“We have no text to explain [this] for us,” said Gisli Sigurdsson, a professor in the department of folklore at the University of Iceland’s Magnusson Institute.

We will speculate about that horn in a bit, but first, a crash course in Norse mythology and the role a squirrel plays in it: The Viking age began around A.D. 800 and ended about 300 years later. During that time, Norsemen (and women) poured forth from Scandinavia, pillaging and colonizing their way across Britain, through the scattered islands of the North Atlantic, into Iceland and Greenland and venturing as far as North America.

Most of what we know about the stories Vikings told each other comes from Snorri Sturluson, who was an Icelandic poet and lawyer, a combination not quite so rare then as now. Snorri (1179-1241) was ambitious. He journeyed from Iceland to Norway to ingratiate himself with leaders there and pick up skills.

Snorri wrote something known as the “Prose Edda,” which collected various Norse myths and organized them so they could be learned and used by trainee poets. He described how the World Tree included such lands as Asgard, where gods lived, and Midgard — or Middle Earth — where humans lived. He mentioned such figures as Odin, Thor and Loki.

Sitting in the topmost branch of the World Tree is an eagle. At the bottom, gnawing on a root, is a monstrous snake. And scurrying up and down the tree, carrying messages between the eagle and the snake like a bushy-tailed Mercury, is Ratatoskr.

“It’s a curious thing,” said Jesse Byock, professor of Old Norse and medieval Scandinavian studies at UCLA. “It’s a minor character and it’s mentioned only once in the ‘Prose Edda,’ then in some of the poems. But it plays a strange and mysterious role connecting the heavens and the lower worlds.”

Of course, it makes sense that if you have a tree, you have a squirrel. What’s fascinating about the tree is that it isn’t planted in the ground. It floats through the sky.

For years, scholars wondered what that meant. Then in 1994, a medical doctor in Canada named Bjorn Jonsson presented his theory: The World Tree and its elements were astronomical. Its various features represented stars and planets in the night sky, each corresponding to a different god or character.

“It’s a funny game to play, but without Snorri by our side pointing things out, we are more or less shooting in the dark — except for the Milky Way,” Sigurdsson said. “If you are sitting in a hot tub on a dark winter’s night and someone is telling you that there is a World Tree holding up the sky, and it’s white and transparent like the membrane inside an eggshell, you would quickly focus on the Milky Way.”

Marvel’s Thor movies have brought Norse mythology to a larger audience, something Snorri would probably have understood.

“There is no such thing as the correct version of any myth,” Sigurdsson said. “Myths are constantly being reshaped and retold and reformulated to serve the purposes of the present. Snorri is writing his version for a particular purpose: as a textbook for poets. . . . Now you have Hollywood taking all these elements and motifs and shaking them in the bag once again for the purpose of moviemaking.”

Said Byock: “I’m a professor of Old Norse and an archaeologist. It was obscure when I started out. Now I’ve come into the mainstream here.”

But what about Ratatoskr’s horn?

Well, squirrels don’t live in Iceland. It’s likely whoever created that drawing had never seen one. Snorri probably saw them when he went to Norway, where red squirrels are common.

What’s common around Iceland are narwhals. The Icelandic artist may have used one of those sea creatures as inspiration. Or perhaps he simply felt Ratatoskr — the name means “drill tooth” — deserved some embellishment, making it as much unicorn as squirrel.

Snorri did his own embellishing. Earlier mentions of Ratatoskr described the squirrel as ferrying messages, nothing more, nothing less. But Snorri introduced another element: His Ratatoskr conveys not just information but insults and slanderous gossip.

Joked Sigurdsson: “Because of this message, Ratatoskr should really be a guardian squirrel of journalists.”

Now wait just a minute. Where did I put my hammer?

Tomorrow: Squirrel Week continues with the winners of the Squirrel Week Squirrel Photography Contest.