One big tooth: Let’s talk about the narwhal’s tusk

In the far far north, close to the sea ice, lives a mysterious creature often called the unicorn of the sea: the narwhal. This animal owes its name to its tusk. In old Norse, the term nafarr means “auger”, which is a type of drill that resembles the narwhal’s spiraled tusk. Narwhals are quite elusive and live close to the Arctic, which means they are hard to study. Why do they have this massive tusk, and what is its purpose? In this article, we dive into the icy waters of the North Pole and explain what scientists have discovered so far.

Credit: Wikimedia Commons

An elongated tooth

This might surprise you, but the tusk is essentially an overgrown tooth. Narwhals possess only two teeth, and only one of them can develop into an up to 3m-long tusk (10ft). This overgrown tooth is off-centered and causes the head to grow asymmetrically. In rare cases, both teeth can overgrow, and narwhals develop two tusks. However, this is quite a rare event; having one tusk is much more frequent. Only male narwhals develop a tusk, although there are some rare cases where also females develop one. So if you come across a narwhal with a tusk, chances are it’s a male.

Did you know that the narwhal’s tusk is flexible? It can bend up to one foot (30 cm) in all directions. In fact, narwhals use their tusk to only stun their prey instead of stabbing them.

Ten million nerve endings

Recent studies have revealed that the narwhal tusk possesses up to 10 million nerve endings. It makes their tusk extremely sensitive, just like our teeth. Scientists have experimented with exposing the narwhal’s tusk to changing salt concentrations. Increasing salt concentration in the water is linked to ice formation in the natural environment (the saltier the water, the colder). They found that the animal’s heart rate increased when exposed to higher salt concentrations. The sensation must be similar to when we bite something icy, like ice cream.

The tusk as a weather forecast: sensing ice formation

If narwhals cannot use their tusk to stab their prey, what is the tusk’s function? This question has long been debated, and scientists are still trying to answer it. Lots of theories have been proposed, from hunting to breaking the ice and even as a method of communication. Due to the tusk’s sensory ability, some studies suggested that their function is to sense when ice cover is forming by detecting the changes in salt concentration. This ability could enhance narwhals’ survival: ice can form abruptly and cover the few breathing holes that narwhals use to survive. By predicting where and when the ice is forming, they can avoid entrapment and swim towards ice-free waters. This theory, however, does not explain why females do not develop a tusk and still manage to survive.

A big tusk equals big testes

A study conducted in 2020 showed that the size of the narwhal’s tusk is directly linked to its sexual dominance. A male with a bigger tusk is likely to scare off other males when competing for a female. A large tusk conveys the message “I am stronger than you, you better stay away” and discourages potential opponents from engaging in a fight. The study also found that males with bigger tusks also had bigger testes. This is evidence that the tusk is a secondary sex trait, similar to the mane of a lion or the antlers of a deer. The size of the tusk is thus an indication of males’ overall fitness, and the bigger the tusk the fitter the individual. Keeping a large tusk is energetically costly; therefore, only healthy and well-fed males can afford to develop it. Consequently, females are attracted to males with big tusks; it is a sign of a healthy and strong individual who will produce healthy offspring.

In the end, the narwhal’s tusk could have evolved as a secondary sex trait but still be used for other functions. Research is ongoing, and scientists still do not have a full answer about the tusk’s range of functions. If males use it for purposes other than sexual selection, it does not give them an advantage over females in terms of survival. In fact, females still manage to thrive and live longer than males. Hopefully, future research will give us more clues about the full function of these impressively long teeth.

A recap of this post on the narwhal’s tusk – Credit: Anaïs Remili / Whale Scientists

Sources and further reading

  • Graham, Zackary A., et al. “The longer the better: evidence that narwhal tusks are sexually selected.” Biology letters 16.3 (2020): 20190950.
  • Dietz, Rune, et al. “Analysis of narwhal tusks reveals lifelong feeding ecology and mercury exposure.” Current Biology 31.9 (2021): 2012-2019.

Did you enjoy this post about the narwhal’s tusk? Find out more about the narwhal’s cousin, the beluga!

The beluga whale – February 2021

Chiara obtained her Honours Bsc in Marine Biology from the University of Aberdeen. She had the chance to volunteer in Galicia at the BDRI, where she helped with dolphins Photo-ID and boat surveys. She also spent one year in Tromsø as an Erasmus student. She was lucky enough to participate in an expedition in West Ice to study harp and hooded seals.
She enjoys science communication and has a passion for seals. Her honours project focused on the timing of moult in harbour and grey seals, in relation to sex and reproductive status.

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