In Norse mythology Ratatoskr (in Old Norse: "piercing tooth" or "drilling tooth") is a squirrel that runs up and down the Yggdrasil world tree carrying messages between the nameless eagle perched in the canopy and the dragon Nidhogg, who resides under one of the tree's three branches.
An account of Ratatoskr is given in the poetic Edda, compiled in the 13th century from earlier traditional sources, and in the prosaic Edda, written in the same century by Snorri Sturluson. Various scholars have proposed theories about the implications of the squirrel.
The name Ratatoskr is formed by two elements: rata- and -tosk. The latter is mostly estimated to mean "tusk". Guðbrandur Vigfússon theorizes that the element rati- means "the traveler". Vigfússon claims that the name of the legendary drill Rati could feature the same term. According to Vigfússon, Ratatoskr means "the traveling fang" or "the climbing fang."
Sophus Bugge in turn theorizes that the name Ratatösk is a loan from Old English meaning "Rat-tooth", literally "rat-tooth". This theory is based on the fact that the -tosk element of the compound appears nowhere else in Old Norse.
Bugge proposes that the said element is a reformation of the Old English word tūsc (Old Frisian tusk) and that, instead, the Rata-element represents the Old English word ræt ("rat") as well.
According to Albert Sturtevant, "as far as the Rata-element is concerned, Bugge's hypothesis has no valid foundation in view of the fact that the word [in Old Norse] Rata (form of Rati*) is used in Hávamál (106, 1) to name the instrument used by Odin to pierce through the rocks in search of the mead of the poets [. ...]", and that "Rati* must be considered a native word [in Old Norse] meaning "the driller, rodent" [...]".
Sturtevant explains that Bugge's theory of the -tosk element seems to be supported by the fact that the word appears nowhere else in Old Norse.
He takes issue with this theory, however, as he claims that the Old Norse proper name Tunne (derived from the Protonordic *Tunþē) refers to "a person who is characterized as bearing a peculiar kind of tooth," and speculates a Proto-Germanic form of -tosk.
Sturtevant concludes that "the fact that the word occurs only in the name Rata-toskr is not valid evidence against this assumption, since there are many hapax legomena of native origin [in Old Norse], as proven in modern Scandinavian dialect equivalents."
Modern scholars have accepted this etymology, leaving the meaning of Ratatoskr as "drilling tooth," as Jesse Byock, Andy Orchard, and Rudolf Simek, or "drilling tooth," as John Lindow.
According to Rudolf Simek, "the squirrel probably represents only a decorative detail of the mythological image of the world ash tree in Grímnismál".
H. R. Ellis Davidson, describing the world tree, states that the squirrel also gnawed it, which continues the cycle of destruction and reconstruction, and proposes the tree as a symbolization of constant existential change.
John Lindow notes that Yggdrasil is described as rotten on one side and in turn chewed by four deer and by Nidhogg, and that, according to Gylfaginning's account, it also endures verbal hostility from the fauna it supports.
Lindow adds that "in the sagas, a person who helps stoke and keep alive a feud by carrying malicious words between the sides is rarely someone of high status, which would explain why a relatively insignificant animal is assigned this role in mythology."
Richard W. Thorrington Jr. and Katie Ferrell in turn propose that "Ratatoskr's role probably derived from the habit of European tree squirrels (Sciurus vulgaris) to have a confrontational response to danger. It doesn't take much imagination to think that the squirrel is saying nasty things about you."
