Social Organization of a Low Density Spring Population of the Bank Vole, Clethrionomys glareolus

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Abstract
Social organization of the bank vole, Clethrionomys glareolus Schreber, was studied in March and April in 1985 and 1986 near Uppsala, Sweden. Cohesion within groups, stable in size and composition, was demonstrated by live-trapping and radio-tracking and lasted from the cessation of reproduction in autumn until after the onset of breeding in the following spring. On average one third of the coordinate points in the study area used within groups was communally used by group members. The corresponding proportion of overlap between neighbouring voles belonging to different groups was less than 10%. Group members more often than was expected by chance overlapped at coordinates within commonly used areas. The corresponding overlap of individuals from different groups only occurred randomly. While no female vacancies were filled, male vacancies were rapidly filled by males without nest-mates after the disappearance or removal of the ordinary group male. As some groups are composed of only males and others by one male plus females, difference in initial male mating tactics is suggested, with males in the former group competing for any potential mating partner and those in the latter having access to their own nest-mates for the first mating of the season.