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Big tusks are sexy, if you're a narwhal

I like big tusks and I cannot lie. You other narwhals can deny.

The narwhal tusk is actually a tooth.
Zack Graham, Arizona State University

When it comes to facing down rivals and attracting mates, narwhals with big tusks might have an advantage. 

Arizona State University researcher Zackary Graham led a study on narwhal tusks, published Tuesday in the journal Biology Letters. The research suggests that tusks, which are elongated, spiraling teeth that emerge from the whales' heads, are a sexual trait. Put simply, a big tusk signals a stronger, more attractive narwhal.  

Graham has a special interest in sexual selection, which he said "is responsible for creating some of the craziest traits in biology." The narwhal's distinctive unicorn-like tusk was a perfect candidate for a deeper investigation.

Typically, male narwhals have tusks, but the size of the tusk can vary.

Zack Graham, Arizona State University

Narwhal behavior is still very mysterious, but the tusks may be used in battles between males while also acting as a sexual signal for females.   

The team collected data on 245 adult male narwhals and looked at tusk growth compared to body size and the size of the narwhals' tails (called "flukes"). 

"If the narwhal tusk is sexually selected, we expect greater variation in tusk length compared to the variation in fluke width," said Graham in an Arizona State University release on Tuesday. 

The study found a wide amount of variation in tusk length for males with the same body size, with tusks ranging from 1.5-feet to 8.2-feet long. In comparison, the tails, which aren't used to prove a narwhal's sexual prowess, showed very little variation among similarly sized narwhals. So the tails were sized as expected, but the tusks were wildly different. 

So what's the point of sporting a longer tusk? The tusk may act as a signal to other males. "The information that the tusk communicates is simple: 'I am bigger than you,'" said Graham.  

Scientists refer to physical traits that truly indicate the worth of a mate as "honest signals." If this research holds up, then a female narwhal sizing up two similar males would likely be more into the one with the larger tusk as an honest signal of quality, health and good genes.

According to the university, this is one of those tough nature truths: "...many sexual traits are highly sensitive to nutrient and body condition, such that only the biggest and strongest individuals can afford the energy to produce extremely large traits."  

The new tusk research dovetails nicely with a 2014 study that found narwhals with longer tusks had larger testicles. Graham and his colleagues hope future observations of narwhals in the wild will help better determine the role of the tusk in the lives of the "unicorns of the sea."