Evaluation of swimming ability as a means of island invasion by small mammals in coastal Virginia

Small mammals were live-trapped on the mainland and on an island located in the southeast corner of Virginia. White-footed mice (Peromyscus leucopus) were trapped on the mainland, but not on the island, whereas meadow voles (Microtus pennsylvanicus) inhabited both the island and mainland areas located to the south and west of the island. A series of swimming tests was performed on both of these species in both field and laboratory to determine if swimming abilities could be a factor preventing P. leucopus from invading the island. Both species of small mammals showed good swimming endurance in water of 30C, but meadow voles had significantly better swimming endurance in water of 20 and 10C than did white-footed mice. The pelage of M. pennsylvanicus repelled water better in water of 30 and 20C than did that of P. leucopus. The repellent properties of the fur of meadow voles may be due in part to their grooming habits. After being removed from water, meadow voles began grooming more quickly than did white-footed mice. Ninety-five % of M. pennsylvanicus tested voluntarily crossed an artificial water barrier, whereas only 50% of P. leucopus crossed the barrier of their own volition. When released 200 m from shore, meadow voles swam directly to land; however, only a few white-footed mice could orient toward land from a minimal distance of 50 m from shore. The comparatively poor swimming ability of P. leucopus is thought to be an important factor accounting for its meager representation on islands in North America.