I wrote about narwhals yesterday and figured that would hold me for a while. It did not. Because the same research has a second act, and it's the part I keep circling back to at 2 AM: a team of biologists, chemists and physicists just used advanced 3D X-ray imaging to map the interior of a tusk at a level of detail nobody had managed before, and what they found settles one argument while quietly opening better ones. The Tribune has the rundown.

The short version for anyone who missed it: the tusk is a tooth. One tooth, the left canine of the upper jaw usually, that in males grows out through the lip and just keeps going. And for as long as people have studied it, the thing has been a puzzle. Look at a tusk and you see one spiral groove winding down its length, which is how it got engraved, illustrated and described for centuries. The new imaging says that's an illusion.

There are two spirals in there, and they run against each other. The outer layer of the tusk twists left. The inner layer twists right. Two counter-rotating helical structures, stacked inside a single tooth, and until the 3D X-rays cut through the material nobody had ever seen the second one. Everyone, scientists included, had been reading the surface and assuming that was the whole story, while the deeper structure ran the opposite direction the entire time.

Think about that too long and it gets strange. The most examined tooth in the animal kingdom had a second spiral hiding in it, invisible, until somebody finally looked through it instead of at it.

And then there's the question the new imaging doesn't close, which is what the tusk is actually for. That debate has been running for centuries and it's still refusing to die. The old answer was display, a lance for jousting over mates. Then came footage of narwhals tapping fish with the tusk, stunning them before a strike, which suggested a hunting tool. Then researchers proposed the tusk as a sensory organ, an exposed antenna reading the water around it, possibly detecting conditions like shifting salinity in ways that could matter for an animal navigating changing sea ice. It may be all three. It may be none. The females mostly don't have one and they survive fine, which complicates every confident theory ever proposed.

What I like about this study is what I like about the best rabbit holes: the imaging answered the structural question and left the functional one more interesting than before. We now know the tusk is a double spiral. We still don't know why the animal bothered. The good kind of gap, the kind that keeps the lights on and the tabs open...

The tusk has been called the unicorn horn for five hundred years, sold for fortunes, ground into medicine by people who believed almost nothing true about it. And the real object turns out to be stranger than the myth: a tooth wearing one spiral on its sleeve and hiding a second one underneath, wound the other way. People stared at this thing for centuries and saw half of it. Makes you wonder what else we're only seeing the surface of.