Although the narwhal (Monodon monoceros) is well-known throughout popular culture, the purpose of its iconic tusk is not. The appendage—which is actually a single tooth that protrudes from the whale’s upper left jaw—can grow up to 2 to 3 meters in length and is found almost exclusively in males. Many explanations have been offered up, including its potential use in defense, foraging, male competition, and breaking of sea ice; however, support for many of the proposed functions has been limited to isolated observations. Now, new findings published online this month in Marine Mammal Science provide evidence that the tusk may serve as a visible feature that females use to identify the most fertile males when choosing a mate. To make the discovery, researchers collected detailed anatomical measurements from more than 100 whales taken during hunts by aboriginal Inuits in the Canadian Arctic between 1990 and 2008. They found that tusk length was significantly related to the testes mass—an indicator of fertility—suggesting that males with longer tusks are likely also the most fertile and best mates. That makes the narwhal tusk similar to sexual traits in other animals, such as a stag’s antlers or a peacock’s feathers, that are used to attract females. Although the tusk may serve other functions for males, such as sensing changes in water temperature and salinity, the fact that it is not needed by females indicates it is not critical for survival. Instead, it may actually be a detriment for some males, as narwhals with large tusks are preferentially targeted during Inuit hunts.