Scientists in the United States have discovered the secret of the narwhal's long tusk, which they say is something unique in the animal world.

Researchers working in Canadian Arctic with the sea mammal say the tusk is actually a sensory probe delivering information to the animal in a distinctive way.

The narwhal's tusk, a 1.5-metre-long tooth emerging from the left side of the upper jaw, has long been a source of fascination. Its spiral nature led to it being marketed for princely sums in medieval Europe as a unicorn's horn.

In the past the tusk has been judged a weapon, a mating display and a fishing spear.

It turns out, the truth is stranger than the fiction.

Scientists studying the animal in Canada's Arctic have found that more than 10 million tiny nerve connections tunnel their way from the tusk's core to its outer surface.

These give the tusk an extremely sensitive surface, capable of detecting changes in water temperature, pressure and particle gradients, scientists say. It also allows the whales to detect water particles characteristic of the fish that constitute their diet.

And when Narwhals display "tusking" behaviour, or rub tusks, they're likely experiencing a unique sensation, say scientists.

The researchers say there is no other animal with a comparable ability in nature, and certainly no comparable tooth with that kind of functional adaptation.

"Now that we know the sensory capabilities of the tusk, we can design new experiments to describe some of the unique and unexplained behaviours of this elusive and extraordinary whale," said Martin Nweeia of the Harvard School of Dental Medicine in Boston.

The research into the nature and function of the narwhal's tooth may also lead dental researchers to develop better materials for tooth restoration in humans, says Nweeia.

The research was partly funded by the federal Department of Fisheries and Oceans. The findings were presented Tuesday at the Conference on the Biology of Marine Mammals in San Diego.