New research discovers the twisty secret behind the narwhal's unusually straight tusk

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.

Opposing spirals

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.

This closeup of a narwhal's head shows the tusk protruding from its mouth, as well as the distinctive spirals on the outside. (Mads Peter Heide-Jørgensen)

And what they found wasn’t just a simple spiral.

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.

A sensory organ

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.

Martin Nweeia is narwhal biologist and specialist in marine mammal teeth at Harvard University. He said his research has found that the tusk's main function is primarily as a sensory organ. (Paul Nicklen)

“I'm not sure where they got this,” he said.

Nweeia contends that teeth are multifunctional. But in the case of the narwhal, his research has shown the tusk's main function is primarily as a sensory organ, rather than being used for defence or for feeding.

“Do we use our teeth for self-defence? No. But if someone grabbed us around the neck, might we bite them? Sure,” he said.

That said, Nweeia commended the study authors for devoting the energy and effort to learning more about narwhals, and says he is always cheering on anyone who tries to discover more about them.

Future research

When looking ahead to future research projects about narwhals, Nweeia said he hopes to see more off-season studies and collaboration with Indigenous communities.

The Canadian Arctic is home to most of these mammals in the summer. The off-season, he said, is when Inuit knowledge becomes critical because scientists typically only go up for a month or two when there’s more availability to study the animals, while they are not under the sea ice cover.

“To have that link that ties everything together, you take science, you take traditional knowledge, and you take the value that the Inuit observations have,” said Nweeia.

He was a part of a study in 2024 that showed the capability of narwhal tusks to bend and flex 12 degrees in every direction.

Narwhals during spring migration make their way to Baffin Island in northern Canada. There are few opportunities to study the whales, which live under sea ice for more than half the year. (Glenn Williams)

“If you've ever seen a narwhal tusk, you would never imagine that thing could bend and be flexible,” said Nweeia.

He didn’t think it was possible when he first heard about it through interviews with Inuit elders. Yet through science and study, they found it to be true.

“These are the components I think that really make a study what I call integrative — that you take different fields of science and that you look at something from multiple perspectives.”

Birkedal agrees that interdisciplinary work is crucial. His recent paper was only possible because of the involvement of teams in Greenland and Sweden.

“It's also a good example of how when disciplines meet, you can really learn from each other to do something that you wouldn't be able to do on your own," he said.