Unit 3 Lesson 4 How Do Animals Reproduce? Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 3 Lesson 4 How Do Animals Reproduce?

Copyright © Houghton Mifflin Harcourt Publishing Company


Life in Full Circle


• Like plants, animals have life cycles. Most animals reproduce sexually.

• During sexual reproduction, sperm from a male joins an egg from a female.

• The fertilized egg can then develop into a new animal.


Life in Full Circle


• In some animals, such as many kinds of fish, eggs are fertilized outside the female’s body.

• In other animals, such as birds, eggs are fertilized inside the female’s body.

• After the eggs are fertilized, birds lay the eggs.

• Bird parents protect the eggs until they have hatched.


Life in Full Circle


• After the young are born, they grow and change.

• Over time, newborns develop into youths. Youths continue to develop until they grow into adults.

• Adult animals made with one another to produce offspring.

• An animal’s life cycle ends when the animal dies.


Bringing Up Baby


• Some animals, such as turtles, are on their own as soon as they hatch from their eggs.

• Other animals, such as penguins, give their young a great deal of care.

• Penguins keep their young warm and fed until they are strong enough to take care of themselves.


Bringing Up Baby


• Animals such as deer, bears, rabbits, cats, and kangaroos take care of their young by feeding them milk.

• These animals care for their young for months or years until the young can live on their own.


Growing Up


• Just like other animals, humans go through stages of development. It takes many years for a human baby to grow into an adult.

• After a human egg is fertilized, it grows inside the mother. After nine months, the baby is born.

• When babies are born, they have no teeth. They are not able to walk on their own.

• Babies grow into toddlers. Toddlers learn how to walk and how to speak.


Growing Up


• As a child grows, they develop many physical and mental skills.

• During the teenage years, boys and girls start looking more like adults.

• During adulthood, people reach maturity.

• As an adult ages, they lose some of their physical abilities.


My, How You’ve Changed!


• In complete metamorphosis, an animal goes through four different stages in its life cycle.

• An egg hatches into a larva.

• A larva develops into a pupa.

• A pupa develops into an adult.




• A butterfly goes through complete metamorphosis.

• The larva of a butterlfy is called a caterpillar.

• The caterpillar grows into a pupa.

• Inside a chrysalis, the pupa becomes an adult butterfly.




• What are the stages in a butterfly’s life cycle?




• Some insects, such as dragonfiles and termites, go through a different series of changes.

• In incomplete metamorphosis, an animal goes through three different stages in its life cycle.

• First, the animal hatches from an egg as a nymph.

• Nymphs look like tiny adults without wings.




• As the nymph grows larger, it molts.

• Molting happens when an insect sheds its hard outer skeleton.

• After several moltings, the insect, which now has wings, reaches its adult stage.




• What are the stages in the life cycle of a grasshopper?


Saving the Sea Turtles


• Some kinds of animals, such as sea turtles, are endangered. There are not many of them left.

• Scientists study the life cycles of these endangered animals to try to save them and help them increase their numbers.

• To rear sea turtles, scientists collect the eggs.

• When the eggs hatch, the young turtles are cared for until they can be released into the wild.

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Unit 3 Lesson 4 How Do Animals Reproduce? Copyright © Houghton Mifflin Harcourt Publishing Company