The Common frog is a smooth skinned anuran with adult females usually reaching up to 80mm in length; males are slightly smaller than this.
The snout is blunt and the iris of the eye is golden. There is a dorso-lateral ridge of skin beginning behind the eye and running along the length of the body on either side. The back legs are long and powerful and the long toes of the back feet are strongly webbed.
The colouration is extremely variable and can change with temperature or lighting conditions. The dorsal ground colour of most individuals is some shade of grey or brown which is overlaid with a pattern of dark spots and blotches. The ground colour together with both the pattern and the intensity of the dark blotches vary between individuals and it is not uncommon to find specimens with yellow, golden or red ground colours. Additionally, specimens may lack dark spots and markings altogether or they may be so heavily spotted that they appear almost entirely black. The ventral surface is dirty white.
Characteristic markings which are found in the majority of individuals and which may assist in identification include a pattern of dark bands along the back legs and a dark patch between the eye and the shoulder, forming a mask across the tympanum or eardrum.
The frog’s year begins in early spring when they emerge from hibernation and immediately make their way to their breeding sites to spawn.
The date on which frogs begin to spawn has always been a topic that generates considerable excitement, as spawning is widely viewed as a harbinger of spring, and first spawning dates can vary widely from one location to another in response to significant local influences. The earliest record of spawn in Somerset was collected from Exmoor on the 8th of January, although the average date for records of spawn across the county is the 7th of March.
Frogs do not have very specific requirements when it comes to choosing a pond in which to spawn and they do not exhibit the fidelity to a particular pond that is found in some other amphibian species. In fact frogs will spawn in a wide diversity of water bodies from large lakes to the smallest of ponds or ditches, although they rarely breed in ponds occupied by great crested newts which are efficient predators on frog spawn and tadpoles. It is thought that frogs may be able to follow scents emitted by algae, enabling them to identify water bodies that contain the necessary algae on which the tadpoles will feed.
The males arrive at the breeding pond a few days before the females and it is at this time that frogs become very conspicuous. The pond can become very active with large numbers of frogs congregating and calling in a low pitched growl.
As the females arrive competition to reproduce becomes very intense and frogs become oblivious to observers and can be easily approached. As soon as a female is spotted, a male will climb onto her back wrapping his stout muscular front legs around her body underneath her armpits, in a position known as amplexus. At this time of year the skin on the flanks of the female frog takes on a rough, granulated texture, while the males develop a dark rough callous on each thumb known as a nuptial pad. The effect of these developments is to assist the male in maintaining a grip on the female. A good grip is essential as males will compete fiercely for females, each attempting to dislodge one another. These amplexus disputes can result in large balls of frogs jostling and clinging to a female and on occasion may lead to the drowning of the female if she is prevented from reaching the surface. So excitable can the males become that they frequently grab anything within reach, and stories abound of frogs grasping fish around the head preventing the flow of water through the gills, resulting in the death of the fish. While this may occasionally occur, its prevalence is probably widely exaggerated; yet ornamental fish-keepers sometimes regard the frog as a threat to their livestock during the breeding season!
Fertilisation of the eggs is external and the object of amplexus is to align the cloacae of the two animals so that the male can eject his sperm over the eggs, which may number up to 1500, as the female produces them.
The position in the pond where this occurs is carefully chosen. A spot in shallow water and full sun is favoured as it provides the warm conditions necessary for egg development.
The spawn is laid in a clump, unlike the strings of spawn produced by a toad, and is surrounded by a gelatinous substance that rapidly absorbs water to swell into the familiar jelly matrix that surrounds each egg and probably buffers the eggs from any large temperature swings.
Once spawning is complete the frogs leave the pond.
The common frog is sometimes referred to as the grass frog, which reflects their behaviour rather well. The spawning season is brief and most of their time is spent on land among grass and herbage. With their permeable skin they need to remain in damp habitats to avoid desiccation, but nevertheless during the summer months they can be found long distances from any ponds. Throughout the summer they may periodically visit ponds, particularly in dry weather, but breeding season aside, they lead a predominantly terrestrial existence.
Once laid, the spawn begins to develop. The rate at which development progresses is temperature dependent, but usually the eggs start to hatch within a couple of weeks. On occasion the pond may freeze, after the spawn has been deposited. The spawn can tolerate this so long as it remains below the ice and the eggs themselves avoid freezing. Any eggs exposed at the surface that succumb to freezing will cease development and die. However, the principal effect of reduced temperature on spawn is simply to slow development down.
