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20.2: Reproduction in Mammals

Difficulty Level: At Grade Created by: CK-12

Lesson Objectives

  • Describe female reproductive structures of therian mammals.
  • Outline reproduction in placental mammals.
  • Explain how marsupials reproduce.
  • Describe monotreme reproduction.


  • marsupial
  • monotreme
  • placenta
  • placental mammal
  • therian mammal
  • uterus (plural, uteri)
  • vagina


Most mammals are viviparous. Their young are born live. They are born either as relatively large, well-developed fetuses or as tiny, immature embryos. Mammals that are viviparous are called therian mammals. Only a few mammals lay eggs instead of giving birth to an infant or embryo.

Therian Mammals

Like other female vertebrates, all female mammals have ovaries. These are the organs that produce eggs (see Figure below). Therian mammals also have two additional female reproductive structures that are not found in other vertebrates. They are the uterus and vagina.

  • The uterus (plural, uteri) is a pouch-like, muscular organ. The embryo or fetus develops inside the uterus. Muscular contractions of the uterus push the offspring out during birth.
  • The vagina is a tubular passageway through which the embryo or fetus leaves the mother’s body during birth. The vagina is also where the male deposits sperm during mating.

Female Reproductive System of a Therian Mammal (Human). The female reproductive system of all therian mammals is similar to that of humans.

Therian mammals are divided into two groups: placental mammals and marsupial mammals. Each group has a somewhat different reproductive strategy.

Placental Mammals

Placental mammals are therian mammals in which a placenta develops during pregnancy. The placenta sustains the fetus while it grows inside the mother’s uterus. Placental mammals give birth to relatively large and mature infants. Most mammals are placental mammals.

The Placenta

The placenta is a spongy structure. It consists of membranes and blood vessels from both mother and embryo (see Figure below). The placenta passes oxygen, nutrients, and other useful substances from the mother to the fetus. It also passes carbon dioxide and other wastes from the fetus to the mother. The placenta lets blood from the fetus and mother exchange substances without actually mixing. Thus, it protects the fetus from being attacked by the mother’s immune system as a “foreign parasite.”

Placenta of a Placental Mammal (Human). The placenta allows the exchange of gases, nutrients, and other substances between the fetus and mother.

Pros and Cons of Placental Reproduction

The placenta permits a long period of fetal growth in the uterus. As a result, the fetus can become large and mature before birth. This increases its chances of surviving.

On the other hand, supporting a growing fetus is very draining and risky for the mother. The mother has to eat more food to nourish the fetus. She also becomes heavier and less mobile as the fetus gets larger. As a result, she may be less able to escape from predators. Because the fetus is inside her, she can’t abandon it to save her own life if she is pursued or if food is scarce. Giving birth to a large infant is also risky. It may even result in the mother’s death.


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 in Figure below.

Marsupials. Marsupials include the kangaroo, koala, and opossum.

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 (Figure below). Eventually, the offspring is mature enough to remain outside the pouch on its own.

The marsupial embryo finishes development in the mother's pouch. Here, a joey is shown in the mother's pouch.

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.

KQED: The North American Marsupial: The Opossum

Most people think of Opossums as scary creatures. Is this because they look kind of funny, walk kind of funny, have beady eyes and sharp teeth, and can emit a very foul odor? Maybe. But what is so different about opossums is that they are the only marsupial in North America.

But opossums can be beneficial to humans. They use their sharp teeth to crush bone – which means that they are good getting rid of unwanted rodents in your neighborhood. They have excellent immune systems and they emit that terrible odor for protection. Learn more about opossums at http://www.kqed.org/quest/blog/2009/03/31/producers-notes-for-cool-critters-opossums/.


Only five living species of mammals are not therian mammals. They are called monotremes. Monotremes are mammals that reproduce by laying eggs. The only living monotreme species are the platypus and echidnas (see Figure below and Figure below). They are found solely in Australia and New Guinea (an island not far from Australia).

Platypus. The platypus is a monotreme, a mammal that reproduces by laying eggs.

Echidna. Like the platypus, the echnida is a monotreme. The only living monotreme species inhabit Australia and New Guinea.

Eggs and Lactation in Monotremes

Female monotremes lack a uterus and vagina. Instead, they have a cloaca with one opening, like the cloacas of reptiles and birds. The opening is used to excrete wastes as well as lay eggs.

Monotreme eggs have a leathery shell, like the eggs of reptiles. The eggs are retained inside the mother’s body for at least a couple of weeks. During that time, the mother provides the eggs with nutrients. Platypus females lay their eggs in a burrow. Echidna females have a pouch in which they store their eggs. Female monotremes have mammary glands but lack nipples. Instead, they “sweat” milk from a patch on their belly.

Pros and Cons of Monotreme Reproduction

The mother’s risks are less in monotremes than in therian mammals. The mother doesn’t need to eat more or put herself at risk by carrying and delivering a fetus or an embryo. On the other hand, externally laid eggs are more difficult to protect than an embryo in a pouch or a fetus in a uterus. Therefore, monotreme offspring may be less likely to survive than the offspring of therian mammals.

Lesson Summary

  • Therian mammals are viviparous. They give birth to an embryo or infant rather than laying eggs. The female reproductive system of a therian mammal includes a uterus and a vagina. There are two groups of therian mammals: placental mammals and marsupials.
  • Placental mammals give birth to a relatively large and mature fetus. This is possible because they have a placenta to nourish the fetus and protect it from the mother’s immune system. This allows for a long period of growth and development before birth. Because the offspring is relatively large and mature at birth, it has a good chance of surviving. However, carrying and giving birth to a large fetus is risky for the mother. It also requires her to eat more food.
  • Marsupials give birth to a tiny, immature embryo. The embryo then continues to grow and develop in a pouch on the mother’s belly. This is less risky for the mother. However, the embryo is fragile, so it may be less likely to survive than the fetus of a placental mammal.
  • Monotremes reproduce by laying eggs. They have a cloaca instead of a uterus and vagina. The eggs pass through the opening of the cloaca. This form of reproduction is the least risky for the mother. However, eggs are harder to protect than is an embryo or a fetus in a pouch or uterus. Therefore, monotreme offspring may have a lower chance of surviving than the offspring of therian mammals.

Lesson Review Questions


1. What are the functions of the uterus and vagina in therian mammals?

2. What is the placenta? What is its role?

3. Where does a marsupial embryo develop? How is it nourished?

4. Describe eggs and egg laying in monotremes.

5. How does lactation differ in monotremes and therian mammals?

Apply Concepts

6. Create a chart that you could use to explain to a younger student the different ways that mammals reproduce.

Think Critically

7. Compare and contrast the advantages and disadvantages of the three forms of reproduction in mammals.

8. Placental mammals greatly outnumber the other two groups of mammals. Infer why placental mammals have been so successful.

Points to Consider

Monotremes are less similar to therian mammals than the two groups of therian mammals are to each other.

  • How might the different groups of mammals have evolved?
  • Which group of mammals do you think evolved first?

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