Rana temporaria - Common Frog

Common Frog

Taxonomy

Phylum: Chordata

Class: Amphibia

Order: Anura

Family: Ranidae

These lively amphibians are very common around streams, rivers, ponds and lakes. Sometimes they can even be found breeding in ditches that dry up completely in summer. The backs and flanks of adult frogs vary considerably in colour, with brown, olive green or olive-brown occurring most often; however some frogs are almost grey while others (albinos) are yellow with red eyes, and very occasionally a rufous (reddish) frog is reported. Frogs have the ability to lighten or darken their skin colours to blend in with the background of their surroundings.

Common Frogs are more angular in body shape than Common Toads and unlike other amphibians they do not have a distinct dorsal stripe. In any case confusing the two species is most unlikely because a frog's moist smooth skin readily distinguishes it from the dry, warty-skinned Common Toad. In size frogs and toads are quite similar. A full-grown adult Common Frog has a body length of 6 to 9cm, with (as is the norm with amphibians) the females on average slightly larger than the males.

Life cycle

Frogspawn

Frogs mate in early spring. In Britain and Ireland you may see paired frogs around the margins of ponds and slow-flowing rivers in late February, March and early April, and soon afterwards the frogspawn appears in large blobs. (Toads shed their spawn later in the spring; they leave it in long ribbons and so it is easily distinguished from frogspawn). After spawning the adult frogs leave the water, where they are vulnerable to predators, and head back to cover in ditches, damp grassland or wet woodland.

In a few weeks, depending on how cold the water is, tadpoles emerge from the spawn. They swim near the edges of lakes, often in vast numbers. (It is just as well that frogs produce much more than an heir and a spare, because all sorts of other creatures including fish, snakes, mammals and birds eat a lot of tadpoles!) After a while, little gold spots appear on the tadpoles - toad tadpoles remain black, so it is easy to tell which are which - and then tiny legs are formed and soon afterwards the tail drops off.

Tadpoles

During the summer months the tadpole loses its rounded shape and develops the general appearance of an adult frog. The colour also changes to a greenish-brown. Fish, dragonfly larvae and the larvae and adults of large water beetles (for example the Great Diving Beetle) thin out the tadpole population, and in small ponds it is not unusual to lose every one to these predators. By late summer it is time for any surviving froglets to leave the water and begin their treacherous journey to wet habitats that offer more cover. Some stay close to the pond, while others may trek several hundred metres to take up residence where there is less competition for food.

During the cold winter months, frogs hibernate. They have the ability to breathe through their skins, and so they can lie dormant in mud; alternatively, they can burrow down into rotting vegetation on the bed of a pond, ditch or slow-flowing stream, which affords protection from frost and conceals them from predators.

Frogs do not live as long as toads do, and in the wild they have a typical life expectancy of up to eight years. Of course, the world would be overrun with frogs if most of the tadpoles survived to beciome breeding adults, and so apart from those kept as pets in captivity, old frogs are very rare animals indeed.

Predators

Frogs do not secrete toxins from their skins, as toads do, and so they are palatable to a wider range of predators. Herons and otters account wreak great carnage on frog populations, especially when the young froglets begin leaving the ponds and lakes of their birth. Many other birds, including ducks, crows and gulls, will readily seize any frog that they come across, while mammals such as badgers, stoats, weasels and polecats are also fond of amphibian snacks when they can get them. The only defence that frogs have is to hide, and so they tend to hunt in long grass and in wet woodland with plenty of leaf cover rather than out in the open on bare earth.

Prey

Apart from during the early spring breeding season, adult frogs spend most of their time in damp places on land. They eat insects, spiders, worms and other small invertebrates, which they catch mainly at night.

Unlike the dry warty skin of a toad, the skin of a frog is smooth and has to be kept moist at all times. Frogs are therefore less likely to be found in wood piles and old sheds - just the sort of places you might come across a toad.