The narwhal, often called the “unicorn of the sea,” is instantly recognizable by the long, spiraled projection extending from its head. This striking feature is not a horn, but an enormously elongated canine tooth, or tusk. The absence of this famous tusk is primarily a matter of sexual difference and developmental biology, resulting from differing evolutionary pressures between the sexes.
The Tusk: Anatomy of a Specialized Tooth
The narwhal tusk typically grows from the upper left jaw, erupting through the lip to project forward. It can reach up to 10 feet in length and features a distinctive counter-clockwise spiral pattern. Unlike most mammal teeth, it lacks hard enamel, instead featuring porous cementum.
The dentin layer contains millions of tiny, fluid-filled channels, or tubules, which extend from the central nerve-rich pulp to the tusk’s surface. This structure allows the tusk to function as a highly sensitive organ, creating a direct pathway from the Arctic water to the narwhal’s nervous system.
The tusk detects subtle changes in its environment, sensing variations in temperature, pressure, and salt concentration. This sensory ability provides the whale with valuable information about its surroundings, including the formation of sea ice and the presence of prey.
Sexual Dimorphism and Tusk Distribution
The primary reason some narwhals lack tusks is sex, reflecting sexual dimorphism. Nearly 99% of adult male narwhals possess the long, spiraled tusk. In contrast, most females remain tuskless throughout their lives, as do juvenile males who have not yet developed the feature. Approximately 15% of female narwhals may grow a tusk, though it is typically shorter and less pronounced than those found on males.
A narwhal can rarely possess two tusks, which occurs when the typically embedded right canine tooth also erupts. This double-tusk phenomenon is seen in less than 1% of males. The standard is for only the left tooth to develop, highlighting the asymmetry of the narwhal’s dental structure.
Evolutionary Purpose of the Narwhal Tusk
Because the tusk is heavily skewed toward males, its primary evolutionary function is linked to reproduction. It serves as a secondary sexual characteristic, similar to the antlers of a male deer, signaling a male’s fitness and dominance to potential mates. Length and robustness signal the male’s health and ability to survive.
The tusk is used in male-on-male competition through ritualized sparring, known as “tusking.” Narwhals rub their tusks together, likely to establish rank and social hierarchy. Broken tusk fragments and scars on males support the idea that the feature is used as a weapon in aggressive encounters.
While the sensory function is established, the tusk’s evolutionary role is debated. The ability to detect environmental changes, such as salinity, may aid in foraging. However, since tusked females are uncommon, the sensory benefit alone does not explain the male-biased trait. It is likely a multi-functional trait, driven by sexual selection for size and maintained by natural selection for sensory capability.