The narwhal, a medium-sized whale with a long tooth that looks like a unicorn's horn, feeds on fish that swim in Arctic seas such as Greenland halibut, arctic cod, polar cod, squid, shrimp and cuttlefish. Because narwhals do not have well-developed teeth, they swim close to their prey and suck it into their mouths.
Narwhals feed in deep Arctic waters often covered in sea ice, and for this reason it has been difficult to observe them feeding. Information on narwhal feeding habits has been obtained from examining their stomach contents. From these, it has been determined that they follow a very restricted and specific diet of only a few species of fish. Unlike most sub-arctic whales, narwhals feed mostly in winter. During the ice-free months of summer their stomachs are nearly empty. To feed, they make long, deep dives of up to 25 minutes into the total darkness at depths up to 4,500 feet. Physiological adaptations such as a compressible rib cage and the ability to carry a large amount of oxygen in their lungs, muscles and blood enable them to make the dives.
In medieval times, narwhal tusks were more valuable than gold, as they were believed to be unicorn horns. People attributed magical powers to them. In fact, the narwhal tusk is a front tooth that grows right through the narwhal's lip up to about 9 feet in length. Usually only males have tusks, and if females grow them, they are not as long as those of the males. Scientists have speculated about the purpose of the tusks, but the most prominent belief is that they have something to do with narwhal mating rituals.