Does the narwhal have a horn or a tooth?

Nicknamed the “unicorn of the seas”, the narwhal (Monodon monoceros) has intrigued observers since the Middle Ages. Its long spiralled protrusion, which can reach three metres, fed medieval legends about unicorns, to the point that kings and emperors paid fortunes to acquire what they believed to be the horn of the fabulous animal. The biological reality is more surprising still than the legend: it is neither a horn nor a tusk in the traditional sense, but a tooth, and this tooth defies everything we know about dental anatomy.

A canine grown giant

Scientifically, the narwhal’s appendage is a hypertrophied upper left canine. The animal’s scientific name already gives the observation away: Monodon monoceros means literally “one tooth, one horn”, a neat illustration of the historical confusion. This canine begins to grow out of all proportion in young males at around one year of age, eventually passes through the upper lip, and keeps growing for years.

The dimensions are striking: up to three metres long and about ten kilograms for the largest documented tusks. The shape is highly recognisable: twisted into a spiral, and always anticlockwise (to the left, seen from the animal). This uniform twist remains largely unexplained, no other tooth in the world shows this regular structure.

A structure inverted compared with our own teeth

The most extraordinary aspect of this tooth lies in its internal anatomy, which runs counter to every other mammalian tooth. In us, as in almost all vertebrates, enamel: the hardest substance in the body, covers the outside of the tooth, protecting the dentine and the pulp at the centre. In the narwhal it is exactly the other way round: the hard layer is on the inside, and the dentine, rich in nerves and blood vessels, is exposed at the surface.

This configuration has two remarkable consequences. First, the tusk has a surprising flexibility for a structure of this size: a 2.40 metre tusk can bend by about thirty centimetres without breaking, which is extraordinary for an object so long and so slender. Second, and above all, this inverted anatomy makes the tusk a genuine sensory organ. The thousands of dentinal tubules connect sea water directly to the nerve endings, turning the tusk into a giant sensor able to detect variations in temperature, salinity and pressure, and probably the presence of prey.

What is it actually for?

The function of the tusk was debated for a long time. Several hypotheses have been put forward: a weapon for fights between males, a hunting tool for stunning fish, an ice-breaker for opening holes in the sea ice, a sexual signal. Today the body of studies suggests multiple uses.

Its sensory function is now regarded as central. Recent drone observations have also shown that narwhals use their tusk to strike and stun cod before swallowing them, a hunting behaviour documented for the first time in 2017. Males, for their part, engage in tusking, a display in which they cross and rub their tusks, probably for purposes of social hierarchy and sexual selection, in the manner of deer antlers.

Males, females, exceptions

Almost all adult males have a tusk. But the rule admits a few notable exceptions. About 15% of females also develop one, generally smaller. Rarer still, narwhals are sometimes seen with two tusks: the right canine, normally arrested, then develops as well. These double-tusked individuals are so rare that they became highly prized historical trophies. The Natural History Museum in Hamburg holds one of the most famous specimens in the world.

A threatened emblem of the Arctic

The narwhal lives only in Arctic waters, mainly around Greenland, northern Canada and Russia. A species perfectly adapted to ice, it is particularly vulnerable to the warming that is rapidly transforming its habitat. The retreat of the sea ice is changing its hunting grounds, exposing its populations to new predators such as the killer whale moving further north, and disrupting the migration routes it has followed for millennia. More than its famous tusk, it is this uncertain future that concerns biologists today.

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