Function of the Compressed Tail of Surface Swimming Muskrats (Ondatra zibethicus)

The movement and function of the laterally flattened tail of surface swimming muskrats (Ondatra zibethicus) were studied by forcing individual animals to swim against a constant water flow of velocity ranging from 0.2 to 0.75 m/s. Simultaneous lateral and ventral views of the swimming muskrats were filmed. During swimming the tail produces a series of traveling waves that moved posteriorly with increasing amplitude at a velocity faster than the muskrat was swimming. The characteristics of the traveling waves were analogous to the body and tail undulations observed in the swimming of anguilliform fish. Calculations showed that the tail provided a maximum of 1.4% of the total thrust power necessary to propel the muskrat at 0.75 m/s. Amputation of a muskrat tail causes the animal to yaw while swimming. The compressed tail of the muskrat may allow for the effective generation of thrust in surface swimming by producing small amounts of thrust in conjunction with the prevention of yaw.