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Frogs - an Introduction

Frog catching fly
For illustrations to accompany this article see Amphibia, an Introduction

The frogs are some of the few remaining members of the amphibia, a group which flourished 250 million years ago. Other present-day members of the group are the toads, newts and salamanders. The amphibia are adapted, in general, to moving, feeding and breathing on land as well as in fresh water. At certain times of their life history or at particular seasons, however, they show a dependence on, or preference for one or the other.

External features

The frog is poikilothermic i.e. its body temperature varies with that of its surroundings. It has a loose-fitting moist skin. Its eyes protrude in such a way that they are above water when the rest of the body is immersed. The eyes have movable lids but, in addition, the whole eyeball can be withdrawn farther into the head by muscles. This can be seen to happen sometimes when the frog is swallowing. Its nostrils are situated so that air can be breathed while the frog is swimming at the surface; they can also be closed. Behind the eyes are circular ear-drums. Sounds in the air or water set these thin membranes vibrating, the vibration being transmitted by a small bone to a sensory region which sends nervous impulses to the brain.

Locomotion

The frog's powerful hind legs are adapted for both swimming and leaping. The strong extensor muscles of the thigh contract, extending the limb and thrusting the foot against the ground or against the water. The thrust is transmitted through the body of the frog by the pelvic girdle and the spine so that the whole animal is pushed forward. In the water the webbed hind feet provide a greater surface area for pushing backwards on the water. The smaller fore-limbs help to steer when the frog is swimming and absorb the shock of landing after a jump on land. On moving from water to land or over rough ground the frog will crawl rather than leap.

Breathing

The frog's skin is smooth and moist, fairly thin, and well supplied with blood vessels which branch into a fine network of thin-walled capillaries. Oxygen from the air or water, dissolves in the film of moisture over the skin, diffuses through the skin, through the walls of the blood capillaries and into the blood. Here it combines with the red pigment, haemoglobin, and is carried away in the circulation, back to the heart and then round the rest of the body. Excess carbon dioxide is eliminated from the blood in a similar way, diffusing out of the blood vessels, through the skin and into the atmosphere. The skin is effective for breathing on land or in water and is in use continuously.

When the frog is inactive the skin absorbs enough oxygen to meet its needs. During and after activity it may breathe air into its lungs by gulping movements of the floor of its mouth. The lungs can be used only when the frog is on land or swimming at the surface. The nostrils have valves which prevent the entry of water and control the flow of air into the lungs. Unlike the mammals, amphibia and reptiles do not make regular and rhythmic breathing movements but gulp air into their lungs spasmodically as the need arises. Air is forced into the lungs by raising the floor of the mouth. The lungs lie in the body cavity and, unlike those of mammals, are not separated from the other organs by a diaphragm. The lungs can be inflated to many times their relaxed size, so apparently inflating the entire frog. The moist lining of the large mouth is also a respiratory surface. Like the skin, it is in constant use, except when submerged, but the movements of the mouth-floor can be used to exchange the air in it.

Feeding

Adult frogs are carnivorous, feeding on worms, beetles, flies and other insects. Worms and beetles may simply be picked up by the mouth but flying insects can be caught on the wing. On occasions the frog will leap towards the insect and trap it in its wide, gaping mouth; on other occasions its tongue is used. The tongue is attached to the front of the mouth and can be rapidly extended by muscles. It is shot out in a half circle, and the insect is trapped by the sticky saliva covering the surface. Insects can be picked off the ground or vegetation in a similar way. The prey is swallowed whole but there are rows of tiny, closely set teeth in the upper jaw and in the roof of the mouth which prevent the prey escaping. In swallowing, the eyes are often pulled farther into the head and press down on the prey.

Skin and colour

The predominant colours in the common frog are green, yellow, brown and black, with whitish areas on the underside. The mottled patterning of these colours is thought to be of protective value in camouflaging the animal in its natural surroundings. The frog's colour can change to some extent, expansion or contraction of pigment cells in the skin making the frog darker or lighter. Such tones may correspond more closely to the frog's background in different circumstances and help to conceal it, but in many amphibia, temperature and humidity play a part in causing colour changes. The colour change operates through the sense organs and brain. A hormone from the pituitary gland circulates in the blood stream and has an effect on the pigment in certain cells.

In the skin are mucous glands which make the slimy fluid that covers the body. The sliminess makes the frog difficult to catch and keeps the skin moist. In the toad, a special group of glands behind the eyes produces an unpleasant poisonous substance that may serve as protection against enemies.

Life Cycle

(a) Fertilisation

Nearly all amphibians must return to water to breed although some tropical species make a "pond" in a rolled up leaf or hollow tree, the "pond" being derived from liquefaction of the jelly round the eggs. In one species the female places the fertilised eggs in pouches on her back where they develop through all the tadpole stages into tiny but fully formed frogs.

In this country in Spring, usually during March, male and female frogs move out of their winter quarters to the nearest pond. The females are usually larger, their bodies swollen with mature eggs, while the males have developed black, horny pads on their thumbs. These pads enable the male to grip the female behind the fore-limbs, and in this way the male may be carried about on the female's back, mostly in the water, for several days.

When the female lays the eggs, the male produces a seminal fluid containing sperms. This pours on to the eggs as they leave the female's body, and the sperms fertilize the eggs. Fertilization occurs when the nuclei of the sperm and egg meet and join together or fuse.

The eggs or sperms leave the body through an opening, the cloaca, just above the region where the hind legs join the body. Since the jelly, or albumen, round the eggs swells on contact with the water, fertilization would be impossible unless carried out at the moment the eggs leave the female's body. Thus, although fertilization is external, the pairing of frogs ensures that this happens.

(b) Development

1. The jelly round the eggs in the familiar frog-spawn has several advantages. It sticks them together and prevents their being swept away or eaten. It protects them from mechanical injury, from drying up and, probably, from attacks by fungi and bacteria.

2. The egg itself is a small sphere of semi-liquid cytoplasm in a tough, black egg membrane. There is a nucleus, and the lower cytoplasm contains yolky granules that are the only food supply for the first weeks of development. Sufficient oxygen must be able to diffuse through the jelly and the egg to allow the vital processes to go on.

3. Shortly after fertilization, the time depending partly on the temperature, the nucleus of the egg divides into two smaller nuclei which separate. The cytoplasm then divides to include each nucleus in a separate unit of cytoplasm so that there now appear two smaller cells, each with a nucleus.

4. A similar division takes place again in each cell but at right angles to the first division, making four smaller, roughly equal cells.

5. A third division takes place in the four cells, this time at right angles to the other two, round the "equator", forming eight cells of which the lower four are slightly larger than the upper four.

6. The cells continue to divide again and again until the "egg" becomes a hollow ball of tiny cells. The cells are very numerous and too small to be seen even with a hand lens. There is little increase in size during this division or cleavage but a great increase in the number of cells and nuclei. The egg has become an embryo, but to a casual observer it is still a spherical black ball and there is little evidence of the vigorous activity that has been going on.

7. All this happens in the first day or two. Later the sphere begins to elongate and develop a distinct head and tail. Meanwhile the cells are being organized internally to form the structures and organs of the tadpole. The energy and raw materials for this process come from the yolk.

8. After about ten days the jelly immediately round the tadpole liquefies and the tadpole can be seen moving about inside. The liquefaction also makes it easier for the tadpole to wriggle out of the jelly and into the water. At this stage its mouth has not yet opened and it is still digesting and using the remains of the yolk in its intestine. It clings to waterweed or to the surface of its jelly by its mucous glands which produce a sticky secretion. The tadpole is quite black, and the rudimentary external gills are visible, but it breathes through its skin at this stage.

Although the tadpoles are merely attached to, and not feeding on the waterweed, a good deal of spasmodic wriggling takes place in the clusters of tadpoles.

9. In two or three days the mouth has opened and the tadpoles can scrape the coatings of microscopic plants and other deposits from the surface of pondweeds using a pair of horny jaws and frilly lips. The three pairs of external, branched gills have developed and the blood can be seen circulating in them under the low power of a microscope. These gill filaments are thin-walled and present a fairly large surface area to the water. Oxygen dissolved in the water passes through the filament walls and into the blood close to the surface.

10. After about three weeks the mucous glands ‘disappear’ and a distinct division into body and tail occurs, together with a rapid increase in size. Internal gills are formed, opening to the outside by a single hole, the spiracle, on the left side. The fold of skin enclosing the space outside the gills is called the operculum.

By now the external gills have shrivelled and been reabsorbed into the body. In breathing, water is taken in through the mouth, passed over the internal gills, through the gill slits into the gill chamber formed by the operculum and, finally, out through the spiracle. As the water passes over the gill filaments, dissolved oxygen diffuses into the blood.

11. The tail elongates and develops a broad transparent web along its dorsal and ventral surfaces. Vigorous wriggling movements of the body and tail propel the tadpole through the water in a similar way to a fish but with less speed and precision.

A long, coiled intestine has developed and can be seen through the skin of the abdomen. The long intestine is adapted to the digestion of an exclusively vegetable diet. Eyes and nostrils are easily seen at this time. In this stage the tadpole grows considerably in size with little pronounced change in form for two or three weeks.

12. At two months from hatching, the tadpole comes to the surface frequently to gulp air into its lungs, which have begun to form. The hind-limb buds near the junction of the body and tail begin to grow and develop into perfect legs. The front legs also grow but do not yet appear because they are covered by the operculum; nevertheless they can be seen bulging beneath the skin in this region. The hind limbs are not yet used for locomotion but hang limply by the side of the body while the fish-like wriggling movements take place. The diet changes from vegetation, and the tadpoles nibble preferentially at dead animals or raw meat, at least in the aquarium, and associated with this is the shortening of the intestine and, later, the narrowing of the abdominal region.

13. Metamorphosis

At about three months, the front legs break through the operculum, the left leg appearing first by pushing through the spiracle while the right has to rupture the operculum. The tail shortens, being internally digested and absorbed, so providing a source of nutriment for the tadpole which has stopped feeding. The skin is shed, taking with it the larval lips and horny jaws, leaving a much wider mouth. Finally, the young frog climbs out of the pond on to the land, still with a tail stump, but using its legs for jumping and swimming. These changes take place within about four weeks. The young frogs remain in the damp vegetation and long grass in the vicinity of the pond, catching and eating small insects. In four years the frog will be old enough to breed.

The times given for development are only approximate since the temperature of the pond-water can alter the rate of metamorphosis from days to weeks and vice versa. It is more reliable to refer to the phase of development, e.g. hind-leg stage or external-gill stage, rather than age in days.

Habitat

In early spring, during the breeding season, frogs-spend their time in ponds and lakes with a steady flow of water. They are not usually found in swiftly running water. After egg-laying they are more likely to be encountered in damp vegetation than in water. They are unlikely to be found in any dry situation where their skins could lose water and dry up and so seriously impair their breathing. In winter they hibernate in the sense that they are dormant and do not feed. They lie up in the mud at the bottom of ponds, in damp moss or holes in the ground and their eyes, mouth and nostrils are closed.

For illustrations to accompany this article see Characteristics of Amphibia, an Introduction

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