Reproduction of the root vole (Microtus oeconomus) at the edge of its distribution range

Data on the reproduction of the root vole (Microtus oeconomus) in Lithuania, the margin of its geographic range, are presented in the context of litter size analysis. In Lithuania, the breeding season lasts from April until the end of October. The average litter size was 5.93 +/- 0.11 (2-14), decreasing from a mean of 8.0 embryos in June to 4.3 in October, with 2-3 litters per year. The average body mass of breeding females was 40.4 +/- 0.6 (16.5-77.0) g. However, 6.25% of females start breeding with a body mass of under 25 g. Litter size was found to increase from south to north in the geographical distribution range of the species and is described by the regression Litter size = 0.11147 x latitude - 0.2646 (R-2 = 0.79). The small litter size and the comparatively small number of litters in the Lithuanian population indicate that reproduction is similar to that of other populations in the southern part of the geographical range of the species.