What is a Monotreme?
Image Citation: Pen_Ash (photographer). (2018). Platypus Monotreme Mammal [photography]. Retrieved from
In elementary school we usually learn that mammals are differentiated from the rest of the animal kingdom because they have hair, produce milk from mammary glands, and give birth to live young. However, there are some mammals that don’t play by these rules.
Monotremes are mammals that lay eggs like reptiles. Mammals originally evolved from reptile-like ancestors, so at one point in our evolutionary history all mammals probably laid eggs. Nowadays, egg laying in mammals is limited to a single species of platypus and several species of echidna, all of which can be found in either Australia or New Guinea.
In addition to laying eggs, monotremes are also unique in several other ways. For example, they only have one opening (called the cloaca) for the digestive and reproductive systems. That means they mate, give birth, and excrete wastes from the same hole, just like reptiles.
Female monotremes lack the nipples that we usually associate with mammary glands. They still produce milk, but it is excreted from a patch of pores on the female’s belly.
When baby monotremes hatch they are pink, naked, and about the size of a large jelly bean. Newly hatched echidnas haven’t even developed their hind legs yet. These tiny babies are entirely reliant on their mother for warmth, food, and protection. Echidnas carry their young in a temporary pouch on their belly, while mother platypuses carry them between their tail and their body.
As if they weren’t strange enough, male platypuses also have another very unique trick up their sleeve: They are one of the only venomous mammals currently known to science. Male platypuses have two venomous spurs on their hind feet, which they use to inject toxins into predators or competing male platypuses.