RYBCZYNSKI, Natalia*; FISH , F; MCLELLAN, WA; PABST, DA; Canadian Museum of Nature, Ottawa; West Chester University, PA ; University of North Carolina – Wilmington, NC; University of North Carolina – Wilmington, NC: THE BEAVER TAIL: FUNCTION IN SWIMMING AND CONNECTIVE-TISSUE STRUCTURE

The beaver (Castor canadensis) is more specialized for swimming than any other living rodent and represents a good model for investigating questions pertaining to the evolutionary transition from terrestrial to aquatic habits. Aquatic specializations in the beaver include a streamlined body, enlarged webbed hind-feet, and a specialized tail. The tail is comprised of a muscular base and a distal �paddle�, which is dorsoventrally flattened and possesses a scaly integument. Most of the �paddle� is composed of fibrous connective-tissues embedded in a “matrix” of adipose tissue. Lateral to the caudal vertebrae crossed structural fibers are attached to the dorsal and ventral dermis, and are oriented between 90 and 45 ^o to the horizontal. Previous workers have suggested that crossed-array patterns of connective fibers are associated with structures that exhibit stereotyped movements. Stereotyped movements by the beaver tail during swimming can occur, but have not been described as few studies have examined swimming by this aquatic rodent. Observations of two captive beavers found that the tail is not held rigidly during swimming, but undulates. Posteriorly traveling waves in the tail were observed as the animals were simultaneously stroking with the hind feet. The waves moved on average 72% faster than the forward velocity of the animal, indicating generation of thrust by the broad tail (N=25, Standard deviation=55). These findings are consistent with the hypothesis that the beaver tail is specialized to produce undulatory propulsion. The addition of undulatory propulsion to hind foot paddling is consistent with proposed evolutionary stages associated with transition to fully aquatic specializations in mammals.