Reproduction of the bank vole, Clethrionomys glareolus, in northern and southern Sweden during several seasons and in different phases of the vole population cycle

The breeding biology of the bank vole was studied in northern and southern populations in Sweden in the years 1973–76. Trapped specimens supplied organ weights and histological data, which were utilized to describe the breeding season, number and size of litters, reproductive losses, spermatogenesis, and sexual maturation. In the north the populations are cyclic. During the increase phase of the cycle the breeding season was long, litter sizes and gonad weights were large, and there was maturation of yearlings in the summer. During the peak phase reproduction was impaired in all these respects. In southern Sweden there was no cyclicity, and the reproductive events varied less, but in 1976 the litters were larger than usual and the season was much extended.
In the south the voles born during the early part of the season regularly became sexually mature during their first summer. The differences in reproductive biology are related to population dynamics and discussed in terms of different hypotheses concerning vole population cyclicity.