Bank vole, Myodes (Clethrionomys) glareolus, serves as an object of research in many scientific disciplines. Both wild and laboratory-reared animals are used in such studies but the latter, contrary to the wild individuals, reproduce throughout the year. The aim of this study was to investigate the effect of the season outside the breeding chambers on the reproductive characteristics of bank voles kept under constant, optimal breeding conditions for 30 years. The comparison was made between summer, a breeding season, and winter, a non-breeding season, in four consecutive years.

Significant differences in reproduction and development were observed. More females gave birth in summer than in winter. The number of pups born were similar in both seasons but more pups survived the first day of life in summer than in winter. Moreover, more young survived to the time of weaning (19 days) and reached higher level of body weight in summer than in winter. Season influenced the rate of morphological development of the reproductive tract of weaned males and those individuals born in summer had significantly heavier testes than those born in winter. Differences in sexual maturation were observed also in 6 week old males. Sperm concentration, as well as the proportion of viable sperm, motile sperm and not swollen sperm were higher in males born in summer. Our results may have important implications for bank vole breeders and scientists working on other captive bred rodents.