The tusks have fascinated scientists ever since, but they have proven extremely difficult to study because these odd-looking, blubbery whales spend most of their lives under the Arctic ice.

But on Tuesday, a large international team of researchers announced they had solved a long-running mystery about the tusks: how they get their uniquely spiraled yet arrow-straight form.

The left canine tooth of male narwhals grows through the jaw and upper lip, jutting out into the animal kingdom's only single tusk. The right canine stays buried in the head.

"Imagine that you had to ride on a bicycle with a broom strapped to your head," said Henrik Birkedal, the senior author of a study in Nature Communications describing the new discovery.

That is how it must feel for male narwhals to dive deep underwater, the chemist at Aarhus University in Denmark told AFP. Yet despite this huge "Monty Pythonesque" protuberance, the animals seem to get about just fine, he added.

To solve what they called the "narwhal tusk puzzle," the team of biologists, chemists and physicists used advanced 3D X-ray technology, including three European particle accelerators called synchrotrons.

The tusks are made of two kinds of dental tissue: dentin on the inside and cementum on the outside.

At a molecular level, the inner dentin forms a right-handed spiral while the outer cementum forms a left-handed spiral, the powerful X-ray scans revealed.