On hatching the tadpoles are largely immobile. They spend their time aggregating around the remaining jelly for several days while they absorb any remaining yolk, before dispersing around the pond. Unlike toad tadpoles those of frogs are secretive and inconspicuous, spending their time at the bottom of the pond grazing on algae and trying to avoid predators. The rate of growth and development, while dependent on water temperature, can be rapid. By the fifth week the hind legs appear as small buds while simultaneously, beneath the skin, the front legs begin to form and the lungs develop. As the lungs develop and the gills are reabsorbed the tadpoles begin to make regular visits to the surface for a gulp of air. The legs will be fully developed by the time the tadpole is about three months old at which time the front legs, which have been developing beneath the skin, will rupture through. The larvae now more closely resemble miniature frogs although the tail is still present and will be reabsorbed over the next few days, at which point the ‘metamorphs’ will leave the pond.
Tadpole survivorship is low; in his 1961 monograph of the common frog, R. Maxwell Savage estimated that only about 1% of frog tadpoles survive to metamorphosis. During dry springs and summers, many of the ponds in which frogs have spawned will dry out before the tadpoles have reached metamorphosis, killing them all. Additionally, predation takes a large toll. Unlike toad tadpoles, the tadpoles of frogs lack any toxins, allowing a wide range of creatures to prey upon them, including aquatic birds, insects, fish and newts. There is also strong competition between the tadpoles in a pond for the best feeding opportunities and all the tadpoles in a cohort do not grow at a uniform rate. Those that grow quickly dominate the best feeding opportunities at the expense of those that grow slowly. This perpetuates the inequality and allows the rapid growers to metamorphose earlier, consequently increasing their chances of survival.
At metamorphosis the froglets are about 10 – 15 mm long, but growth is rapid and after living in the undergrowth for two or three years they will mature and be ready to breed themselves.
On the 5th of March 2004 a bizarre story appeared in the national media that catapulted Somerset’s frogs into the spotlight. It was reported that a three headed frog had been caught in Weston-super-Mare. Photographs and a short video showed the ‘creature’ that had been caught in the garden of the Green Umbrella pre-school and nursery.
To anyone familiar with frogs the pictures and the footage showed quite clearly that it was two male frogs in an amplexus dispute over a single female, however, to those unfamiliar with frogs it would perhaps have looked rather alarming. The story escalated after a presenter at the BBC Natural History Unit in Bristol commented that he had never come across such an unusual animal and that it could result from environmental pollution.
Needless to say the animal(s) escaped before the press crews arrived on the scene, but speculation ensued about whether the ‘mutant’ was the result of leaks from the nearby Hinckley Point atomic power station. Somebody somewhere could not have imagined how successful their hoax would become!
Common frogs are widespread across Britain and Europe, from Scandinavia in the north, south as far as Italy and northern Greece. They are absent from the Iberian Peninsula in the west, but from the Pyrenees they extend eastward throughout Europe across central Asia, all the way to Japan.
Locally, they are common and widespread throughout Somerset and the unitary authorities. Their ability to tolerate a wide range of environmental conditions means they can be found at all altitudes and in most habitats. They can breed either in acidic or alkaline ponds and have been found to be present in the majority of ponds that have been surveyed in the county, including the fire ponds and mire washes on the high moorlands and heaths. However, they exist at greater densities in the damper lowlands, in ditches, rhynes and farm ponds, being particularly prevalent in garden ponds. In fact, during the second half of the twentieth century, when ponds were disappearing from the countryside and frogs were suffering a serious decline in abundance, garden ponds provided something of a lifeline for the species. Thankfully, that situation has now stabilised and in Somerset frogs remain ubiquitous.
Historically, frogs may have experienced several fluctuations in abundance and distribution in and around the county. Writing in 1887 Charbonnier suggests that frogs were not as common in the Bristol area as they had been previously. Similarly, in 1900 a correspondent to ‘Notes and Queries for Somerset and Dorset’ mentions the mysterious absence of frogs in the vicinity of Weston-super-Mare, despite plenty of ponds and rhynes providing ample habitat. Residents seemed to believe that they became locally extinct during a prolonged drought in 1844, suggesting they may have been absent from the area for over half a century. If this was really the case, rather than simply a perception, it is no longer so and frogs are abundant throughout the North Somerset levels, including in and around Weston-super-Mare. Intriguingly, Brent Knoll, the characteristic landmark on the clay-belt, just south of Weston-super-Mare, was known to the Romans as the Isle of Frogs!