Home range size and overlap in female root voles: effects of season and density

In small mammals living in highly seasonal environments, observational studies show that female home range size and exclusiveness are smaller in the nonbreeding winter season than in the breeding summer season. This has led to the notion that nonbreeding females are more social and decrease territorial behavior during winter. However, because territoriality decreases with increasing population density, and density normally increases during the breeding season, the effects of density and season on social structure are usually confounded. To find out which of the 2 factors explains space use, we experimentally established 3 high-density and 3 low-density root vole (Microtus oeconomus) populations in late spring and monitored the populations into the nonbreeding winter season. Population sizes were controlled throughout the breeding period to minimize seasonal variation in density. Home range sizes were larger in founder females than in field-born females but did not change with season or density. Area exclusively used by individual females was lower in winter than summer, and founder females decreased exclusiveness as density increased. We argue that this seasonal pattern of space use might be caused by variation in benefits of group living, whereas founder females also responded to density-dependent competition by reducing area exclusively used.