The narwhal (Monodon monoceros) is a medium-sized toothed whale that lives year-round in the Arctic. One of two species of whale in the Monodontidae family, along with the Beluga whale, the narwhal males are distinguished by a characteristic long, straight, helical tusk extending from their upper left jaw. Found primarily in Canadian Arctic and Greenlandic waters rarely south of 65°N latitude, the narwhal is a uniquely specialized Arctic predator. In the winter, it feeds on benthic prey, mostly flatfish, at depths of up to 1500 m under dense pack ice. Narwhal have been harvested for over a thousand years by Inuit people in Northern Canada and Greenland for meat and ivory and a regulated subsistence hunt continues to this day. While populations appear stable, the narwhal has been deemed particularly vulnerable to climate change due to a narrow geographical range and specialized diet.
Male narwhals weigh up to 1,600 kilograms (3,500 lb), and the females weigh around 1,000 kilograms (2,200 lb). The pigmentation of the narwhal is a mottled black and white pattern. They are darkest when born and become whiter in color with age.
The most conspicuous characteristic of the male narwhal is its single 2–3 meter (7–10 ft) long tusk. It is an incisor tooth that projects from the left side of the upper jaw and forms a left-handed helix. The tusk can be up to 3 meters (9.8 ft) long—compared with a body length of 4–5 meters (13–16 ft)—and weigh up to 10 kilograms (22 lb). About one in 500 males has two tusks, which occurs when the right incisor, normally small, also grows out. A female narwhal may also produce a tusk, but this occurs rarely, and there is a single recorded case of a female with dual tusks.
The most broadly accepted theory for the role of the tusk is as a secondary sexual characteristic, similar to the mane of a lion or the tail feathers of a peacock. This hypothesis was notably discussed and defended at length by Charles Darwin, in The Descent of Man, and Selection in Relation to Sex (1871). It may help determine social rank, maintain dominance hierarchies or help young males develop skills necessary for performance in adult sexual roles. Narwhals have rarely been observed using their tusk for fighting or other aggressive behavior or for breaking sea ice in their Arctic habitat.