42 L5-Animal life cycles

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Oxford Read and Discover level 5 Animal Life Cycles by Rachel Bladon

Introduction Animals grow and change during their life. To produce young, some animals have babies, and others lay eggs. These young animals then grow up and produce young, too. This journey from being born to producing young is called a life cycle. Animal life cycles are amazing. How do male birds find a mate? How big is a baby kangaroo when it's born? How does a crocodile carry her babies? What does a tadpole grow into? Discover! Now read and discover more about some amazing animal life cycles!

Chapter 1 Life Cycles There are millions of different species of animal in the world. Some animals, like the blue whale, are very big. Others are so small that we almost can't see them. These animals all have very different life cycles. Animal Groups Scientists put animals into groups. The animals in each group are the same in many ways, and they often have very similar life cycles. The two biggest animal groups are invertebrates and vertebrates. Invertebrates are animals that have no backbone. More than 95% of the animal species in the world are invertebrates, and most of them are very small. There are many different groups of invertebrates. The biggest group is called arthropods. Insects, spiders, and animals like crabs that have a hard shell, are all arthropods. Animals that have a backbone are called vertebrates. Only a few percent of the animals in the world are vertebrates, and most of them are much bigger than invertebrates. Because of this, we see vertebrates more often than invertebrates. Scientists put vertebrates into five big groups they are called fish, amphibians, reptiles, birds, and mammals.

Staying Alive All animals need to find food to stay alive. Some animals are carnivores. This means that they eat other animals. Herbivores eat plants, and omnivores eat plants and animals. There are many dangers for animals. They have to keep safe from predators— animals that hunt and eat other animals. People hunt birds and other animals. People also cut down trees and build houses and roads on land. When they do this, animals lose their homes and their food. Many animals also become sick or die because of pollution of the land, oceans, rivers, and air. Life is very dangerous for animals. Many animals die before they are old enough to produce young. Discover! Different animals live for different amounts of time. A Galapagos tortoise can live for 150 years, but an adult mayfly usually lives for just a few hours. Breeding Most species of animal have males and females. Males produce sperm, and females produce eggs. Baby animals are made when sperm from the male joins together with an egg from the female. This is called fertilization. Some animals give birth to baby animals. Other animals, like reptiles and birds, lay eggs. Their babies hatch— they break the egg open and come out. Most reptiles and birds care for their babies. Most insects and fish do not care for their eggs or their babies. All mammals care for their babies and feed them milk. Some big mammals stay with their babies for many years.

Chapter 2 Insects Insects are small animals like butterflies, beetles, flies, bees, and ants. There are more than a million species of insect in the world. Many insects change in amazing ways during their life. Larvae and Nymphs Almost all insects start their life in an egg. The young insect grows inside the egg and then it hatches. Some baby insects, like bees, look very different from their parents. These baby insects are called larvae. Other baby insects, like locusts, look more like their parents. These baby insects are called nymphs. Larvae and nymphs are very small when they hatch. Growing Up A young insect eats a lot of food and grows very quickly. It has a hard cover, called an exoskeleton. When the exoskeleton is too small for the young insect, it breaks open and comes off. A new exoskeleton grows under the old one. This is called molting. Some insects grow wings that get bigger every time they molt. After the insect molts for the last time, it becomes an adult that can fly and produce young. This is called incomplete metamorphosis. Most insects change by complete metamorphosis. When the larva is ready to change into an adult, its exoskeleton comes off. Under the exoskeleton there's a pupa— it's like a closed case. Inside the pupa, an amazing change happens. The larva changes into an adult that looks completely different!