Beavers are mammals. Mammals cannot utilize the cellulose found in trees, at least not directly. But beavers eat a lot of bark and wood--how do they obtain nourishment? The answer is that they use the same indirect methods that cows and other ruminants do, though they don't have a huge fermentation vat (the rumen), that cows do. The beaver has bacteria living in its digestive tract that DO eat the cellulose; the beaver then digests the bacteria, obtaining nutrition that way. They also have an adaptation common in animals such as rabbits--they eat their food twice. The food passes through the digestive tract, where it picks up the bacteria. The beaver then eats the food for a second time. So, gross, yes, but the beaver eats its poop.