The life cycle of a frog
About three hundred million years ago a very great change took place in the animal kingdom. Up until that time all the animals lived in water. There were many kinds of water snails, lobsters and fish. Some fish were unlike the rest. Instead of graceful flowing fins they had stumpy fins. These fins were so strong that the fish used them to climb out of the water. In time these fish developed into animals that spent their young lives in water and their adult lives on land. Animals with this kind of life cycle are still present on the Earth today. We call them amphibians. There are about four thousand different kinds of amphibian alive today. A common example is the frog.

Every year young frogs re-enact the great change that took place millions of years ago. They begin life in water then change into an animal that can live on land. Scientists have a word for the process in which an animal, like the frog, completely changes its form as it grows. This word is metamorphosis. Some other animals undergo metamorphosis in their life cycle. The butterfly and beetle are examples.

A frog begins life not as a green, croaking, slimy skinned hopper but as a black, fish-like animal about the size of a tea leaf. This stage in the frog's life is called the tadpole. It forms in a jelly-covered egg. The jelly protects the tadpole from predators but when the tadpole is ready to hatch the jelly turns watery. This lets the tadpole escape from the egg. The newly hatched tadpole is very weak but has a pair of suckers to hold it in place as it feeds. The first tadpole food is the jelly around the egg. Later, as the tadpole builds up its strength, it moves on to feed on algae that cover water plants and rocks. The tadpole does not just have a fish-like shape. It has gills to breathe oxygen in the water ? just as fish do. The tadpole has two sets of gills. The first ones grow out from the side of its head like tiny shrubs. They are called external gills. As the tadpole grows, it becomes more active and the external gills are in greater danger of being damaged as the tadpole moves about. At this time the tadpole develops gills inside its body. They are called internal gills. The body protects them from damage as the tadpole swims around rocks and water plants. The internal gills last about five weeks then a pair of lungs replaces them. The tadpole can no longer remain submerged in the water all the time now. It must swim to the surface to gulp in air to breathe.

Although the tadpole has a streamlined body like a fish to help it move through the water, it does not have all the fins that a fish has. It has a tail fin to give it power to move forward but it does not have fins on its sides and back. These fins help a fish swim in a straight line through the water. Without these fins the tadpole's swimming is clumsy and it tends to thrash about in the water as it moves along.

A fish has two pairs of fins on its sides. One pair is at the front and the other pair is at the back. The tadpole does not develop these side fins. It develops legs instead. At about eight weeks old the frog develops a pair of back legs. About two weeks later the left front leg grows out and about a week later the right front leg grows.

As the last of the legs appears, a strange thing happens. The frog stops feeding and absorbs its tail into its body. The frog uses the materials and energy stored in its tail to grow a wider mouth and a new frog-like skin. When these changes are complete the tiny frog, or froglet, makes its first steps on land just like the fish with stumpy limbs did, millions of years ago.

Why do frogs produce so many tadpoles?
They produce a large number so that there is a chance that some may complete their life cycle and breed. Tadpoles do not really have any defences. They are black which can help them hide near mud but when they have to swim to the water surface to breathe, the colour makes them easy for predators to see. They are also poor swimmers so cannot move quickly away if a predator attacks them. A tadpole has many predators such as dragonfly nymphs, fish and water birds.

Can you tell frog spawn from the spawn of toads and newts?
Yes, you can. Toads and newts are also members of the amphibian group. Frog spawn is in the form of a clump of eggs. Toad spawn forms strings and newts lay their eggs singly on the leaves of water plants. They bend the leaves so predators cannot see the eggs.

Are there any other amphibians besides frogs, toads and newts?
Yes, there are. Some amphibians are like large newts and are called salamanders. A well-known salamander is the fire salamander. It has a black body with yellow spots. These colours are warning colours because the salamander produces a poison from the skin on its head if a predator attacks it. It is called a fire salamander because it hibernates in logs. Sometimes these logs are gathered and put on a fire. When this happens, the salamanders wake up and climb out of the logs and move away from the fire. There are also some amphibians which look like snakes. They live in the moist soil of tropical rainforests. These amphibians are called caecilians.

Do all amphibians lay their eggs in ponds?
No. The male midwife toad carries the eggs on his back while the tadpoles develop inside them. When the tadpoles are ready to hatch the toad finds a pond and climbs into the water. The tadpoles then escape from their eggs and swim away. There are many kinds of frogs which live in rainforests. There are plants in the rainforest which grow on the branches of trees. They do not have roots to collect water but grow their leaves in the shape of a bowl to collect the rain. Small pools form in the leafy bowls and some frogs lay their eggs in them. The tadpoles live in the bowls until they change into frogs.