Narwhals, finally explained.
Narwhal tusks are the result of some fascinatingly odd evolutionary anatomical migration. The tusk is actually a single canine tooth that made its way to the forehead of an otherwise completely toothless whale. Males can have two tooth-tusks on rare occasions, while females usually have no tusks (but sometimes grow a small one).
These tusks can be up to 10 feet long, and their use, if there is any, is unknown. They are hollow, spiral, and are perhaps to only straight tusks in the animal world. One of the most fascinating parts of narwhal tusk evolution is that they always end up on the left side of the forehead. Somehow this gene can sense when it is on the left side of the body and be activated only there.
Check out Why Evolution is True to read more about the amazingly odd forehead-tooth of Monodon monoceros, the “one tusk, one tooth” whale.
(image via Ed Yong on Twitter)