Abstract
The swimming ability of cotton rats and rice rats from several localities in Florida and Mississippi was compared. Rice rats freely swam a 2-m barrier separating two small “islands,” making better than 20 times the number of crossings made by cotton rats. Some cotton rats (26%) never swam the barrier during the 2-day test period, but all rice rats did. In general, cotton rats from Florida, especially ones from a barrier island, swam more frequently than did cotton rats from Mississippi. Rice rats from all localities were approximately equal in their tendency to swim. All crossings by rice rats and most by cotton rats were made during darkness. The fur of rice rats was more water repellent than that of cotton rats. Air trapped between hairs increased the buoyancy of the rice rats and reduced heat loss in water. Even with twice the body weight, cotton rats exhibited a significantly greater decrease in core body temperature when placed in 15°C water for 40 min than did rice rats. While swimming, both species used mainly the hind feet for propulsion and held the front feet close to the body. Rice rats swam faster than cotton rats, frequently swimming underwater for more than 10 m. Cotton rats never swam underwater, although they would momentarily submerge when they jumped from a platform. Semiaquatic adaptations appear to be important components of niche segregation in these sympatric rodents.
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