The common frog (Rana temporaria) has a deceiving name as they aren’t nearly as common as they once were in the UK. Throughout the 20th century common frog populations were declining due to habitat loss and fragmentation. This combined with newly discovered rana-viruses has caused a population decline of the common frog by up-to 95% in some areas.
Adult male frogs measure upto 9cm from the tip of their snout to the bottom of their spine with females slightly larger, measuring upto 13cm. The hind limbs are long and powerful, leading to five spindly, webbed toes. The forearms are short and muscular with four toes. The inner most toes of males grow a dark, leathery growth throughout the spring. These are known as nuptial pads which assist the males in holding onto their mates throughout amplexus.
The nostrils and eyes are found on top of the head and these are often all that is above the water level throughout summer when the frogs are basking in shallow ponds and pools. Colour can vary from almost black/dark olive brown through pale greens and yellows to bright pink/red. Some individuals are one colour throughout but most have patches of darker colour down their backs and legs, this may create a banded pattern on the hind limbs. The underside is lighter throughout the sexes and is often a creamy yellow colour with less darker patches. The throat colour of common frog males varies throughout the year with those in breeding condition often demonstrating a washed out bluey-green throat colouring.
The adult frogs are mainly insectivorous and will use its protruding tongue to stick and retrieve any desired prey which includes worms, small slugs and beetles. Newly hatched tadpoles feed predominantly on aquatic algae and vegetation. They gradually become more carnivorous as they grow and may eat aquatic snails, leeches, insect larvae and even other tadpoles.
Common frogs are found throughout the UK including Ireland and the northern most areas of Scotland. The geographical range of this species extends throughout most of Europe into northern Spain, Norway and as far east as Russia.
The reproductive cycle of the Common frog begins in springtime. As the temperature and daylight hours increase, Adult frogs begin to migrate to their breeding pools. This often occurs over a period of weeks, unlike the concentrated migration of the Common toad.
Males are the first to reach the breeding pools and are often present in larger numbers throughout the breeding season. This is partly due to them being able to breed from two years (Compared to the females that aren’t sexually mature until their third year), and that they can mate multiple times in a single spring. Once they arrive at the breeding pools, the males begin to gather in shallow, warm areas and “Croak” to entice the females.
The females swell with eggs over the winter and upon reaching the breeding pools, can be swarmed by many males. The males attempt to use their strong fore-arms to grab hold of the females around the waist. This is assisted by special nuptial pads that develop on males thumbs and is known as amplexus. This can last for 3-4 days when the female eventually releases upto 2000 eggs.
The ball of eggs is roughly the size of a golf ball and the male must fertilise them before the protective jelly swells and forms what is commonly recognised as frogspawn. Depending upon water temperature the spawn will hatch between 2-4 weeks time. When the tadpoles hatch they measure 8-10mm. For the first 2-3 days they cling to the remaining spawn and consume what remains of the nutrients. The tadpoles then become free swimming and a layer of skin grows over their external gills. At first the tadpoles diet consists mainly of aquatic vegetation, algae and micro plankton. As they grow they become more carnivorous, eating aquatic invertebrates, snails, worms and even each other.
After 6 weeks the first tadpoles begin to grow back legs. These are held tucked in close to the tail and do not contribute to the tadpoles movement initially. Over the next several weeks the tadpoles begin to grow their front legs and start using their hind legs to assist their swimming. The tadpoles start to look like miniature frogs at this point. Their tails shrink to small stumps, their lungs become functional and their gills useless. Throughout summer the small froglets leave the breeding pools to run the gauntlet as terrestrial miniatures of their parents.
Threats and Impact
The Common frog does not have any significant negative impact upon other wildlife. Although they appear superficially similar to the Common toad, Natterjack toad and Pool frog, these species occupy different environmental niches. They can therefore live side by side with little impact upon one another.
In the modern world there are many threats to the common frog. These include: habitat destruction and fragmentation, traffic collisions and over recent years severe population declines caused by rapidly spreading, contagious diseases.