Platypuses have traits of birds and reptiles, but also have traits of mammals. Rather puzzling to scientists picking over pickled specimens was their internal reproductive system was more like a bird than a mammal.

Female platypuses don't have vaginas. They have a cloaca, which is a single hole for pee, poo, sex and eggs. Eggs. Which they lay.

The female platypus has two ovaries, but only the left one is functional. They also have two organs which can be called uteruses or oviducts. The embryo isn't in there very long, and the uterus puts a shell on it before laying the egg.

The weird inner junk aside, platypuses also don't have nipples, even though they produce milk, like other mammals. They secrete it through the pores in their skin, which then pools on their belly. Their young lap it up from these little milk bowls.

These discoveries throughout the nineteenth century were quite an existential crisis for the burgeoning field of taxonomy, who were trying to neatly define and group animals, then this pesky creature with its peculiar birdy inner junk and no nipples but milk comes along.

There were arguments. Lots of arguments about what to do about platypuses. Also, huge questions about whether they were using their bird-like internal organs to make eggs or live young because nobody had seen it in action yet.

Because telegrams from Australia to London were very expensive, and charged by the word, Caldwell wrote up the fruits of his research in four words: "Monotremes oviparous, ovum meroblastic." (In lay terms, "yep they lay eggs, they're like reptile eggs")

Due to the pricing by the word in telegrams, Caldwell didn't add "what the fuck, what the fucking fuck". Plus, he didn't really need to say that, because what the fuck, what the fucking fuck.

Now, the fun thing about platypuses is another century later, their reproductive biology continues to yield "what the fuck, what the fucking fuck" moments.

Genetic research in the 2000s found that they have FIVE pairs of sex chromosomes.

Mammals have one pair of sex chromosomes, dubbed X and Y, with XX corresponding roughly with female and XY with male. Birds also have a pair, Z and W, with ZZ being male and ZW being female.

In platypuses, XXXXXXXXXX is female and XYXYXYXYXY is male.

Platypuses have a neat little party trick so they don't end up producing young that are any other combination such as XYXXXXXXXY, which would just be *silly*.

Essentially, sperm select every other chromosome to produce either XXXXX or YYYYY sperm.

Oh, and some of their sex genes have more in common with birds than with mammals. And that's provoked another existential crisis because it has huge implications for how sex chromosomes evolved in birds *and* mammals.