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Friday 18 February 2022

Why do Narwhals have tusks?

Narwhals, the unicorns of the sea, have large tusks (which are actually large canine teeth) protruding from their foreheads. Each tusk holds 10 million nerve endings. But what do they do? Narwhals have not been observed fencing with them for territory, food or mates, or using them to cut things or defend against enemies. Scientists think they may be sensors, sensing changes in pressure, salinity and water chemistry.
Narwhal illustration. Strangely, there are few good pictures of these animals as they spend most of their time partially submerged.
"Where's Mr Narwhal?"
I’ve just been reading “Where’s Mr Narwhal?” to my 1-year-old, and it occurred to me that it was asking the wrong question.
Narwhals might be scarce and hard-to-spot, but we do know where to find them. On the other hand, their impressive and distinct tusks still remain a mystery – and so here I am, writing about it.

Because the tusk is a primarily male structure, scientists believe that it is a secondary sexual characteristic for nonviolent assessment of hierarchical status… but the tusks do occur in 15% of female narwhals. For the females, however, they tend to be shorter and less spiral. And there's other variation: some males have two tusks. Although they haven't been seen fighting, 40-60% have damaged tusks, leading scientists to think they do fight with them – unseen.

Other ideas for the function of the tusk include an acoustic probe, a thermoregulator, a swimming rudder, a breathing organ, a hunting or defence tool, or a tool for breaking or mounting ice sheets… This could be particularly useful as ice sheets have the potential to trap narwhals and are a serious environmental threat.

Narwhal tusk showing spiral structure. Science Museum Group.
There is good evidence to support the tusk-sensor hypothesis too[1]. Examining its structure shows a sensory pathway all along the tusk to the narwhal brain. Cementum channels (holes in the surface layer of the tusk) connect deep inside the tusk, where nerves known as pulpal nerves transmit signals to the tusk base and connect to cranial nerves that reach the brain. Putting alternating high and low salt concentration signals down the tusks of male narwhals changes their heart rates. Heart rates increase in response to high salt concentrations as expected, since salinity increases in the remaining water when ice forms nearby, and ice, as we know, is dangerous and could trap the narwhals. Scientists think the tusk could also act as a sensory for other stimuli such as temperature and pressure.

It’s likely the females don’t have this sensory ability, as their tusks, when they have them, are so different. But we're yet to test them.

Overall, it’s likely a range of uses for the tusks exist, and their evolutionary history and purpose remain mysterious. More uses may yet be discovered.

why don't all references have links?

[1] Nweeia, Martin T., et al. "Sensory ability in the narwhal tooth organ system." The anatomical record 297.4 (2014): 599-617. doi: 10.1002/ar.22886.

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