Monotremes vs Marsupials: A strange battle

While most mammals are placental, meaning they give birth to live young, these two infraclasses of mammals decided to do things a little differently.


Monotremes are some of the most ancient mammals. They technically fill the mammalian category, with the classic mammalian jaw hinge, inner ear bones, and diaphragm. They have fur and produce milk to feed their young.

But unlike most other mammals, monotremes do not produce live young, instead laying eggs from which baby monotremes will emerge. They also have slightly different jaw muscles, a reptilian gate, and extra bones in the shoulder girdle that other mammals don’t possess.

The only extant monotremes are the platypus and the echidna. Platypuses are pretty iconic for their weirdness in the mammalian world. Platypuses have duck-like bills, webbed feet, biofluorescent fur, venomous leg spurs, and can locate prey using electroception. No wonder we are so fascinated by them!

While there is only one species of platypus, echidnas are really making their mark with a full four separate species! They are sometimes called “spiny anteaters” and it’s not hard to see why, as they have skinny snouts and long sticky tongues that they use to scoop up ants and other insects that make up their diet. However, anteaters and echidnas are extremely distant cousins, with their only ancestral commonality being that they are both mammals.

Like platypuses, echidnas can also process electrical signals, which they use to locate prey. They also have spurs which were likely once used to emit venom, but that ability has been lost to evolution.


Marsupials are slightly less on the fringes of mammalian society, but still have some unique oddities that are not shared by the rest of the placental mammalian kingdom.

The most memorable distinguishing feature of marsupials is of course their pouches. Marsupials birth underdeveloped young, which then crawl into a pouch on their mother’s belly, where they live until they have developed enough to thrive on their own. All marsupial young are called “joeys.”

Marsupials also have some other mammalian anomalies. Most have a rudimentary knee-cap, small skulls with a different dental formation from placental mammals. They also have a double set of reproductive organs, unlike the placentals.

Marsupials have quite a bit more diversity than monotremes, with 335 extant species across 7 orders. These include kangaroos, wallabies, koalas, wombats, Tasmanian devils, opossums, bandicoots, marsupial moles, and marsupial mice (amongst others!). The extinct Tasmanian tiger was a marsupial as well.

Together in Weirdness

Both monotremes and marsupials also share some commonalities with each other, but not with placental mammals. They have slightly lower metabolic rates then other mammals, and lack a corpus callosum – the bridge that joins the two hemispheres of the brain.

Now it’s time to pick your mammalian outsider and let the competition begin! Let’s get weird!

The competition starts at 11:00 AM ET on my 8/21 and goes for 48 hours. Bonus information is detailed in your notifications.

Swag: The top scoring player on the winning team wins the choice of a notebook or a mug, plus a sticker/magnet set! Second and third place will each also win a sticker/magnet set.