This is a beautiful example of classic Stretch and Squash. The beaver opens his mouth extremely wide as he holds the top part of Lady's muzzle right between his upper and lower teeth. This is top notch staging because you want the audience to focus on the upcoming bite. The beaver's stretched open mouth is an anticipation in animation terms, but the viewer is also left anticipating and wondering if the bite will be successful. And you guessed it, the animator is Milt Kahl.
To see a few rough animation drawings of this character by Eric Larson, go here: