When it comes to sex between the “unicorn of the sea,” size really does matter – but we’re not talking about down there. Male narwhals have evolved longer, bigger tusks (get your mind out of the gutter) in a show for courtship, a new study suggests.

The tusks of male narwhals (Monodon Monoceros) have been called one of the most charismatic structures in biology, yet little is known about their function as the elusive whales spend much of their life under Arctic ice. Females do occasionally grow small tusks but they are usually not as prominent as the males'. Like walruses and elephants, the tusks of the narwhals are not a horn but simply modified teeth. As the narwhals mature, their left tooth erupts from the head and grows in a spiral pattern reminiscent of an oceanic unicorn.

Previous observations aimed to describe the purpose of narwhal tusks have reported head scarring and broken tusks, as well as some males who were impaled in their sides, all of which suggest that the teeth may be used as a form of aggression. Other reports have described a “tusking” behavior where two individuals rub and cross their tusks, which could be a form of communication during sexual interactions.

Sexual selection is responsible for some of the “craziest traits in biology,” says doctoral student and evolutionary biologist Zackary Graham, lead author of a new study, in a statement.

"One way we try to understand these traits is by looking at the morphology, or the size and shape of them. I immediately became obsessed with trying to think of some interesting animals to study. I was Googling everything; maybe I can find a dinosaur in a museum. Eventually, I found the narwhal tusk,” said Graham.  

To understand whether the tusk plays a role in sex and choosing a mate, an international team of researchers headed by Graham, whose work is being conducted out of Arizona State University, studied the relationship between tusk size and body size by collecting morphology data on 245 adult male narwhals over 35 years. This dataset was then used to compare individuals of the same age and determine whether there were large variations in their tusk size, a typical characteristic of traits evolved for sexual purposes.

Publishing their findings in the journal Biology Letters, the team compared the growth, or scaling, of tusks against the size of a feature not likely to have any sexual purposes, like the fluke (tail). Tusks were found to have more than four-fold variation in length, ranging from 0.5 to 2.5 meters long (1.5 to 8.2 feet) but fluke hardly varies at all with widths just between 0.5 to 1 meter wide (1.5 to 3 feet). Furthermore, males of the same size can have different sizes of tusks, suggesting that they may be used as a sort of “contest” between individuals.

"By combining our results on tusk scaling with known material properties of the tusk, we suggest that the narwhal tusk is a sexually selected signal that is used during the male-male tusking contests," said Graham. "The information that the tusk communicates is simple: 'I am bigger than you.'"

This disproportionate growth of tusk in combination with varying lengths suggest that the elongated tooth is sexually selected. In short, the most desirable males will have the longest tusk, signaling to females that they are of the highest reproduction quality.

"Overall, our evidence supports the hypothesis that the tusk functions both as a sexually selected weapon and sexually selected signal during male-male contests," said Graham. "However, further evaluations of the narwhal's ecology are warranted."