Ratatoskr, the Squirrel in Yggdrasil

excerpt from the article "Creatures in Norse Mythology" via Life in Norway

Ratatoskr

Ratatoskr, drill-tooth or bore-tooth, is a squirrel who runs up and down Yggdrasil, the world tree, ferrying messages between the eagle Veðrfölnir, who perches atop Yggdrasil, and the serpent Níðhöggr, living beneath one of the three roots of the tree.

Most theories believe that this is to stir up hostilities between the two, spreading slander and gossip. In turn the two attack the tree, leading to a cycle of decay and rebirth that refreshed the tree.