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# 20.2: Reproduction in Mammals

Difficulty Level: At Grade Created by: CK-12

## Lesson 20.2: True or False

Name___________________ Class______________ Date________

Write true if the statement is true or false if the statement is false.

_____ 1. Most mammals are viviparous.

_____ 2. Mammals that are viviparous are called therian mammals.

_____ 3. There are mammals that lay eggs instead of giving birth to an infant or embryo.

_____ 4. The vagina is a pouch-like, muscular organ where the baby develops.

_____ 5. Therian mammals are divided into three groups: placental mammals, monotreme mammals, and marsupial mammals.

_____ 6. The uterus sustains the fetus while it grows inside the mother’s placenta.

_____ 7. The placenta allows the exchange of gases, nutrients, and other substances between the fetus and mother.

_____ 8. Kangaroo and koala are marsupials.

_____ 9. The marsupial embryo is nourished inside the placenta with food from a yolk sac instead of through the uterus.

_____ 10. Because the mother produces a placenta, a fetus can become large and mature before birth.

_____ 11. Marsupials live mainly in Australia.

_____ 12. Therian females have reproductive structures that are not found in other vertebrates.

_____ 13. The only living monotreme specie is the platypus.

_____ 14. Female monotremes are like reptiles and birds, with a cloaca with one opening.

_____ 15. Only five living species of mammals are therian mammals.

Name___________________ Class______________ Date________

Read these passages from the text and answer the questions that follow.

Marsupials

Marsupials have a different way of reproducing that reduces the mother’s risks. A marsupial is a therian mammal in which the embryo is born at an early, immature stage. The embryo completes its development outside the mother’s body in a pouch on her belly. Only a minority of therian mammals are marsupials. They live mainly in Australia. Examples of marsupials are pictured below.

Marsupials. Marsupials include the kangaroo, koala, and opossum. (From left to right, images courtesy of Fir0002/Flagstaffotos and under GNU-FDL 1.2, koala courtesy of David Iliff and under the Creative Commons license CC-BY-SA 3.0, and courtesy of Drcyrus and under the Creative Commons license CC-BY-SA 2.1 Australia. Composite created by CK-12 Foundation.)

The Marsupial Embryo

The marsupial embryo is nourished inside the uterus with food from a yolk sac instead of through a placenta. The yolk sac stores enough food for the short period of time the embryo remains in the uterus. After the embryo is born, it moves into the mother’s pouch, where it clings to a nipple. It remains inside the pouch for several months while it continues to grow and develop. Even after the offspring is big enough to leave the pouch, it may often return to the pouch for warmth and nourishment. Eventually, the offspring is mature enough to remain outside the pouch on its own.

Pros and Cons of Marsupial Reproduction

In marsupials, the short period of development within the mother’s uterus reduces the risk of her immune system attacking the embryo. In addition, the marsupial mother doesn’t have to eat extra food or carry a large fetus inside her. The risks of giving birth to a large fetus are also avoided. Another pro is that the mother can expel the embryo from her pouch if she is pursued by a predator or if food is scarce. On the other hand, a newborn marsupial is tiny and fragile. Therefore, it may be less likely to survive than a newborn placental mammal.

Questions

1. What is a marsupial? Give an example.

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2. What is unique about the marsupial embryo?

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3. How is the marsupial embryo nourished?

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4. Describe an advantage of marsupial development.

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5. Describe a disadvantage of marsupial development.

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## Lesson 20.2: Multiple Choice

Name___________________ Class______________ Date________

Circle the letter of the correct choice.

1. Therian mammals
1. are viviparous.
2. have young that are born live.
3. have young that are born either as relatively large, well-developed fetuses or as tiny, immature embryos.
4. all of the above
2. All female mammals have
1. ovaries, which are the organs that produce eggs.
2. a uterus, which is a tubular passageway through which the embryo or fetus leaves the mother’s body during birth.
3. a vagina, a pouch-like, muscular organ where the embryo or fetus develops until birth.
4. all of the above.
3. Which statement is true of a placenta?
1. The placenta passes oxygen, nutrients, and other useful substances from the fetus to the mother.
2. The placenta passes oxygen, nutrients, and other useful substances from the mother to the fetus.
3. The placenta mixes blood from the mother and fetus together.
4. The placenta protects the mother from being attacked by the fetal immune system.
4. Advantages to placental reproduction include
1. reduced mobility of the mother as the baby grows.
2. the ability to abandon the baby to save the mother's life if necessary.
3. a long period of fetal growth, allowing the fetus to become large and mature before birth.
4. all of the above.
5. Marsupials include
1. the kangaroo.
2. the platypus.
3. humans.
4. all whales.
6. Among mammals, female monotremes are unique in that they
1. have a pouch where the fetus completes development.
2. have a cloaca with one opening.
3. “sweat” milk from a patch on their mammary glands.
4. live mainly in Australia.
7. Female monotremes
1. lay eggs.
2. have a placenta.
3. lack a vagina but have a uterus.
4. lay eggs and have a placenta.
8. An advantage to marsupial reproduction is that
1. the marsupial mother has to eat extra food, and marsupials love to eat.
2. there is a short period of development within the mother’s uterus.
3. a newborn marsupial is small, making delivery and development easy on the mother.
4. due to their size, newborn marsupials have a very high survival rate.

## Lesson 20.2: Vocabulary I

Name___________________ Class______________ Date________

Match the vocabulary word with the proper definition.

Definitions

_____ 1. viviparous mammals

_____ 2. the organ that produces eggs

_____ 3. opening that is used to excrete wastes as well as lay eggs

_____ 4. a tubular passageway through which the embryo or fetus leaves the mother’s body during birth

_____ 5. a pouch-like, muscular organ where the fetus develops

_____ 6. mammals that reproduce by laying eggs

_____ 7. a therian mammal in which the embryo is born at an early, immature stage

_____ 8. therian mammals in which a placenta develops during pregnancy

_____ 9. sustains the fetus while it grows inside the mother’s uterus

Terms

a. cloaca

b. marsupial

c. monotreme

d. ovary

e. placenta

f. placental mammal

g. therian mammal

h. uterus

i. vagina

## Lesson 20.2: Vocabulary II

Name___________________ Class______________ Date________

Fill in the blank with the appropriate term.

1. Therian mammals are divided into two groups: ____________ mammals and marsupial mammals.

2. A placenta sustains the ____________ while it grows inside the mother’s uterus.

3. The placenta passes ____________, nutrients, and other useful substances from the mother to the fetus.

4. A ____________ is a therian mammal in which the embryo is born at an early, immature stage.

5. ____________ are mammals that reproduce by laying eggs.

6. The only living monotreme species are the ____________ and echidnas.

7. Female monotremes have a ____________ with only one opening.

8. The ____________ embryo is nourished inside the uterus with food from a yolk sac.

9. ____________ mammals give birth to relatively large and mature infants.

10. Female therian mammals have an ____________ where the embryo or fetus develops.

11. All female mammals have ovaries, the organs that produce ____________.

12. ____________ are born either as relatively large, well-developed fetuses or as tiny, immature embryos.

## Lesson 20.2: Critical Writing

Name___________________ Class______________ Date________

Thoroughly answer the question below. Use appropriate academic vocabulary and clear and complete sentences.

Explain how marsupials reproduce.

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