For centuries, the purpose of a narwhal’s tusk has eluded explanation. Now, researchers suggest that these small whales use their tusks as sensory organs and speculate that sensing changes in seawater salinity might help male narwhals stay safe, and locate fish or females.

Narwhals are a little bit like Arctic unicorns. At least, the males are. They’re the beasts that swim around wielding giant, spiraling tusks that can grow to nearly 9 feet long. But unlike the mythical horned horse, narwhals are a) real and b) their horns aren’t centered on their faces. Instead, their tusks protrude from the left of their mouths – they’re actually big, twisted canine teeth (the right canine usually remains embedded in the whale’s jaw).

Since at least the 15th century, scientists have been mulling over the purpose of the narwhal’s super-long tooth, proposing roles in defense, attracting a mate, hunting, hearing, breathing, and ice-breaking, among others. Now, it seems clear that the tooth is capable of acting like an enormous sensory organ, says Harvard University’s Martin Nweeia, a marine mammal dental specialist. Nweeia and his colleagues have been studying narwhals in the Arctic for more than a decade, and published a paper describing the tooth’s sensory capabilities today in The Anatomical Record.

“It takes a tremendous amount of energy and devotion to get that thing to grow,” Nweeia says. “To expend that much energy in such a harsh environment – there has to be a pretty compelling reason to do it.”

Photo: Gretchen Freund )

Nweeia and his colleagues collected narwhal tusks from Inuit hunters near Baffin Island, then studied those tusks for anatomical clues to their function. Turns out, narwhal tusks are filled with a nerve-rich pulp that’s similar to the stuff in human teeth that can sometimes make drinking coffee or eating ice cream a painful experience.

Next, the team looked to see if there were any genes expressed by the pulp that would indicate a role in relaying sensory information to the brain. And there were: two genes expressed in sensory signaling pathways were present at much higher levels in tusk pulp than in muscle or jaw tissues.

Nweeia then decided to test whether the tusks helped convey information about salt concentration in the surrounding sea. To do this, he fitted narwhals with a “tusk jacket” – a clear plastic tube that encloses the tusk, from one end to the other. He attached electrodes to the animals so that he could measure their heart rate. Then he bathed the jacketed tusks in solutions with either high or low concentrations of dissolved salt – a situation that mimics changes to seawater as icebergs form (high salt) or melt (low salt).

He found that narwhal heart rates rose in response to high salt concentrations, presumably because these concentrations normally suggest that the sea is freezing and entrapment is possible. The animals' heart rates dropped when the tusks were washed with fresh water, suggesting they could detect this change. But, Nweeia says, salt is just one of many environmental stimuli the tusks could be sensing. "Our premise was just to open the pathway for people understand that this is a sensory organ," he says. "Now the pathway is open for people, including ourselves, to look at other variables it might also detect."