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.

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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:

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]

Header image: Public domain/Wikimedia Commons