What are amphibians?
Amphibians are animals that spend part of their life in water and part on land. The word comes from two Greek words that mean 'both lives'.
Examples: frog, toad, newt, salamander and the legless caecilian.
- They have a backbone, so they are vertebrates.
- They are cold-blooded: the body is as warm as the air or water around them.
- Their skin is thin, bare and wet. No scales, no feathers, no hair.
- Their heart has three chambers.
Life in water: the egg and the tadpole
The female lays hundreds of soft eggs in water. The eggs have only jelly around them, so they would dry out in air. This is why amphibians must go back to water to breed.
The tadpole has a tail for swimming and gills for breathing. Its heart has two chambers and it eats water plants.
Metamorphosis: changing into a land animal
Metamorphosis means a big change of body shape. In a frog it takes a few weeks to months.
- Back legs grow.
- Front legs grow.
- The tail shrinks and its material is reused by the body.
- Gills disappear and lungs grow.
- The mouth gets wider and the animal begins to eat insects.
Life on land: breathing and skin
An adult frog breathes in three ways: through lungs, through the thin wet skin, and through the lining of its mouth. Oxygen can only pass through skin if the skin is wet. So frogs stay near water or in damp places.
In winter or in very hot summers, many frogs rest underground. Sleeping through cold is called hibernation, and through heat it is aestivation.
Try it
Use the slider in the 3D. Before you move it, guess: at which stage will the gills disappear? Then check. At home, look for frogspawn in a pond in the rainy season (do not touch it with dry or soapy hands) or draw the five stages in a row and label what changes.
Key formulas and definitions
- Amphibian = amphi (both) + bios (life)
- Life cycle: egg โ tadpole โ tadpole with legs โ young frog โ adult
- Young: gills + tail. Adult: lungs + skin + legs.
- Cold-blooded, 3-chambered heart, wet bare skin, soft eggs laid in water
Worked examples
1. Why must a frog lay its eggs in water?
A frog's egg has no hard shell, only a soft jelly layer. In air it would dry up, so the eggs need water.
2. Name two body parts a tadpole has that an adult frog does not.
A tail and gills. The tail is for swimming and the gills are for breathing in water.
3. A frog sits in a dry sunny place for many hours. What can go wrong and why?
Its skin dries. Gas exchange through skin needs a wet surface, so it would get less oxygen and could die.
Common mistakes
- Thinking toads are not amphibians. Toads are amphibians too; they just have drier, bumpy skin.
- Saying a tadpole is a fish. A tadpole breathes with gills like a fish but it grows into a four-legged animal with lungs.
- Thinking amphibians can live far from water. They need water or a damp place to keep skin wet and to breed.
- Calling the tail a part that falls off. In metamorphosis the tail is slowly absorbed into the body.