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12.31: Monotremes

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    f-d:b4327501dee783622c46f3ccbf555925a725bfff378ead719afeb47a IMAGE_TINY IMAGE_TINY.1

    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.

    Echinda closeup

    Like the platypus, the echidna 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.


    1. What are monotremes?
    2. Describe eggs and egg laying in monotremes.
    3. How does lactation differ in monotremes and therian mammals?
    4. Create a chart that you could use to explain to a younger student the different ways that mammals reproduce.

    12.31: Monotremes is shared under a CK-12 license and was authored, remixed, and/or curated by CK-12 Foundation via source content that was edited to conform to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.

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