The narwhal tusk is a true oddity in the natural world.
Reaching up to three metres in length, the tusk is an upper tooth that grows through the whale's lip on the left side. And it is the only mammal tusk that grows perfectly straight, with no curves.
A team of international researchers believe they have figured out why it grows so straight — and the answer lies in its unique pattern of opposing spirals.
“This was really the first time that we could prove that this was the case and that there were two opposing spirals … that was really quite surprising to us,” said study author Henrik Birkedal from Aarhus University in Denmark.
New research published in the journal Nature Communications breaks down how the two opposing spirals — one going right, the other going left — allow the tusk to grow straight.
The project began when Birkedal was approached by an organic chemistry colleague who was looking at why some molecules are right-handed and some left-handed.
The colleague had been visiting Greenland and heard that narwhal tusk spirals all go in the same direction, to the left. Birkedal decided to look into it.
Birkedal’s team collaborated with biologists from Greenland and physicists from Sweden to better understand the molecular structure of the tusks and spirals.
“We really set out to try and understand how the spiralling tusk is related to the underlying structure of the tusk,” he said.
The tusks, which are mostly found on male narwhals, are made of dentine and cementum, similar to human teeth but without the hard layer of enamel.
Through advanced imaging techniques that Birkedal described as being similar to CT scans, the researchers found two opposing spirals — a left-handed spiral on the outer layer of cementum, and a right-handed one in the inner layer of dentine.
It's an important discovery, not just in solving the mystery of the narwhal's straight tusk but for its potential to help shed light on the structural principles of human bones on other similar instances, Birkedal said.
Martin Nweeia, a narwhal biologist and specialist in marine mammal teeth at Harvard University, said he was delighted to see the research's deep-dive into what he calls "the most extraordinary tooth on the planet."
“Naturally, when a curious dentist meets the most extraordinary tooth, there's an irresistible urge to figure out what's going on here,” he told CBC News.
However, Nweeia — who was not associated with this study — said he was surprised to see the researchers characterize the tusk’s function as “speculative” in the initial pages of the paper.