Narwhal tusk – size matters

According to popular belief, the narwhal is inextricably linked with the fabled unicorn, and for good reason: the long, spiraling tusk of the adult male came closer to “proving” the existence of the unicorn than anything else in nature. In fact, since the Middle Ages, traders and chemists have conspired to keep the narwhal’s existence a secret, while selling its tusk as “unicorn horn” for immense profit. Although no longer associated with the mythical horned horse, the narwhal remains an irresistible creature due to the remoteness and harshness of its Arctic environment, as well as its unusual appearance.
Now, a new study confirms that the male’s tusk is the result of sexual selection.

Magnificent peacock feathers, extravagant antlers in deer and powerful crab claws are just a few examples of the ostentatious extremes with which animals compete for mates and attract them. A process known as sexual selection.

Thanks to Zackary Graham, a researcher at Arizona State University, and his colleagues, the “unicorn of the seas”, the narwhal, can now be added to the list.

“Broadly speaking, I’m interested in sexual selection, which is responsible for giving rise to some of the weirdest traits in biology. As an evolutionary biologist, I’m trying to understand why some animals have these bizarre traits and others don’t,” said Graham, who is a doctoral student in ASU’s School of Life Sciences.
“One way to understand these traits is to look at the morphology, or the size and shape of the animals. I immediately became obsessed with picking some interesting animals to study. I googled everything; maybe I could find a dinosaur in a museum. Eventually I found the narwhal tusk.”

Graham is the lead author of a new study that provides the best evidence yet that the narwhal tusk functions as a sexual trait, published in the journal Biology Letters.

Similar to walruses and elephants, male narwhals(Monodon monoceros) grow tusks. In narwhals, the left of two teeth in the upper jaw breaks through the lip at the age of two or three years and grows up to 2.70 meters long in some individuals. The tusk grows out in a spiral, giving it the appearance of a unicorn living in the sea.

As narwhals spend most of their lives hidden under the Arctic ice, there has been much speculation as to what exactly the tusk is used for: hunting, fighting or perhaps something more amorous in nature?

Graham mentions that there are reports of scars on the head, broken tusks and lateral injuries from tusks on males that may have been the target of some aggression. Other anecdotal observations describe “tusking” behavior, where two narwhals are facing each other, crossing their tusks and rubbing them together. This suggests that the tusk is used for communication during intra- or intersexual interactions.

Graham has studied sexual selection in many different species, including the crayfish he is studying for his PhD. He realized that to prove the sexual selection of the tusk, he could use the relationship between tusk size and body size to understand this mysterious trait. To this end, his team collected morphology data from 245 adult male narwhals recorded over a 35-year period.

Together with colleagues Alexandre V. Palaoro from LUTA do Departamento de Ecologia e Biologia Evolutiva, UNIFESP, Brazil, and Mads Peter Heide-Jørgensen and Eva Garde from the Greenland Institute of Natural Resources, they compiled a large data set from the carefully collected narwhal data from field observations.

When comparing individuals of the same age, sexually selected traits often show disproportionate growth, i.e. for a given body size, sexually selected traits are often larger than expected in the largest individuals. Importantly, they compared the growth of the tusk with the size ratio between body size and a trait that probably has no sexual functions. They used the tail fin of narwhals for this purpose.

“We also predicted that we would expect to see greater variation in tusk length compared to variation in caudal fin width under sexual selection of the tusk,” Graham said. This is because many sexual traits are very sensitive to nutrients and body condition, so only the largest and strongest individuals can muster the energy to produce extremely large traits

According to Graham, they found that the tusks can vary more than fourfold in length: with the same body size, males can have tusks from 45 centimeters to 2.5 meters in length. The caudal fin, however, hardly varies; in individuals of the same body size, it is between 45 and 90 centimeters wide. They also found a disproportionate growth of the tusk compared to the caudal fin. Due to the disproportionate growth and the large variation in tusk length, they have provided the best evidence to date that narwhal tusks are indeed sexually selected.

“By combining our results from the scaling with known material properties of the tusk, we suggest that the narwhal tusk is a sexually selected signal used during fights between males to clarify rank order,” Graham said. “The information conveyed by the tusk is simply: ‘I’m bigger than you’.”

And if only the fittest and strongest males produce the biggest tusks, then the tusk probably serves as an honest signal of quality to the females or males.

Graham hopes that future researchers will use aerial and aquatic drones to provide concrete evidence of the tusk’s function in nature and clarify the exact role of the tusk as an aggressive weapon, a sexual signal, or both. Perhaps one day we can look forward to a nature documentary “Great love: narwhals under the ice”.

“Overall, our evidence supports the hypothesis that the tusk functions both as a sexually selected weapon and as a sexually selected signal during rank bouts among males,” Graham said. “However, further assessment of narwhal ecology is warranted.”

Source: Arizona State University