In Norse mythology, Ratatoskr is a squirrel who runs up and down the world tree Yggdrasil to carry messages between Hraesvelger, the eagle perched atop Yggdrasil, and the wyrm Nidhoggr, who dwells beneath one of the three roots of the tree. Ratatoskr is attested in the Poetic Edda, compiled in the 13th century from earlier traditional sources, and the Prose Edda, written in the 13th century by Snorri Sturluson. Scholars have proposed theories about the implications of the squirrel.