The narwhal's distinctive tusk is actually a tooth, one that can grow to nearly 10 feet in length. It's almost never found on females, and researchers have a new theory as to why: It may communicate a male's fertility to its tusk-less counterparts. Researchers pored over anatomical measurements taken of 144 narwhals that were legally killed by Inuits between 1997 and 2008. What they discovered was what Phys.org terms a "very clear correlation" between the mass of their testicles and their tusk length. Quite simply, bigger testicles and bigger tusks seem to go together. As Science explains, testicle mass is an "indicator of fertility." So by extension, the narwhals with the longest tusks are likely the most fertile.
The team suggests the tusk could be used to attract females, who would be able to judge which male would make the best mate. And in terms of mating, the size correlation was found only among mature males, not juveniles. As further evidence of their theory, the researchers note (via iO9) that "captive female narwhal have been observed becoming excited by the presence of a tusk-shaped object such as a pole or broom handle in their pool, butting the object, and jockeying for position close to it." Science notes that the tusk may serve other functions—it's been suggested as a sensory organ and weapon—but writes that "the fact that it is not needed by females indicates it is not critical for survival." (Scientists recently defrosted an incredible sea creature.)