Tusk Love

For narwhals, a big tusk means a sexual advantage

When it comes to romance, size really does matter.

For one of the most mysterious animals in the oceans, size really does matter.

That's the upshot of a new study that finally answers why, unlike every other whale alive today, narwhals evolved their showy tusks. It's all about showing the ladies who is boss.

The new study contradicts several other theories about these creatures' unusual teeth. For most of their lives, narwhals live in Arctic waters, and scientists had thought the tusks — which are actually long, spindly teeth that can grow up to 8 feet — may help them bust through the ice. There’s was also some speculation that these lance-like tusks may come in handy during hunting or fighting.

But the new evidence shows the actual reason has to do with love — or, at least, mating.

Basically, a big tusk means a sexual advantage over other males, the research suggests.

The study, led by Zackary Graham, a graduate student at Arizona State University, calculates the relationship between tusk and body size using data from 245 adult male narwhals collected across a 35-year span.

A big tusk establishes one male narwhal's dominance over other males, making them the more attractive to the ladies, the researchers found.

But that is not all they can be used for, the study suggests.

Narwhal tusks show other males who's the boss, making them more attractive mates.Zack Graham

The toothy protrusion also plays a role in male-on-male combat.

"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," Graham said in a statement.

"The information that the tusk communicates is simple: ‘I am bigger than you,'" Graham said.

With “contest” winners decided by largest tusk, the male narwhals signal their quality to female mates, too, the researchers say.

The results were published Tuesday in the journal Biology Letters.

Mysteries of the narwhal, explained

Narwhal tusks are actually overgrown teeth. Their left tooth breaks out of the top of their head and grows in a spiral pattern, unicorn-style.

The unusual horn earns narwhals the distinction of being the "unicorns of the sea." And much like unicorns, they are peculiar, singular creatures.

Curiously, male narwhals’ tusks vary four times as much as their body size. That's evidence of just how important these distinguishing features are for narwhal courtship, the researchers say.

The narwhal's tusk is actually an overgrown left tooth.Zack Graham

When they are not duking it out in biggest-tusk competitions, these creatures spend most of their time in the waters of Canada, Greenland, Norway, and Russia. For five months of the year, most of the world’s narwhals hang out under the sea ice in the Baffin Bay-Davis Strait area, according to the World Wildlife Fund. They breathe through cracks in the ice, and can dive extremely deep in the water, up to a mile and a half down.

Favorite foods of narwhals include Greenland halibut, along with other fish, squid, and shrimp.

And now, one more finding about these underwater unicorns: When it comes to tusks, size matters.

Abstract: Once thought to be the magical horn of a unicorn, narwhal tusks are one of the most charismatic structures in biology. Despite years of speculation, little is known about the tusk’s function, because narwhals spend most of their lives hidden [underneath] the Arctic ice. Some hypotheses propose that the tusk has sexual functions as a weapon or as a signal. By contrast, other hypotheses propose that the tusk functions as an environmental sensor. Since assessing the tusks function in nature is difficult, we can use the morphological relationships of tusk size with body size to understand this mysterious trait. To do so, we collected morphology data on 245 adult male narwhals over the course of 35 years. Based on the disproportional growth and large variation in tusk length we found, we provide the best evidence to date that narwhal tusks are indeed sexually selected. 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 male–male contests.
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