Consistency and change in bank vole (Clethrionomys glareolus) home ranges across a decade
Minimum distance moved (MDM) and home-range sizes were studied in overwintering bank voles (Clethrionomys glareolus) during March and April in 1985, 1986, and 1996 near Uppsala, Sweden. In 1985 and 1986, the average snow depth was 50 and 35 cm in March and 7 and 18 cm in April, respectively. In 1996 the snow depth was only 16 cm in March and 2 cm in April. Bank voles were found to have winter nests at exactly the same grid coordinates in 1985, 1986, and 1996. Among females there was a high degree of consistency in home-range sizes, both between months and between years. Daily MDM of females was shorter in March 1996 than in 1985 and 1986, while no significant difference occurred during April. Males' home-range sizes, on the other hand, increased significantly from March to April in all 3 years, i.e., at the onset of the mating period. There was no significant difference in males' home-range sizes in March across years, but in April, home ranges were significantly larger 1985 and 1986 than in 1996. Males' average home-range sizes in April showed a positive correlation with snow depth. MDM of males in both March and April was also significantly higher during the snowy years of 1985 and 1986 than in 1996, when there was much less snow. These observations suggest that adequate snow cover is a favourable factor, allowing the males to expand their home ranges, while lack of snow or a thin, patchy snow cover prevents them from travelling longer distances during the mating period and consequently from visiting a large number of potential mates. These differences in the behaviour of male bank voles between snow-rich and snow-poor years may offer a partial explanation of the hypothetical connection between the less severe winter conditions during the past decade and the decline in population density and cyclicity in boreal bank vole populations.