Beaver

Beavers are semi-aquatic rodent native to North America and Europe. They are the only living members of the family Castoridae, which contains a single genus, Castor. Genetic research has shown the European and North American beaver populations to be distinct species and that hybrid is unlikely. Beavers are best known for their natural trait of building dam in river and stream, and building their homes (known as beaver lodges) in the resulting pond. They are the second-largest rodent in the world (after the capybara). They are also known for their "danger signal": when startled or frightened, a swimming beaver will rapidly dive while forcefully slapping the water with its broad tail. This creates a loud 'slap', audible over large distances above and below water. This noise serves as a warning to other beavers in the area. Once a beaver has made this danger signal, all nearby beavers will dive and may not reemerge for some time. Although this happens rarely, a frightened beaver may... full article at wikipedia