The mythical unicorn continues to gallop its way across the fields of human imagination, from the 4,000-year-old Indus Valley civilization carvings to the Starbucks Unicorn Frappuccino. But as Argentinian writer and dreamer Jorge Luis Borges pointed out, "The first version of the Unicorn is nearly identical with the latest."
In other words, the unicorn is rarely little more than a one-horned horse. As Joe McCormick and I discuss in our podcast episode "Unicorns of the Mythic World," the fairy-tale beast has come to symbolize everything from sinful lust to Jesus Christ himself, and the purpose of its single horn is rarely clear. Luckily, our episode provides some tantalizing biological suggestions as to why these fanciful beasts would boast such a fabulous protrusion.
For starters, we can cast aside the more magical properties. Natural world organisms don't purify lakes or heal wounded paladins with their bony growths. It's also tempting to cast aside the notion presented in Ridley Scott's 1985 film "Legend," that a unicorn's horn is "a single horn reaching straight to heaven," as if it functions as a sort of divine antenna.
Yet the idea of a horn antenna isn't that far-fetched. The most famous unicorn of the natural world is the narwhal (Monodon monoceros), though its lone tusk is actually an oversized canine tooth that grows straight out of the creature's face. The exact purpose of the tusk remains a topic of scientific dispute, but it might well function as a sense organ. After all, it's loaded with sensitive nerve endings.
Some researchers theorize that narwhals use the tusk to focus their potent echolocation powers, which they use to hunt fish, shrimp and squid. Martin Nweeia of Harvard's School of Dental Medicine also suggests that it could be used to detect changes in water salinity. Still, sensory explanations for narwhal tusks are undercut by the fact that females rarely grow them. As in most cases of sexual dimorphism, we can usually assume that mating plays a key role in the difference.
This brings us back to the unicorn. Perhaps only the males of the species boast horns as a means of sparring with romantic rivals or communicating sexual fitness to potential mates. The latter reason, at least, would jibe with the writings of fifth-century B.C.E. Greek historian Ctesias, in which the unicorn's horn is red, black and white. That sounds like a festive mating display to me.
The other obvious possibility is that they provide a means for the unicorn to defend itself against predators, such as human hunters or its legendary rival, the lion. This at least matches up with one of the creature's most closely associated real-world counterpart: the rhinoceros.
The unicorns of medieval European art were a docile-looking bunch, but older texts described a more fearsome creature. First-century Roman author Pliny the Elder wrote that a unicorn could not be taken alive, and others depicted it as a creature capable of besting lions. Sometimes, it would seem, the most violently obvious guess is the most valid.