Demography of a Cyclic Population of the Bank Vole Clethrionomys glareolus

Body weights of overwintered males of Clethrionomys glareolus (Schreber) were lowest during the year of increase and highest during the peak year. These weight changes showed a relationship with variations in the commencement of breeding. All animals born during the spring and summer of the year of increase became mature during the season of their birth whereas during the pre-peak and peak years there was a suppression of maturation in all males and most females of the year. During the peak year, breeding commenced late and terminated early, and the crash which followed was complete by January. The late summer cohort survived winter best and accounted for the bulk of the overwintered population.