The narwhal, Monodon monoceros, is an Arctic whale known as the “unicorn of the sea” due to its single, long, spiraled tusk. This structure, which can reach lengths of up to 10 feet, projects forward from the animal’s head. For centuries, the purpose of this unique appendage remained a mystery, sparking myths and speculation. Modern science has revealed that the tusk is actually a highly specialized tooth, serving multiple complex roles in the narwhal’s life.
Anatomy and Composition: Defining the Tusk
The narwhal tusk is not a horn, but an elongated canine tooth, typically the upper left one, that erupts through the whale’s upper lip. The core is primarily dentin, a calcified tissue that forms the bulk of mammalian teeth and is a type of ivory. Unlike most mammalian teeth, the narwhal tusk lacks a hard outer layer of enamel where it is exposed to the ocean environment.
Instead, the tusk’s exterior is covered by cementum, a bone-like tissue normally found below the gumline. This structural arrangement makes the tusk an “inside-out” tooth, featuring a softer, flexible exterior and a harder core. The tusk grows in a left-handed spiral and is hollow, containing a central pulp cavity that runs its entire length. This composition provides a remarkable combination of flexibility and toughness, allowing it to bend without fracturing.
The Tusk’s Role as a Sensory Organ
The absence of enamel and the exposed cementum layer link the tusk directly to its function as a highly sensitive sensory organ. The dentin within the tusk is threaded with millions of microscopic channels, called dentinal tubules, which extend from the outer cementum layer to the inner pulp cavity. These tubules are open to the external environment through porous channels, allowing seawater to flow inside.
This pathway provides a direct connection between the Arctic water and the central nerve pulp. Nerve endings within the pulp are connected to the narwhal’s brain, enabling the detection of subtle environmental changes. Experiments confirm the tusk acts as a chemosensor: the narwhal’s heart rate changes significantly when the tusk is exposed to alternating solutions of high-salt and fresh water. This capability assists the whale in detecting variations in water salinity and temperature, which helps locate food or navigate ice formations.
Development and Sexual Dimorphism
Narwhals are born with two teeth in their upper jaw; the tusk develops from one of these canine teeth, usually the left one. The tooth begins to erupt through the lip between two and three years old and continues to grow throughout the animal’s life. Rarely, a narwhal may develop two tusks if both canine teeth erupt, or sometimes have no tusk at all.
The tusk is a clear example of sexual dimorphism, as it is found in almost all males but only about 15% of females. Female tusks are typically smaller and less spiraled than a male’s. The variation in male tusk length suggests it is a sexually selected trait, signaling social dominance and fitness. Males engage in “tusking,” crossing and rubbing their tusks, which is hypothesized to be a form of communication or a way to establish rank during contests.