Narwhal tusks are teeth

The narwhal's tusk, which may appear to be in the middle of its head, is an enlarged front left tooth that protrudes from the upper lip. The tiny right front tooth typically stays in the mouth. Even stranger, narwhal teeth are the opposite of most teeth (including human teeth) which have a hard outside and a soft, sensitive inner.

"No real shock there. After learning this, Martin Nweeia, a clinical instructor at the Harvard School of Dental Medicine, told NPR that it had been the opposite in every other manner. "But it was quite strange to discover a tooth that is soft on the outside and densest near the pulp."

Narwhal tusks' sensory capabilities were very recently identified. Researchers from Harvard Medical School discovered in 2014 that a narwhal's heart rate changes depending on whether its tusk is exposed to high or low levels of salt in ocean water.

The tusks of narwhals can grow to astonishing lengths of up to 2.6 meters (8.53 ft) throughout the course of their whole lives. A closer inspection reveals that it spirals to the left as it expands.