internal yolk sac attached to the stomach. At approximately 2 days old the external gills, mouth and eyes are formed. At this stage it moves like a fish and begins to eat algae. At 12 days spiracles and internal gills are formed. At 5 weeks the hind legs are showing and the lungs are forming. It then has to swim to the surface of the water to gulp air. The tadpole has fleshy lips with rows of teeth for rasping away at water plants and by seven weeks it also eats insects and even other tadpoles.
At 10 weeks the forelegs are growing. The hind legs are fully grown and the tail is reducing. At 14 weeks the tail is nearly fully absorbed. At this stage the froglets are usually starting to spend time on rocks or in nearby damp grass.
The Frog feeds on slugs, insects, worms, spiders and similar prey, but does not predate aquatic organisms (BLACKSMITH & SPEIGHT 1974). Scarcity of food or severe cold may delay metamorphosis and overwintering tadpoles are not uncommon in northern countries.
In winter frogs hide in frost-free refuges, under tree stumps, in stacks of turf, or in rock piles where they enter torpor until the following spring.
Young frogs usually double in size by the following autumn and they reach sexual maturity in their third year. The y can live for 7-8 years.
Frogs like to be near ponds which have plenty of algae and plants near the edge, usually with shallow edges so that they can easily climb out. In general the common frog seems to prefer ponds which have water flowing in and out of them (HUGHES 1981). But they also use garden ponds, streams, bog pools, drains and ditches as breeding sites.
The terrestrial habitat of frogs is also important. The land around the breeding site or pond needs to be rough with long grass and some scrub to give cover for terrestrial foraging. Frogs also require habitats for hibernation. Large stones, old logs and hedgerows offer just such accommodation.
The Common Frog is considered to be widespread and common in Ireland but vulnerable in the rest of Europe. Rana temporaria has an extensive range of habitat - from sea level to nearly as high as the snow line on mountains 760m up. The 'Habitats Directive' of the European Community recommends that its exploitation should be subject to a management plan (WHILDE 1993).
IPCC Hop To It Irish Frog Survey 1997
The Hop To It Irish Frog Survey was undertaken by the Irish Peatland Conservation Council in 1997. The survey aimed to provide base-line information about where frogs occur, the habitats they prefer, when they start breeding and their breeding success in Ireland. School children in primary and secondary schools were invited to undertake the survey in order to capitalise on the potential resource of student researchers throughout the country. The participants of the survey looked for frog spawn, tadpoles and juvenile frogs and filled in postcards with information about the location, the habitat and the date of expectation.
A total of 832 frog survey cards were returned completed. The survey showed that frogs occurred and reproduced in every county in the Republic of Ireland in 1997. The majority of frogs were recorded between elevations from 0 to 200 meters above sea level. This included all four stages in the life-cycle. Garden ponds, farm ponds, streams, bog pools, drains and ditches are the most important habitats for breeding frogs. The habitats surrounding breeding sites were natural and in keeping with the breeding habitat.
Threats to the Common Frog
Amphibians play an important role in the food chain. During spring and summer many thousands are killed and eaten daily to nourish predators such as otters, foxes or herons. For a typical lump of spawn containing 2,000 eggs, 95% of the eggs may hatch. Only 1-5% of the remaining tadpoles make it through the metamorphosis and only a handful of the original 2,000 reach sexual maturity. If spawn is laid early in the season, hard frost may kill it, especially in shallow water. Tadpoles may also die if their aquatic habitat dries out before they have metamorphosed.
Over 50% of Ireland's amphibian wetlands have been lost to drainage, industrial peat extraction, pollution and natural senescence in the past 100 years (MARNELL 1997). The terrestrial habitat of frogs is also important. Unfortunately just as the wetlands are being drained, hedgerows are also being destroyed to make way for industrial farming methods.
Water polluted with heavy metals such as Aluminium, Cadmium, Zinc, Copper and Iron are toxic to frogs. lead from car exhausts may be important even in rural areas. Acid rain can also increase the toxicity of metals in ponds causing further threats to frog populations.
UV radiation has become a prime candidate for blame in the world-wide decline in frog numbers in recent years. In 1989 herpetologists from around the world reported declines in amphibian numbers. UV radiation damages DNA causing cell mutations and death. Frogs have very low levels of the necessary enzyme, photlayse, to repair the damage, and it is believed that this is a large contributor to their apparent demise.
What can you do?
Garden ponds play an important role in maintaining frog populations. Common frogs often find new garden pons unaided and successful populations can develop within a few years (MARNELL 1997). The best way for an individual to help the Irish frog is to create a garden pond. IT must have a minimum depth of 60cm and should have shallow edges.
Do not introduce fish or exotic amphibians like the American bullfrog into your garden pond. Some of these species are highly efficient predators and can eat large numbers of amphibians and other animals.
References and Further Reading
ANONYMOUS (1991) Hot frogs fall prey to pesticides.
New Scientist 12: p18.
ANONYMOUS (1991a) Worlds frogs are killed off by pesticides.
Environmental Digest 52: p5.
ANONYMOUS (1996) Bhs Frog Watch.
Wildlife Watch, Royal Society for nature Conservation. Lincoln: 24pp
BISCARE, M., CARLISLE, R., ROBERTSON, D. & RUCK, J. (1979) The Marshall Cavendish Illustrated Encyclopaedia of Plants and Animals.
MARSHALL CAVENDISH, London: 439 pp
BLACKITH, R.M. &SPEIGHT, M.C.D (1974) Food and feeding habitats of the frog Rana temporaria in bogland habitats in the West of Ireland.
J. Zool., Lond. 172:67-79
CIHAR, J., DOBROVSKI, J. HARCUBA, P. & KHOLOVA, H. (1994) The Encyclopaedia of Animals.
Published by Sunurst Books, London.
DALLINGER, j. & JOHNSON, S.A. (1972) Frogs & Toads.
Lerner Publciations Company. Minneapolis: 48pp
FOSS, P. AND O'CONNELL, C. (1997) Hop to it Irish Frog Survey.
Irish Peatland Conservation Council, Dublin: 40pp
GIBBONS, M.M & McCARTHY, T.K. (1984) Growth , maturation and survival of frogs Rana Temporaria
L. Holartic Ecology 7: p419-427. Copenhagen.
HUGHES, D. (1981) Croaking for their mates.
The Living Countryside 1 (6): p101-103.
MARNELL, F.(1997) Amphibians.
Irish Wildlife Trust, Fact Files on Nature: 4pp
MACQUEEN, J. (1976) Success in Biology
John Murray (Publishers)Ltd.: p374-385.
PEARCE, F. (1996) Amphibians Alert.
BBC Wildlife. March: p58.
WHILDE, A. (1993) Threatened Mammals, Birds, Amphibians and Fish in Ireland.
Irish Red Data Book 2 - Vertebrates. HMSO, Belfast: 224pp
More information on frogs and activities relating to frogs are online at http://www.ipcc.ie/