Nature

21
Mar

Narwhal Tusks Might Be Nerve-Rich Ocean Sensors

These tusks weren’t just made for jousting.

Since the 15th century, people have come up with many theories as to why this “sea unicorn”—whose relatives include beluga whales, orcas, and dolphins—sprouted such an obscenely large appendage. Some have called it an acoustic probe, others have labelled it an ice picker, and still others have cited its possible role in fending off predators. These hypotheses didn’t gain much traction, though, because they were based on mere observation.

Now Martin Nweeia, a Connecticut dentist and a clinical instructor at the Harvard School of Dental Medicine, and his colleagues believe they have an answer. They’ve concluded that the tusk is a sensory organ that helps it navigate and perceive the ocean.

narwhals-beach
Are these narwhals battling each other, or sensing their environment? A new study claims it's the latter.

Nweeia’s background as a dentist helped make him an authority on narwhal tusks, in part because the tusks actually massive canine teeth. Nweeia says that while most mammals have protective sheets of enamel on their teeth, narwhals’ tusks are instead coated in fine grooves that channel water closer to the nerve endings in the tusks’ interior. Learning that wasn’t easy, though.

Here’s Nadia Drake, writing for Wired Science:

Nweeia and his colleagues collected narwhal tusks from Inuit hunters near Baffin Island, then studied those tusks for anatomical clues to their function. Turns out, narwhal tusks are filled with a nerve-rich pulp that’s similar to the stuff in human teeth that can sometimes make drinking coffee or eating ice cream a painful experience.

Next, the team looked to see if there were any genes expressed by the pulp that would indicate a role in relaying sensory information to the brain. And there were: two genes expressed in sensory signaling pathways were present at much higher levels in tusk pulp than in muscle or jaw tissues.

Nweeia and his team also discovered that the tusks can detect the difference between salt water and fresh water. Since the ability is only apparent in male narwhals, though, it’s possible that they’re using it to find suitable mates—either by sampling the kinds of chemicals that female narwhals release or by measuring the salinity of the water where females are likely to feed.

narwhal-anatomy
Narwhal horns and anatomy, from a 1746 drawing

While some scientists contest this new theory, instead claiming that narwhals use their tusks to fight each other for mates, others say that both theories could be correct. Douglas Emlen of the University of Montana cites a study of the Japanese giant rhinoceros beetle’s horn to back up that claim, which Carl Zimmer has more on at his blog, The Loom.