I like the duck-billed platypus
Because it is anomalous.
I like the way it raises its family,
Partly birdly, partly mammaly.
I like its independent attitude.
Let no one call it a duck-billed platitude.

-Ogden Nash, “The Duck-billed Platypus,” in Beastly Poetry

Duck-billed platypus

(Duck-billed platypus; Image filched from WikiMedia Commons)

The duck-billed platypus is special. No, really. It’s special. And not just because it lays eggs, has venomous feet, and hunts using electric fields. Kate Jones of the Zoological Society of London and her colleagues developed a quantitative method to rank how “special” a mammalian species is, and the duck-billed platypus is number one on the list. Of all mammals. That’s right, the platypus is the most special mammal of all.

How is specialness calculated? Well, the technical term for special, in this context, is “evolutionarily distinct.”

[DISCLAIMER: if you don’t read on, you’ll miss the four-headed spiny anteater penis. Just so you know.]

The score is calculated based on a new family tree that includes almost all known extant species of mammals (check out the paper, or see the tree in a BBC article). If I could turn this tree into a poster, I would — it’s amazing, and it’s hard to take in just how amazing it is on a tiny little computer screen. It’s stretched into a circle such that the root (almost 170 million years ago) is at the center and all the modern-day species are arrayed along the circumference. Each species has its own branch, and the tree shows how they’re all related — when different lineages emerged and who’s in whose family. If you’re intrigued, pick up a copy of the November issue of Discover and flip to page 24 to read my article about this “mammalian supertree.” Or wait for them to post the story online, which they will eventually do.

In any case, using this phylogenetic tree — which contains quantitative information about how all the species are related — Jones and her colleagues can calculate how evolutionarily distinct a species is. Evolutionary distinctness is a measure of how unlike other mammals a given species is. So the monotremes (the order the platypus belongs to), whose branch is really really long, are really really unlike other mammals. The last common ancestor between monotremes and the rest of us was 166 million years ago. By contrast, the last common ancestor between humans and chimpanzees was 5 to 8 million years ago.

Besides the platypus, there are just two other species of monotremes: the long-beaked and the short-beaked echidna. Echidnas are also known as spiny anteaters, although they’re not even in the same order as non-spiny anteaters. The long-beaked echidna ranks #2 on the special scale. But it’s #3, the short-beaked echidna, that’s made the news lately.

Echidna

(Short-beaked echidna; Image borrowed from prb on Flickr)

Like the lady platypus, the lady echidna lays eggs and leaks milk directly through her skin (who needs nipples, when you can just ooze?). But the gentleman echidna is even weirder, according to a recent-ish article in New Scientist. It has a four-headed penis. And that’s especially weird given that the lady echidna’s vagina only has two branches. It is, quite simply, a problem of a four-pronged peg in a two-pronged hole.

As puzzled as scientists were, captive echidnas were far too modest to offer up any answers, refusing to mate for nosey onlookers. The researchers even tried zapping the echidna’s penis to get it to ejaculate, but, as University of Queensland biology Steve Johnston told New Scientist, “not only did we not get a single drop, but the whole penis swelled up to a four-headed monster.”

Then Johnston met with a stroke of good luck. A spiny anteater was retired from the zoo when it, er, responded enthusiastically to being handled. Such an exhibitionist echidna was just what Johnston and his team needed. Sure enough, the little guy let the researchers see just what an echidna erection looked like. Turns out only two of the four prongs become erect at once — creating a perfect fit for the two-pronged vagina. And they trade off, erection to erection, so that no prong need perform twice in a row.

Kate Jones’ evolutionary distinctness scale is incredibly useful, particularly for conservation efforts. But after reading about the four-pronged penis-monster of the exhibitionist echidna, I’m not sure we needed her to tell us that the monotremes are special.

[A video is worth 1,0002 words. Watch it, via New Scientist, here.]

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