The long, spiral tusks of narwhals could be used by these Arctic whales to stun or even kill prey, according to wildlife researchers who have spied on these elusive creatures using drones.
They captured footage of narwhals using their tusks to mess with fish, and even saw a narwhal repeatedly strike a fish with its tusk.
But narwhal experts don't agree on what these observations mean.
A report in the journal Frontiers in Marine Science says the footage provides "clear evidence of narwhals chasing fish and using their tusks to interact directly with the fish and to influence the fish's behavior."
And members of the research team like Cortney Watt, a scientist with Fisheries and Oceans Canada, as well as the University of Manitoba, believe that narwhal tusks could get used to hunt fish.
She notes that female narwhals, which usually don't have tusks, manage to find food just fine, so these tusks can't be essential. Still, she says, they might be helpful.
"I don't think it's a necessary foraging adaptation, but I think it's a sort of secondary use for their tusks that maybe some whales have learned and are utilizing to their advantage," says Watt.
Given that there are very few underwater observations of narwhals and so little is understood about this whale's behavior, however, another researcher who wasn't part of the study team is skeptical.
"I think these are short term videos that show a few narwhals at the surface, you know, moving their heads around in the vicinity of a fish," says Kristin Laidre, a polar biologist with the University of Washington. "And whether it's play behavior or pursuit of that fish to consume it, nobody knows."
Unicorn horns
Scientists have long been fascinated by narwhal tusks, which can grow up to ten feet long.
Zoologist Greg O'Corry-Crowe with Florida Atlantic University, has gotten to see this tusk up close, while working to tag narwhals with satellite trackers.
"It is quite an overwhelming experience to stand there in the water and help hold a live narwhal and feel the tusk," he says. "It almost looks like it has been fashioned and carved. It has a lovely spiral, scrolled pattern to it. So it's mesmerizing, really, in some ways."
Hundreds of years ago, seafarers brought these tusks home and passed them off as unicorn horns. "It almost seems like it was the strongest proof that unicorns really did exist," says O'Corry-Crowe.
It's now clear that the tusks play a role when male narwhals compete for mates. A long tusk tells females that a male is large and successful, and the length of a male's tusk corresponds to the size of its internal testes, revealing its potential fertility. As one research paper put it, "the longer the better."
"We've converged on that as the primary use, but it looks now like it has other uses," says O'Corry-Crowe, who was part of the team that captured videos of narwhals pursuing fish with their tusks.
When the drones hovered over the whales, the water was clear enough to see down 15 feet or so beneath the surface. The narwhals seemed to be relaxed, just gently socializing, as this is a time of the year that seems less critical for feeding or mating.
"Females were nursing calves. Males were hanging out in these sort of bachelor herds," says O'Corry-Crowe.
Some of the narwhals with tusks seemed to use them in a playful way to explore and manipulate fish such as Arctic char, he says.
They'd mess with it with their tusk and "flip it a few times," says O'Corry-Crowe. "Then they would turn to each other, as if saying 'You have a go' or 'This is what you do.'"
They could hit the fish, stunning them. Other times, it seemed that the whales were just inquisitive.
Watching the narwhals interacting with the fish and each other, O'Corry-Crowe thought it looked like some kind of social learning, such as older whales teaching younger ones how to handle fish.
"We don't know if that's what's happening. But, you know, these kinds of observations make you wonder," he says.
"The dexterity and use of the tusk was what was really striking," he adds. "It's like they're not wielding a broadsword, they're using a very precise surgical instrument."
Diving deep
Past research has suggested that male and female narwhals might have slightly different diets, which "certainly piqued some curiosity" about what role tusks might have in that difference, says Watt.
"I was definitely interested in how the tusk might play a role in foraging," she says.
One study found narwhals swimming upside down near the seafloor, suggesting that perhaps the tusk gets used almost like a shovel to scare or root out prey near the bottom.
And in 2017, researchers said they'd captured video of narwhals seemingly using their tusks to go after Arctic cod.
That study prompted headlines such as, "Video solves mystery of how narwhals use their tusks." But Laidre says it's already clear how narwhals use their tusks: females use them to select the best possible mate.
"There's so much scientific evidence showing that this is a sexual trait," she says. "I don't think there's a very big mystery."
On the other hand, most of a narwhal's life is spent deep beneath the Arctic ice, where they feed in darkness at great depths, diving down a mile or even more.
"There's so much we could learn if we could, you know, have images or video or something down there, seeing their behavior and being able to record it," says Laidre. "That would be amazing, and incredibly difficult to get."
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