common frog

Fabulous Frogs

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Organised by amphibian charity Save the Frogs, World Frog Day is celebrated every year on March 20th. Around the world, frogs, and their amphibian relatives, are under huge pressure from a range of threats including disease, climate change, pollution and habitat loss. World Frog Day aims to highlight these threats, as well as the range of incredible amphibian species around the world. Here in the UK, our most familiar species is the aptly named common frog. However, despite their relative abundance, they too are declining. So, with spring undoubtedly the best time to see common frogs, it is also the perfect time to find out more about these fascinating amphibians and how we can help them.

common frog
A beautiful common frog

The Frog Prince

Many of us are probably familiar with the famous fairy tale, The Frog Prince. In the story, an uppity princess reluctantly befriends the frog who rescues her lost golden ball. At the tale’s end, she finally kisses him, transforming him back into the handsome prince he was before an evil curse struck him. The implication throughout is that the frog is an ugly and slimy thing to have to kiss or even look at. Despite the princess’ disgust, however, common frogs are beautiful creatures. Far from slimy, they can be quite variable in appearance. Although usually a mixture of olive greens and browns, some are red or yellow in colour. All, though, have a dark patch behind each eye, a range of dark blotches and a pale throat and belly. They use their long legs to help them hop when out of water and swim when submerged. Their striking eyes have a large black central area surrounded by a mixture of brown and gold. Males are smaller than females.

Frog
Common frogs can be quite variable in colour, as this yellowish individual shows
Frog
A croaking male frog

To the uninitiated, the main confusion species in the UK is the common toad. However, there are some easy ways to tell the two apart. Toads are generally larger and chunkier looking than frogs and have much drier looking skin with a warty appearance. Frogs, meanwhile, secrete a substance to keep their skin moist, making them look wet and smooth. This helps them to breathe through their skin, something toads don’t do to the same extent. Toads have shorter legs than frogs which means they tend to walk on land rather than hop. They also spend much less time than frogs in or around water and usually only return to it during the spring breeding season. The two animals’ spawn, more of which later, is also different. Frogs lay their spawn in large clumps while toads do so in long strings. Both eat a range of invertebrates, including slugs, snails, worms and beetles.

Toad
Toads tend to walk rather than hop

The Frog’s Year

Unlike mammals, frogs are cold-blooded so can’t generate their own body heat. This means that once temperatures start to drop in autumn, they must take refuge and effectively hibernate for the colder months, although probably not in a complete sleep like some animals. Good wintering spots include old burrows, log piles or compost heaps, as long as they are fairly moist. Some individuals choose to hibernate at the bottom of ponds, buried within the sediments or leaf litter there. There is a risk the pond will freeze, cutting off the frog’s oxygen supply. But the advantages outweigh the risks. They are not only first on the scene when breeding begins, but pond hibernation also means that they don’t have to face the risky journey between hibernation site and mating ponds. Many a frog (or toad, for that matter) meets its end on the dangerous journey to reproduce in spring.

Frog on snow
Frogs can sometimes get caught out by snow if a cold snap follows a warm spell in late winter

Triggered by the spring’s rise in temperatures, frogs emerge from hibernation to head to shallow breeding ponds, usually in February or March. There is some evidence to suggest they are emerging earlier every year as our climate warms. Huge aggregations of animals can build up at some ponds, with hundreds of frogs crowding the water. Males attract females by using their single inflatable vocal sacs to croak loudly. Once the females arrive, males will clamber on top of them. This in turn stimulates the females to release their eggs into the water, which the males fertilise externally. Sometimes, the multiple males will attempt to climb on the same female in their desperation to breed. The clumps of fertilised eggs, or spawn, hatch into tadpoles between two and four weeks later depending on the weather. Born with gills, the tadpoles then undergo a huge metamorphosis as they develop lungs, grow hind, then fore legs and finally lose their gills and tails to become tiny froglets.

