You might think you know why the platypus is a bizarre beast.

Maybe you know it lays eggs. Maybe you know that the males have venomous spurs on their ankles.

And maybe you know that they can sense electricity with their bills and don’t have nipples. But do you know the most bizarre thing about their anatomy? Riddle me this: what major organ is the platypus missing? [♪♪ INTRO ♪♪] The platypus lives in rivers, streams, and ponds in the wetter parts of eastern Australia and Tasmania.

They mostly eat aquatic invertebrates, but will also consume small fish and fish eggs, if they’re around. As adults, they have special grinding plates within their duck-like bills that they use to mash their food up. And they locate their prey using three types of receptor cells on those bills.

One set senses changes in the motion and pressure of the water around them, while the other two pick up on the subtle electrical signals given off when their prey moves a muscle. This is called electroreception and only something like two other kinds of mammals have this ability. The duck-bill of the platypus is one of its most obviously odd features, just by the virtue of being attached to a body that otherwise looks kind of like a chubby otter or a beaver, complete with a paddle tail.

But that furry body is also hiding a secret, or, really, a bunch of secrets. And we’re going to tackle some of them, going from the outside in. Under UV light, the platypus glows a neon blue-green.

You might remember the term biofluorescence from our very first episode, about the glowing bones of chameleons. But the platypus’ glow comes from its fur, not from within. It’s not the only biofluorescent mammal, Virginia opossums and North American flying squirrels also glow under UV light, along with a few other Australian marsupials.

And all of these species, including the platypus, are nocturnal. The researchers speculated that absorbing UV light may help camouflage these critters from UV-sensitive nocturnal predators, but they need to do more research to test this hypothesis. Another thing you might notice if you inspected a platypus is that, like we said before, they don’t have nipples.

Now, platypuses are mammals, which, by definition, produce milk for their young. Along with the echidna, platypuses are part of a special group of egg-laying mammals called monotremes. So, how does the milk thing work with no nipples?

Well, platypus mammary glands just secrete milk onto a special ‘milk pad’ on their bellies for their babies to lap up. This has led to the myth that platypuses “sweat” milk, even though that’s not really what’s going on. And that milk has some incredible antimicrobial and antibacterial properties, including one uniquely-folded protein that looks like a ringlet or a very long spiral curly fry.

The scientists who found the new protein think that platypus milk is so antimicrobial because they don’t have nipples. This means the milk is exposed to the environment before the baby platypuses can drink it, so it can pick up contaminants. And milk-drinking brings us to the next amazing thing about platypus anatomy, the thing we teased in the intro to this video.

The major organ that the platypus is missing is its stomach. The platypus has no stomach. It eats, but has no stomach.

The food just goes straight from its esophagus to its small intestine, without a stop in the middle in a sac that secretes digestive enzymes and acids. Most of the actual food breakdown seems to take place in the platypus’s mouth, where it’s ground up very very fine, and then in the small intestine, no stomach required. The platypus has even lost the genes that would code for things like stomach acid and the protein-digestion enzymes called pepsinogens.

And the researchers suggest that this probably comes down to their diet. Platypuses eat a lot of aquatic invertebrates with shells, things like mollusks, and shells are chemically basic, which can neutralize stomach acid. So it doesn’t make a lot of sense, evolutionarily, to keep using energy making stomach acid if it’s just going to be neutralized.

Pepsinogens need an acidic environment to work, so no stomach acid means no need for these enzymes, either. The platypus isn’t even the only animal that’s lost their stomach and stomach genes, the echidna doesn’t have them either and neither do many fish, including carp and lungfish! The rest of the genome of the platypus is also filled with things that defy our expectations about genomes, too, but you’ll have to follow us on our socials for those extra facts.

The platypus is one of those critters that, when we started doing the research for this episode, we realized the rabbit hole is deeper than we could’ve ever imagined. There is basically no part of this animal that, upon closer study, does not seem strange in some way and the more papers we read, the stranger it seemed. And I’m going to be a little meta here for a second, but it’s worth pointing out that we started this channel as a way to explore what makes animals seem weird to us, humans.

Because “weird” is just a way that we frame things that are unfamiliar or rare or somehow don’t fit our expectations, it’s not objective. And, for whatever reason, in saying that the platypus is weird, it seems like we still have expectations about it, when really, at this point, it seems like a safer bet to just not. There is nothing else on this planet that is quite like it, even though it was produced by the same evolutionary processes that generated every other living thing, so maybe it’s time to stop calling the platypus weird.

The Bizarre Beasts pin club subscription window is open from now until the end of June 12th! The platypus pins are so fun. One of them does glow in the dark, and one of them is very sparkly.

And they are all just really cute. To find us for all those extra weird animal facts, we’re on Twitter @BizarreBeasts, and on Instagram and Facebook @BizarreBeastsShow! And profits from the pin club and all of our merch go to support our community’s efforts to decrease maternal mortality in Sierra Leone. [♪♪ OUTRO ♪♪]