The most familiar of our amphibians, the Common Frog Rana temporaria grows up to 65 mm long, the females being very slightly larger than the males. The colours are variable shades of brown and green above, with paler grey, white or lemon underneath. Frogs with yellow or red-brown above and lemon below are more likely to be females. No particular colour distribution patterns have been identified in the North East region. The back, head and legs have darker bars and spots, which help to camouflage the animal from its many predators. The skin is smooth, in contrast to the rough and warty skin of the Common Toad Bufo bufo (Arnold, 1978).
The hind legs are much larger than the front legs, and very muscular, enabling frogs to jump to escape predators and to swim strongly in the water. Common Toads have smaller hind legs than frogs, rarely jump, and swim less strongly. In the mating season male frogs develop “mating pads” on the “thumbs” of their front legs to assist in gripping the female frogs (Beebee and Griffiths, 2000).
Frogs are the earliest of our amphibians to breed, the whole population at each site spawning in one go, in contrast to newts, which spread the breeding season over several months. Early spawning may be linked to the tendency for frogs to spawn in ponds that are likely to dry out in the summer, making an early start a wise option. The timing of frog spawning has been getting earlier in recent years, due to climate change. Spawning usually follows the first two or three days when the night-time temperature is three or four degrees above freezing. This has been the second week in February in several recent years. In the North Pennines the date of spawning is later than in any other part of the British Isles, including the north of Scotland (Savage, 1961).
Some males will have hibernated under water in the breeding pond, others on land in crevices, rabbit holes etc. Females rarely hibernate underwater and usually arrive at the pond later than the males, in response to the chorus of croaking made by the males. They are often carrying a male in the mating position before they reach the water. The spawning animals tend to bunch together in one pond, even where there are several apparently suitable ponds close together. Each clump of spawn has one mother, and most of the eggs will be fertilised by the mating male, but at least some of the eggs may be fertilised by nearby males. Like most of our amphibians and reptiles, the males are sexually mature a year earlier than the females. Often the same pond is chosen each year. Typical spawning numbers in our region are 10 to 20 females and a larger number of males, which would be a “low” population by the Natural England Site of Special Scientific Interest (SSSI) criteria for amphibians. Fifty to 500 spawn clumps/females is a “Good” population. The largest of our spawning populations have about 500 females, which is the “Exceptional” rating in the SSSI criteria (Nature Conservancy Council, 1998). Most of the exceptional sites in our region occur in quarry ponds and disused mining reservoirs on the edges of moorland. There are few accurate counts for our large frog populations, though this would be an interesting area of research for someone.
Predation by carnivorous mammals and large birds can be quite high during the spawning period. Adult frogs, gathered together for spawning, are a good source of food for predators at the end of the winter, when finding food can be difficult. Many frogs are in poor condition after spawning and die shortly afterwards. The high mortality of both adults and tadpoles gives frogs an important place in the pond food chain.
The frogspawn is usually deposited in one place in the pond, each female’s contribution merging together into a large clump. A sunny spot in water 10 to 20 cm deep, so that the spawn sits on the bottom, is usually chosen. The eggs are laid with the “jelly” part highly condensed. It expands rapidly by absorbing water, to produce the familiar frogspawn. The transparent jelly protects the dark-coloured egg and embryo from predators and also provides insulation and a greenhouse warming effect. The spawn is several degrees warmer than the surrounding water, and both frogs and newts can be found sheltering underneath it on frosty nights. Frogs often spawn in sites which seem, to us, to be totally unsuitable and certain to dry out too quickly. This is the frog breeding strategy as such sites will probably fail, but if they succeed then they will be highly productive because the tadpoles will have few aquatic competitors and predators.
As the jelly dissolves and the young tadpoles emerge, predation by fish, dragonfly larvae, water beetle larvae and newts can be quite high. Once the tadpoles are mobile, they disperse and become elusive. They are dark brown with copper coloured spots, in contrast to toad tadpoles, which look plain black. They feed on plant material, such as algae, and small invertebrates, but dead amphibians and smaller, live tadpoles may also be eaten. Tadpoles take 10 to 15 weeks to develop and to leave the pond, during which time the 1,000-2,000 tadpoles from each mother frog are reduced by predators to perhaps 10 or 20 survivors. In dry summers, all of the year’s productivity may be lost if the pond dries out too soon. This has been a regular occurrence in the North East since around 2000, due to long periods of dry weather followed by occasional downpours, which could be a feature of climate change.
After the spring spawning some adults disperse up to one km from the pond, while others remain close by. They feed on land, mostly on warm, wet nights. They have many predators, so adult survival is less than 50% each year. A large frog population can be an important food source for some of the predator species.
The Common Frog is our most common and most widespread amphibian. They can be found breeding in moorland pools, woodland ponds and ponds in lowland agricultural areas, as well as disused quarries, reservoirs, ditches, small streams and garden ponds even in urban areas. Surveys in South Tyneside, Sunderland and Tees Valley have shown that a high proportion of garden ponds have amphibians and that these ponds are a significant part of the overall amphibian populations. Though frog numbers have declined considerably in the last century they have maintained their range and are probably still present in every kilometre square in the region, except for the Farne and Coquet Islands. The blank areas of the distribution map are unsurveyed areas, not areas where frogs are absent (Durkin, 2010A).
Frogs form the basic amphibian community. Often only frogs are present, and where any of the newt species are present, there are almost always frogs as well. Frogs are often the first colonists of new ponds, followed by toads if the pond is large enough, and then the newts. Frog tadpoles are often hatching just as newts return to ponds for breeding, and the tadpoles provide an easy source of food for newts, particularly while they are clumped together and not yet free swimming.
by John Durkin