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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). The Anura are the largest and most widely distributed amphibian group with over 3,500 species world-wide.

IRISH AMPHIBIANS

There are three species of 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 Counties Kerry and Wexford. 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. 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.

SPECIES PROFILE	Common Frog
Food	Slugs, worms, flies and other insects
Habitat	Damp vegetation, camouflaged ponds, hedgerows
Reproduction	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

Look Out: The large eyes bulge out of the top of the head so the frog can keep a sharp lookout for food and danger even when most of its body is under water. Frog's Hearing: Frogs have good hearing. Just behind the eye is a large eardrum which leads to the rest of the ear inside the head. Strong Muscles and Webbed Feet: The frog's hind legs are used for swimming in the water and leaping on land. The feet on a frog's hind legs are webbed like flippers to help it swim fast. Coat of Many Colours: The colourful patterns on the frog's skin help to disguise it from enemies such as rats, herons and hedgehog. It takes a frog 2 hours to change the colour of its skin to that of its surroundings. Stick Tongue: The frog's long sticky tongue is attached to the front part of the mouth, so that it can flick out to catch flies. Vocal Sac: Each time a frog croaks, the loose skin on its throat expands. Common Frog (Rana temporaria) has four fingers and five toes. Artwork: © IPCC

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 in 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).

Tadpole
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.

Froglet
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. 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. They can live for 7-8 years.

Growing up - from egg to tadpole to froglet to adult, the life-cycle of the Common Frog (Rana temporaria) takes over a year to complete. Artwork: © IPCC

FROG HABITATS

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. Frogs use garden ponds, farm ponds, wildlife 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.

FROG DISTRIBUTION

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.

Distribution of The Common Frog in Ireland based on all frog surveys to date: Ní Lamhna (1979), Foss and O'Connell (1997), Marnell (1997), Russ (2001) and McLoughlin (2003). Each dot represents the presence of the frog within that 10km square. While frogs have been found in every county in Ireland there are still some areas where there are no records or where records are sparse. Map © IPCC

HOP TO IT IRISH FROG SURVEYS

The first 'Hop To It Frog Survey' was undertaken in 1997 by the Irish Peatland Conservation Council. The survey provided base-line information on frogs in Ireland, such as their biogeographical distribution, preferred habitats, breeding habits and success. Prior to this, surveys on the distribution of the frog in Ireland were undertaken by researchers in An Foras Forbartha, the Ulster Museum and Trinity College Dublin.

The Hop to It Frog Survey 2003 aimed to reassess the distribution of the frog and to identify any changes in its habitat preferences or breeding.

A total of 1,502 records for frog spawn, tadpoles and frogs were returned completed between December 2002 and June 2003. These records yielded 74 10km grid squares where frogs had not been recorded before. In total, 416 10km grid squares were covered by this survey. The survey also showed that frogs occurred and reproduced in every county in Ireland in 2003. The majority of frogs were recorded between elevations of 0 to 200 meters above sea level. This included all five stages in the life cycle. Garden ponds were the most important habitats for breeding frogs, accounting for 47% of all records received. Other important habitats were bog pools, ditches, drains and puddles.

Pie chart showing the breeding habitats used in the life cycle of the frog from the Hop To It Irish Frog Survey 2003. Data © IPCC

WHAT KILLS FROGS?

Natural Mortality
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.

Habitat Loss
Over 50% of Ireland's amphibian wetlands have been lost to drainage, industrial peat extraction, pollution and natural senescence in the past 100 years. 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.

Fire
An extensive danger to frogs is that of accidental fires. In any hot dry summer there are inevitably going to be accidental fires which can result in the loss of habitat. Another threat for the common frog is from deliberate regular burning of bogland in the belief that this improves the grazing for farm stock.

Pollution
Exposure to chemical fertilisers, pesticides and herbicides can cause frogs agonising deaths. Ammonium nitrate granules can kill a frog within 5 minutes. It is thought that the chemicals are absorbed into the skin and affect the balance of chemicals in the moist tissue. They then suffer a massive toxic attack. 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
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.

Fatal Infections
The fungus Batrachochytrium dendrobatidis paratises amphibians and has caused frog and toad population declines throughout the world since the 1980's. The fungus destroys the skin pigment keratin in amphibians. Frogs infected appear emaciated and lethargic, often with abnormalities of the skin or eyes. The fungus has caused the extinction of the Costa Rican golden toad (Bufo periglenes) and serious frog kills in Panama, Australia and the U. S. The rapid spread of the fungus at 42km per year, has been blamed on ozone depletion and loss of forest cover, which change the habitats of sensitive amphibians. Several mass deaths of frogs have been blamed on a disease known as "red leg". A British study has uncovered a new virus - Ranavirus which is responsible for killing frogs by the hundred.

Road Migration Deaths
During a few warm, damp nights in spring, thousands of amphibians follow traditional migration routes on their way to spawning ponds. Unfortunately, hundreds can be squashed and killed by traffic on intervening roads as they make for a suitable pond.

Food and Pets
Europe imports hundreds of millions of frogs from Indonesia, India and Bangladesh, threatening some species. Millions of South African frogs have been exported to the U.S. as pets. The American bull frog, a pet in Britain is a danger to native frogs. Once these or others escape they displace natural amphibian populations by hunting them or mating and producing hybrids. The release of exotic species into the wild is an offense under the Wildlife Act.

WHAT CAN YOU DO TO HELP FROGS?

1. Make a garden pond to encourage frogs to breed.
2. Frogs spend most of their lives on land so give them long grass, leaf and log piles, trees and shrubs in your garden to feed and hibernate under.
3. Pass on your knowledge of frogs to others.
4. Do not keep endangered frog species as pets and never release a pet frog into the wild.
5. Organise a clean up of rubbish from local ponds and streams.
6. Take part in the Hop To It Irish Frog Survey each year and help us learn more about the status of frogs in Ireland. Follow this link to the Hop To It! - Irish Frog Survey