The magnificent horn of the Narwhal is actually a tusk - the left canine tooth, to be exact. It grows throughout the animal’s life, and can reach lengths of up to 10 feet (3m). Despite this huge size, it’s hollow, and so surprisingly lightweight. It’s rarely used for fighting, the way antlers are in deer, or for other percussive uses like breaking ice cover.
Researchers have recently shown that it’s full of nerve endings and is highly sensory, useful for detecting small changes in things like temperature and salinity of their environment. The tusk is the Narwhal’s only tooth (except for a very small percentage - about 1 in 500 - of males that may grow a second tusk); the rest of its mouth is toothless. Despite this, they are fish-eaters, and since they can’t grab their prey with teeth, it’s thought that they suck it into their mouth when they get close.