Emigration to new habitats by voles: the cost of dispersal paradox

We tested the hypothesis that dispersal and philopatry are components of a mixed evolutionarily stable strategy (ESS). The hypothesis predicts that fitness of dispersers should be equal to that of philopatric individuals. Alternatively, fitness of dispersers could be lower (the resident fitness hypothesis) or greater (the cost of dispersal hypothesis) than that of philopatric individuals. We compared fitness of individuals that moved to new habitats (emigrants) and those that remained within habitat boundaries (residents) in populations of the prairie vole, Microtus ochrogaster, and the meadow vole, M. pennsylvanicus. We established vole populations in four enclosures (66 * 78 m). Within each enclosure, voles were free to move between four types of habitats that varied in the availability of supplemental food and the amount of vegetative cover. We analysed two fitness components: the survival rates of all individuals, and pregnancy rates of females. Our study showed that emigrants generally had greater fitness than residents and that the difference in fitness was habitat dependent (i.e. was greater when individuals were emigrating from low-quality habitats than from high-quality habitats). High-food, highcover habitats were the only habitat types for which fitness of emigrants was lower than that of residents. Similar patterns occurred in both prairie voles and meadow voles. Our results support the cost of dispersal hypothesis.