Population turnover and wintering of the bank vole, Clethrionomys glareolus (Schreb.), in southern and central Finland

The material comprises 1 214 specimens snap-trapped in Finland, south of 65°N, over a period of 14 years. Though sexually mature males were trapped in January in certain years, no winter breeding has been established; the first litters are born in May, the last in September. After living through one winter, mature voles die at the beginning of the next; the longest-lived were trapped in mid-December. Of the voles which reached sexual maturity during the summer of their birth, the last were trapped the following March. Thus the vast majority of wintering bank voles are immature. The body length of immature bank voles increases slowly throughout the winter. Mature individuals, too, continue to grow in length after the breeding season. Their weight, on the other hand, drops almost to that of the immatures by the end of the breeding season. In winter, both age groups reach their minimum weight in February, and the last individuals that were reproducing the previous summer die soon after this. A lesser drop in weight occurs in November.