The common, heron’s or grass frog, Rana temporaria, is probably our most common amphibian. Those of us of a certain age will remember it being a staple of the dissection table in school and college and, indeed, populations suffered locally to meet the demands for specimens by education institutes. These days, with the advent of 3D models and virtual dissection labs, dissection in UK schools is rare and many schools in both Europe and North America have voluntarily abandoned the practice.
Rana have been around for some 30-40 million years and are a more recent evolutionary appearance than Bufo toads.
The binomial name alludes to its sudden appearance and disappearance during spawning in the spring. Rana is the Latin for “frog”, while temporaria is “temporary”; so Rana temporaria is quite literally “the temporary frog”.
Size: Adults have a body length of up to 13cm (5 in.), usually 6-9cm (2.4-3.5 in.), with females typically larger than males. Typically weigh ~22g (0.8 oz.), but varies considerably with age, season and temperature of water; range for adults appears to be ca. 15-35g (0.5-1.2 oz.).
Colour/Appearance: A robust-looking frog with large eyes situated on the top of the head sporting circular black pupils surrounded by golden iris. Considerable variation in snout profile, although older individuals tend to have rounder noses. Short front legs hinged at elbow and with un-webbed toes. Hind legs long and folded back under hips with long webbed toes. Vast variation in colour. Commonly dark olive with mottled dark brown patches behind the eyes, along back and on legs. Flanks are mottled and pale area of skin found around mouth and make up two parallel stripes along either side of back. Often present with dark chevron-shaped mark between shoulders. Small lumps sparsely distributed across the skin tend to be dark brown in colour. Underside pale to white. Red and orange morphs common; leucistic/albino individuals are reasonably common. Occasionally melanistic animals are recorded, with a near-black back and head with dark green (typically spotted) flanks and belly.
Distribution: A cosmopolitan frog widely distributed throughout the UK and Ireland. In Europe range extends from Ireland in west eastwards to the Urals in western Russia. They're found north into the Arctic Circle to northern Scandinavia and south into Iberia, northern Italy and the southern Balklands. Also found in parts of Asia as far east as Japan.
Habitat: Contrary to popular misconception, while some may live in or near ponds for much of the year (particularly further south in their range), throughout much of their distribution common frogs are primarily terrestrial and spend most of their life on land. They'll return to ancestral ponds and ditches to spawn and to ponds or other water bodies during hot weather or (sometimes) to hibernate. They inhabit a wide variety of habitats, from moorland to farmland and deciduous woodland, and can be found in almost any damp, shaded areas even in the middle of large towns and cities.
Mortality/Longevity: Only ~5% of spawn clutch likely to survive to metamorphosis and mortality can be as high as 95% within the first 3 years following metamorphosis. Adults probably live 5-10 years in the wild, although can live up to 14 years. Reports of individuals surviving for 27 years in captivity.
Predators: While comparatively few species prey on adult frogs, some are taken by tawny owls, otters, foxes, badgers, stoats, weasels and even hedgehogs. Herons are probably most significant predator. Spawn and tadpoles fall prey to a wide range of animals, including dragonfly larvae.
Sexing: During breeding season the males develop dark brown nuptial pads on their front feet (used for gripping the female) and females exhibit pearly granules on their flanks and hind legs.
Activity: Primarily nocturnal, although can be active in and around breeding ponds during the late winter and early spring. May spend time resting underwater or on land under logs and in long vegetation during the day. Newly-metamorphosed froglets frequently encountered during the late afternoon in summertime, particularly in damp habitats. Hibernate during winter, although will emerge during mild and wet interludes.
Territory/Home Range: Poorly known. Observations suggest frogs are non-territorial, even males in the presence of females during the breeding season. Scant tracking data suggests may regularly move reasonable distances of 30-100m (100-328 ft.) and perhaps up to 10km (6 mi.) during the breeding season.
Diet: Active predators taking a wide variety of live prey including insects, spiders, woodlice, earthworms, slugs and snails, and sometimes tadpoles and froglets of their own and other species. Prey captured with aid of long sticky tongue that extends rapidly from lower jaw.
Reproduction: “Explosive breeder” depositing rafts of spawn. Each female lays 1 or 2 clumps containing ca. 2,000 eggs (range 700-4,500) around vegetation, typically in shallow (<30cm / 12 in. deep) water. Each small black egg surrounded by gelatinous capsule that swells to ~1cm (0.4 in.) in diameter upon contact with water. Male latches onto female (state called amplexus), his arms around her chest, using specialise nuptial pad on thumb, releasing sperm as female releases eggs. Most eggs fertilised by 'latched' male, but others nearby may contribute. Sometimes several males latch onto and can drown single female. Males will also latch on surrounding objects, including fish.
Spawn remains at surface for few days and develops rapidly, sinking as tadpoles eat the yolk. Development heavily influenced by water temperature - in cold regions, may overwinter as tadpoles. Nucleus starts as small round dot, then divides and lengthens to about 5mm long at which point tadpoles leave residual jelly to feed. Emerge and begin grazing ~2-3 wks, develop speckles by ~6 wks and back leg buds appear ~8 wks. Feed on algae, detritus, plants and small invertebrates, with diet becoming increasingly carnivorous once hind limbs begin developing. Full-grown tadpoles often largely inactive “sunbathing” in shallows. Metamorphosis complete (i.e. front legs fully developed and tail lost) at 12-16 wks. Newly metamorphosed froglets ca. 1-1.5cm (0.6 in.) long and emerge on damp nights between June and September. Sexually mature at 2-3 yrs, around 6cm (2.4 in.).
Behaviour and Sociality: Largely solitary outside breeding season, although may congregate to feed at prime spots. Low 'purring' or 'rasping' croak emitted day and night during breeding season late winter/early spring, also sometimes during autumn. Most vocalisation occurs at surface, although occasionally underwater. Occasionally scream when caught or threatened.
Threats: Broadly a species of least concern, although local populations can be unstable and susceptible to disease and habitat disturbance, particularly construction, pollution and fragmentation. Rural populations have declined significantly in recent decades, largely a result of disease (red-legged virus) and agricultural intensification. Some data suggest 68% decline over 1985-2016. Urban gardens with ponds, even small ones, may have helped stabilise/boost some populations, although more recently sightings appear to have declined.
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