This tusk consists of a hollow, left canine that projects out of the male narwhal's upper lip to a length of between 1.5 and 3 metres, and weighs only 10 kilograms.
Being such an unusual appendage, the tusk has been the subject of many legends. One such legend tells of an Inuit woman with a harpoon who was pulled into the sea by a whale, turning into a narwhal in the process. Her spirally-knotted hair became the famous tusk.
During the medieval period, it was believed that the narwhal's tusk came from a unicorn and had the ability to cure numerous ailments. The Vikings took advantage of these legends, selling them for many times their weight in gold while Queen Elizabeth bought a carved and jewel-encrusted tusk for £10,000 - quite a sum in the 16th century!
However, not everyone was fooled. In 1555, Olaus Magnus, the Swedish monk who incorrectly drew underwater-hibernating swallows, correctly identified the "narwal" in a drawing of a horned fish-like animal.
But what are their tusks used for?
A Matter of Sex
Although female narwhals have a much shorter and straighter tusk, the longer, spiralled tusks are usually only found in males. As such one explanation could be that it is a secondary sexual characteristic, much like that of the a male deer's antlers or a male peacock's tail.
Deer use their antlers as a display of physical fitness in order to secure breeding rights with females. The overall size serves as a visual cue to deters potential rivals with smaller antlers, while they can also be used physically against any challengers. Meanwhile, the peacock's tail is an extraordinarily large and cumbersome, showing how strong the individual bird must be to cope with this highly visual burden.
Perhaps narwhal's use their tusks in a similar way?
Bull narwhals are known to engage in "tusking", where individuals rub their tusks together, which could help to gauge the length of their tusk and determine social rank. In situations where the individual with the longer tusk cannot be determined there is the possibility that the tusk be used as a joust or sword. Or could it?
Sword or Joust
Jules Verne mentions what would seem to be a gigantic narwhal in his 1870 novel Twenty Thousand Leagues Under the Sea which punctures ships with its tusk. Although this hasn't been reported in reality, perhaps it could function as a weapon?
It seems unlikely.
Narwhals have rarely been observed using their tusk to aggressively fight other narwhals and the act of "tusking" between male narwhals seems to be a passive interaction.
The tusk is also too long and slender to be functional for chipping away at sea ice and it would be useless as a spear as the removal of speared items from the tusk would prove tough.
A Sixth Sense
As the tusk lacks an enamel covering and with a porous outer surface means that it is exposed to the narwhal's environment. As such the tusk could be sensitive to temperature and chemical cues. The centre of the tusk is made up of pulp, just like our teeth, which contains nerve endings, allowing for the relay of messages to the brain.
New research has suggested that the tusk may be a sensory organ. It was shown that when a narwhal is exposed to water of different salt levels there is a noticeable change to its heart rate, possibly identifying a reaction that could help narwhals to locate fish, find mates or predict when the sea is freezing, preventing entrapment.
As narwhals are relatively obscure they haven't been subject to much scientific scrutiny over the past few years - leaving us with this enduring question.
In the meantime, what do you think?