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12.29: Monotremes

Difficulty Level: At Grade Created by: CK-12
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Can a mammal lay an egg?

This is a mammal. But it is unlike placental or marsupial mammals. If a mammal does not develop within a placenta or a pouch, what type of development is left? Eggs!


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).

This egg-laying, venomous, duck-billed, beaver-tailed, otter-footed mammal is a platypus, a monotreme mammal that reproduces by laying eggs.

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.


  • Monotremes reproduce by laying eggs.
  • Monotremes have a cloaca instead of a uterus and vagina. The eggs pass through the opening of the cloaca.
  • Monotreme 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.


Use this resource to answer the questions that follow.

  1. How do monotremes reproduce?
  2. Contrast the reproductive strategies of the platypus and the echidna.
  3. What is a cloaca? Describe its function in monotremes.


1. Describe eggs and egg laying in monotremes.

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

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

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

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monotreme Type of mammal that reproduces by laying eggs.
cloaca Body cavity with a single opening in amphibians, reptiles, birds, and monotreme mammals that collects and excretes wastes from the digestive and excretory systems and gametes from the reproductive system.

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At Grade

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Date Created:
Feb 24, 2012
Last Modified:
Aug 28, 2016
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