A narwhal’s “horn” is actually a tooth. Specifically, it’s an elongated left canine that spirals out through the upper lip and can grow up to 3 meters (about 10 feet) long. Unlike a rhino horn or an antler, it has a living nerve supply, a blood-filled pulp cavity, and the same basic tissues found in your own teeth.
It’s a Tooth, Not a Horn
Every narwhal has two canine teeth embedded in its upper jaw. In males, the left canine erupts through the gum and lip during adolescence and keeps growing for decades, spiraling counterclockwise into the iconic tusk. The right canine normally stays buried in the skull. Females have both canines embedded and unerupted, though about 1.5 to 2.7 percent of females do grow a visible tusk. In rare cases (roughly 0.9 percent of males), both canines erupt, producing a double-tusked narwhal.
The tusk lacks enamel, the hard outer coating on most mammalian teeth. Instead, it’s built from three layers: a soft, porous outer cementum, a thick middle layer of dentin shot through with millions of microscopic tubules, and a central pulp cavity filled with nerves and blood vessels. That porous outer surface is a key feature. It means the tusk is permeable to seawater in a way no other tooth in nature is.
How the Tusk Grows
The tusk begins elongating rapidly in young males. By age six or seven, it can add roughly 100 millimeters (about 4 inches) per year. Growth continues for most of the animal’s life but gradually slows. The dentin layers deposited each year become thinner and more compressed as the narwhal ages, much like tree rings narrowing in a slow-growth year. External tusk length reaches an asymptote around 162 to 186 centimeters at roughly 50 years of age, though exceptional tusks have been measured at 240 centimeters.
Growth eventually stops when a layer of cementum seals over the root tip, cutting off the pulp cavity’s ability to add new material. Researchers can count the annual dentin layers under a microscope to estimate a narwhal’s age, the same principle dentists use when studying cross-sections of human teeth.
A Tooth That Can Sense the Ocean
The tusk’s most surprising feature is that it works as a sensory organ. Seawater seeps through the porous cementum and flows into the network of dentinal tubules, where it contacts nerve endings deep inside the tooth. Researchers confirmed this by exposing narwhal tusks to alternating rinses of salt water and fresh water. The animals’ heart rates changed significantly with each switch, proving the tusk was transmitting information to the brain. The signal travels along the same cranial nerve (the trigeminal, or fifth nerve) that carries sensation from your own teeth and face.
This means narwhals can likely detect changes in salinity, temperature, and pressure through their tusks. The mechanism follows the same principle dentists use to explain tooth sensitivity in humans: fluid movement inside tiny tubules triggers nerve responses. In a narwhal, that sensitivity isn’t a problem to fix. It’s a built-in instrument for reading the Arctic Ocean.
Why Males Grow Tusks
For decades, the tusk’s purpose was debated. Proposed explanations ranged from ice-breaking tool to echolocation aid. The strongest current evidence points to sexual selection: the tusk functions as both a weapon and a signal during competition between males.
Tusk size scales disproportionately with body size, a pattern biologists call hyperallometry. A narwhal that’s slightly bigger than average will have a tusk that’s significantly bigger than average. This kind of exaggerated scaling is a hallmark of sexually selected traits, structures whose primary job is broadcasting a simple message to rivals: “I am larger than you.” Tusking, a behavior where two males cross and rub their tusks together, appears to be a ritualized way of sizing each other up without resorting to a full fight.
That said, fights do happen. Field observations have documented head scarring, broken tusks, and even tusks embedded in the flanks of other males. Biomechanical analysis shows tusks can’t withstand a direct ramming stab, but they can handle lateral strikes without snapping, which matches the pattern of scars found on narwhal heads. The tusk’s sensory abilities and occasional use in stunning prey likely represent secondary functions layered on top of its primary social role.
The Unicorn Connection
For centuries, most Europeans had no idea narwhals existed. Viking and Inuit traders sold narwhal tusks to medieval markets, where they were presented as unicorn horns. A single “unicorn horn” was worth ten times its weight in gold, prized for its supposed power to neutralize poison. Pope Clement VII gifted one to King Francis I of France. Queen Elizabeth I, fearing assassination, reportedly offered a lavish reward to anyone who could procure one for her. An explorer eventually found a decomposing narwhal on the Canadian coast, tusk still attached, and brought it back as proof of a “sea unicorn.”
The trade thrived through the Renaissance, and narwhal tusks found their way into royal treasuries and cathedral collections across Europe. It wasn’t until Arctic exploration improved in the 17th and 18th centuries that the true origin of these “horns” became widely known.
Female and Anomalous Tusks
Although tusks are primarily a male trait, the occasional female tusk is not dramatically different in structure. One measured female tusk was estimated at 146 to 151 centimeters, a length that falls within the normal range for males of similar body size. The difference in typical tusk size between sexes largely reflects the fact that female narwhals have smaller bodies overall, not a fundamentally different growth pattern.
Tuskless males also occur, making up about 2.8 percent of sampled males in Greenland. Combined with double-tusked males and tusked females, fewer than 3 percent of narwhals display any tusk anomaly. These variations suggest that the genetic controls over tusk eruption aren’t absolute, leaving room for occasional departures from the norm.