Frogspawn
A typical clump of frogspawn
Froglets
Some of these tiny froglets still have their tadpole tails

Frogs Far and Wide

The only other frog native to the UK is the northern pool frog, although it became extinct here in the 1990s due to habitat loss in the fens and brecks of eastern England. Now reintroduced at a few sites in Norfolk, there are hopes it will become reestablished. Pool frogs are similar in size to common frogs but have a distinctive yellow or cream stripe down the back. Males also have two vocal sacs, one on either side of the head, unlike common frogs who have one central one. Pool frogs breed much later in the year than our other native amphibians, only starting in late May or early June.

Iberian water frog
Iberian water frogs are now well-established in the Somerset Levels

Five non-native frog species have been introduced to the UK at various times over the last 90 years and are largely confined to the south of the England. These include Europe’s largest species, the marsh frog. Edible frogs, another non-native, are actually the product of hybridisation between marsh and pool frogs. As their name suggests, the species is that usually used for the quintessentially French dish, frogs’ legs. Other non-native species in the UK are the Iberian water frog, European tree frog and the North American bullfrog.

Iberian water frog
Iberian water frogs have also been introduced to the Azores

In contrast to the UK, Continental Europe has a large number of native frog species. These include seven tree frogs and the incredible moor frog. In the coldest part of its range, it can survive freezing solid and then thawing due to antifreeze substances in its body. Further afield, South America is home to over 100 poison dart species of frog, including one of the most toxic animals on the planet, the golden poison frog. Just one milligram of its poison can kill 10 humans. Brazil also has two venomous species. This means that they actually inject poison into another animal rather than just relying on touch alone to pass on toxins. 

Blue poison arrow frogs
The blue poison arrow frog is one of many poisonous South American species

Threats from all Sides

Amphibians worldwide face huge threats, with many species facing extinction. Habitat loss, climate change, predation by invasive species, the exotic pet trade and diseases such as chytridiomycosis have all taken their toll. Chytridiomycosis alone is thought to be responsible for the extinction of at least 90 amphibian species globally over the last five decades. Our own frogs aren’t immune to these threats either, as seen with the extinction here of the northern pool frog. Although still abundant, common frogs are declining, too, although more data is needed to ascertain exactly what is going on. Changes in land use and drainage are one factor almost certainly affecting all amphibians in the UK. Over the last 50 years, a large number of ponds have been filled in for development projects or to increase farming areas. Climate change is also likely to be having an impact, with recent droughts extremely bad news for frogs.

Water is vital for common frogs meaning increasing droughts are a threat

Increased traffic on our roads means that frogs face an even greater threat than ever when travelling back to their breeding pools in spring. Development also puts up barriers across these routes, preventing frogs from reaching suitable breeding habitat. Meanwhile, globalisation and animal imports from abroad mean that diseases from further afield have reached the UK, as well. This includes the aforementioned chytridiomycosis, which affects frogs’ ability to breathe and take in water by causing various skin problems. Another disease from abroad, this time probably North America, is the Ranavirus genus of viruses. These can cause internal bleeding and skin ulcers. In warm weather, they can result in mass die offs of frogs and other amphibians.

Frogs
Common frogs at their breeding pool

Saving Frogs

Common frogs in the UK face a number of threats, just as their relatives further afield do. Those of us with gardens can take a few simple steps to help them out, though. If you have room to create a wildlife pond, you will be providing valuable breeding habitat for them. And you will be helping a host of other creatures too. Even if you can’t install a pond, however, you can still help by leaving messier areas of your garden. Leaf and wood piles left alone in winter provide vital hibernation spots for frogs, as well as other animals. This one simple action can make a huge difference to this fascinating amphibian.

A male frog piggybacking on a female during breeding

Further Reading

Find out more about Save the Frogs work here.

Like all of the RSPB’s Spotlight books, this volume is full of fascinating information. Focusing on the UK’s four native species of frog and toad, photographs and informative text combine to provide the perfect introduction to this suite of species.

Covering all of the reptiles and amphibians occurring in Britain, Ireland and the Channel Islands, this field guide also contains useful distribution maps and ID tips for confusion species.

Cold Blood is author Richard Kerridge’s wonderful natural history of the amphibians and reptiles that captivated him as a child. Kerridge introduces us to an array of species, while also revealing just what it is about these cold blooded creatures that fascinates him so much.

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