The narwhal has a thickset body with a short, blunt snout, small upcurved flippers, and convex to concave tail flukes. Adults measure in body-to-tail length from 3.0 to 5.5 m (9.8 to 18.0 ft) and weigh 800 to 1,600 kg (1,800 to 3,500 lb.). Male narwhals attain sexual maturity at 12 to 20 years of age, reaching a length of 3.5 to 4.0 m (11.5 to 13.1 ft). Females become sexually mature at a younger age, between 8 and 9 years old, when they are about 3.4 m (11 ft) long. On average, males are about 70 cm (28 in) longer and more than 75% heavier than females.
The pigmentation of the narwhal is a mottled pattern, with blackish-brown markings over a white background. At birth, the skin is light grey, and when sexually mature, white patches grow on the navel and genital slit. Whitening occurs throughout life, resulting in aged narwhals that are almost pure white. Unlike most whales, the narwhal has a shallow dorsal ridge, rather than a dorsal fin. This is possibly an evolutionary adaptation to make swimming under ice easier, or to facilitate rolling. The neck vertebrae are jointed, instead of being fused together as in most whales; this allows a greater range of neck flexibility. These characteristics—a dorsal ridge and jointed neck vertebrae—are shared by the beluga whale. Male and female narwhals have different tail flukes; the former are bent inward, while the latter have a sweep-back on the front margins. This is thought to be an adaptation for reducing drag caused by the tusk.
Compared with most marine mammals, the narwhal has a higher amount of myoglobin in its body, which facilitates deeper dives. It has a dense layer of blubber, around 50 to 100 mm (2.0 to 3.9 in) thick. This fat accounts for a third of the body mass and helps insulate from cold ocean temperatures. Skeletal muscles in the narwhal are highly adapted for prolonged periods of deep-sea foraging. During such activities, oxygen is reserved in the muscles, which are typically slow-twitch, allowing for endurance and maneuverable motion.
The most conspicuous characteristic of the male narwhal is a long, spiraled tusk, which is a canine tooth that projects from the left side of the upper jaw. Both sexes have a pair of tusks embedded in the upper jaw, which in males erupts at two or three. The tusk grows throughout the animal’s life, reaching lengths of 1.5 to 3 m (4.9 to 9.8 ft). It is hollow and weighs up to 7.45 kg (16.4 lb.). Some males may grow two tusks, occurring when the right canine also protrudes through the lip. Females rarely grow tusks: when they do, the tusks are typically smaller than those of males, with less noticeable spirals.
The function of the narwhal tusk is debated. Some biologists suggest that narwhals use their tusks in fights, while others argue that they may be of use in feeding. There is, however, a scientific consensus that tusks are secondary sexual characteristics which indicate social status. The tusk is also a highly innervated sensory organ with millions of nerve endings that connect seawater stimuli to the brain, allowing the narwhal to sense temperature variability in its surroundings. These nerves may be able to pick up the slightest increase or decrease in the magnitude of particles and water pressure. According to Dr. Martin Nweeia, male narwhals may rid themselves of encrustations on their tusks by rubbing them together, as opposed to posturing displays of aggressive male-to-male rivalry. Drone footage from August 2016 in Tremblay Sound, Nunavut, revealed that narwhals used their tusks to tap and stun small Arctic cod, making them easier to catch for feeding. Females, who usually do not have tusks, live longer than males, hence the tusk cannot be essential to the animal’s survival. It is generally accepted that the primary function of the narwhal tusk is associated with sexual selection.