CBSE Class 4 Science Animals and Their Young Ones Notes Set 01

Why animals lay eggs or give birth

Every animal begins life in one of two ways. Some animals lay eggs, and the young ones hatch outside the mother's body. Others give birth to babies that have grown inside the mother's body. The way a young animal grows into an adult is called its life cycle.

Fish

A fish lays a very large number of eggs in water. The eggs are called spawn. They have no hard shell and are covered with a jelly-like layer. Many eggs are eaten by other animals, so only a few grow into baby fish. The young fish that hatches from an egg is called a fry. A fry slowly grows into an adult fish.

Life cycle of a fish: 1. Egg, 2. Fry, 3. Adult fish.

Frog and metamorphosis

A frog also lays its eggs in water, in a cluster called frog spawn. A tiny creature called a tadpole comes out of each egg. A tadpole looks very different from its parent. Over time it grows legs, loses its tail and becomes an adult frog. A big change in body shape from a young one to an adult is called metamorphosis.

Life cycle of a frog: 1. Egg, 2. Tadpole, 3. Adult frog.

Butterfly

The life cycle of a butterfly has four stages.

1. Egg: the butterfly lays eggs on leaves.

2. Larva: a worm-like caterpillar hatches and eats leaves to grow.

3. Pupa: the caterpillar forms a protective covering around itself and changes inside.

4. Adult: the butterfly comes out of the pupa.

Some young animals shed their old skin as they grow. This is called moulting.

Grasshopper and cockroach

These insects have three stages: egg, nymph and adult. A nymph hatches from the egg. It looks like a small copy of its parent but has no wings. The nymph moults several times and finally becomes an adult.

Reptiles

Snakes, lizards and crocodiles are reptiles. They lay eggs on land, even if they live near rivers. The warmth of the sun helps the eggs to develop. Reptiles rarely look after their young ones, and some eggs are eaten by other animals.

Mammals

Animals that give birth to babies and feed them with their own milk are called mammals. Cows, dogs, cats, elephants, lions and humans are mammals. The baby grows inside the mother's body. Most mammals have hair on the body and breathe through lungs. Dolphins and whales are mammals that live in water. They come to the surface to breathe air. The spiny anteater and the platypus lay eggs, but they are still mammals because they feed milk to their young ones.

Parenting

Looking after the young is called parenting. Mammals care for their babies for a long time. The mother feeds, cleans and warms the baby, protects it from enemies and teaches it to find food. Fish, frogs and reptiles usually lay eggs in a safe place and leave the young to manage on their own.

Key points

1. Fish and frogs lay eggs in water; reptiles lay eggs on land.

2. Metamorphosis is a complete change in body form, as in frog and butterfly.

3. Mammals give birth, feed milk and show strong parenting.

Worked examples

Example 1. Arrange the stages of a butterfly: pupa, egg, adult, larva. Answer: egg, larva, pupa, adult.

Example 2. Why do fish lay thousands of eggs? Answer: Many eggs are eaten by other animals, so laying many gives some chance that a few will grow into adult fish.

Example 3. Is a whale a fish? Answer: No. A whale gives birth, feeds its baby with milk, and breathes air through lungs, so it is a mammal.

Quick Check Questions