Frightened or surprised, a beaver (Castor canadensis) will customarily slap its powerful tail on the surface of the water in close proximity to an intruder. The behavior startles (and soaks) a potential predator, and also warns other beavers, who respond to the sound by swimming towards deep waters or retreating into lodges. The signaling beaver then commonly resurfaces to escort an intruder off the lake with more slapping.
Kits as young as 2 weeks try repeatedly and unsuccessfully to make a slapping sound with their tiny tails when imitating their mothers. When they finally succeed they seem to startle themselves.