Once thought to be the magical horn of a unicorn, narwhal tusks are one of the most charismatic structures in biology. However, narwhals spend most of their lives hidden underneath the Arctic ice, which makes testing the function of the tusks nearly impossible. Over the course of 35 years, we collected morphology data on 245 adult male narwhals to shed light on the function and selection pressures that have acted on this exaggerated trait. Based on the disproportional growth and immense variation in tusk length that we found, we demonstrate that narwhal tusks are indeed sexually selected, and believe that they likely function as signals during male-male aggression or mate choice.