Common frogs have a robust body and relatively short hind limbs with webbed toes. Males tend to be slightly smaller and darker than females, and can also be distinguished by the dark bluish-black nuptial pads (swellings) on their first fingers. These pads become more pronounced during the breeding season, helping males to grip on to females during mating.
The common frog's smooth skin varies in colour from grey, olive green and yellow to various shades of brown and is covered with irregular dark blotches. Common frogs have a dark ‘mask’ enclosing their eyes and eardrums, and often have barred markings on their limbs and flanks. Their undersides are white or yellow, sometimes orange in females, and are often covered with brown or orange speckles. Completely red or black individuals are occasionally found in Scotland, and some individuals may turn blue during the breeding season. Albino common frogs have been found with yellow skin and red eyes. Common frogs also have the ability to lighten or darken their skin to match their environment. Common frogs have brown eyes with black horizontal pupils, and transparent inner eyelids that protect their eyes while they are underwater
Although common frogs are active both day and night, they tend to be more active at night. During the winter they hibernate in compost heaps, under stones and logs, or underwater beneath piles of mud and decaying leaves.
Common frogs do not feed at all throughout the breeding season, but when they are active they will feed on any moving invertebrates of a suitable size, such as insects, snails, slugs and worms, which they catch with their long, sticky tongues. Adult frogs feed entirely on land, whereas younger frogs will also feed in the water. Tadpoles are herbivorous and feed on algae but become carnivores when they mature into adult frogs.
Common frogs become sexually mature at around three years of age. During February and March they begin to emerge from hibernation and make their way to the breeding grounds. Common frogs have been seen to return annually to the sites where they originally developed from spawn into adult frogs. The males arrive first and attempt to attract a mate by producing a low purring croak. A successful male will wrap his forelimbs around the female in a mating embrace known as 'amplexus'. Each female lays 1000-4000 eggs at a time, which are fertilised by the male as they are released. Frogs can spawn as early as December and as late as April, depending on how warm the weather is. They prefer to lay their eggs in shallow, still water. Frogspawn is surrounded with a clear jelly-like substance, which swells up in the water to protect the fragile embryos. The spawn floats to the surface in large round clumps so that the sun can warm the eggs. After 30 to 40 days, tadpoles begin to emerge from the jelly-like spawn. The tadpoles feed on the spawn for the first few days until they begin to eat algae. Tadpoles change into frogs through a process called ‘metamorphosis’, which takes between 12 and 14 weeks. Both spawn and tadpoles are extremely vulnerable, and many get eaten by predators such as fish, birds and grass snakes. On average, only 5 out of every 2000 eggs will survive to become adult frogs. When tadpoles hatch they have gills that allow them to breathe underwater. After 9 weeks they have lost their gills and developed lungs, and therefore must swim to the surface to breathe. As they grow, tadpoles begin to feed on insects as well as plants. Hind legs develop between 6 and 9 weeks, and front legs are fully developed after about 11 weeks. The tail begins to be absorbed by the developing tadpole, and by 12 weeks it has practically disappeared, leaving a tiny froglet. At this stage the tadpoles are less dependent on water and will hide in long grass in and around the pond.