“The whole thing that is great about the teeth of the narwhal is that nothing makes sense,” Nweeia adds. “The tusks are an extreme example of dental asymmetry. They exhibit uncharacteristic dimorphic or sexual expressions since females do not exhibit erupted tusks as commonly as males. Also, the tusk has a straight axis and a spiraled morphology. Conventional mechanisms of evolution do not help explain these expressions of teeth.”
From Wikipedia:
https://en.wikipedia.org/wiki/Sexual_selection
Sexual selection can, typically, lead males to extreme efforts to demonstrate their fitness to be chosen by females, producing sexual dimorphism in secondary sexual characteristics, such as the ornate plumage of birds such as birds of paradise and peafowl, or the antlers of deer, or the manes of lions, caused by a positive feedback mechanism known as a Fisherian runaway, where the passing-on of the desire for a trait in one sex is as important as having the trait in the other sex in producing the runaway effect.
https://en.wikipedia.org/wiki/Fisherian_runaway
As another example, musical talent is probably an example of "Fisherian runaway" in humans. It doesn't make a person more likely to survive, but it makes a person more likely to mate.
The key point is that music can be broadcast to a large audience. And the audience can instantly tell if the music is "good" or not. It can't be faked.
You make patterns of noise that indicate 1) working sensory perception 2) motor skills 3) cognitive skills like memory and 4) the ability to cooperate with other humans (i.e. a band).
Hence it indicates fitness. It's selected for sexually, but not "naturally".
(edit: Now that I google it, it seems like a lot of scientists think that too (including Darwin), which is not surprising. It seems hard to prove, even though it makes a lot of sense.)