EVER since Europeans began trading the whorled ivory tusk of the narwhal in the 12th century, people have puzzled over its purpose. Early research was hampered by the widespread myth that the tusk came from the head of a unicorn. Its supposed magic properties made it a prized item: Queen Elizabeth I bought one for £10,000, the price of a castle. Yet even after it was firmly established that the tusk was the left-front tooth of the narwhal, an Arctic whale unappealingly named the "corpse whale" by Scandinavians because of its mottled colouring (nar means corpse and hvalr means whale in Old Norse), conjecture continued as to why a whale would have developed such an appendage. What is its purpose? Herman Melville, the author of "Moby Dick", joked that the tusk, which can grow close to three metres in length, was a letter opener. Charles Darwin thought it had something to do with sexual selection, because with rare exceptions it is only found in males. Others have suggested that it could be used as a weapon, a tool to stir the ocean floor in search of food, a means of propping the whale’s head on an ice floe to sleep, or as a spear to catch fish—although how the narwhal would get its prey off its tusk and into its mouth remained a riddle. Research by a dentist from Harvard School of Dental Medicine published earlier this year said the massive tooth was a sensor for changes in water salinity. Most of these theories founder on the crucial point that if the tusk were necessary for the whale’s survival, females would have them too.

The speculation circled back to Darwin last month when a team led by Trish Kelley of the University of Manitoba published research in Marine Mammal Science that indicates the tusk’s main purpose is to attract females and display male status, much like feathers on a peacock or antlers on a deer. The team arrived at this conclusion after measuring the length of tusks and the weight of testes on male narwhals caught in an 11-year period ending in 2008. It seems that narwhals with big tusks were also well endowed at the other end. One look at the tusk would alert females to what they could expect. The tusk as a symbol of virility fits with observations by Inuit, who hunt narwhal for food. (The vitamin C in narwhal skin and blubber helps ward off scurvy.) Inuit say males with the biggest tusks lead whale pods and that tusking displays, when two males rub their tusks together, appear more playful than aggressive. As for those broken tips, which some researchers took as evidence of fighting, they are probably caused by narwhals hitting the bottom in panicked flight from killer whales or human hunters, according to Inuit.

Is the science settled, then? “The fun thing about science is that nothing is ever really definitive,” says Ms Kelley. “You amass evidence to support the theory.” To that end, her team is planning new research into the relationship between testosterone levels, testes size and tusk length in the hope it will bolster their case. There are a surprising number of researchers working on various tusk theories. And it may be that more than one is true, says Ms Kelley. “I don’t want to say there is absolutely no other function that the tusk can serve because you are never really sure.” But the consensus is growing that Darwin was right: the male narwhal grows his spectacular tusk mainly to attract members of the opposite sex.

Dig deeper:

The Inuit take on greens and animal rightists over hunting (February 2013)

The origins of the myth of the unicorn (June 2013)

Damien Hirst sticks a plastic narwhal horn on a pony in the name of art (September 2008)