Narwhals grow long tusks, if they have the balls to do it. The spear at the end of the male sea unicorn has been variously thought to be a sensory organ, a device for battling other males, or an elaborate reproductive signal. New evidence shows they're the whale version of peacock feathers. 
 
 The latest research, published this week in the journal Marine Mammal Science, began with the detailed anatomical measurements taken from 144 narwhals legally killed in Inuit subsistence hunts, in the Canadian Arctic between 1990 and 2008. 
 
 The researchers, led by University of Manitoba researcher Trish C. Kelley, discovered a positive correlation between the size of male narwhal's tusks, which are really just long teeth, and the mass of their testicles. The researchers suspect that tusks evolved as an "honest signal" of reproductive status and fertility. They write: 
 
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 Larger males, with proportionately larger tusks, would be likely to successfully deter smaller males from engaging in conflict, or be more likely to be successful in an intra-specific contest for mates. Pairs of narwhals have been observed crossing their tusks and striking them together, and males have been found with tips of broken tusk embedded in their jaws near the base of their own tusk. In addition to fighting with males, the tusk may also serve to attract females, as 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. 
 
 While they acknowledge that the tusk has evolved other functions as well, such as being a sensory organ, the relationship of tusk length with testicle size only applies to mature males, not to juveniles, which suggests that it probably has a critical role to play in mating. It places the narwhal's tusk firmly alongside deer antlers and peacock tails as sexually selected status signals. 
 
 The problem is that larger tusks are also favored by hunters. On one hand, according to an idea in biology called Zahavi's Handicapping Principle, that's an even greater indication that narwhal tusks evolved as sexual displays. That's because it's really a bad idea to fake a large tusk, unless you have the balls to back it up. On the other hand, it may make narwhal conservation harder. Kelley explains: 
 
 The skewed sex-ratio of the harvest...may be an additive stress on the conservation of [narwhals], as larger tusks seem to be favored both by hunters, and potentially by female narwhal, as they appear to be honest advertisements of male fecundity. A more complete understanding of the importance of the tusk to females is necessary to ensure hunting of large-tusked males does not disrupt narwhal mating ecology. 
 
 And you thought size didn't matter. I guess now we know what adolescent narwhals compare in their high school locker rooms. 
 
 [Marine Mammal Science] ||||| 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.
– 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.)