Flexi Says: Rodents are gnawing animals that include beavers, mice, and squirrels. They have a single pair of continuously growing incisors (teeth) in each of the upper and lower jaws that must be kept short by gnawing. They can use their teeth to grind the wood. This action mechanically breaks down wood into smaller pieces. However, saprotrophic fungi are the only organisms that can biochemically decompose wood.