A swimming rudder, thermal regulator, ice breaker, weapon, foraging tool, sensor—that’s the list of ideas so far that scientists have proposed the iconic narwhal tusk could be used for. But due to limited observations, it’s been difficult to be confident about any of these explanations. Now, scientists have thrown yet another idea into the mix: it could have evolved as a status signal used by females to guide mate choice. The bigger the tusk, the more attractive the male. This doesn’t necessarily discredit the other ideas, but suggests that the tusk may indicate fertility to females, much like brightly colored bird plumage or impressive deer antlers. The work has been published in Marine Mammal Science.

Narwhals, sometimes described as the “unicorns of the ocean,” are toothed whales related to bottlenose dolphins, orcas and belugas. These arctic dwellers possess just two teeth; in males, the left canine protrudes through the jaw into a sword-like helical tusk that can reach almost three meters in length. Some females also grow tusks, but they’re not as prominent.

The purpose of this tusk has been a matter of debate for some time now and many different ideas have been thrown around in the past. Males have been observed using them as weapons to fight over females, and some lines of evidence have suggested they may be used to stir up sediment in search of food. More recently, one study proposed that they may actually provide a sensory function. But given that most females don’t have one, it’s unlikely that they are crucial for survival. Now, newly gathered evidence suggests that they could serve as a “Hey ladies, I’m a good mate,” sign, much like peacock feathers.

To make this discovery, a team of researchers took various body part measurements from almost 150 narwhals legally killed in the Canadian Arctic between 1997 and 2008. They discovered a significant association between male tusk length and testes mass, which is an indicator of fertility. This indicates that the tusk may be an important factor in female mate choice and is thus sexually selected for. Bigger tusks likely signal to the female that the male is more fertile and a good mate, and would also have the added bonus of helping the male to win a fight over potential mates.

The researchers acknowledge that the tusk probably plays other roles as well, such as acting as an environmental sensor. However, since the correlation between testicle size and tusk length is only observed in mature males and not juveniles, it seems likely that it does play an important role in mating.

Unfortunately, being well endowed also comes at a cost as those with larger tusks are targeted by hunters. This could potentially disturb narwhal mating ecology, so the researchers are keen to investigate the importance of tusk length in female mate selection further.

[Via Marine Mammal Science, io9 and Science]