Now We Know the Reason for the Narwhal's Tusk

Known as "the unicorns of the sea," narwhals are unique for the solitary tusk that protrudes through the tops of their heads. The horn is actually a canine front tooth that can reach as long as nine feet, similar to an elephant's tusks. But until recently scientists weren't sure what, if any, purpose it had.

Research has pinpointed many possibilities, suggesting the tusk is used as a sensory organ, helping the narwhal pick up changes in its environment. Males of the species may even use the horns to look for food or find mates.

Narwhals = Peacocks?

The newest theory is one that doesn't feel too far-fetched: It's a way to show off to females and to warn off competing males.

Arizona State University researcher Zackary A. Graham, who authored the study published in the journal Biology Letters, focuses on sexual selection in his work. As he hunted around for new examples, the narwhal caught his eye. Its tusk grows out in a spiral pattern, making it look like a marine unicorn of sorts.

"Broadly, I'm interested in sexual selection, which is responsible for creating some of the craziest traits in biology. As an evolutionary biologist, I try to understand why some animals have these bizarre traits, and why some don't," Graham said in a university news release.

Graham has studied sexual selection in many species, and he realized that to demonstrate that the tusk is sexually selected, he could use the relationship between tusk size with body size.

He and his team looked at data on 245 adult male narwhals, most of which were captured by Inuit hunters over the course of 35 years. Whenever narwhals are caught, the Greenland Institute of Natural Resources asks for data to be shared.

By looking at growth of the tail or fluke versus growth of the tusk, it was relatively easy to see outsized growth of the tusk, and only the biggest and strongest narwhals can afford to have such large tusks. That helps with males—"The information that the tusk communicates is simple: 'I am bigger than you,'" says Graham—and also with attracting a mate.

As Scientific American reported on Graham's work, "The top tusks thus appear to be like a billboard that shouts, 'Look at me. I’m the biggest.' After all, only the strongest, best fed individuals can afford to produce such an ostentatious ornament. Of course, tusks can do more than just say, 'Hey, how you doin’?'"

If you're going to put that much effort into a body part, it had better be worth it.

Is It a Really Weird Tooth?

By combing through studies with different filters—from anatomy to genetics to diet—one team of researchers took a more comprehensive view of past research, and that led them to the theory about sensory ability mentioned above.

"This tooth is almost like a piece of skin in the sense that it has all these sensory nerve endings," the study's lead author, Dr. Martin Nweeia from the Harvard School of Dental Medicine, told BBC Earth. The narwhal tusk is "essentially built inside out."

He assembled a team of international investigators to understand the function of the narwhal's unusual protuberance. To do so, they captured several of the elusive animals and anchored them using a net anchored perpendicular to shore.

The researchers found that the outer cementum layer of the tusk is porous, the inner dentin layer has microscopic tubes that channel toward the middle, and the pulp in the center has nerve endings that connect to the animal's brain. The structure makes the tusk sensitive to temperature and chemical differences in the environment.

The results of their work were published in the journal "The Anatomical Record."

When the tusk was exposed to different levels of salt in the surrounding water, for example, the researchers noticed a change in the narwhal's heart rate.

The animals can basically "taste" the concentrations of chemicals in the water. Because of that, researchers believe males may use the tusk to find food. They also appear to be able to find females that are ready to mate. Some suggest that the tusk's sensitivity to salinity concentrations enable a narwhal to read and navigate ice formations in Arctic waters: "The whale’s migration and behavior patterns may thus be potential indicators of arctic climate and environmental change."

Nweeia told the BBC that he's fascinated that narwhals put all their energy into growing a single tusk rather than having a set of teeth to help them eat their diet of large fish.

"If you were looking for an ideal and fascinating tooth to study, there's no question this would be it."

Is the Tusk for Stunning Fish?

Footage from Canada may support one of the tentative conclusions made in Nweeia's study, that narwhals use the tusks to find food. One additional quirk? The horns may also help the narwhals prepare to eat that food, too.

The video above, shot using drones by the WWF in Canada in 2017, shows narwhals in Tremblay Sound, Nunavut, striking Arctic cod with their tusks to stun them before gobbling them up.

Steve Ferguson of Fisheries and Oceans Canada explained in a video for the agency that the drone footage shows male narwhals "kind of tracking the cod with the tusk [...] and as the cod was positioned close to the tip of the tusk, the narwhal sort of gave it a quick, hard tap that likely stunned the fish — it looked like it was momentarily not moving — and then the narwhal would move in with its mouth and suck in the prey."

Given that we're only seeing this behavior now, in no small thanks to the general unobtrusiveness of drones, researchers are eager to learn what other possible uses there are for the tusks. It's also a surprisingly flexible protuberance, capable of bending up to one foot (30 centimeters) in every direction.

A dual-purpose sensory organ, a way to attract females, and cod stunner are already exciting, so what other uses could these creatures of the deep have for this horn-looking tooth?